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Configuring basic system settings

Red Hat Enterprise Linux 8

A guide to configuring basic system settings in Red Hat Enterprise Linux 8

Red Hat Customer Content Services

Abstract

This document describes basics of system administration on Red Hat Enterprise Linux 8. The title focuses on: basic tasks that a system administrator needs to do just after the operating system has been successfully installed, installing software with yum, using systemd for service management, managing users, groups and file permissions, using chrony to configure NTP, working with Python 3 and others.

Preface

Chapter 1. Getting started with system administration

The following sections provide an overview of basic administration tasks on the installed system.

Note

The following basic administration tasks may include items that are usually done already during the installation process, but they do not have to be done necessarily, such as the registration of the system. The sections dealing with such tasks provide a summary of how you can achieve the same goals during the installation.

For information on Red Hat Enterprise Linux installation, see Performing a standard RHEL installation.

Although you can perform all post-installation tasks through the command line, you can also use the RHEL 8 web console to perform some of them.

1.1. Getting started using the RHEL web console

Install the web console in Red Hat Enterprise Linux 8 and learn how to add remote hosts and monitor them in the RHEL 8 web console.

Prerequisites

1.1.1. What is the RHEL web console

The RHEL web console is a Red Hat Enterprise Linux 8 web-based interface designed for managing and monitoring your local system, as well as Linux servers located in your network environment.

cockpit overview page PF4

The RHEL web console enables you a wide range of administration tasks, including:

  • Managing services
  • Managing user accounts
  • Managing and monitoring system services
  • Configuring network interfaces and firewall
  • Reviewing system logs
  • Managing virtual machines
  • Creating diagnostic reports
  • Setting kernel dump configuration
  • Configuring SELinux
  • Updating software
  • Managing system subscriptions

The RHEL web console uses the same system APIs as you would in a terminal, and actions performed in a terminal are immediately reflected in the RHEL web console.

You can monitor the logs of systems in the network environment, as well as their performance, displayed as graphs. In addition, you can change the settings directly in the web console or through the terminal.

1.1.2. Installing and enabling the web console

To access the RHEL 8 web console, first enable the cockpit.socket service.

Red Hat Enterprise Linux 8 includes the RHEL 8 web console installed by default in many installation variants. If this is not the case on your system, install the cockpit package before enabling the cockpit.socket service.

Procedure

  1. If the web console is not installed by default on your installation variant, manually install the cockpit package:

    # yum install cockpit
  2. Enable and start the cockpit.socket service, which runs a web server:

    # systemctl enable --now cockpit.socket
  3. If the web console was not installed by default on your installation variant and you are using a custom firewall profile, add the cockpit service to firewalld to open port 9090 in the firewall:

    # firewall-cmd --add-service=cockpit --permanent
    # firewall-cmd --reload

Verification steps

  1. To verify the previous installation and configuration, open the web console.

1.1.3. Logging in to the web console

Use the steps in this procedure for the first login to the RHEL web console using a system user name and password.

Prerequisites

  • Use one of the following browsers for opening the web console:

    • Mozilla Firefox 52 and later
    • Google Chrome 57 and later
    • Microsoft Edge 16 and later
  • System user account credentials

    The RHEL web console uses a specific PAM stack located at /etc/pam.d/cockpit. Authentication with PAM allows you to log in with the user name and password of any local account on the system.

Procedure

  1. Open the web console in your web browser:

    • Locally: https://localhost:9090
    • Remotely with the server’s hostname: https://example.com:9090
    • Remotely with the server’s IP address: https://192.0.2.2:9090

      If you use a self-signed certificate, the browser issues a warning. Check the certificate and accept the security exception to proceed with the login.

      The console loads a certificate from the /etc/cockpit/ws-certs.d directory and uses the last file with a .cert extension in alphabetical order. To avoid having to grant security exceptions, install a certificate signed by a certificate authority (CA).

  2. In the login screen, enter your system user name and password.

    cockpit login page PF4

  3. Optionally, click the Reuse my password for privileged tasks option.

    If the user account you are using to log in has sudo privileges, this makes it possible to perform privileged tasks in the web console, such as installing software or configuring SELinux.

  4. Click Log In.

After successful authentication, the RHEL web console interface opens.

1.1.4. Connecting to the web console from a remote machine

It is possible to connect to your web console interface from any client operating system and also from mobile phones or tablets.

Prerequisites

  • Device with a supported internet browser, such as:

    • Mozilla Firefox 52 and later
    • Google Chrome 57 and later
    • Microsoft Edge 16 and later
  • RHEL 8 server you want to access with an installed and accessible web console. For more information about the installation of the web console see Installing the web console.

Procedure

  1. Open your web browser.
  2. Type the remote server’s address in one of the following formats:

    1. With the server’s host name: server.hostname.example.com:port_number
    2. With the server’s IP address: server.IP_address:port_number
  3. After the login interface opens, log in with your RHEL machine credentials.

1.1.5. Logging in to the web console using a one-time password

If your system is part of an Identity Management (IdM) domain with enabled one-time password (OTP) configuration, you can use an OTP to log in to the RHEL web console.

Important

It is possible to log in using a one-time password only if your system is part of an Identity Management (IdM) domain with enabled OTP configuration. For more information about OTP in IdM, see One-time password in Identity Management.

Prerequisites

Procedure

  1. Open the RHEL web console in your browser:

    • Locally: https://localhost:PORT_NUMBER
    • Remotely with the server hostname: https://example.com:PORT_NUMBER
    • Remotely with the server IP address: https://EXAMPLE.SERVER.IP.ADDR:PORT_NUMBER

      If you use a self-signed certificate, the browser issues a warning. Check the certificate and accept the security exception to proceed with the login.

      The console loads a certificate from the /etc/cockpit/ws-certs.d directory and uses the last file with a .cert extension in alphabetical order. To avoid having to grant security exceptions, install a certificate signed by a certificate authority (CA).

  2. The Login window opens. In the Login window, enter your system user name and password.
  3. Generate a one-time password on your device.
  4. Enter the one-time password into a new field that appears in the web console interface after you confirm your password.
  5. Click Log in.
  6. Successful login takes you to the Overview page of the web console interface.

1.1.6. Restarting the system using the web console

You can use the web console to restart a RHEL system that the web console is attached to.

Prerequisites

Procedure

  1. Log into the RHEL 8 web console.

    For details, see Logging in to the web console.

  2. Click Overview.
  3. Click the Restart restart button.

    cockpit system restart pf4

  4. If any users are logged into the system, write a reason for the restart in the Restart dialog box.
  5. Optional: In the Delay drop down list, select a time interval.

    cockpit restart delay pf4

  6. Click Restart.

1.1.7. Shutting down the system using the web console

You can use the web console to shut down a RHEL system that the web console is attached to.

Prerequisites

Procedure

  1. Log into the RHEL 8 web console.

    For details, see Logging in to the web console.

  2. Click Overview.
  3. In the Restart drop down list, select Shut Down.

    cockpit system shutdown pf4

  4. If any users are logged in to the system, write a reason for the shutdown in the Shut Down dialog box.
  5. Optional: In the Delay drop down list, select a time interval.
  6. Click Shut Down.

1.1.8. Configuring time settings using the web console

You can set a time zone and synchronize the system time with a Network Time Protocol (NTP) server.

Prerequisites

Procedure

  1. Log in to the RHEL 8 web console.

    For details, see Logging in to the web console.

  2. Click the current system time in Overview.

    cockpit time settings pf4

  3. In the Change System Time dialog box, change the time zone if necessary.
  4. In the Set Time drop down menu, select one of the following:

    Manually
    Use this option if you need to set the time manually, without an NTP server.
    Automatically using NTP server
    This is a default option, which synchronizes time automatically with the preset NTP servers.
    Automatically using specific NTP servers
    Use this option only if you need to synchronize the system with a specific NTP server. Specify the DNS name or the IP address of the server.
  5. Click Change.

    cockpit time change pf4

Verification steps

  • Check the system time displayed in the System tab.

1.1.9. Joining a RHEL 8 system to an IdM domain using the web console

You can use the web console to join the Red Hat Enterprise Linux 8 system to the Identity Management (IdM) domain.

Prerequisites

  • The IdM domain is running and reachable from the client you want to join.
  • You have the IdM domain administrator credentials.

Procedure

  1. Log into the RHEL web console.

    For details, see Logging in to the web console.

  2. Open the System tab.
  3. Click Join Domain.

    idm cockpit join domain

  4. In the Join a Domain dialog box, enter the host name of the IdM server in the Domain Address field.
  5. In the Authentication drop down list, select if you want to use a password or a one-time password for authentication.

    idm cockpit join psswd

  6. In the Domain Administrator Name field, enter the user name of the IdM administration account.
  7. In the password field, add the password or one-time password according to what you selected in the Authentication drop down list earlier.
  8. Click Join.

    idm cockpit join

Verification steps

  1. If the RHEL 8 web console did not display an error, the system has been joined to the IdM domain and you can see the domain name in the System screen.
  2. To verify that the user is a member of the domain, click the Terminal page and type the id command:

    $ id
    euid=548800004(example_user) gid=548800004(example_user) groups=548800004(example_user) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

1.1.10. Disabling SMT to prevent CPU security issues using the web console

Disable Simultaneous Multi Threading (SMT) in case of attacks that misuse CPU SMT. Disabling SMT can mitigate security vulnerabilities, such as L1TF or MDS.

Important

Disabling SMT might lower the system performance.

Prerequisites

Procedure

  1. Log in to the RHEL 8 web console.

    For details, see Logging in to the web console.

  2. Click System.
  3. In the Hardware item, click the hardware information.

    cockpit smt hardware

  4. In the CPU Security item, click Mitigations.

    If this link is not present, it means that your system does not support SMT, and therefore is not vulnerable.

  5. In the CPU Security Toggles, switch on the Disable simultaneous multithreading (nosmt) option.

    cockpit smt disable

  6. Click on the Save and reboot button.

After the system restart, the CPU no longer uses SMT.

1.1.11. Adding a banner to the login page

Companies or agencies sometimes need to show a warning that usage of the computer is for lawful purposes, the user is subject to surveillance, and anyone trespassing will be prosecuted. The warning must be visible before login. Similarly to SSH, the web console can optionally show the content of a banner file on the login screen. To enable banners in your web console sessions, you need to modify the /etc/cockpit/cockpit.conf file. Note that the file is not required and you may need to create it manually.

Prerequisites

Procedure

  1. Create the /etc/issue.cockpit file in a text editor of your preference if you do not have it yet. Add the content you want to display as the banner to the file.

    Do not include any macros in the file as there is no re-formatting done between the file content and the displayed content. Use intended line breaks. It is possible to use ASCII art.

  2. Save the file.
  3. Open or create the cockpit.conf file in the /etc/cockpit/ directory in a text editor of your preference.

    $ sudo vi cockpit.conf
  4. Add the following text to the file:

    [Session]
    Banner=/etc/issue.cockpit
  5. Save the file.
  6. Restart the web console for changes to take effect.

    # systemctl try-restart cockpit

Verification steps

  • Open the web console login screen again to verify that the banner is now visible.

Example 1.1. Adding an example banner to the login page

  1. Create an /etc/issue.cockpit file with a desired text using a text editor:

    This is an example banner for the RHEL web console login page.
  2. Open or create the /etc/cockpit/cockpit.conf file and add the following text:

    [Session]
    Banner=/etc/issue.cockpit
  3. Restart the web console.
  4. Open the web console login screen again.

    cockpit login page banner

1.1.12. Configuring automatic idle lock in the web console

By default, there is no idle timeout set in the web console interface. If you wish to enable an idle timeout on your system, you can do so by modifying the /etc/cockpit/cockpit.conf configuration file. Note that the file is not required and you may need to create it manually.

Prerequisites

Procedure

  1. Open or create the cockpit.conf file in the /etc/cockpit/ directory in a text editor of your preference.

    $ sudo vi cockpit.conf
  2. Add the following text to the file:

    [Session]
    IdleTimeout=X

    Substitute X with a number for a time period of your choice in minutes.

  3. Save the file.
  4. Restart the web console for changes to take effect.

    # systemctl try-restart cockpit

Verification steps

  • Check if the session logs you out after a set period of time.

1.2. Configuring the host name in the web console

Learn how to use the RHEL 8 web console to configure different forms of the host name on the system that the web console is attached to.

1.2.1. Host name

The host name identifies the system. By default, the host name is set to localhost, but you can change it.

A host name consists of two parts:

Host name
It is a unique name which identifies a system.
Domain
Add the domain as a suffix behind the host name when using a system in a network and when using names instead of just IP addresses.

A host name with an attached domain name is called a fully qualified domain name (FQDN). For example: mymachine.example.com.

Host names are stored in the /etc/hostname file.

1.2.2. Pretty host name in the web console

You can configure a pretty host name in the RHEL web console. The pretty host name is a host name with capital letters, spaces, and so on.

The pretty host name displays in the web console, but it does not have to correspond with the host name.

Example 1.2. Host name formats in the web console

Pretty host name
My Machine
Host name
mymachine
Real host name - fully qualified domain name (FQDN)
mymachine.idm.company.com

1.2.3. Setting the host name using the web console

This procedure sets the real host name or the pretty host name in the web console.

Prerequisites

Procedure

  1. Log into the RHEL 8 web console.

    For details, see Logging in to the web console.

  2. Click Overview.
  3. Click edit next to the current host name.

    cockpit hostname pf4

  4. In the Change Host Name dialog box, enter the host name in the Pretty Host Name field.
  5. The Real Host Name field attaches a domain name to the pretty name.

    You can change the real host name manually if it does not correspond with the pretty host name.

  6. Click Change.

    cockpit hostname change pf4

Verification steps

  1. Log out from the web console.
  2. Reopen the web console by entering an address with the new host name in the address bar of your browser.

    cockpit hostname change verify pf4

1.3. Red Hat web console add-ons

Install add-ons in the RHEL 8 web console and learn what add-on applications are available for you.

1.3.1. Installing add-ons

The cockpit package is a part of Red Hat Enterprise Linux 8 by default. To be able to use add-on applications you must install them separately.

Prerequisites

  • Installed and enabled cockpit package. If you need to install web console first, check the installation section.

Procedure

  • Install an add-on.

    # yum install <add-on>

1.3.2. Add-ons for the RHEL 8 web console

The following table lists available add-on applications for the RHEL 8 web console.

Feature namePackage nameUsage

Composer

cockpit-composer

Building custom OS images

Dashboard

cockpit-dashboard

Managing multiple servers in one UI

Machines

cockpit-machines

Managing libvirt virtual machines

PackageKit

cockpit-packagekit

Software updates and application installation (usually installed by default)

PCP

cockpit-pcp

Persistent and more fine-grained performance data (installed on demand from the UI)

podman

cockpit-podman

Managing podman containers (available from RHEL 8.1)

Session Recording

cockpit-session-recording

Recording and managing user sessions

1.4. Optimizing the system performance using the web console

Learn how to set a performance profile in the RHEL 8 web console to optimize the performance of the system for a selected task.

1.4.1. Performance tuning options in the web console

Red Hat Enterprise Linux 8 provides several performance profiles that optimize the system for the following tasks:

  • Systems using the desktop
  • Throughput performance
  • Latency performance
  • Network performance
  • Low power consumption
  • Virtual machines

The tuned service optimizes system options to match the selected profile.

In the web console, you can set which performance profile your system uses.

Additional resources

1.4.2. Setting a performance profile in the web console

This procedure uses the web console to optimize the system performance for a selected task.

Prerequisites

Procedure

  1. Log into the RHEL 8 web console.

    For details, see Logging in to the web console.

  2. Click Overview.
  3. In the Performance Profile field, click the current performance profile.

    cockpit performance profile pf4

  4. In the Change Performance Profile dialog box, change the profile if necessary.
  5. Click Change Profile.

    cockpit performance profile change pf4

Verification steps

  • The Overview tab now shows the selected performance profile.

1.5. Getting started with RHEL System Roles

This section explains what RHEL System Roles are. Additionally, it describes how to apply a particular role through an Ansible playbook to perform various system administration tasks.

1.5.1. Introduction to RHEL System Roles

RHEL System Roles is a collection of Ansible roles and modules. RHEL System Roles provide a configuration interface to remotely manage multiple RHEL systems. The interface enables managing system configurations across multiple versions of RHEL, as well as adopting new major releases.

On Red Hat Enterprise Linux 8, the interface currently consists of the following roles:

  • kdump
  • network
  • selinux
  • storage
  • certificate
  • kernel_settings
  • logging
  • metrics
  • nbde_client and nbde_server
  • timesync
  • tlog

All these roles are provided by the rhel-system-roles package available in the AppStream repository.

Additional resources

1.5.2. RHEL System Roles terminology

You can find the following terms across this documentation:

System Roles terminology

Ansible playbook
Playbooks are Ansible’s configuration, deployment, and orchestration language. They can describe a policy you want your remote systems to enforce, or a set of steps in a general IT process.
Control node
Any machine with Ansible installed. You can run commands and playbooks, invoking /usr/bin/ansible or /usr/bin/ansible-playbook, from any control node. You can use any computer that has Python installed on it as a control node - laptops, shared desktops, and servers can all run Ansible. However, you cannot use a Windows machine as a control node. You can have multiple control nodes.
Inventory
A list of managed nodes. An inventory file is also sometimes called a “hostfile”. Your inventory can specify information like IP address for each managed node. An inventory can also organize managed nodes, creating and nesting groups for easier scaling. To learn more about inventory, see the Working with Inventory section.
Managed nodes
The network devices, servers, or both that you manage with Ansible. Managed nodes are also sometimes called “hosts”. Ansible is not installed on managed nodes.

1.5.3. Applying a role

The following procedure describes how to apply a particular role.

Prerequisites

  • The rhel-system-roles package is installed on the system that you want to use as a control node:

    # yum install rhel-system-roles
  • The Ansible Engine repository is enabled, and the ansible package is installed on the system that you want to use as a control node. You need the ansible package to run playbooks that use RHEL System Roles.

    • If you do not have a Red Hat Ansible Engine Subscription, you can use a limited supported version of Red Hat Ansible Engine provided with your Red Hat Enterprise Linux subscription. In this case, follow these steps:

      1. Enable the RHEL Ansible Engine repository:

        # subscription-manager refresh
        # subscription-manager repos --enable ansible-2-for-rhel-8-x86_64-rpms
      2. Install Ansible Engine:

        # yum install ansible
    • If you have a Red Hat Ansible Engine Subscription, follow the procedure described in How do I Download and Install Red Hat Ansible Engine?.
  • You are able to create an Ansible playbook.

    Playbooks represent Ansible’s configuration, deployment, and orchestration language. By using playbooks, you can declare and manage configurations of remote machines, deploy multiple remote machines or orchestrate steps of any manual ordered process.

    A playbook is a list of one or more plays. Every play can include Ansible variables, tasks, or roles.

    Playbooks are human-readable, and they are expressed in the YAML format.

    For more information about playbooks, see Ansible documentation.

Procedure

  1. Create an Ansible playbook including the required role.

    The following example shows how to use roles through the roles: option for a given play:

    ---
    - hosts: webservers
      roles:
         - rhel-system-roles.network
         - rhel-system-roles.timesync

    For more information on using roles in playbooks, see Ansible documentation.

    See Ansible examples for example playbooks.

    Note

    Every role includes a README file, which documents how to use the role and supported parameter values. You can also find an example playbook for a particular role under the documentation directory of the role. Such documentation directory is provided by default with the rhel-system-roles package, and can be found in the following location:

    /usr/share/doc/rhel-system-roles/SUBSYSTEM/

    Replace SUBSYSTEM with the name of the required role, such as selinux, kdump, network, timesync, or storage.

  2. Execute the playbook on targeted hosts by running the ansible-playbook command:

    # ansible-playbook -i name.of.the.inventory name.of.the.playbook

    An inventory is a list of systems against which Ansible works. For more information on how to create and inventory, and how to work with it, see Ansible documentation.

    If you do not have an inventory, you can create it at the time of running ansible-playbook:

    If you have only one targeted host against which you want to run the playbook, use:

    # ansible-playbook -i host1, name.of.the.playbook

    If you have multiple targeted hosts against which you want to run the playbook, use:

    # ansible-playbook -i host1,host2,....,hostn name.of.the.playbook

Additional resources

  • For more detailed information on using the ansible-playbook command, see the ansible-playbook man page.

1.5.4. Additional resources

1.6. Changing basic environment settings

Configuration of basic environment settings is a part of the installation process. The following sections guide you when you change them later. The basic configuration of the environment includes:

  • Date and time
  • System locales
  • Keyboard layout
  • Language

1.6.1. Configuring the date and time

Accurate timekeeping is important for a number of reasons. In Red Hat Enterprise Linux, timekeeping is ensured by the NTP protocol, which is implemented by a daemon running in user space. The user-space daemon updates the system clock running in the kernel. The system clock can keep time by using various clock sources.

Red Hat Enterprise Linux 8 uses the chronyd daemon to implement NTP. chronyd is available from the chrony package. For more information, see Using the chrony suite to configure NTP.

1.6.1.1. Displaying the current date and time

To display the current date and time, use either of these steps.

Procedure

  1. Enter the date command:

    $ date
    Mon Mar 30 16:02:59 CEST 2020
  2. To see more details, use the timedatectl command:

    $ timedatectl
    Local time: Mon 2020-03-30 16:04:42 CEST
    Universal time: Mon 2020-03-30 14:04:42 UTC
      RTC time: Mon 2020-03-30 14:04:41
     Time zone: Europe/Prague (CEST, +0200)
    System clock synchronized: yes
    NTP service: active
    RTC in local TZ: no

Additional resources

  • For more information, see the date(1) and timedatectl(1) man pages.

1.6.1.2. Additional resources

1.6.2. Configuring the system locale

System-wide locale settings are stored in the /etc/locale.conf file, which is read at early boot by the systemd daemon. Every service or user inherits the locale settings configured in /etc/locale.conf, unless individual programs or individual users override them.

This section describes how to manage system locale.

Procedure

  1. To list available system locale settings:

    $ localectl list-locales
    C.utf8
    aa_DJ
    aa_DJ.iso88591
    aa_DJ.utf8
    ...
  2. To display the current status of the system locales settings:

    $ localectl status
  3. To set or change the default system locale settings, use a localectl set-locale sub-command as the root user. For example:

    # localectl set-locale LANG=en-US

Additional resources

  • For more information, see the localectl(1), locale(7), and locale.conf(5) man pages.

1.6.3. Configuring the keyboard layout

The keyboard layout settings control the layout used on the text console and graphical user interfaces.

Procedure

  1. To list available keymaps:

    $ localectl list-keymaps
    ANSI-dvorak
    al
    al-plisi
    amiga-de
    amiga-us
    ...
  2. To display the current status of keymaps settings:

    $ localectl status
    ...
    VC Keymap: us
    ...
  3. To set or change the default system keymap, use a localectl set-keymap sub-command as the root user. For example:

    # localectl set-keymap us

Additional resources

  • For more information, see the localectl(1), locale(7), and locale.conf(5) man pages.

1.6.4. Changing the language using desktop GUI

This section describes how to change the system language using the desktop GUI.

Prerequisites

  • Required language packages are installed on your system

Procedure

  1. Open the GNOME Control Center from the System menu by clicking on its icon.

    cs system menu

  2. In the GNOME Control Center, choose Region & Language from the left vertical bar.
  3. Click the Language menu.

    cs language menu

  4. Select the required region and language from the menu.

    cs select region language

    If your region and language are not listed, scroll down, and click More to select from available regions and languages.

    cs available region language

  5. Click Done.
  6. Click Restart for changes to take effect.

    cs restart region language

Note

Some applications do not support certain languages. The text of an application that cannot be translated into the selected language remains in US English.

Additional resources

  • For more information on how to launch the GNOME Control Center, see approaches described in Launching applications

1.6.5. Additional resources

1.7. Configuring and managing network access

This section describes different options on how to add Ethernet connections in Red Hat Enterprise Linux.

1.7.1. Configuring the network and host name in the graphical installation mode

Follow the steps in this procedure to configure your network and host name.

Procedure

  1. From the Installation Summary window, click Network and Host Name*.
  2. From the list in the left-hand pane, select an interface. The details are displayed in the right-hand pane.
  3. Toggle the ON/OFF switch to enable or disable the selected interface.

    Note

    The installation program automatically detects locally accessible interfaces, and you cannot add or remove them manually.

  4. Click + to add a virtual network interface, which can be either: Team, Bond, Bridge, or VLAN.
  5. Click - to remove a virtual interface.
  6. Click Configure to change settings such as IP addresses, DNS servers, or routing configuration for an existing interface (both virtual and physical).
  7. Type a host name for your system in the Host Name field.

    Note
    • There are several types of network device naming standards used to identify network devices with persistent names, for example, em1 and wl3sp0. For information about these standards, see the Configuring and managing networking document.
    • The host name can be either a fully-qualified domain name (FQDN) in the format hostname.domainname, or a short host name with no domain name. Many networks have a Dynamic Host Configuration Protocol (DHCP) service that automatically supplies connected systems with a domain name. To allow the DHCP service to assign the domain name to this machine, specify only the short host name. The value localhost.localdomain means that no specific static host name for the target system is configured, and the actual host name of the installed system is configured during the processing of the network configuration, for example, by NetworkManager using DHCP or DNS.
  8. Click Apply to apply the host name to the environment.

Additional resources and information

  • For details about configuring network settings and the host name when using a Kickstart file, see the corresponding appendix in Performing an advanced RHEL installation.
  • If you install Red Hat Enterprise Linux using the text mode of the Anaconda installation program, use the Network settings option to configure the network.

1.7.2. Configuring a static Ethernet connection using nmcli

This procedure describes adding an Ethernet connection with the following settings using the nmcli utility:

  • A static IPv4 address - 192.0.2.1 with a /24 subnet mask
  • A static IPv6 address - 2001:db8:1::1 with a /64 subnet mask
  • An IPv4 default gateway - 192.0.2.254
  • An IPv6 default gateway - 2001:db8:1::fffe
  • An IPv4 DNS server - 192.0.2.200
  • An IPv6 DNS server - 2001:db8:1::ffbb
  • A DNS search domain - example.com

Procedure

  1. Add a new NetworkManager connection profile for the Ethernet connection:

    # nmcli connection add con-name Example-Connection ifname enp7s0 type ethernet

    The further steps modify the Example-Connection connection profile you created.

  2. Set the IPv4 address:

    # nmcli connection modify Example-Connection ipv4.addresses 192.0.2.1/24
  3. Set the IPv6 address:

    # nmcli connection modify Example-Connection ipv6.addresses 2001:db8:1::1/64
  4. Set the IPv4 and IPv6 connection method to manual:

    # nmcli connection modify Example-Connection ipv4.method manual
    # nmcli connection modify Example-Connection ipv6.method manual
  5. Set the IPv4 and IPv6 default gateways:

    # nmcli connection modify Example-Connection ipv4.gateway 192.0.2.254
    # nmcli connection modify Example-Connection ipv6.gateway 2001:db8:1::fffe
  6. Set the IPv4 and IPv6 DNS server addresses:

    # nmcli connection modify Example-Connection ipv4.dns "192.0.2.200"
    # nmcli connection modify Example-Connection ipv6.dns "2001:db8:1::ffbb"

    To set multiple DNS servers, specify them space-separated and enclosed in quotes.

  7. Set the DNS search domain for the IPv4 and IPv6 connection:

    # nmcli connection modify Example-Connection ipv4.dns-search example.com
    # nmcli connection modify Example-Connection ipv6.dns-search example.com
  8. Activate the connection profile:

    # nmcli connection up Example-Connection
    Connection successfully activated (D-Bus active path: /org/freedesktop/NetworkManager/ActiveConnection/13)

Verification steps

  1. Display the status of the devices and connections:

    # nmcli device status
    DEVICE      TYPE      STATE      CONNECTION
    enp7s0      ethernet  connected  Example-Connection
  2. To display all settings of the connection profile:

    # nmcli connection show Example-Connection
    connection.id:              Example-Connection
    connection.uuid:            b6cdfa1c-e4ad-46e5-af8b-a75f06b79f76
    connection.stable-id:       --
    connection.type:            802-3-ethernet
    connection.interface-name:  enp7s0
    ...
  3. Use the ping utility to verify that this host can send packets to other hosts.

    • Ping an IP address in the same subnet.

      For IPv4:

      # ping 192.0.2.3

      For IPv6:

      # ping 2001:db8:2::1

      If the command fails, verify the IP and subnet settings.

    • Ping an IP address in a remote subnet.

      For IPv4:

      # ping 198.162.3.1

      For IPv6:

      # ping 2001:db8:2::1
      • If the command fails, ping the default gateway to verify settings.

        For IPv4:

        # ping 192.0.2.254

        For IPv6:

        # ping 2001:db8:1::fffe
  4. Use the host utility to verify that name resolution works. For example:

    # host client.example.com

    If the command returns any error, such as connection timed out or no servers could be reached, verify your DNS settings.

Troubleshooting steps

  1. If the connection fails or if the network interface switches between an up and down status:

    • Make sure that the network cable is plugged-in to the host and a switch.
    • Check whether the link failure exists only on this host or also on other hosts connected to the same switch the server is connected to.
    • Verify that the network cable and the network interface are working as expected. Perform hardware diagnosis steps and replace defect cables and network interface cards.

Additional resources

  • See the nm-settings(5) man page for more information on connection profile properties and their settings.
  • For further details about the nmcli utility, see the nmcli(1) man page.
  • If the configuration on the disk does not match the configuration on the device, starting or restarting NetworkManager creates an in-memory connection that reflects the configuration of the device. For further details and how to avoid this problem, see NetworkManager duplicates a connection after restart of NetworkManager service.

1.7.3. Adding a connection profile using nmtui

The nmtui application provides a text user interface to NetworkManager. This procedure describes how to add a new connection profile.

Prerequisites

  • The NetworkManager-tui package is installed.

Procedure

  1. Start the NetworkManager text user interface utility:

    # nmtui
  2. Select the Edit a connection menu entry, and press Enter.
  3. Select the Add button, and press Enter.
  4. Select Ethernet, and press Enter.
  5. Fill the fields with the connection details.

    add connection in nmtui
  6. Select OK to save the changes.
  7. Select Back to return to the main menu.
  8. Select Activate a connection, and press Enter.
  9. Select the new connection entry, and press Enter to activate the connection.
  10. Select Back to return to the main menu.
  11. Select Quit.

Verification steps

  1. Display the status of the devices and connections:

    # nmcli device status
    DEVICE      TYPE      STATE      CONNECTION
    enp1s0      ethernet  connected  Example-Connection
  2. To display all settings of the connection profile:

    # nmcli connection show Example-Connection
    connection.id:              Example-Connection
    connection.uuid:            b6cdfa1c-e4ad-46e5-af8b-a75f06b79f76
    connection.stable-id:       --
    connection.type:            802-3-ethernet
    connection.interface-name:  enp1s0
    ...

Additional resources

1.7.4. Managing networking in the RHEL 8 web console

In the web console, the Networking menu enables you:

  • To display currently received and sent packets
  • To display the most important characteristics of available network interfaces
  • To display content of the networking logs.
  • To add various types of network interfaces (bond, team, bridge, VLAN)

Figure 1.1. Managing Networking in the RHEL 8 web console

cs getting started networking new

1.7.5. Managing networking using RHEL System Roles

You can configure the networking connections on multiple target machines using the network role.

The network role allows to configure the following types of interfaces:

  • Ethernet
  • Bridge
  • Bonded
  • VLAN
  • MacVLAN
  • Infiniband

The required networking connections for each host are provided as a list within the network_connections variable.

Warning

The network role updates or creates all connection profiles on the target system exactly as specified in the network_connections variable. Therefore, the network role removes options from the specified profiles if the options are only present on the system but not in the network_connections variable.

The following example shows how to apply the network role to ensure that an Ethernet connection with the required parameters exists:

Example 1.3. An example playbook applying the network role to set up an Ethernet connection with the required parameters

# SPDX-License-Identifier: BSD-3-Clause
---
- hosts: network-test
  vars:
    network_connections:

      # Create one ethernet profile and activate it.
      # The profile uses automatic IP addressing
      # and is tied to the interface by MAC address.
      - name: prod1
        state: up
        type: ethernet
        autoconnect: yes
        mac: "00:00:5e:00:53:00"
        mtu: 1450

  roles:
    - rhel-system-roles.network

For more information on applying a system role, see Introduction to RHEL System Roles.

1.7.6. Additional resources

1.8. Registering the system and managing subscriptions

Subscriptions cover products installed on Red Hat Enterprise Linux, including the operating system itself.

You can use a subscription to Red Hat Content Delivery Network to track:

  • Registered systems
  • Products installed on your systems
  • Subscriptions attached to the installed products

1.8.1. Registering the system after the installation

Use the following procedure to register your system if you have not registered it during the installation process already.

Prerequisites

Procedure

  1. Register and automatically subscribe your system in one step:

    # subscription-manager register --username <username> --password <password> --auto-attach
    Registering to: subscription.rhsm.redhat.com:443/subscription
    The system has been registered with ID: 37to907c-ece6-49ea-9174-20b87ajk9ee7
    The registered system name is: client1.idm.example.com
    Installed Product Current Status:
    Product Name: Red Hat Enterprise Linux for x86_64
    Status:       Subscribed

    The command prompts you to enter your Red Hat Customer Portal user name and password.

    If the registration process fails, you can register your system with a specific pool. For guidance on how to do it, proceed with the following steps:

    1. Determine the pool ID of a subscription that you require:

      # subscription-manager list --available

      This command displays all available subscriptions for your Red Hat account. For every subscription, various characteristics are displayed, including the pool ID.

    2. Attach the appropriate subscription to your system by replacing pool_id with the pool ID determined in the previous step:

      # subscription-manager attach --pool=pool_id

Additional resources

1.8.2. Registering subscriptions with credentials in the web console

Use the following steps to register a newly installed Red Hat Enterprise Linux using the RHEL 8 web console.

Prerequisites

  • A valid user account on the Red Hat Customer Portal.

    See the Create a Red Hat Login page.

  • Active subscription for your RHEL system.

Procedure

  1. Type subscription in the search field and press the Enter key.

    cockpit subscription icon

    Alternatively, you can log in to the RHEL 8 web console. For details, see Logging in to the web console.

  2. In the polkit authentication dialog for privileged tasks, add the password belonging to the user name displayed in the dialog.

    cockpit subscription password

  3. Click Authenticate.
  4. In the Subscriptions dialog box, click Register.

    cockpit subscription notregistered

  5. Enter your Customer Portal credentials.

    cockpit subscription register cred

  6. Enter the name of your organization.

    If you have more than one account on the Red Hat Customer Portal, you have to add the organization name or organization ID. To get the org ID, go to your Red Hat contact point.

  7. Click the Register button.

At this point, your Red Hat Enterprise Linux 8 system has been successfully registered.

cockpit subscription registered

1.8.3. Registering a system using Red Hat account on GNOME

Follow the steps in this procedure to enroll your system with your Red Hat account.

Prerequisites

Procedure

  1. Go to the system menu, which is accessible from the top-right screen corner and click the Settings icon.
  2. In the DetailsAbout section, click Register.
  3. Select Registration Server.
  4. If you are not using the Red Hat server, enter the server address in the URL field.
  5. In the Registration Type menu, select Red Hat Account.
  6. Under Registration Details:

    • Enter your Red hat account user name in the Login field,
    • Enter your Red hat account password in the Password field.
    • Enter the name of your organization in the Organization field.
  7. Click Register.

1.8.4. Registering a system using an activation key on GNOME

Follow the steps in this procedure to register your system with an activation key. You can get the activation key from your organization administrator.

Prerequisites

  • Activation key or keys.

    See the Activation Keys page for creating new activation keys.

Procedure

  1. Go to the system menu, which is accessible from the top-right screen corner and click the Settings icon.
  2. In the DetailsAbout section, click Register.
  3. Select Registration Server.
  4. Enter URL to the customized server, if you are not using the Red Hat server.
  5. In the Registration Type menu, select Activation Keys.
  6. Under Registration Details:

    • Enter Activation Keys.

      Separate multiple keys by a comma (,).

    • Enter the name or ID of your organization in the Organization field.
  7. Click Register

1.9. Making systemd services start at boot time

Systemd is a system and service manager for Linux operating systems that introduces the concept of systemd units.

This section provides information on how to ensure that a service is enabled or disabled at boot time. It also explains how to manage the services through the web console.

1.9.1. Enabling or disabling the services using the CLI

You can determine which services are enabled or disabled at boot time already during the installation process. You can also enable or disable a service on an installed operating system.

This section describes the steps for enabling or disabling those services on an already installed operating system:

Prerequisites

  • You must have root access to the system.

Procedure

  1. To enable a service, use the enable option:

    # systemctl enable service_name

    Replace service_name with the service you want to enable.

    You can also enable and start a service in a single command:

    # systemctl enable --now service_name
  2. To disable a service, use the disable option:

    # systemctl disable service_name

    Replace service_name with the service you want to disable.

Warning

You cannot enable a service that has been previously masked. You have to unmask it first:

# systemctl unmask service_name

1.9.2. Managing services in the RHEL 8 web console

This section describes how you can also enable or disable a service using the web console. You can manage systemd targets, services, sockets, timers, and paths. You can also check the service status, start or stop services, enable or disable them.

Prerequisites

  • You must have root access to the system.

Procedure

  1. Open https://localhost:9090/ in a web browser of your preference.
  2. Log in to the web console with your root credentials on the system.
  3. To display the web console panel, click the Host icon, which is in the upper-left corner of the window.

    managing services web console
  4. On the menu, click Services.

    You can manage systemd targets, services, sockets, timers, and paths.

  5. For example, to manage the service NFS client services:

    1. Click Targets.
    2. Select the service NFS client services.
    3. To enable or disable the service, click the Toogle button.
    4. To stop the service, click the button and choose the option 'Stop'.

      stopping service web console

1.10. Configuring system security

Computer security is the protection of computer systems and their hardware, software, information, and services from theft, damage, disruption, and misdirection. Ensuring computer security is an essential task, in particular in enterprises that process sensitive data and handle business transactions.

This section covers only the basic security features that you can configure after installation of the operating system. For detailed information on securing Red Hat Enterprise Linux, see the Security section in Product Documentation for Red Hat Enterprise Linux 8.

1.10.1. Enhancing system security with a firewall

A firewall is a network security system that monitors and controls incoming and outgoing network traffic according to configured security rules. A firewall typically establishes a barrier between a trusted secure internal network and another outside network.

The firewalld service, which provides a firewall in Red Hat Enterprise Linux, is automatically enabled during installation.

1.10.1.1. Enabling the firewalld service

To enable the firewalld service, follow this procedure.

Procedure

  1. Display the current status of firewalld:

    $ systemctl status firewalld
    ● firewalld.service - firewalld - dynamic firewall daemon
       Loaded: loaded (/usr/lib/systemd/system/firewalld.service; disabled; vendor preset: enabled)
       Active: inactive (dead)
    ...
  2. If firewalld is not enabled and running, switch to the root user, and start the firewalld service and enable to start it automatically after the system restarts:

    # systemctl enable --now firewalld

Verification steps

  1. Check that firewalld is running and enabled:

    $ systemctl status firewalld
    ● firewalld.service - firewalld - dynamic firewall daemon
       Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor preset: enabled)
       Active: active (running)
    ...

Additional resources

  • For more information, see the firewalld(1) man page.

1.10.1.2. Managing firewall in the RHEL 8 web console

To configure the firewalld service in the web console, navigate to NetworkingFirewall.

By default, the firewalld service is enabled.

Procedure

  1. To enable or disable firewalld in the web console, switch the Firewall toggle button.

    cs getting started firewall new
Note

Additionally, you can define more fine-grained access through the firewall to a service using the Add services…​ button.

1.10.1.3. Additional resources

1.10.2. Managing basic SELinux settings

Security-Enhanced Linux (SELinux) is an additional layer of system security that determines which processes can access which files, directories, and ports. These permissions are defined in SELinux policies. A policy is a set of rules that guide the SELinux security engine.

1.10.2.1. SELinux states and modes

SELinux has two possible states:

  • Disabled
  • Enabled

When SELinux is enabled, it runs in one of the following modes:

  • Enabled

    • Enforcing
    • Permissive

In enforcing mode, SELinux enforces the loaded policies. SELinux denies access based on SELinux policy rules and enables only the interactions that are explicitly allowed. Enforcing mode is the safest SELinux mode and is the default mode after installation.

In permissive mode, SELinux does not enforce the loaded policies. SELinux does not deny access, but reports actions that break the rules to the /var/log/audit/audit.log log. Permissive mode is the default mode during installation. Permissive mode is also useful in some specific cases, for example when troubleshooting problems.

Additional resources

1.10.2.2. Ensuring the required state of SELinux

By default, SELinux operates in enforcing mode. However, in specific scenarios, you can set SELinux to permissive mode or even disable it.

Important

Red Hat recommends to keep your system in enforcing mode. For debugging purposes, you can set SELinux to permissive mode.

Follow this procedure to change the state and mode of SELinux on your system.

Procedure

  1. Display the current SELinux mode:

    $ getenforce
  2. To temporarily set SELinux:

    1. To Enforcing mode:

      # setenforce Enforcing
    2. To Permissive mode:

      # setenforce Permissive
      Note

      After reboot, SELinux mode is set to the value specified in the /etc/selinux/config configuration file.

  3. To set SELinux mode to persist across reboots, modify the SELINUX variable in the /etc/selinux/config configuration file.

    For example, to switch SELinux to enforcing mode:

    # This file controls the state of SELinux on the system.
    # SELINUX= can take one of these three values:
    #     enforcing - SELinux security policy is enforced.
    #     permissive - SELinux prints warnings instead of enforcing.
    #     disabled - No SELinux policy is loaded.
    SELINUX=enforcing
    ...
    Warning

    Disabling SELinux reduces your system security. Avoid disabling SELinux using the SELINUX=disabled option in the /etc/selinux/config file because this can result in memory leaks and race conditions causing kernel panics. Instead, disable SELinux by adding the selinux=0 parameter to the kernel command line as described in Changing SELinux modes at boot time .

Additional resources

1.10.2.3. Switching SELinux modes in the RHEL 8 web console

You can set SELinux mode through the RHEL 8 web console in the SELinux menu item.

By default, SELinux enforcing policy in the web console is on, and SELinux operates in enforcing mode. By turning it off, you switch SELinux to permissive mode. Note that this selection is automatically reverted on the next boot to the configuration defined in the /etc/sysconfig/selinux file.

Procedure

  1. In the web console, use the Enforce policy toggle button in the SELinux menu item to turn SELinux enforcing policy on or off.

    cs getting started selinux on

1.10.2.4. Next steps

1.10.3. Next steps

1.11. Getting started with managing user accounts

Red Hat Enterprise Linux is a multi-user operating system, which enables multiple users on different computers to access a single system installed on one machine.

Every user operates under its own account, and managing user accounts thus represents a core element of Red Hat Enterprise Linux system administration.

1.11.1. Overview of user accounts and groups

This section provides an overview of user accounts and groups. The following are the different types of user accounts:

  • Normal user accounts:

    Normal accounts are created for users of a particular system. Such accounts can be added, removed, and modified during normal system administration.

  • System user accounts

    System user accounts represent a particular applications identifier on a system. Such accounts are generally added or manipulated only at software installation time, and they are not modified later.

    Warning

    System accounts are presumed to be available locally on a system. If these accounts are configured and provided remotely, such as in the instance of an LDAP configuration, system breakage and service start failures can occur.

    For system accounts, user IDs below 1000 are reserved. For normal accounts, you can use IDs starting at 1000. However, the recommended practice is to assign IDs starting at 5000.

  • Group

    A group in an entity which ties together multiple user accounts for a common purpose, such as granting access to particular files.

  • For more information, see
  • For assigning IDs, see the /etc/login.defs file.

1.11.2. Managing accounts and groups using command-line tools

This section describes basic command-line tools to manage user accounts and groups.

  • To display user and group IDs:

    $ id
    uid=1000(example.user) gid=1000(example.user) groups=1000(example.user),10(wheel) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
  • To create a new user account:

    # useradd example.user
  • To assign a new password to a user account belonging to example.user:

    # passwd example.user
  • To add a user to a group:

    # usermod -a -G example.group example.user

Additional resources

  • The useradd(8), passwd(1), and usermod(8) man pages.

1.11.3. System user accounts managed in the web console

With user accounts displayed in the RHEL web console you can:

  • Authenticate users when accessing the system.
  • Set them access rights to the system.

The RHEL web console displays all user accounts located in the system. Therefore, you can see at least one user account just after the first login to the web console.

After logging into the RHEL web console, you can perform the following operations:

  • Create new users accounts.
  • Change their parameters.
  • Lock accounts.
  • Terminate user sessions.

1.11.4. Adding new accounts using the web console

Use the following steps for adding user accounts to the system and setting administration rights to the accounts through the RHEL web console.

Prerequisites

Procedure

  1. Log in to the RHEL web console.
  2. Click Accounts.
  3. Click Create New Account.

    cockpit create new account pf4

  4. In the Full Name field, enter the full name of the user.

    The RHEL web console automatically suggests a user name from the full name and fills it in the User Name field. If you do not want to use the original naming convention consisting of the first letter of the first name and the whole surname, update the suggestion.

  5. In the Password/Confirm fields, enter the password and retype it for verification that your password is correct. The color bar placed below the fields shows you security level of the entered password, which does not allow you to create a user with a weak password.

    cockpit user accounts pf4

  6. Click Create to save the settings and close the dialog box.
  7. Select the newly created account.
  8. Select Server Administrator in the Roles item.

cockpit terminate session pf4

Now you can see the new account in the Accounts settings and you can use the credentials to connect to the system.

1.12. Dumping a crashed kernel for later analysis

To analyze why a system crashed, you can use the kdump service to save the contents of the system’s memory for later analysis.

This section provides a brief introduction to kdump, and information about configuring kdump using the RHEL web console or using the corresponding RHEL system role.

1.12.1. What is kdump

kdump is a service providing a crash dumping mechanism. The service enables you to save the contents of the system’s memory for later analysis. kdump uses the kexec system call to boot into the second kernel (a capture kernel) without rebooting; and then captures the contents of the crashed kernel’s memory (a crash dump or a vmcore) and saves it. The second kernel resides in a reserved part of the system memory.

Important

A kernel crash dump can be the only information available in the event of a system failure (a critical bug). Therefore, ensuring that kdump is operational is important in mission-critical environments. Red Hat advise that system administrators regularly update and test kexec-tools in your normal kernel update cycle. This is especially important when new kernel features are implemented.

1.12.2. Configuring kdump memory usage and target location in web console

The procedure below shows you how to use the Kernel Dump tab in the Red Hat Enterprise Linux web console interface to configure the amount of memory that is reserved for the kdump kernel. The procedure also describes how to specify the target location of the vmcore dump file and how to test your configuration.

Prerequisites

Procedure

  1. Open the Kernel Dump tab and start the kdump service.
  2. Configure the kdump memory usage through the command line.
  3. Click the link next to the Crash dump location option.

    web console initial screen
  4. Select the Local Filesystem option from the drop-down and specify the directory you want to save the dump in.

    web console crashdump target
    • Alternatively, select the Remote over SSH option from the drop-down to send the vmcore to a remote machine using the SSH protocol.

      Fill the Server, ssh key, and Directory fields with the remote machine address, ssh key location, and a target directory.

    • Another choice is to select the Remote over NFS option from the drop-down and fill the Mount field to send the vmcore to a remote machine using the NFS protocol.

      Note

      Tick the Compression check box to reduce the size of the vmcore file.

  5. Test your configuration by crashing the kernel.

    web console test kdump config
    Warning

    This step disrupts execution of the kernel and results in a system crash and loss of data.

Additional resources

1.12.3. Configuring kdump using RHEL System Roles

RHEL System Roles is a collection of Ansible roles and modules that provide a consistent configuration interface to remotely manage multiple RHEL systems. The kdump role enables you to set basic kernel dump parameters on multiple systems.

Warning

The kdump role replaces the kdump configuration of the managed hosts entirely by replacing the /etc/kdump.conf file. Additionally, if the kdump role is applied, all previous kdump settings are also replaced, even if they are not specified by the role variables, by replacing the /etc/sysconfig/kdump file.

The following example playbook shows how to apply the kdump system role to set the location of the crash dump files:

---
- hosts: kdump-test
  vars:
    kdump_path: /var/crash
  roles:
    - rhel-system-roles.kdump

Additional resources

  • For a detailed reference on kdump role variables, install the rhel-system-roles package, and see the README.md or README.html files in the /usr/share/doc/rhel-system-roles/kdump directory.
  • For more information on RHEL System Roles, see Introduction to RHEL System Roles

1.12.4. Additional resources

1.13. Recovering and restoring a system

To recover and restore a system using an existing backup, Red Hat Enterprise Linux provides the Relax-and-Recover (ReaR) utility.

You can use the utility as a disaster recovery solution and also for system migration.

The utility enables you to perform the following tasks:

  • Produce a bootable image and restore the system from an existing backup, using the image.
  • Replicate the original storage layout.
  • Restore user and system files.
  • Restore the system to a different hardware.

Additionally, for disaster recovery, you can also integrate certain backup software with ReaR.

Setting up ReaR involves the following high-level steps:

  1. Install ReaR.
  2. Create rescue system.
  3. Modify ReaR configuration file, to add backup method details.
  4. Generate backup files.

1.13.1. Setting up ReaR

Use the following steps to install the packages for using the Relax-and-Recover (ReaR) utility, create a rescue system, configure and generate a backup.

Prerequisites

  • Necessary configurations as per the backup restore plan are ready.

    Note that you can use the NETFS backup method, a fully-integrated and built-in method with ReaR.

Procedure

  1. Install ReaR, the genisomage pre-mastering program, and the syslinux package providing a suite of boot loaders:

    # yum install rear genisoimage syslinux
  2. Create a rescue system:

    # rear mkrescue
  3. Modify the ReaR configuration file in an editor of your choice, for example:

    # vi /etc/rear/local.conf
  4. Add the backup setting details to /etc/rear/local.conf. For example, in the case of the NETFS backup method, add the following lines:

    BACKUP=NETFS
    BACKUP_URL=backup.location

    Replace backup.location by the URL of your backup location.

  5. To configure ReaR to keep the previous backup archives when the new ones are created, also add the following line to the configuration file:

    NETFS_KEEP_OLD_BACKUP_COPY=y
  6. To make the backups incremental, meaning that only the changed files are backed up on each run, add the following line:

    BACKUP_TYPE=incremental
  7. Take a backup as per the restore plan.

1.14. Troubleshooting problems using log files

Log files contain messages about the system, including the kernel, services, and applications running on it. These contain information that helps troubleshoot issues or monitor system functions. The logging system in Red Hat Enterprise Linux is based on the built-in syslog protocol. Particular programs use this system to record events and organize them into log files, which are useful when auditing the operating system and troubleshooting various problems.

1.14.1. Services handling syslog messages

The following two services handle syslog messages:

  • The systemd-journald daemon
  • The Rsyslog service

The systemd-journald daemon collects messages from various sources and forwards them to Rsyslog for further processing. The systemd-journald daemon collects messages from the following sources:

  • Kernel
  • Early stages of the boot process
  • Standard and error output of daemons as they start up and run
  • Syslog

The Rsyslog service sorts the syslog messages by type and priority and writes them to the files in the /var/log directory. The /var/log directory persistently stores the log messages.

1.14.2. Subdirectories storing syslog messages

The following subdirectories under the /var/log directory store syslog messages.

  • /var/log/messages - all syslog messages except the following
  • /var/log/secure - security and authentication-related messages and errors
  • /var/log/maillog - mail server-related messages and errors
  • /var/log/cron - log files related to periodically executed tasks
  • /var/log/boot.log - log files related to system startup

1.14.3. Inspecting log files using the web console

Follow the steps in this procedure to inspect the log files using the web console.

Procedure

  1. Log into the Red Hat Enterprise Linux 8 web console.

    For details, see Logging in to the web console.

  2. Click Logs.

Figure 1.2. Inspecting the log files in the RHEL 8 web console

cs viewing logs web console

1.14.4. Viewing logs using the command line

The Journal is a component of systemd that helps to view and manage log files. It addresses problems connected with traditional logging, closely integrated with the rest of the system, and supports various logging technologies and access management for the log files.

You can use the journalctl command to view messages in the system journal using the command line, for example:

$ journalctl -b | grep kvm
May 15 11:31:41 localhost.localdomain kernel: kvm-clock: Using msrs 4b564d01 and 4b564d00
May 15 11:31:41 localhost.localdomain kernel: kvm-clock: cpu 0, msr 76401001, primary cpu clock
...

Table 1.1. Viewing system information

CommandDescription

journalctl

Shows all collected journal entries.

journalctl FILEPATH

Shows logs related to a specific file. For example, the journalctl /dev/sda command displays logs related to the /dev/sda file system.

journalctl -b

Shows logs for the current boot.

journalctl -k -b -1

Shows kernel logs for the current boot.

Table 1.2. Viewing information on specific services

CommandDescription

journalctl -b _SYSTEMD_UNIT=foo

Filters log to see ones matching the "foo" systemd service.

journalctl -b _SYSTEMD_UNIT=foo _PID=number

Combines matches. For example, this command shows logs for systemd-units that match foo and the PID number.

journalctl -b _SYSTEMD_UNIT=foo _PID=number + _SYSTEMD_UNIT=foo1

The separator “+” combines two expressions in a logical OR. For example, this command shows all messages from the foo service process with the PID plus all messages from the foo1 service (from any of its processes).

journalctl -b _SYSTEMD_UNIT=foo _SYSTEMD_UNIT=foo1

This command shows all entries matching either expression, referring to the same field. Here, this command shows logs matching a systemd-unit foo or a systemd-unit foo1.

Table 1.3. Viewing logs related to specific boots

CommandDescription

journalctl --list-boots

Shows a tabular list of boot numbers, their IDs, and the timestamps of the first and last message pertaining to the boot. You can use the ID in the next command to view detailed information.

journalctl --boot=ID _SYSTEMD_UNIT=foo

Shows information about the specified boot ID.

1.14.5. Additional resources

1.15. Accessing the Red Hat support

This section describes how to effectively troubleshoot your problems using Red Hat support and sosreport.

To obtain support from Red Hat, use the Red Hat Customer Portal, which provides access to everything available with your subscription.

1.15.1. Obtaining Red Hat Support through Red Hat Customer Portal

The following section describes how to use the Red Hat Customer Portal to get help.

Prerequisites

  • A valid user account on the Red Hat Customer Portal. See Create a Red Hat Login.
  • An active subscription for the RHEL system.

Procedure

  1. Access Red Hat support:

    1. Open a new support case.
    2. Initiate a live chat with a Red Hat expert.
    3. Contact a Red Hat expert by making a call or sending an email.

1.15.2. Troubleshooting problems using sosreport

The sosreport command collects configuration details, system information and diagnostic information from a Red Hat Enterprise Linux system.

The following section describes how to use the sosreport command to produce reports for your support cases.

Prerequisites

  • A valid user account on the Red Hat Customer Portal. See Create a Red Hat Login.
  • An active subscription for the RHEL system.
  • A support-case number.

Procedure

  1. Install the sos package:

    # yum install sos
    Note

    The default minimal installation of Red Hat Enterprise Linux does not include the sos package, which provides the sosreport command.

  2. Generate a report:

    # sosreport
  3. Attach the report to your support case.

    See the How can I attach a file to a Red Hat support case? Red Hat Knowledgebase article for more information.

    Note that when attaching the report, you are prompted to enter the number of the relevant support case.

Additional resources

Chapter 2. Managing software packages

2.1. Software management tools in Red Hat Enterprise Linux 8

In RHEL 8, software installation is enabled by the new version of the YUM tool (YUM v4), which is based on the DNF technology.

Note

Upstream documentation identifies the technology as DNF and the tool is referred to as DNF in the upstream. As a result, some output returned by the new YUM tool in RHEL 8 mentions DNF.

Although YUM v4 used in RHEL 8 is based on DNF, it is compatible with YUM v3 used in RHEL 7. For software installation, the yum command and most of its options work the same way in RHEL 8 as they did in RHEL 7.

Selected yum plug-ins and utilities have been ported to the new DNF back end, and can be installed under the same names as in RHEL 7. Packages also provide compatibility symlinks, so the binaries, configuration files, and directories can be found in usual locations.

Note that the legacy Python API provided by YUM v3 is no longer available. You can migrate your plug-ins and scripts to the new API provided by YUM v4 (DNF Python API), which is stable and fully supported. See DNF API Reference for more information.

2.2. Application streams

Red Hat Enterprise Linux 8 introduces the concept of Application Streams. Multiple versions of user space components are now delivered and updated more frequently than the core operating system packages. This provides greater flexibility to customize Red Hat Enterprise Linux without impacting the underlying stability of the platform or specific deployments.

Components made available as Application Streams can be packaged as modules or RPM packages, and are delivered through the AppStream repository in Red Hat Enterprise Linux 8. Each Application Stream has a given life cycle, either the same as RHEL 8 or shorter, more suitable to the particular application. Application Streams with a shorter life cycle are listed in the Red Hat Enterprise Linux 8 Application Streams Life Cycle page.

Modules are collections of packages representing a logical unit: an application, a language stack, a database, or a set of tools. These packages are built, tested, and released together.

Module streams represent versions of the Application Stream components. For example, two streams (versions) of the PostgreSQL database server are available in the postgresql module: PostgreSQL 10 (the default stream) and PostgreSQL 9.6. Only one module stream can be installed on the system. Different versions can be used in separate containers.

Detailed module commands are described in the Installing, managing, and removing user-space components document. For a list of modules available in AppStream, see the Package manifest.

2.3. Searching for software packages

yum allows you to perform a complete set of operations with software packages.

The following section describes how to use yum to:

  • Search for packages.
  • List packages.
  • List repositories.
  • Display information about the packages.
  • List package groups.
  • Specify global expressions in yum input.

2.3.1. Searching packages with yum

  • To search for a package, use:

    # yum search term

    Replace term with a term related to the package.

    Note that yum search command returns term matches within the name and summary of the packages. This makes the search faster and enables you to search for packages you do not know the name of, but for which you know a related term.

  • To include term matches within package descriptions, use:

    # yum search --all term

    Replace term with a term you want to search for in a package name, summary, or description.

    Note that yum search --all enables a more exhaustive but slower search.

2.3.2. Listing packages with yum

  • To list information on all installed and available packages, use:

    # yum list --all
  • To list all packages installed on your system, use:

    # yum list --installed
  • To list all packages in all enabled repositories that are available to install, use:

    # yum list --available

Note that you can filter the results by appending global expressions as arguments. See Section 2.3.6, “Specifying global expressions in yum input” for more details.

2.3.3. Listing repositories with yum

  • To list all enabled repositories on your system, use:

    # yum repolist
  • To list all disabled repositories on your system, use:

    # yum repolist --disabled
  • To list both enabled and disabled repositories, use:

    # yum repolist --all
  • To list additional information about the repositories, use:

    # yum repoinfo

Note that you can filter the results by passing the ID or name of repositories as arguments or by appending global expressions. See Section 2.3.6, “Specifying global expressions in yum input” for more details.

2.3.4. Displaying package information with yum

  • To display information about one or more packages, use:

    # yum info package-name

    Replace package-name with the name of the package.

Note that you can filter the results by appending global expressions as arguments. See Section 2.3.6, “Specifying global expressions in yum input” for more details.

2.3.5. Listing package groups with yum

  • To view the number of installed and available groups, use:

    # yum group summary
  • To list all installed and available groups, use:

    # yum group list

    Note that you can filter the results by appending command line options for the yum group list command (--hidden, --available). For more available options see the man pages.

  • To list mandatory and optional packages contained in a particular group, use:

    # yum group info group-name

    Replace group-name with the name of the group.

Note that you can filter the results by appending global expressions as arguments. See Section 2.7.4, “Specifying global expressions in yum input” for more details.

2.3.6. Specifying global expressions in yum input

yum commands allow you to filter the results by appending one or more glob expressions as arguments. Global expressions must be escaped when passed as arguments to the yum command. To ensure global expressions are passed to yum as intended, use one of the following methods:

  • Double-quote or single-quote the entire global expression.

    # yum provides "*/file-name"

    Replace file-name with the name of the file.

  • Escape the wildcard characters by preceding them with a backslash (\) character.

    # yum provides \*/file-name

    Replace file-name with the name of the file.

2.4. Installing software packages

The following section describes how to use yum to:

  • Install packages.
  • Install a package group.
  • Specify a package name in yum input.

2.4.1. Installing packages with yum

  • To install a package and all the package dependencies, use:

    # yum install package-name

    Replace package-name with the name of the package.

  • To install multiple packages and their dependencies simultaneously, use:

    # yum install package-name-1 package-name-2

    Replace package-name-1 and package-name-2 with the names of the packages.

  • When installing packages on a multilib system (AMD64, Intel 64 machine), you can specify the architecture of the package by appending it to the package name:

    # yum install package-name.arch

    Replace package-name.arch with the name and architecture of the package.

  • If you know the name of the binary you want to install, but not the package name, you can use the path to the binary as an argument:

    # yum install /usr/sbin/binary-file

    Replace /usr/sbin/binary-file with a path to the binary file.

    yum searches through the package lists, finds the package which provides /usr/sbin/binary-file, and prompts you as to whether you want to install it.

  • To install a previously-downloaded package from a local directory, use:

    # yum install /path/

    Replace /path/ with the path to the package.

Note that you can optimize the package search by explicitly defining how to parse the argument. See Section 2.4.3, “Specifying a package name in yum input” for more details.

2.4.2. Installing a package group with yum

  • To install a package group by a group name, use:

    # yum group install group-name

    Or

    # yum install @group-name

    Replace group-name with the full name of the group or environmental group.

  • To install a package group by the groupID, use:

    # yum group install groupID

    Replace groupID with the ID of the group.

2.4.3. Specifying a package name in yum input

To optimize the installation and removal process, you can append -n, -na, or -nerva suffixes to yum install and yum remove commands to explicitly define how to parse an argument:

  • To install a package using it’s exact name, use:

    # yum install-n name

    Replace name with the exact name of the package.

  • To install a package using it’s exact name and architecture, use:

    # yum install-na name.architecture

    Replace name and architecture with the exact name and architecture of the package.

  • To install a package using it’s exact name, epoch, version, release, and architecture, use:

    # yum install-nevra name-epoch:version-release.architecture

    Replace name, epoch, version, release, and architecture with the exact name, epoch, version, release, and architecture of the package.

2.5. Updating software packages

yum allows you to check if your system has any pending updates. You can list packages that need updating and choose to update a single package, multiple packages, or all packages at once. If any of the packages you choose to update have dependencies, they are updated as well.

The following section describes how to use yum to:

  • Check for updates.
  • Update a single package.
  • Update a package group.
  • Update all packages and their dependencies.
  • Apply security updates.
  • Automate software updates.

2.5.1. Checking for updates with yum

  • To see which packages installed on your system have available updates, use:

    # yum check-update

    The output returns the list of packages and their dependencies that have an update available.

2.5.2. Updating a single package with yum

  • To update a package, use:

    # yum update package-name

    Replace package-name with the name of the package.

Important

When applying updates to kernel, yum always installs a new kernel regardless of whether you are using the yum update or yum install command.

2.5.3. Updating a package group with yum

  • To update a package group, use:

    # yum group update group-name

    Replace group-name with the name of the package group.

2.5.4. Updating all packages and their dependencies with yum

  • To update all packages and their dependencies, use:

    # yum update

2.5.6. Automating software updates

To check and download package updates automatically and regularly, you can use the DNF Automatic tool that is provided by the dnf-automatic package.

DNF Automatic is an alternative command-line interface to yum that is suited for automatic and regular execution using systemd timers, cron jobs and other such tools.

DNF Automatic synchronizes package metadata as needed and then checks for updates available. After, the tool can perform one of the following actions depending on how you configure it:

  • Exit
  • Download updated packages
  • Download and apply the updates

The outcome of the operation is then reported by a selected mechanism, such as the standard output or email.

2.5.6.1. Installing DNF Automatic

The following procedure describes how to install the DNF Automatic tool.

Procedure

  • To install the dnf-automatic package, use:

    # yum install dnf-automatic

Verification steps

  • To verify the successful installation, confirm the presence of the dnf-automatic package by running the following command:

    # rpm -qi dnf-automatic

2.5.6.2. DNF Automatic configuration file

By default, DNF Automatic uses /etc/dnf/automatic.conf as its configuration file to define its behavior.

The configuration file is separated into the following topical sections:

  • [commands] section

    Sets the mode of operation of DNF Automatic.

  • [emitters] section

    Defines how the results of DNF Automatic are reported.

  • [command_email] section

    Provides the email emitter configuration for an external command used to send email.

  • [email] section

    Provides the email emitter configuration.

  • [base] section

    Overrides settings from the main configuration file of yum.

With the default settings of the /etc/dnf/automatic.conf file, DNF Automatic checks for available updates, downloads them, and reports the results as standard output.

Warning

Settings of the operation mode from the [commands] section are overridden by settings used by a systemd timer unit for all timer units except dnf-automatic.timer.

Additional resources

2.5.6.3. Enabling DNF Automatic

To run DNF Automatic, you always need to enable and start a specific systemd timer unit. You can use one of the timer units provided in the dnf-automatic package, or you can write your own timer unit depending on your needs.

The following section describes how to enable DNF Automatic.

Prerequisites

  • You specified the behavior of DNF Automatic by modifying the /etc/dnf/automatic.conf configuration file.

For more information on DNF Automatic configuration file, see Section 2.5.6.2, “DNF Automatic configuration file”.

Procedure

  • Select, enable and start a systemd timer unit that fits your needs:

    # systemctl enable --now <unit>

    where <unit> is one of the following timers:

    • dnf-automatic-download.timer
    • dnf-automatic-install.timer
    • dnf-automatic-notifyonly.timer
    • dnf-automatic.timer

For downloading available updates, use:

# systemctl enable dnf-automatic-download.timer
# systemctl start dnf-automatic-download.timer

For downloading and installing available updates, use:

# systemctl enable dnf-automatic-install.timer
# systemctl start dnf-automatic-install.timer

For reporting about available updates, use:

# systemctl enable dnf-automatic-notifyonly.timer
# systemctl start dnf-automatic-notifyonly.timer

Optionally, you can use:

# systemctl enable dnf-automatic.timer
# systemctl start dnf-automatic.timer

In terms of downloading and applying updates, this timer unit behaves according to settings in the /etc/dnf/automatic.conf configuration file. The default behavior is similar to dnf-automatic-download.timer: it downloads the updated packages, but it does not install them.

Note

Alternatively, you can also run DNF Automatic by executing the /usr/bin/dnf-automatic file directly from the command line or from a custom script.

Verification steps

  • To verify that the timer is enabled, run the following command:

    # systemctl status <systemd timer unit>

Additional resources

2.5.6.4. Overview of the systemd timer units included in the dnf-automatic package

The systemd timer units take precedence and override the settings in the /etc/dnf/automatic.conf configuration file concerning downloading and applying updates.

For example if you set:

download_updates = yes

in the /etc/dnf/automatic.conf configuration file, but you have activated the dnf-automatic-notifyonly.timer unit, the packages will not be downloaded.

The dnf-automatic package includes the following systemd timer units:

Timer unitFunctionOverrides settings in the /etc/dnf/automatic.conf file?

dnf-automatic-download.timer

Downloads packages to cache and makes them available for updating.

Note: This timer unit does not install the updated packages. To perform the installation, you have to execute the dnf update command.

Yes

dnf-automatic-install.timer

Downloads and installs updated packages.

Yes

dnf-automatic-notifyonly.timer

Downloads only repository data to keep repository cache up-to-date and notifies you about available updates.

Note: This timer unit does not download or install the updated packages

Yes

dnf-automatic.timer

The behavior of this timer concerning downloading and applying updates is specified by the settings in the /etc/dnf/automatic.conf configuration file.

Default behavior is the same as for the dnf-automatic-download.timer unit: it only downloads packages, but does not install them.

No

Additional resources

  • For more information on the dnf-automatic timers, see the man dnf-automatic manual pages.
  • For more information on the /etc/dnf/automatic.conf configuration file, see Section 2.5.6.2. DNF Automatic configuration file

2.6. Uninstalling software packages

The following section describes how to use yum to:

  • Remove packages.
  • Remove a package group.
  • Specify a package name in yum input.

2.6.1. Removing packages with yum

  • To remove a particular package and all dependent packages, use:

    # yum remove package-name

    Replace package-name with the name of the package.

  • To remove multiple packages and their dependencies simultaneously, use:

    # yum remove package-name-1 package-name-2

    Replace package-name-1 and package-name-2 with the names of the packages.

Note

yum is not able to remove a package without removing depending packages.

Note that you can optimize the package search by explicitly defining how to parse the argument. See Section 2.6.3, “Specifying a package name in yum input” for more details.

2.6.2. Removing a package group with yum

  • To remove a package group by the group name, use:

    # yum group remove group-name

    Or

    # yum remove @group-name

    Replace group-name with the full name of the group.

  • To remove a package group by the groupID, use:

    # yum group remove groupID

    Replace groupID with the ID of the group.

2.6.3. Specifying a package name in yum input

To optimize the installation and removal process, you can append -n, -na, or -nerva suffixes to yum install and yum remove commands to explicitly define how to parse an argument:

  • To install a package using it’s exact name, use:

    # yum install-n name

    Replace name with the exact name of the package.

  • To install a package using it’s exact name and architecture, use:

    # yum install-na name.architecture

    Replace name and architecture with the exact name and architecture of the package.

  • To install a package using it’s exact name, epoch, version, release, and architecture, use:

    # yum install-nevra name-epoch:version-release.architecture

    Replace name, epoch, version, release, and architecture with the exact name, epoch, version, release, and architecture of the package.

2.7. Managing software package groups

A package group is a collection of packages that serve a common purpose (System Tools, Sound and Video). Installing a package group pulls a set of dependent packages, which saves time considerably.

The following section describes how to use yum to:

  • List package groups.
  • Install a package group.
  • Remove a package group.
  • Specify global expressions in yum input.

2.7.1. Listing package groups with yum

  • To view the number of installed and available groups, use:

    # yum group summary
  • To list all installed and available groups, use:

    # yum group list

    Note that you can filter the results by appending command line options for the yum group list command (--hidden, --available). For more available options see the man pages.

  • To list mandatory and optional packages contained in a particular group, use:

    # yum group info group-name

    Replace group-name with the name of the group.

Note that you can filter the results by appending global expressions as arguments. See Section 2.7.4, “Specifying global expressions in yum input” for more details.

2.7.2. Installing a package group with yum

  • To install a package group by a group name, use:

    # yum group install group-name

    Or

    # yum install @group-name

    Replace group-name with the full name of the group or environmental group.

  • To install a package group by the groupID, use:

    # yum group install groupID

    Replace groupID with the ID of the group.

2.7.3. Removing a package group with yum

  • To remove a package group by the group name, use:

    # yum group remove group-name

    Or

    # yum remove @group-name

    Replace group-name with the full name of the group.

  • To remove a package group by the groupID, use:

    # yum group remove groupID

    Replace groupID with the ID of the group.

2.7.4. Specifying global expressions in yum input

yum commands allow you to filter the results by appending one or more glob expressions as arguments. Global expressions must be escaped when passed as arguments to the yum command. To ensure global expressions are passed to yum as intended, use one of the following methods:

  • Double-quote or single-quote the entire global expression.

    # yum provides "*/file-name"

    Replace file-name with the name of the file.

  • Escape the wildcard characters by preceding them with a backslash (\) character.

    # yum provides \*/file-name

    Replace file-name with the name of the file.

2.8. Handling package management history

The yum history command allows you to review information about the timeline of yum transactions, dates and times they occurred, the number of packages affected, whether these transactions succeeded or were aborted, and if the RPM database was changed between transactions. yum history command can also be used to undo or redo the transactions.

The following section describes how to use yum to:

  • List transactions.
  • Revert transactions.
  • Repeat transactions.
  • Specify global expressions in yum input.

2.8.1. Listing transactions with yum

  • To display a list of all the latest yum transactions, use:

    # yum history
  • To display a list of all the latest operations for a selected package, use:

    # yum history list package-name

    Replace package-name with the name of the package. You can filter the command output by appending global expressions. See Section 2.8.4, “Specifying global expressions in yum input” for more details.

  • To examine a particular transaction, use:

    # yum history info transactionID

    Replace transactionID with the ID of the transaction.

2.8.2. Reverting transactions with yum

  • To revert a particular transaction, use:

    # yum history undo transactionID

    Replace transactionID with the ID of the transaction.

  • To revert the last transaction, use:

    # yum history undo last

Note that the yum history undo command only reverts the steps that were performed during the transaction. If the transaction installed a new package, the yum history undo command uninstalls it. If the transaction uninstalled a package, the yum history undo command reinstalls it. yum history undo also attempts to downgrade all updated packages to their previous versions, if the older packages are still available.

2.8.3. Repeating transactions with yum

  • To repeat a particular transaction, use:

    # yum history redo transactionID

    Replace transactionID with the ID of the transaction.

  • To repeat the last transaction, use:

    # yum history redo last

Note that the yum history redo command only repeats the steps that were performed during the transaction.

2.8.4. Specifying global expressions in yum input

yum commands allow you to filter the results by appending one or more glob expressions as arguments. Global expressions must be escaped when passed as arguments to the yum command. To ensure global expressions are passed to yum as intended, use one of the following methods:

  • Double-quote or single-quote the entire global expression.

    # yum provides "*/file-name"

    Replace file-name with the name of the file.

  • Escape the wildcard characters by preceding them with a backslash (\) character.

    # yum provides \*/file-name

    Replace file-name with the name of the file.

2.9. Managing software repositories

The configuration information for yum and related utilities are stored in the /etc/yum.conf file. This file contains one or more [repository] sections, which allow you to set repository-specific options.

It is recommended to define individual repositories in new or existing .repo files in the /etc/yum.repos.d/ directory.

Note that the values you define in individual [repository] sections of the /etc/yum.conf file override values set in the [main] section.

The following section describes how to:

  • Set [repository] options.
  • Add a yum repository.
  • Enable a yum repository.
  • Disable a yum repository.

2.9.1. Setting yum repository options

The /etc/yum.conf configuration file contains the [repository] sections, where repository is a unique repository ID. The [repository] sections allows you to define individual yum repositories.

Note

Do not give custom repositories names used by the Red Hat repositories to avoid conflicts.

For a complete list of available [repository] options, see the [repository] OPTIONS section of the yum.conf(5) manual page.

2.9.2. Adding a yum repository

To define a new repository, you can:

  • Add a [repository] section to the /etc/yum.conf file.
  • Add a [repository] section to a .repo file in the /etc/yum.repos.d/ directory.

    yum repositories commonly provide their own .repo file.

Note

It is recommended to define your repositories in a .repo file instead of /etc/yum.conf as all files with the .repo file extension in this directory are read by yum.

  • To add a repository to your system and enable it, use:

    # yum-config-manager --add-repo repository_URL

    Replace repository_url with URL pointing to the repository.

Warning

Obtaining and installing software packages from unverified or untrusted sources other than Red Hat certificate-based Content Delivery Network (CDN) constitutes a potential security risk, and could lead to security, stability, compatibility, and maintainability issues.

2.9.3. Enabling a yum repository

2.9.4. Disabling a yum repository

2.10. Configuring yum

The configuration information for yum and related utilities are stored in the /etc/yum.conf file. This file contains one mandatory [main] section, which enables you to set yum options that have global effect.

The following section describes how to:

  • View the current yum configurations.
  • Set yum [main] options.
  • Use yum plug-ins.

2.10.1. Viewing the current yum configurations

  • To display the current values of global yum options specified in the [main] section of the /etc/yum.conf file, use:

    # yum config-manager --dump

2.10.2. Setting yum main options

The /etc/yum.conf configuration file contains one [main] section. The key-value pairs in this section affect how yum operates and treats repositories.

You can add additional options under the [main] section heading in /etc/yum.conf.

For a complete list of available [main] options, see the [main] OPTIONS section of the yum.conf(5) manual page.

2.10.3. Using yum plug-ins

yum provides plug-ins that extend and enhance its operations. Certain plug-ins are installed by default.

The following section describes how to enable, configure, and disable yum plug-ins.

2.10.3.1. Managing yum plug-ins

The plug-in configuration files always contain a [main] section where the enabled= option controls whether the plug-in is enabled when you run yum commands. If this option is missing, you can add it manually to the file.

Every installed plug-in has its own configuration file in the /etc/dnf/plugins/ directory. You can enable or disable plug-in specific options in these files.

2.10.3.2. Enabling yum plug-ins

  • To enable all yum plug-ins:

    1. Ensure a line beginning with plugins= is present in the [main] section of the /etc/yum.conf file.
    2. Set the value of plugins= to 1.

      plugins=1

2.10.3.3. Disabling yum plug-ins

  • To disable all yum plug-ins:

    1. Ensure a line beginning with plugins= is present in the [main] section of the /etc/yum.conf file.
    2. Set the value of plugins= to 0.

      plugins=0
      Important

      Disabling all plug-ins is not advised. Certain plug-ins provide important yum services. In particular, the product-id and subscription-manager plug-ins provide support for the certificate-based Content Delivery Network (CDN). Disabling plug-ins globally is provided as a convenience option, and is advisable only when diagnosing a potential problem with yum.

  • To disable all yum plug-ins for a particular command, append --noplugins option to the command.

    # yum --noplugins update
  • To disable certain yum plug-ins for a single command, append --disableplugin=plugin-name option to the command.

    # yum update --disableplugin=plugin-name

    Replace plugin-name with the name of the plug-in.

Chapter 3. Managing services with systemd

3.1. Introduction to systemd

Systemd is a system and service manager for Linux operating systems. It is designed to be backwards compatible with SysV init scripts, and provides a number of features such as parallel startup of system services at boot time, on-demand activation of daemons, or dependency-based service control logic. Starting with Red Hat Enterprise Linux 7, systemd replaced Upstart as the default init system.

Systemd introduces the concept of systemd units. These units are represented by unit configuration files located in one of the directories listed in the following table.

Table 3.1. Systemd unit files locations

DirectoryDescription

/usr/lib/systemd/system/

Systemd unit files distributed with installed RPM packages.

/run/systemd/system/

Systemd unit files created at run time. This directory takes precedence over the directory with installed service unit files.

/etc/systemd/system/

Systemd unit files created by systemctl enable as well as unit files added for extending a service. This directory takes precedence over the directory with runtime unit files.

The units encapsulate information about:

  • System services
  • Listening sockets
  • Other objects that are relevant to the init system

For a complete list of available systemd unit types, see the following table.

Table 3.2. Available systemd unit types

Unit TypeFile ExtensionDescription

Service unit

.service

A system service.

Target unit

.target

A group of systemd units.

Automount unit

.automount

A file system automount point.

Device unit

.device

A device file recognized by the kernel.

Mount unit

.mount

A file system mount point.

Path unit

.path

A file or directory in a file system.

Scope unit

.scope

An externally created process.

Slice unit

.slice

A group of hierarchically organized units that manage system processes.

Socket unit

.socket

An inter-process communication socket.

Swap unit

.swap

A swap device or a swap file.

Timer unit

.timer

A systemd timer.

Overriding the default systemd configuration using system.conf

The default configuration of systemd is defined during the compilation and it can be found in the systemd configuration file at /etc/systemd/system.conf. Use this file if you want to deviate from those defaults and override selected default values for systemd units globally.

For example, to override the default value of the timeout limit, which is set to 90 seconds, use the DefaultTimeoutStartSec parameter to input the required value in seconds.

DefaultTimeoutStartSec=required value

For further information, see Example 3.11, “Changing the timeout limit”.

3.1.1. Main features

The systemd system and service manager provides the following main features:

  • Socket-based activation — At boot time, systemd creates listening sockets for all system services that support this type of activation, and passes the sockets to these services as soon as they are started. This not only allows systemd to start services in parallel, but also makes it possible to restart a service without losing any message sent to it while it is unavailable: the corresponding socket remains accessible and all messages are queued.

    Systemd uses socket units for socket-based activation.

  • Bus-based activation — System services that use D-Bus for inter-process communication can be started on-demand the first time a client application attempts to communicate with them. Systemd uses D-Bus service files for bus-based activation.
  • Device-based activation — System services that support device-based activation can be started on-demand when a particular type of hardware is plugged in or becomes available. Systemd uses device units for device-based activation.
  • Path-based activation — System services that support path-based activation can be started on-demand when a particular file or directory changes its state. Systemd uses path units for path-based activation.
  • Mount and automount point managementSystemd monitors and manages mount and automount points. Systemd uses mount units for mount points and automount units for automount points.
  • Aggressive parallelization — Because of the use of socket-based activation, systemd can start system services in parallel as soon as all listening sockets are in place. In combination with system services that support on-demand activation, parallel activation significantly reduces the time required to boot the system.
  • Transactional unit activation logic — Before activating or deactivating a unit, systemd calculates its dependencies, creates a temporary transaction, and verifies that this transaction is consistent. If a transaction is inconsistent, systemd automatically attempts to correct it and remove non-essential jobs from it before reporting an error.
  • Backwards compatibility with SysV initSystemd supports SysV init scripts as described in the Linux Standard Base Core Specification, which eases the upgrade path to systemd service units.

3.1.2. Compatibility changes

The systemd system and service manager is designed to be mostly compatible with SysV init and Upstart. The following are the most notable compatibility changes with regards to Red Hat Enterprise Linux 6 system that used SysV init:

  • Systemd has only limited support for runlevels. It provides a number of target units that can be directly mapped to these runlevels and for compatibility reasons, it is also distributed with the earlier runlevel command. Not all systemd targets can be directly mapped to runlevels, however, and as a consequence, this command might return N to indicate an unknown runlevel. It is recommended that you avoid using the runlevel command if possible.
    For more information about systemd targets and their comparison with runlevels, see Section 3.3, “Working with systemd targets”.
  • The systemctl utility does not support custom commands. In addition to standard commands such as start, stop, and status, authors of SysV init scripts could implement support for any number of arbitrary commands in order to provide additional functionality. For example, the init script for iptables could be executed with the panic command, which immediately enabled panic mode and reconfigured the system to start dropping all incoming and outgoing packets. This is not supported in systemd and the systemctl only accepts documented commands.

    For more information about the systemctl utility and its comparison with the earlier service utility, see Table 3.3, “Comparison of the service utility with systemctl”.

  • The systemctl utility does not communicate with services that have not been started by systemd. When systemd starts a system service, it stores the ID of its main process in order to keep track of it. The systemctl utility then uses this PID to query and manage the service. Consequently, if a user starts a particular daemon directly on the command line, systemctl is unable to determine its current status or stop it.
  • Systemd stops only running services. Previously, when the shutdown sequence was initiated, Red Hat Enterprise Linux 6 and earlier releases of the system used symbolic links located in the /etc/rc0.d/ directory to stop all available system services regardless of their status. With systemd , only running services are stopped on shutdown.
  • System services are unable to read from the standard input stream. When systemd starts a service, it connects its standard input to /dev/null to prevent any interaction with the user.
  • System services do not inherit any context (such as the HOME and PATH environment variables) from the invoking user and their session. Each service runs in a clean execution context.
  • When loading a SysV init script, systemd reads dependency information encoded in the Linux Standard Base (LSB) header and interprets it at run time.
  • All operations on service units are subject to a default timeout of 5 minutes to prevent a malfunctioning service from freezing the system. This value is hardcoded for services that are generated from initscripts and cannot be changed. However, individual configuration files can be used to specify a longer timeout value per service, see Example 3.11, “Changing the timeout limit”.

For a detailed list of compatibility changes introduced with systemd, see the Migration Planning Guide for Red Hat Enterprise Linux 7.

3.2. Managing system services

Previous versions of Red Hat Enterprise Linux, which were distributed with SysV init or Upstart, used init scripts located in the /etc/rc.d/init.d/ directory. These init scripts were typically written in Bash, and allowed the system administrator to control the state of services and daemons in their system. Starting with Red Hat Enterprise Linux 7, these init scripts have been replaced with service units.

Service units end with the .service file extension and serve a similar purpose as init scripts. To view, start, stop, restart, enable, or disable system services, use the systemctl command as described in Comparison of the service utility with systemctl, Comparison of the chkconfig utility with systemctl, and further in this section. The service and chkconfig commands are still available in the system and work as expected, but are only included for compatibility reasons and should be avoided.

Table 3.3. Comparison of the service utility with systemctl

servicesystemctlDescription

service name start

systemctl start name.service

Starts a service.

service name stop

systemctl stop name.service

Stops a service.

service name restart

systemctl restart name.service

Restarts a service.

service name condrestart

systemctl try-restart name.service

Restarts a service only if it is running.

service name reload

systemctl reload name.service

Reloads configuration.

service name status

systemctl status name.service

systemctl is-active name.service

Checks if a service is running.

service --status-all

systemctl list-units --type service --all

Displays the status of all services.

Table 3.4. Comparison of the chkconfig utility with systemctl

chkconfigsystemctlDescription

chkconfig name on

systemctl enable name.service

Enables a service.

chkconfig name off

systemctl disable name.service

Disables a service.

chkconfig --list name

systemctl status name.service

systemctl is-enabled name.service

Checks if a service is enabled.

chkconfig --list

systemctl list-unit-files --type service

Lists all services and checks if they are enabled.

chkconfig --list

systemctl list-dependencies --after

Lists services that are ordered to start before the specified unit.

chkconfig --list

systemctl list-dependencies --before

Lists services that are ordered to start after the specified unit.

Specifying service units

For clarity, all command examples in the rest of this section use full unit names with the .service file extension, for example:

# systemctl stop nfs-server.service

However, the file extension can be omitted, in which case the systemctl utility assumes the argument is a service unit. The following command is equivalent to the one above:

# systemctl stop nfs-server

Additionally, some units have alias names. Those names can have shorter names than units, which can be used instead of the actual unit names. To find all aliases that can be used for a particular unit, use:

# systemctl show nfs-server.service -p Names

Behavior of systemctl in a chroot environment

If you change the root directory using the chroot command, most systemctl commands refuse to perform any action. The reason for this is that the systemd process and the user that used the chroot command do not have the same view of the filesystem. This happens, for example, when systemctl is invoked from a kickstart file.

The exception to this are unit file commands such as the systemctl enable and systemctl disable commands. These commands do not need a running system and do not affect running processes, but they do affect unit files. Therefore, you can run these commands even in chroot environment. For example, to enable the httpd service on a system under the /srv/website1/ directory:

# chroot /srv/website1
# systemctl enable httpd.service
Created symlink /etc/systemd/system/multi-user.target.wants/httpd.service, pointing to /usr/lib/systemd/system/httpd.service.

3.2.1. Listing services

To list all currently loaded service units, type the following at a shell prompt:

systemctl list-units --type service

For each service unit file, this command displays its full name (UNIT) followed by a note whether the unit file has been loaded (LOAD), its high-level (ACTIVE) and low-level (SUB) unit file activation state, and a short description (DESCRIPTION).

By default, the systemctl list-units command displays only active units. If you want to list all loaded units regardless of their state, run this command with the --all or -a command line option:

systemctl list-units --type service --all

You can also list all available service units to see if they are enabled. To do so, type:

systemctl list-unit-files --type service

For each service unit, this command displays its full name (UNIT FILE) followed by information whether the service unit is enabled or not (STATE). For information on how to determine the status of individual service units, see Displaying service status.

Example 3.1. Listing services

To list all currently loaded service units, run the following command:

systemctl list-units --type service
UNIT                           LOAD   ACTIVE SUB     DESCRIPTION
abrt-ccpp.service              loaded active exited  Install ABRT coredump hook
abrt-oops.service              loaded active running ABRT kernel log watcher
abrt-vmcore.service            loaded active exited  Harvest vmcores for ABRT
abrt-xorg.service              loaded active running ABRT Xorg log watcher
abrtd.service                  loaded active running ABRT Automated Bug Reporting Tool
…​
systemd-vconsole-setup.service loaded active exited  Setup Virtual Console
tog-pegasus.service            loaded active running OpenPegasus CIM Server

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

46 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'

To list all installed service unit files to determine if they are enabled, type:

$ systemctl list-unit-files --type service
UNIT FILE                                   STATE
abrt-ccpp.service                           enabled
abrt-oops.service                           enabled
abrt-vmcore.service                         enabled
abrt-xorg.service                           enabled
abrtd.service                               enabled
…​
wpa_supplicant.service                      disabled
ypbind.service                              disabled

208 unit files listed.

3.2.2. Displaying service status

To display detailed information about a service unit that corresponds to a system service, type the following at a shell prompt:

systemctl status name.service

Replace name with the name of the service unit you want to inspect (for example, gdm). This command displays the name of the selected service unit followed by its short description, one or more fields described in Table 3.5, “Available service unit information”, and if it is executed by the root user, also the most recent log entries.

Table 3.5. Available service unit information

FieldDescription

Loaded

Information whether the service unit has been loaded, the absolute path to the unit file, and a note whether the unit is enabled.

Active

Information whether the service unit is running followed by a time stamp.

Main PID

The PID of the corresponding system service followed by its name.

Status

Additional information about the corresponding system service.

Process

Additional information about related processes.

CGroup

Additional information about related Control Groups (cgroups).

To only verify that a particular service unit is running, run the following command:

systemctl is-active name.service

Similarly, to determine whether a particular service unit is enabled, type:

systemctl is-enabled name.service

Note that both systemctl is-active and systemctl is-enabled return an exit status of 0 if the specified service unit is running or enabled. For information on how to list all currently loaded service units, see Listing services.

Example 3.2. Displaying service status

The service unit for the GNOME Display Manager is named gdm.service. To determine the current status of this service unit, type the following at a shell prompt:

# systemctl status gdm.service
gdm.service - GNOME Display Manager
   Loaded: loaded (/usr/lib/systemd/system/gdm.service; enabled)
   Active: active (running) since Thu 2013-10-17 17:31:23 CEST; 5min ago
 Main PID: 1029 (gdm)
   CGroup: /system.slice/gdm.service
           ├─1029 /usr/sbin/gdm
           ├─1037 /usr/libexec/gdm-simple-slave --display-id /org/gno…​
           └─1047 /usr/bin/Xorg :0 -background none -verbose -auth /r…​

Oct 17 17:31:23 localhost systemd[1]: Started GNOME Display Manager.

Example 3.3. Displaying services ordered to start before a service

To determine what services are ordered to start before the specified service, type the following at a shell prompt:

# systemctl list-dependencies --after gdm.service
gdm.service
├─dbus.socket
├─getty@tty1.service
├─livesys.service
├─plymouth-quit.service
├─system.slice
├─systemd-journald.socket
├─systemd-user-sessions.service
└─basic.target
[output truncated]

Example 3.4. Displaying services ordered to start after a service

To determine what services are ordered to start after the specified service, type the following at a shell prompt:

# systemctl list-dependencies --before gdm.service
gdm.service
├─dracut-shutdown.service
├─graphical.target
│ ├─systemd-readahead-done.service
│ ├─systemd-readahead-done.timer
│ └─systemd-update-utmp-runlevel.service
└─shutdown.target
  ├─systemd-reboot.service
  └─final.target
    └─systemd-reboot.service

3.2.3. Starting a service

To start a service unit that corresponds to a system service, type the following at a shell prompt as root:

systemctl start name.service

Replace name with the name of the service unit you want to start (for example, gdm). This command starts the selected service unit in the current session. For information on how to enable a service unit to be started at boot time, see Enabling a service. For information on how to determine the status of a certain service unit, see Displaying service status.

Example 3.5. Starting a service

The service unit for the Apache HTTP Server is named httpd.service. To activate this service unit and start the httpd daemon in the current session, run the following command as root:

# systemctl start httpd.service

3.2.4. Stopping a service

To stop a service unit that corresponds to a system service, type the following at a shell prompt as root:

systemctl stop name.service

Replace name with the name of the service unit you want to stop (for example, bluetooth). This command stops the selected service unit in the current session. For information on how to disable a service unit and prevent it from being started at boot time, see Disabling a service. For information on how to determine the status of a certain service unit, see Displaying service status.

Example 3.6. Stopping a service

The service unit for the bluetoothd daemon is named bluetooth.service. To deactivate this service unit and stop the bluetoothd daemon in the current session, run the following command as root:

# systemctl stop bluetooth.service

3.2.5. Restarting a service

To restart a service unit that corresponds to a system service, type the following at a shell prompt as root:

systemctl restart name.service

Replace name with the name of the service unit you want to restart (for example, httpd). This command stops the selected service unit in the current session and immediately starts it again. Importantly, if the selected service unit is not running, this command starts it too. To tell systemd to restart a service unit only if the corresponding service is already running, run the following command as root:

systemctl try-restart name.service

Certain system services also allow you to reload their configuration without interrupting their execution. To do so, type as root:

systemctl reload name.service

Note that system services that do not support this feature ignore this command altogether. For convenience, the systemctl command also supports the reload-or-restart and reload-or-try-restart commands that restart such services instead. For information on how to determine the status of a certain service unit, see Displaying service status.

Example 3.7. Restarting a service

In order to prevent users from encountering unnecessary error messages or partially rendered web pages, the Apache HTTP Server allows you to edit and reload its configuration without the need to restart it and interrupt actively processed requests. To do so, type the following at a shell prompt as root:

# systemctl reload httpd.service

3.2.6. Enabling a service

To configure a service unit that corresponds to a system service to be automatically started at boot time, type the following at a shell prompt as root:

systemctl enable name.service

Replace name with the name of the service unit you want to enable (for example, httpd). This command reads the [Install] section of the selected service unit and creates appropriate symbolic links to the /usr/lib/systemd/system/name.service file in the /etc/systemd/system/ directory and its subdirectories. This command does not, however, rewrite links that already exist. If you want to ensure that the symbolic links are re-created, use the following command as root:

systemctl reenable name.service

This command disables the selected service unit and immediately enables it again. For information on how to determine whether a certain service unit is enabled to start at boot time, see Displaying service status. For information on how to start a service in the current session, see Starting a service.

Example 3.8. Enabling a service

To configure the Apache HTTP Server to start automatically at boot time, run the following command as root:

# systemctl enable httpd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to /usr/lib/systemd/system/httpd.service.

3.2.7. Disabling a service

To prevent a service unit that corresponds to a system service from being automatically started at boot time, type the following at a shell prompt as root:

systemctl disable name.service

Replace name with the name of the service unit you want to disable (for example, bluetooth). This command reads the [Install] section of the selected service unit and removes appropriate symbolic links to the /usr/lib/systemd/system/name.service file from the /etc/systemd/system/ directory and its subdirectories. In addition, you can mask any service unit to prevent it from being started manually or by another service. To do so, run the following command as root:

systemctl mask name.service

This command replaces the /etc/systemd/system/name.service file with a symbolic link to /dev/null, rendering the actual unit file inaccessible to systemd. To revert this action and unmask a service unit, type as root:

systemctl unmask name.service

For information on how to determine whether a certain service unit is enabled to start at boot time, see Displaying service status. For information on how to stop a service in the current session, see Stopping a service.

Example 3.9. Disabling a service

Example 3.6, “Stopping a service” illustrates how to stop the bluetooth.service unit in the current session. To prevent this service unit from starting at boot time, type the following at a shell prompt as root:

# systemctl disable bluetooth.service
Removed symlink /etc/systemd/system/bluetooth.target.wants/bluetooth.service.
Removed symlink /etc/systemd/system/dbus-org.bluez.service.

3.2.8. Starting a conflicting service

In systemd, positive and negative dependencies between services exist. Starting particular service may require starting one or more other services (positive dependency) or stopping one or more services (negative dependency).

When you attempt to start a new service, systemd resolves all dependencies automatically. Note that this is done without explicit notification to the user. If you are already running a service, and you attempt to start another service with a negative dependency, the first service is automatically stopped.

For example, if you are running the postfix service, and you try to start the sendmail service, systemd first automatically stops postfix, because these two services are conflicting and cannot run on the same port.

3.3. Working with systemd targets

Systemd targets are represented by target units. Target units file ends with the .target file extension and their only purpose is to group together other systemd units through a chain of dependencies. For example, the graphical.target unit, which is used to start a graphical session, starts system services such as the GNOME Display Manager (gdm.service) or Accounts Service (accounts-daemon.service) and also activates the multi-user.target unit. Similarly, the multi-user.target unit starts other essential system services such as NetworkManager (NetworkManager.service) or D-Bus (dbus.service) and activates another target unit named basic.target.

This section includes procedures to implement while working with systemd targets.

3.3.1. Difference between SysV runlevels and systemd targets

The previous versions of Red Hat Enterprise Linux were distributed with SysV init or Upstart, and implemented a predefined set of runlevels that represented specific modes of operation. These runlevels were numbered from 0 to 6 and were defined by a selection of system services to be run when a particular runlevel was enabled by the system administrator. Starting with Red Hat Enterprise Linux 7, the concept of runlevels has been replaced with systemd targets.

Red Hat Enterprise Linux 7 was distributed with a number of predefined targets that are more or less similar to the standard set of runlevels from the previous releases. For compatibility reasons, it also provides aliases for these targets that directly maps to the SysV runlevels.

The following table provides a complete list of SysV runlevels and their corresponding systemd targets:

Table 3.6. Comparison of SysV runlevels with systemd targets

RunlevelTarget UnitsDescription

0

runlevel0.target, poweroff.target

Shut down and power off the system.

1

runlevel1.target, rescue.target

Set up a rescue shell.

2

runlevel2.target, multi-user.target

Set up a non-graphical multi-user system.

3

runlevel3.target, multi-user.target

Set up a non-graphical multi-user system.

4

runlevel4.target, multi-user.target

Set up a non-graphical multi-user system.

5

runlevel5.target, graphical.target

Set up a graphical multi-user system.

6

runlevel6.target, reboot.target

Shut down and reboot the system.

The following table compares the SysV init commands with systemctl. Use the systemctl utility to view, change, or configure systemd targets:

Important

The runlevel and telinit commands are still available in the system and work as expected, but are only included for compatibility reasons and should be avoided.

Table 3.7. Comparison of SysV init commands with systemctl

Old CommandNew CommandDescription

runlevel

systemctl list-units --type target

Lists currently loaded target units.

telinit runlevel

systemctl isolate name.target

Changes the current target.

Additional resources

  • man sysv init
  • man upstart init
  • man systemctl

3.3.2. Viewing the default target

The default target unit is represented by the /etc/systemd/system/default.target file.

Procedure

  • To determine which target unit is used by default:

    $ systemctl get-default
    graphical.target
  • To determine the default target using the symbolic link:

    $  ls -l /lib/systemd/system/default.target

3.3.3. Viewing the target units

By default, the systemctl list-units command displays only active units.

Procedure

  • To list all loaded units regardless of their state:

    $ systemctl list-units --type target --all
  • To list all currently loaded target units:

    $ systemctl list-units --type target
    
    UNIT                  LOAD   ACTIVE SUB    DESCRIPTION
    basic.target          loaded active active Basic System
    cryptsetup.target     loaded active active Encrypted Volumes
    getty.target          loaded active active Login Prompts
    graphical.target      loaded active active Graphical Interface
    local-fs-pre.target   loaded active active Local File Systems (Pre)
    local-fs.target       loaded active active Local File Systems
    multi-user.target     loaded active active Multi-User System
    network.target        loaded active active Network
    paths.target          loaded active active Paths
    remote-fs.target      loaded active active Remote File Systems
    sockets.target        loaded active active Sockets
    sound.target          loaded active active Sound Card
    spice-vdagentd.target loaded active active Agent daemon for Spice guests
    swap.target           loaded active active Swap
    sysinit.target        loaded active active System Initialization
    time-sync.target      loaded active active System Time Synchronized
    timers.target         loaded active active Timers
    
    LOAD   = Reflects whether the unit definition was properly loaded.
    ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
    SUB    = The low-level unit activation state, values depend on unit type.
    
    17 loaded units listed.

3.3.4. Changing the default target

The default target unit is represented by the /etc/systemd/system/default.target file. The following procedure describes how to change the default target by using the systemctl command:

Procedure

  1. To determine the default target unit:

    # systemctl get-default
  2. To configure the system to use a different target unit by default:

    # systemctl set-default multi-user.target
    rm /etc/systemd/system/default.target
    ln -s /usr/lib/systemd/system/multi-user.target /etc/systemd/system/default.target

    This command replaces the /etc/systemd/system/default.target file with a symbolic link to /usr/lib/systemd/system/name.target, where name is the name of the target unit you want to use. Replace multi-user with the name of the target unit you want to use by default.

  3. Reboot

    # reboot

3.3.6. Changing the current target

This procedure explains how to change the target unit in the current session using the systemctl command.

Procedure

  • To change to a different target unit in the current session:

    # systemctl isolate multi-user.target

    This command starts the target unit named multi-user and all dependent units, and immediately stops all others.

Replace multi-user with the name of the target unit you want to use by default.

Verification steps

  • Verify the newly created default.target:

    $ systemctl get-default
    multi-user.target

3.3.7. Booting to rescue mode

Rescue mode provides a convenient single-user environment and allows you to repair your system in situations when it is unable to complete a regular booting process. In rescue mode, the system attempts to mount all local file systems and start some important system services, but it does not activate network interfaces or allow more users to be logged into the system at the same time.

Procedure

  • To change the current target and enter rescue mode in the current session:

    # systemctl rescue
    
    Broadcast message from root@localhost on pts/0 (Fri 2013-10-25 18:23:15 CEST):
    
    The system is going down to rescue mode NOW!

    This command is similar to systemctl isolate rescue.target, but it also sends an informative message to all users that are currently logged into the system.

    To prevent systemd from sending a message, run the following command with the --no-wall command-line option: # systemctl --no-wall rescue

3.3.8. Booting to emergency mode

Emergency mode provides the most minimal environment possible and allows you to repair your system even in situations when the system is unable to enter rescue mode. In emergency mode, the system mounts the root file system only for reading, does not attempt to mount any other local file systems, does not activate network interfaces, and only starts a few essential services.

Procedure

  • To change the current target and enter emergency mode:

    # systemctl emergency

This command is similar to systemctl isolate emergency.target, but it also sends an informative message to all users that are currently logged into the system.

To prevent systemd from sending this message, run the following command with the --no-wall command-line option: # systemctl --no-wall emergency

3.4. Shutting down, suspending, and hibernating the system

In Red Hat Enterprise Linux 7, the systemctl utility replaced a number of power management commands used in previous versions of Red Hat Enterprise Linux. The commands listed in Table 3.8, “Comparison of power management commands with systemctl” are still available in the system for compatibility reasons, but it is advised that you use systemctl when possible.

Table 3.8. Comparison of power management commands with systemctl

Old CommandNew CommandDescription

halt

systemctl halt

Halts the system.

poweroff

systemctl poweroff

Powers off the system.

reboot

systemctl reboot

Restarts the system.

pm-suspend

systemctl suspend

Suspends the system.

pm-hibernate

systemctl hibernate

Hibernates the system.

pm-suspend-hybrid

systemctl hybrid-sleep

Hibernates and suspends the system.

3.4.1. Shutting down the system

The systemctl utility provides commands for shutting down the system, however the traditional shutdown command is also supported. Although the shutdown command will call the systemctl utility to perform the shutdown, it has an advantage in that it also supports a time argument. This is particularly useful for scheduled maintenance and to allow more time for users to react to the warning that a system shutdown has been scheduled. The option to cancel the shutdown can also be an advantage.

Using systemctl commands

To shut down the system and power off the machine, type the following at a shell prompt as root:

systemctl poweroff

To shut down and halt the system without powering off the machine, run the following command as root:

systemctl halt

By default, running either of these commands causes systemd to send an informative message to all users that are currently logged into the system. To prevent systemd from sending this message, run the selected command with the --no-wall command line option, for example:

systemctl --no-wall poweroff

Using the shutdown command

To shut down the system and power off the machine at a certain time, use a command in the following format as root:

shutdown --poweroff hh:mm

Where hh:mm is the time in 24 hour clock format. The /run/nologin file is created 5 minutes before system shutdown to prevent new logins. When a time argument is used, an optional message, the wall message, can be appended to the command.

To shut down and halt the system after a delay, without powering off the machine, use a command in the following format as root:

shutdown --halt +m

Where +m is the delay time in minutes. The now keyword is an alias for +0.

A pending shutdown can be canceled by the root user as follows:

shutdown -c

See the shutdown(8) manual page for further command options.

3.4.2. Restarting the system

To restart the system, run the following command as root:

systemctl reboot

By default, this command causes systemd to send an informative message to all users that are currently logged into the system. To prevent systemd from sending this message, run this command with the --no-wall command line option:

systemctl --no-wall reboot

3.4.3. Suspending the system

To suspend the system, type the following at a shell prompt as root:

systemctl suspend

This command saves the system state in RAM and with the exception of the RAM module, powers off most of the devices in the machine. When you turn the machine back on, the system then restores its state from RAM without having to boot again. Because the system state is saved in RAM and not on the hard disk, restoring the system from suspend mode is significantly faster than restoring it from hibernation, but as a consequence, a suspended system state is also vulnerable to power outages.

For information on how to hibernate the system, see Section 3.4.4, “Hibernating the system”.

3.4.4. Hibernating the system

To hibernate the system, type the following at a shell prompt as root:

systemctl hibernate

This command saves the system state on the hard disk drive and powers off the machine. When you turn the machine back on, the system then restores its state from the saved data without having to boot again. Because the system state is saved on the hard disk and not in RAM, the machine does not have to maintain electrical power to the RAM module, but as a consequence, restoring the system from hibernation is significantly slower than restoring it from suspend mode.

To hibernate and suspend the system, run the following command as root:

systemctl hybrid-sleep

For information on how to suspend the system, see Section 3.4.3, “Suspending the system”.

3.5. Working with systemd unit files

This chapter includes the description of systemd unit files. The following sections show you how to:

  • Create custom unit files
  • Convert SysV init scripts to unit files
  • Modify existing unit files
  • Work with instantiated units

3.5.1. Introduction to unit files

A unit file contains configuration directives that describe the unit and define its behavior. Several systemctl commands work with unit files in the background. To make finer adjustments, system administrator must edit or create unit files manually. Table 3.1, “Systemd unit files locations” lists three main directories where unit files are stored on the system, the /etc/systemd/system/ directory is reserved for unit files created or customized by the system administrator.

Unit file names take the following form:

unit_name.type_extension

Here, unit_name stands for the name of the unit and type_extension identifies the unit type, see Table 3.2, “Available systemd unit types” for a complete list of unit types. For example, there usually is sshd.service as well as sshd.socket unit present on your system.

Unit files can be supplemented with a directory for additional configuration files. For example, to add custom configuration options to sshd.service, create the sshd.service.d/custom.conf file and insert additional directives there. For more information on configuration directories, see Modifying existing unit files.

Also, the sshd.service.wants/ and sshd.service.requires/ directories can be created. These directories contain symbolic links to unit files that are dependencies of the sshd service. The symbolic links are automatically created either during installation according to [Install] unit file options or at runtime based on [Unit] options. It is also possible to create these directories and symbolic links manually. For more details on [Install] and [Unit] options, see the tables below.

Many unit file options can be set using the so called unit specifiers – wildcard strings that are dynamically replaced with unit parameters when the unit file is loaded. This enables creation of generic unit files that serve as templates for generating instantiated units. See Working with instantiated units for details.

3.5.2. Unit file structure

Unit files typically consist of three sections:

  • The [Unit] section — contains generic options that are not dependent on the type of the unit. These options provide unit description, specify the unit’s behavior, and set dependencies to other units. For a list of most frequently used [Unit] options, see Table 3.9, “Important [Unit] section options”.
  • The [Unit type] section — if a unit has type-specific directives, these are grouped under a section named after the unit type. For example, service unit files contain the [Service] section.
  • The [Install] section — contains information about unit installation used by systemctl enable and disable commands. For a list of options for the [Install] section, see Table 3.11, “Important [Install] section options”.

3.5.2.1. Important [Unit] section options

The following tables lists important options of the [Unit] section.

Table 3.9. Important [Unit] section options

Option[a]] section, see the systemd.unit(5) manual page.]Description

Description

A meaningful description of the unit. This text is displayed for example in the output of the systemctl status command.

Documentation

Provides a list of URIs referencing documentation for the unit.

After[b]

Defines the order in which units are started. The unit starts only after the units specified in After are active. Unlike Requires, After does not explicitly activate the specified units. The Before option has the opposite functionality to After.

Requires

Configures dependencies on other units. The units listed in Requires are activated together with the unit. If any of the required units fail to start, the unit is not activated.

Wants

Configures weaker dependencies than Requires. If any of the listed units does not start successfully, it has no impact on the unit activation. This is the recommended way to establish custom unit dependencies.

Conflicts

Configures negative dependencies, an opposite to Requires.

[a] For a complete list of options configurable in the [[Unit
[b] In most cases, it is sufficient to set only the ordering dependencies with After and Before unit file options. If you also set a requirement dependency with Wants (recommended) or Requires, the ordering dependency still needs to be specified. That is because ordering and requirement dependencies work independently from each other.

3.5.2.2. Important [Service] section options

The following tables lists important options of the [Service] section.

Table 3.10. Important [Service] section options

Option[a]] section, see the systemd.service(5) manual page.]Description

Type

Configures the unit process startup type that affects the functionality of ExecStart and related options. One of:

* simple – The default value. The process started with ExecStart is the main process of the service.

* forking – The process started with ExecStart spawns a child process that becomes the main process of the service. The parent process exits when the startup is complete.

* oneshot – This type is similar to simple, but the process exits before starting consequent units.

* dbus – This type is similar to simple, but consequent units are started only after the main process gains a D-Bus name.

* notify – This type is similar to simple, but consequent units are started only after a notification message is sent via the sd_notify() function.

* idle – similar to simple, the actual execution of the service binary is delayed until all jobs are finished, which avoids mixing the status output with shell output of services.

ExecStart

Specifies commands or scripts to be executed when the unit is started. ExecStartPre and ExecStartPost specify custom commands to be executed before and after ExecStart. Type=oneshot enables specifying multiple custom commands that are then executed sequentially.

ExecStop

Specifies commands or scripts to be executed when the unit is stopped.

ExecReload

Specifies commands or scripts to be executed when the unit is reloaded.

Restart

With this option enabled, the service is restarted after its process exits, with the exception of a clean stop by the systemctl command.

RemainAfterExit

If set to True, the service is considered active even when all its processes exited. Default value is False. This option is especially useful if Type=oneshot is configured.

[a] For a complete list of options configurable in the [[Service

3.5.2.3. Important [Install] section options

The following tables lists important options of the [Install] section.

Table 3.11. Important [Install] section options

Option[a]] section, see the systemd.unit(5) manual page.]Description

Alias

Provides a space-separated list of additional names for the unit. Most systemctl commands, excluding systemctl enable, can use aliases instead of the actual unit name.

RequiredBy

A list of units that depend on the unit. When this unit is enabled, the units listed in RequiredBy gain a Require dependency on the unit.

WantedBy

A list of units that weakly depend on the unit. When this unit is enabled, the units listed in WantedBy gain a Want dependency on the unit.

Also

Specifies a list of units to be installed or uninstalled along with the unit.

DefaultInstance

Limited to instantiated units, this option specifies the default instance for which the unit is enabled. See Working with instantiated units

[a] For a complete list of options configurable in the [[Install

3.5.3. Creating custom unit files

There are several use cases for creating unit files from scratch: you could run a custom daemon, create a second instance of some existing service (as in Creating a second instance of the sshd service), or import a SysV init script (more in Converting SysV init scripts to unit files). On the other hand, if you intend just to modify or extend the behavior of an existing unit, use the instructions from Modifying existing unit files. The following procedure describes the general process of creating a custom service.

Procedure

  1. Prepare the executable file with the custom service. This can be a custom-created script, or an executable delivered by a software provider. If required, prepare a PID file to hold a constant PID for the main process of the custom service. It is also possible to include environment files to store shell variables for the service. Make sure the source script is executable (by executing the chmod a+x) and is not interactive.
  2. Create a unit file in the /etc/systemd/system/ directory and make sure it has correct file permissions. Execute as root:

    touch /etc/systemd/system/name.service
    
    chmod 664 /etc/systemd/system/name.service

    Replace name with a name of the service to be created. Note that file does not need to be executable.

  3. Open the name.service file created in the previous step, and add the service configuration options. There is a variety of options that can be used depending on the type of service you wish to create, see Unit file structure. The following is an example unit configuration for a network-related service:

    [Unit]
    Description=service_description
    After=network.target
    
    [Service]
    ExecStart=path_to_executable
    Type=forking
    PIDFile=path_to_pidfile
    
    [Install]
    WantedBy=default.target

    Where:

    • service_description is an informative description that is displayed in journal log files and in the output of the systemctl status command.
    • the After setting ensures that the service is started only after the network is running. Add a space-separated list of other relevant services or targets.
    • path_to_executable stands for the path to the actual service executable.
    • Type=forking is used for daemons that make the fork system call. The main process of the service is created with the PID specified in path_to_pidfile. Find other startup types in Table 3.10, “Important [Service] section options”.
    • WantedBy states the target or targets that the service should be started under. Think of these targets as of a replacement of the older concept of runlevels.
  4. Notify systemd that a new name.service file exists by executing the following command as root:

    systemctl daemon-reload
    
    systemctl start name.service
    Warning

    Always run the systemctl daemon-reload command after creating new unit files or modifying existing unit files. Otherwise, the systemctl start or systemctl enable commands could fail due to a mismatch between states of systemd and actual service unit files on disk. Note, that on systems with a large number of units this can take a long time, as the state of each unit has to be serialized and subsequently deserialized during the reload.

3.5.3.1. Creating a custom unit file by using the second instance of the sshd service

System Administrators often need to configure and run multiple instances of a service. This is done by creating copies of the original service configuration files and modifying certain parameters to avoid conflicts with the primary instance of the service. The following procedure shows how to create a second instance of the sshd service.

Procedure

  1. Create a copy of the sshd_config file that will be used by the second daemon:

    # cp /etc/ssh/sshd{,-second}_config
  2. Edit the sshd-second_config file created in the previous step to assign a different port number and PID file to the second daemon:

    Port 22220
    PidFile /var/run/sshd-second.pid

    See the sshd_config(5) manual page for more information on Port and PidFile options. Make sure the port you choose is not in use by any other service. The PID file does not have to exist before running the service, it is generated automatically on service start.

  3. Create a copy of the systemd unit file for the sshd service:

    # cp /usr/lib/systemd/system/sshd.service /etc/systemd/system/sshd-second.service
  4. Alter the sshd-second.service created in the previous step as follows:

    1. Modify the Description option:

      Description=OpenSSH server second instance daemon
    2. Add sshd.service to services specified in the After option, so that the second instance starts only after the first one has already started:

      After=syslog.target network.target auditd.service sshd.service
    3. The first instance of sshd includes key generation, therefore remove the ExecStartPre=/usr/sbin/sshd-keygen line.
    4. Add the -f /etc/ssh/sshd-second_config parameter to the sshd command, so that the alternative configuration file is used:

      ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS
    5. After the above modifications, the sshd-second.service should look as follows:

      [Unit]
      Description=OpenSSH server second instance daemon
      After=syslog.target network.target auditd.service sshd.service
      
      [Service]
      EnvironmentFile=/etc/sysconfig/sshd
      ExecStart=/usr/sbin/sshd -D -f /etc/ssh/sshd-second_config $OPTIONS
      ExecReload=/bin/kill -HUP $MAINPID
      KillMode=process
      Restart=on-failure
      RestartSec=42s
      
      [Install]
      WantedBy=multi-user.target
  5. If using SELinux, add the port for the second instance of sshd to SSH ports, otherwise the second instance of sshd will be rejected to bind to the port:

    # semanage port -a -t ssh_port_t -p tcp 22220
  6. Enable sshd-second.service, so that it starts automatically upon boot:

    # systemctl enable sshd-second.service
  7. Verify if the sshd-second.service is running by using the systemctl status command.
  8. Verify if the port is enabled correctly by connecting to the service:

    ssh -p 22220 user@server

    If the firewall is in use, make sure that it is configured appropriately in order to allow connections to the second instance of sshd.

3.5.3.2. Choosing a target for ordering and dependencies of custom unit files

To learn how to properly choose a target for ordering and dependencies of your custom unit files, see the following articles:

Additional information with some real-world examples of cases triggered by the ordering and dependencies in a unit file is available in Red Hat Knowledgebase article Is there any useful information about writing unit files?

If you want to set limits for services started by systemd, see the Red Hat Knowledgebase article How to set limits for services in RHEL 7 and systemd. These limits need to be set in the service’s unit file. Note that systemd ignores limits set in the /etc/security/limits.conf and /etc/security/limits.d/*.conf configuration files. The limits defined in these files are set by PAM when starting a login session, but daemons started by systemd do not use PAM login sessions.

3.5.4. Converting SysV init scripts to unit files

Before taking time to convert a SysV init script to a unit file, make sure that the conversion was not already done elsewhere. All core services installed on Red Hat Enterprise Linux come with default unit files, and the same applies for many third-party software packages.

Converting an init script to a unit file requires analyzing the script and extracting the necessary information from it. Based on this data you can create a unit file. As init scripts can vary greatly depending on the type of the service, you might need to employ more configuration options for translation than outlined in this chapter. Note that some levels of customization that were available with init scripts are no longer supported by systemd units.

The majority of information needed for conversion is provided in the script’s header. The following example shows the opening section of the init script used to start the postfix service on Red Hat Enterprise Linux 6:

!/bin/bash # postfix Postfix Mail Transfer Agent # chkconfig: 2345 80 30 # description: Postfix is a Mail Transport Agent, which is the program that moves mail from one machine to another. # processname: master # pidfile: /var/spool/postfix/pid/master.pid # config: /etc/postfix/main.cf # config: /etc/postfix/master.cf  BEGIN INIT INFO # Provides: postfix MTA # Required-Start: $local_fs $network $remote_fs # Required-Stop: $local_fs $network $remote_fs # Default-Start: 2 3 4 5 # Default-Stop: 0 1 6 # Short-Description: start and stop postfix # Description: Postfix is a Mail Transport Agent, which is the program that moves mail from one machine to another. # END INIT INFO

In the above example, only lines starting with # chkconfig and # description are mandatory, so you might not find the rest in different init files. The text enclosed between the BEGIN INIT INFO and END INIT INFO lines is called Linux Standard Base (LSB) header. If specified, LSB headers contain directives defining the service description, dependencies, and default runlevels. What follows is an overview of analytic tasks aiming to collect the data needed for a new unit file. The postfix init script is used as an example.

3.5.4.1. Finding the systemd service description

You can find descriptive information about the script on the line starting with #description. Use this description together with the service name in the Description option in the [Unit] section of the unit file. The LSB header might contain similar data on the #Short-Description and #Description lines.

3.5.4.2. Finding the systemd service dependencies

The LSB header might contain several directives that form dependencies between services. Most of them are translatable to systemd unit options, see Table 3.12, “Dependency options from the LSB header”

Table 3.12. Dependency options from the LSB header

LSB OptionDescriptionUnit File Equivalent

Provides

Specifies the boot facility name of the service, that can be referenced in other init scripts (with the "$" prefix). This is no longer needed as unit files refer to other units by their file names.

Required-Start

Contains boot facility names of required services. This is translated as an ordering dependency, boot facility names are replaced with unit file names of corresponding services or targets they belong to. For example, in case of postfix, the Required-Start dependency on $network was translated to the After dependency on network.target.

After, Before

Should-Start

Constitutes weaker dependencies than Required-Start. Failed Should-Start dependencies do not affect the service startup.

After, Before

Required-Stop, Should-Stop

Constitute negative dependencies.

Conflicts

3.5.4.3. Finding default targets of the service

The line starting with #chkconfig contains three numerical values. The most important is the first number that represents the default runlevels in which the service is started. Map these runlevels to equivalent systemd targets. Then list these targets in the WantedBy option in the [Install] section of the unit file. For example, postfix was previously started in runlevels 2, 3, 4, and 5, which translates to multi-user.target and graphical.target. Note that the graphical.target depends on multiuser.target, therefore it is not necessary to specify both. You might find information on default and forbidden runlevels also at #Default-Start and #Default-Stop lines in the LSB header.

The other two values specified on the #chkconfig line represent startup and shutdown priorities of the init script. These values are interpreted by systemd if it loads the init script, but there is no unit file equivalent.

3.5.4.4. Finding files used by the service

Init scripts require loading a function library from a dedicated directory and allow importing configuration, environment, and PID files. Environment variables are specified on the line starting with #config in the init script header, which translates to the EnvironmentFile unit file option. The PID file specified on the #pidfile init script line is imported to the unit file with the PIDFile option.

The key information that is not included in the init script header is the path to the service executable, and potentially some other files required by the service. In previous versions of Red Hat Enterprise Linux, init scripts used a Bash case statement to define the behavior of the service on default actions, such as start, stop, or restart, as well as custom-defined actions. The following excerpt from the postfix init script shows the block of code to be executed at service start.

conf_check() {
    [ -x /usr/sbin/postfix ] || exit 5
    [ -d /etc/postfix ] || exit 6
    [ -d /var/spool/postfix ] || exit 5
}

make_aliasesdb() {
	if [ "$(/usr/sbin/postconf -h alias_database)" == "hash:/etc/aliases" ]
	then
		# /etc/aliases.db might be used by other MTA, make sure nothing
		# has touched it since our last newaliases call
		[ /etc/aliases -nt /etc/aliases.db ] ||
			[ "$ALIASESDB_STAMP" -nt /etc/aliases.db ] ||
			[ "$ALIASESDB_STAMP" -ot /etc/aliases.db ] || return
		/usr/bin/newaliases
		touch -r /etc/aliases.db "$ALIASESDB_STAMP"
	else
		/usr/bin/newaliases
	fi
}

start() {
	[ "$EUID" != "0" ] && exit 4
	# Check that networking is up.
	[ ${NETWORKING} = "no" ] && exit 1
	conf_check
	# Start daemons.
	echo -n $"Starting postfix: "
	make_aliasesdb >/dev/null 2>&1
	[ -x $CHROOT_UPDATE ] && $CHROOT_UPDATE
	/usr/sbin/postfix start 2>/dev/null 1>&2 && success || failure $"$prog start"
	RETVAL=$?
	[ $RETVAL -eq 0 ] && touch $lockfile
        echo
	return $RETVAL
}

The extensibility of the init script allowed specifying two custom functions, conf_check() and make_aliasesdb(), that are called from the start() function block. On closer look, several external files and directories are mentioned in the above code: the main service executable /usr/sbin/postfix, the /etc/postfix/ and /var/spool/postfix/ configuration directories, as well as the /usr/sbin/postconf/ directory.

Systemd supports only the predefined actions, but enables executing custom executables with ExecStart, ExecStartPre, ExecStartPost, ExecStop, and ExecReload options. The /usr/sbin/postfix together with supporting scripts are executed on service start. Converting complex init scripts requires understanding the purpose of every statement in the script. Some of the statements are specific to the operating system version, therefore you do not need to translate them. On the other hand, some adjustments might be needed in the new environment, both in unit file as well as in the service executable and supporting files.

3.5.5. Modifying existing unit files

Services installed on the system come with default unit files that are stored in the /usr/lib/systemd/system/ directory. System Administrators should not modify these files directly, therefore any customization must be confined to configuration files in the /etc/systemd/system/ directory.

Procedure

  1. Depending on the extent of the required changes, pick one of the following approaches:

    • Create a directory for supplementary configuration files at /etc/systemd/system/unit.d/. This method is recommended for most use cases. It enables extending the default configuration with additional functionality, while still referring to the original unit file. Changes to the default unit introduced with a package upgrade are therefore applied automatically. See Extending the default unit configuration for more information.
    • Create a copy of the original unit file /usr/lib/systemd/system/ in /etc/systemd/system/ and make changes there. The copy overrides the original file, therefore changes introduced with the package update are not applied. This method is useful for making significant unit changes that should persist regardless of package updates. See Overriding the default unit configuration for details.
  2. To return to the default configuration of the unit, delete custom-created configuration files in /etc/systemd/system/.
  3. To apply changes to unit files without rebooting the system, execute:

    systemctl daemon-reload

    The daemon-reload option reloads all unit files and recreates the entire dependency tree, which is needed to immediately apply any change to a unit file. As an alternative, you can achieve the same result with the following command, which must be executed under the root user:

    init q
  4. If the modified unit file belongs to a running service, this service must be restarted to accept new settings:

    systemctl restart name.service
Important

To modify properties, such as dependencies or timeouts, of a service that is handled by a SysV initscript, do not modify the initscript itself. Instead, create a systemd drop-in configuration file for the service as described in Extending the default unit configuration and Overriding the default unit configuration. Then manage this service in the same way as a normal systemd service.

For example, to extend the configuration of the network service, do not modify the /etc/rc.d/init.d/network initscript file. Instead, create new directory /etc/systemd/system/network.service.d/ and a systemd drop-in file /etc/systemd/system/network.service.d/my_config.conf. Then, put the modified values into the drop-in file. Note: systemd knows the network service as network.service, which is why the created directory must be called network.service.d

3.5.5.1. Extending the default unit configuration

This section describes how to extend the default unit file with additional configuration options.

Procedure

  1. To extend the default unit file with additional configuration options, first create a configuration directory in /etc/systemd/system/. If extending a service unit, execute the following command as root:

    mkdir /etc/systemd/system/name.service.d/

    Replace name with the name of the service you want to extend. The above syntax applies to all unit types.

  2. Create a configuration file in the directory made in the previous step. Note that the file name must end with the .conf suffix. Type:

    touch /etc/systemd/system/name.service.d/config_name.conf

    Replace config_name with the name of the configuration file. This file adheres to the normal unit file structure, therefore all directives must be specified under appropriate sections, see Unit file structure.

    For example, to add a custom dependency, create a configuration file with the following content:

    [Unit]
    Requires=new_dependency
    After=new_dependency

    Where new_dependency stands for the unit to be marked as a dependency. Another example is a configuration file that restarts the service after its main process exited, with a delay of 30 seconds:

    [Service]
    Restart=always
    RestartSec=30

    It is recommended to create small configuration files focused only on one task. Such files can be easily moved or linked to configuration directories of other services.

  3. To apply changes made to the unit, execute as root:

    systemctl daemon-reload
    systemctl restart name.service

Example 3.10. Extending the httpd.service configuration

To modify the httpd.service unit so that a custom shell script is automatically executed when starting the Apache service, perform the following steps.

  1. Create a directory and a custom configuration file:

    # mkdir /etc/systemd/system/httpd.service.d/
    # touch /etc/systemd/system/httpd.service.d/custom_script.conf
  2. Provided that the script you want to start automatically with Apache is located at /usr/local/bin/custom.sh, insert the following text to the custom_script.conf file:

    [Service]
    ExecStartPost=/usr/local/bin/custom.sh
  3. To apply the unit changes, execute:

    # systemctl daemon-reload
    # systemctl restart httpd.service
Note

The configuration files from configuration directories in /etc/systemd/system/ take precedence over unit files in /usr/lib/systemd/system/. Therefore, if the configuration files contain an option that can be specified only once, such as Description or ExecStart, the default value of this option is overridden. Note that in the output of the systemd-delta command, described in Monitoring overridden units, such units are always marked as [EXTENDED], even though in sum, certain options are actually overridden.

3.5.5.2. Overridding the default unit configuration

This section describes how to override the default unit configuration.

Procedure

  1. To make changes that will persist after updating the package that provides the unit file, first copy the file to the /etc/systemd/system/ directory. To do so, execute the following command as root:

    cp /usr/lib/systemd/system/name.service /etc/systemd/system/name.service

    Where name stands for the name of the service unit you wish to modify. The above syntax applies to all unit types.

  2. Open the copied file with a text editor, and make the desired changes. To apply the unit changes, execute as root:

    systemctl daemon-reload
    systemctl restart name.service

Example 3.11. Changing the timeout limit

You can specify a timeout value per service to prevent a malfunctioning service from freezing the system. Otherwise, timeout is set by default to 90 seconds for normal services and to 300 seconds for SysV-compatible services.

For example, to extend timeout limit for the httpd service:

  1. Copy the httpd unit file to the /etc/systemd/system/ directory:

    cp /usr/lib/systemd/system/httpd.service /etc/systemd/system/httpd.service
  2. Open file /etc/systemd/system/httpd.service and specify the TimeoutStartUSec value in the [Service] section:

    …​
    [Service]
    …​
    PrivateTmp=true
    TimeoutStartSec=10
    
    [Install]
    WantedBy=multi-user.target
    …​
  3. Reload the systemd daemon:

    systemctl daemon-reload
  4. Optional. Verify the new timeout value:

    systemctl show httpd -p TimeoutStartUSec
Note

To change the timeout limit globally, input the DefaultTimeoutStartSec in the /etc/systemd/system.conf file.

3.5.5.3. Monitoring overriden units

This section describes how to display an overview of overridden or modified unit files.

Procedure

  1. To display an overview of overridden or modified unit files, use the following command:

    systemd-delta

    For example, the output of the above command can look as follows:

    [EQUIVALENT] /etc/systemd/system/default.target → /usr/lib/systemd/system/default.target
    [OVERRIDDEN] /etc/systemd/system/autofs.service → /usr/lib/systemd/system/autofs.service
    
    --- /usr/lib/systemd/system/autofs.service      2014-10-16 21:30:39.000000000 -0400
    + /etc/systemd/system/autofs.service  2014-11-21 10:00:58.513568275 -0500
    @@ -8,7 +8,8 @@
     EnvironmentFile=-/etc/sysconfig/autofs
     ExecStart=/usr/sbin/automount $OPTIONS --pid-file /run/autofs.pid
     ExecReload=/usr/bin/kill -HUP $MAINPID
    -TimeoutSec=180
    +TimeoutSec=240
    +Restart=Always
    
     [Install]
     WantedBy=multi-user.target
    
    [MASKED]     /etc/systemd/system/cups.service → /usr/lib/systemd/system/cups.service
    [EXTENDED]   /usr/lib/systemd/system/sssd.service → /etc/systemd/system/sssd.service.d/journal.conf
    
    4 overridden configuration files found.

3.5.6. Working with instantiated units

It is possible to instantiate multiple units from a single template configuration file at runtime. The "@" character is used to mark the template and to associate units with it. Instantiated units can be started from another unit file (using Requires or Wants options), or with the systemctl start command. Instantiated service units are named the following way:

template_name@instance_name.service

Where template_name stands for the name of the template configuration file. Replace instance_name with the name for the unit instance. Several instances can point to the same template file with configuration options common for all instances of the unit. Template unit name has the form of:

unit_name@.service

For example, the following Wants setting in a unit file:

Wants=getty@ttyA.service getty@ttyB.service

first makes systemd search for given service units. If no such units are found, the part between "@" and the type suffix is ignored and systemd searches for the getty@.service file, reads the configuration from it, and starts the services.

For example, the getty@.service template contains the following directives:

[Unit]
Description=Getty on %I
…​
[Service]
ExecStart=-/sbin/agetty --noclear %I $TERM
…​

When the getty@ttyA.service and getty@ttyB.service are instantiated from the above template, Description= is resolved as Getty on ttyA and Getty on ttyB.

3.5.6.1. Important unit specifiers

Wildcard characters, called unit specifiers, can be used in any unit configuration file. Unit specifiers substitute certain unit parameters and are interpreted at runtime. Table 3.13, “Important unit specifiers” lists unit specifiers that are particularly useful for template units.

Table 3.13. Important unit specifiers

Unit SpecifierMeaningDescription

%n

Full unit name

Stands for the full unit name including the type suffix. %N has the same meaning but also replaces the forbidden characters with ASCII codes.

%p

Prefix name

Stands for a unit name with type suffix removed. For instantiated units %p stands for the part of the unit name before the "@" character.

%i

Instance name

Is the part of the instantiated unit name between the "@" character and the type suffix. %I has the same meaning but also replaces the forbidden characters for ASCII codes.

%H

Host name

Stands for the hostname of the running system at the point in time the unit configuration is loaded.

%t

Runtime directory

Represents the runtime directory, which is either /run for the root user, or the value of the XDG_RUNTIME_DIR variable for unprivileged users.

For a complete list of unit specifiers, see the systemd.unit(5) manual page.

3.6. Optimizing systemd to shorten the boot time

There is a list of systemd unit files that are enabled by default. System services that are defined by these unit files are automatically run at boot, which influences the boot time.

This section describes:

  • The tools to examine system boot performance.
  • The purpose of systemd units enabled by default, and circumstances under which you can safely disable such systemd units in order to shorten the boot time.

3.6.1. Examining system boot performance

To examine system boot performance, you can use the systemd-analyze command. This command has many options available. However, this section covers only the selected ones that may be important for systemd tuning in order to shorten the boot time.

For a complete list and detailed description of all options, see the systemd-analyze man page.

Prerequisites

Before starting to examine systemd in order to tune the boot time, you may want to list all enabled services:

$ systemctl list-unit-files --state=enabled
Analyzing overall boot time

Procedure

  • For the overall information about the time that the last successful boot took, use:
$ systemd-analyze
Analyzing unit initialization time

Procedure

  • For the information about the initialization time of each systemd unit, use:
$ systemd-analyze blame

The output lists the units in descending order according to the time they took to initialize during the last successful boot.

Identifying critical units

Procedure

  • To identify the units that took most time to initialize at the last successful boot, use:
$ systemd-analyze critical-chain

The output highlights the units that critically slow down the boot with the red color.

Figure 3.1. The output of the systemd-analyze critical-chain command

systemd analyze critical

3.6.2. A guide to selecting services that can be safely disabled

If you find the boot time of your system long, you can shorten it by disabling some of the services enabled on boot by default.

To list such services, run:

$ systemctl list-unit-files --state=enabled

To disable a service, run:

# systemctl disable service_name

However, certain services must stay enabled in order that your operating system is safe and functions in the way you need.

You can use the table below as a guide to selecting the services that you can safely disable. The table lists all services enabled by default on a minimal installation of Red Hat Enterprise Linux 8, and for each service it states whether this service can be safely disabled.

The table also provides more information about the circumstances under which the service can be disabled, or the reason why you should not disable the service.

Table 3.14. Services enabled by default on a minimal installation of RHEL 8

Service nameCan it be disabled?More information

auditd.service

yes

Disable auditd.service only if you do not need audit messages from the kernel. Be aware that if you disable auditd.service, the /var/log/audit/audit.log file is not produced. Consequently, you are not able to retroactively review some commonly-reviewed actions or events, such as user logins, service starts or password changes. Also note that auditd has two parts: a kernel part, and a service itself. By using the systemctl disable auditd command, you only disable the service, but not the kernel part. To disable system auditing in its entirety, set audit=0 on kernel command line.

autovt@.service

no

This service runs only when it is really needed, so it does not need to be disabled.

crond.service

yes

Be aware that no items from crontab will run if you disable crond.service.

dbus-org.fedoraproject.FirewallD1.service

yes

A symlink to firewalld.service

dbus-org.freedesktop.NetworkManager.service

yes

A symlink to NetworkManager.service

dbus-org.freedesktop.nm-dispatcher.service

yes

A symlink to NetworkManager-dispatcher.service

firewalld.service

yes

Disable firewalld.service only if you do not need firewall.

getty@.service

no

This service runs only when it is really needed, so it does not need to be disabled.

import-state.service

yes

Disable import-state.service only if you do not need to boot from a network storage.

irqbalance.service

yes

Disable irqbalance.service only if you have just one CPU. Do not disable irqbalance.service on systems with multiple CPUs.

kdump.service

yes

Disable kdump.service only if you do not need reports from kernel crashes.

loadmodules.service

yes

This service is not started unless the /etc/rc.modules or /etc/sysconfig/modules directory exists, which means that it is not started on a minimal RHEL 8 installation.

lvm2-monitor.service

yes

Disable lvm2-monitor.service only if you do not use Logical Volume Manager (LVM).

microcode.service

no

Do not be disable the service because it provides updates of the microcode software in CPU.

NetworkManager-dispatcher.service

yes

Disable NetworkManager-dispatcher.service only if you do not need notifications on network configuration changes (for example in static networks).

NetworkManager-wait-online.service

yes

Disable NetworkManager-wait-online.service only if you do not need working network connection available right after the boot. If the service is enabled, the system does not finish the boot before the network connection is working. This may prolong the boot time significantly.

NetworkManager.service

yes

Disable NetworkManager.service only if you do not need connection to a network.

nis-domainname.service

yes

Disable nis-domainname.service only if you do not use Network Information Service (NIS).

rhsmcertd.service

no

 

rngd.service

yes

Disable rngd.service only if you do not need a lot of entropy on your system, or you do not have any sort of hardware generator. Note that the service is necessary in environments that require a lot of good entropy, such as systems used for generation of X.509 certificates (for example the FreeIPA server).

rsyslog.service

yes

Disable rsyslog.service only if you do not need persistent logs, or you set systemd-journald to persistent mode.

selinux-autorelabel-mark.service

yes

Disable selinux-autorelabel-mark.service only if you do not use SELinux.

sshd.service

yes

Disable sshd.service only if you do not need remote logins by OpenSSH server.

sssd.service

yes

Disable sssd.service only if there are no users who log in the system over the network (for example by using LDAP or Kerberos). Red Hat recommends to disable all sssd-* units if you disable sssd.service.

syslog.service

yes

An alias for rsyslog.service

tuned.service

yes

Disable tuned.service only if you do need to use performance tuning.

lvm2-lvmpolld.socket

yes

Disable lvm2-lvmpolld.socket only if you do not use Logical Volume Manager (LVM).

dnf-makecache.timer

yes

Disable dnf-makecache.timer only if you do not need your package metadata to be updated automatically.

unbound-anchor.timer

yes

Disable unbound-anchor.timer only if you do not need daily update of the root trust anchor for DNS Security Extensions (DNSSEC). This root trust anchor is used by Unbound resolver and resolver library for DNSSEC validation.

To find more information about a service, you can run one of the following commands:

$ systemctl cat <service_name>
$ systemctl help <service_name>

The systemctl cat command provides the content of the service file located under /usr/lib/systemd/system/<service>, as well as all applicable overrides. The applicable overrides include unit file overrides from the /etc/systemd/system/<service> file or drop-in files from a corresponding unit.type.d directory.

For more information on drop-in files, see the systemd.unit man page.

The systemctl help command shows the man page of the particular service.

3.7. Additional Resources

For more information on systemd and its usage on Red Hat Enterprise Linux, see the resources listed below.

3.7.1. Installed Documentation

  • systemctl(1) — The manual page for the systemctl command line utility provides a complete list of supported options and commands.
  • systemd(1) — The manual page for the systemd system and service manager provides more information about its concepts and documents available command line options and environment variables, supported configuration files and directories, recognized signals, and available kernel options.
  • systemd-delta(1) — The manual page for the systemd-delta utility that allows to find extended and overridden configuration files.
  • systemd.directives(7) — The manual page named systemd.directives provides detailed information about systemd directives.
  • systemd.unit(5) — The manual page named systemd.unit provides detailed information about systemd unit files and documents all available configuration options.
  • systemd.service(5) — The manual page named systemd.service documents the format of service unit files.
  • systemd.target(5) — The manual page named systemd.target documents the format of target unit files.
  • systemd.kill(5) — The manual page named systemd.kill documents the configuration of the process killing procedure.

3.7.2. Online Documentation

  • systemd Home Page — The project home page provides more information about systemd.

Chapter 4. Introduction to managing user and group accounts

The control of users and groups is a core element of Red Hat Enterprise Linux (RHEL) system administration.

4.1. Introduction to users and groups

Each RHEL user has distinct login credentials and can be assigned to various groups to customize their system privileges.

A user who creates a file is the owner of that file and the group owner of that file. The file is assigned separate read, write, and execute permissions for the owner, the group, and those outside that group. The file owner can be changed only by the root user. Access permissions to the file can be changed by both the root user and the file owner. A regular user can change group ownership of a file they own to a group of which they are a member of.

Each user is associated with a unique numerical identification number called user ID (UID). Each group is associated with a group ID (GID). Users within a group share the same permissions to read, write, and execute files owned by that group.

4.2. Configuring reserved user and group IDs

RHEL reserves user and group IDs below 1000 for system users and groups. You can find the reserved user and group IDs in the setup package. To view reserved user and group IDs, use:

cat /usr/share/doc/setup*/uidgid

It is recommended to assign IDs to the new users and groups starting at 5000, as the reserved range can increase in the future.

To make the IDs assigned to new users start at 5000 by default, modify the UID_MIN and GID_MIN parameters in the /etc/login.defs file.

Procedure

To modify make the IDs assigned to new users start at 5000 by default, use:

  1. Open the /etc/login.defs file in an editor of your choice.
  2. Find the lines that define the minimum value for automatic UID selection.

    # Min/max values for automatic uid selection in useradd
    #
    UID_MIN                  1000
  3. Modify the UID_MIN value to start at 5000.

    # Min/max values for automatic uid selection in useradd
    #
    UID_MIN                  5000
  4. Find the lines that define the minimum value for automatic GID selection.

    # Min/max values for automatic gid selection in groupadd
    #
    GID_MIN                  1000

Note that for users and groups created before you changed the UID_MIN and GID_MIN values, UIDs and GIDs still start at the default 1000.

Warning

Do not raise IDs reserved by the system above 1000 by changing SYS_UID_MAX to avoid conflict with systems that retain the 1000 limit.

4.3. User private groups

RHEL uses the user private group (UPG) system configuration, which makes UNIX groups easier to manage. A user private group is created whenever a new user is added to the system. The user private group has the same name as the user for which it was created and that user is the only member of the user private group.

UPGs simplify the collaboration on a project between multiple users. In addition, UPG system configuration makes it safe to set default permissions for a newly created file or directory, as it allows both the user, and the group this user is a part of, to make modifications to the file or directory.

A list of all groups is stored in the /etc/group configuration file.

Chapter 5. Managing user accounts in the web console

The RHEL web console offers a graphical interface that enables you to execute a wide range of administrative tasks without accessing your terminal directly. For example, you can add, edit or remove system user accounts.

After reading this section, you will know:

  • From where the existing accounts come from.
  • How to add new accounts.
  • How to set password expiration.
  • How and when to terminate user sessions.

Prerequisites

5.1. System user accounts managed in the web console

With user accounts displayed in the RHEL web console you can:

  • Authenticate users when accessing the system.
  • Set them access rights to the system.

The RHEL web console displays all user accounts located in the system. Therefore, you can see at least one user account just after the first login to the web console.

After logging into the RHEL web console, you can perform the following operations:

  • Create new users accounts.
  • Change their parameters.
  • Lock accounts.
  • Terminate user sessions.

5.2. Adding new accounts using the web console

Use the following steps for adding user accounts to the system and setting administration rights to the accounts through the RHEL web console.

Prerequisites

Procedure

  1. Log in to the RHEL web console.
  2. Click Accounts.
  3. Click Create New Account.

    cockpit create new account pf4

  4. In the Full Name field, enter the full name of the user.

    The RHEL web console automatically suggests a user name from the full name and fills it in the User Name field. If you do not want to use the original naming convention consisting of the first letter of the first name and the whole surname, update the suggestion.

  5. In the Password/Confirm fields, enter the password and retype it for verification that your password is correct. The color bar placed below the fields shows you security level of the entered password, which does not allow you to create a user with a weak password.

    cockpit user accounts pf4

  6. Click Create to save the settings and close the dialog box.
  7. Select the newly created account.
  8. Select Server Administrator in the Roles item.

cockpit terminate session pf4

Now you can see the new account in the Accounts settings and you can use the credentials to connect to the system.

5.3. Enforcing password expiration in the web console

By default, user accounts have set passwords to never expire. To enforce password expiration, as administrator, set system passwords to expire after a defined number of days.

When the password expires, the next login attempt will prompt for a password change.

Procedure

  1. Log in to the RHEL 8 web console interface.
  2. Click Accounts.
  3. Select the user account for which to enforce password expiration.
  4. In the user account settings, click Never expire password.
  5. In the Password Expiration dialog box, select Require password change every …​ days and enter a positive whole number representing the number of days when the password expires.

    cockpit password expiration

  6. Click Change.

To verify the settings, open the account settings. The RHEL 8 web console displays a link with the date of expiration.

cockpit password expiration date

5.4. Terminating user sessions in the web console

A user creates user sessions when logging into the system. Terminating user sessions means to log the user out from the system.

It can be helpful if you need to perform administrative tasks sensitive to configuration changes, for example, system upgrades.

In each user account in the RHEL 8 web console, you can terminate all sessions for the account except for the web console session you are currently using. This prevents you from cutting yourself off the system.

Procedure

  1. Log in to the RHEL 8 web console.
  2. Click Accounts.
  3. Click the user account for which you want to terminate the session.
  4. Click the Terminate Session button.

    cockpit password expiration date

If the Terminate Session button is inactive, the user is not logged in to the system.

The RHEL web console terminates the sessions.

Chapter 6. Managing users from the command line

You can manage users and groups using the command-line interface (CLI).

Prerequisites

  • Root access.

6.1. Adding a new user from the command line

This section describes how to use the useradd command to add a new user.

Procedure

  • To add a new user, use:

    # useradd options username

    Replace options with the command-line options for the useradd command, and replace username with the name of the user.

Example

  • To add the user sarah with user ID 5000, use:

    # useradd -u 5000 sarah

Verification steps

  • To verify the new user is added, use the id utility.

    # id sarah

    The output returns:

    uid=5000(sarah) gid=5000(sarah) groups=5000(sarah)

Additional resources

  • For more information about useradd, see the useradd man page.

6.2. Adding a new group from the command line

This section describes how to use the groupadd command to add a new group.

Procedure

  • To add a new group, use:

    # groupadd options group-name

    Replace options with the command-line options for the groupadd command, and replace group-name with the name of the group.

Example

  • To add the group sysadmins with group ID 5000, use:

    # groupadd -g 5000 sysadmins

Verification steps

  • To verify the new group is added, use the tail utility.

    # tail /etc/group

    The output returns:

    sysadmins:x:5000:

Additional resources

  • For more information about useradd, see the groupadd man page.

6.3. Adding a user to a groups from the command line

This section describes how to use the usermod command to add a group to the supplementary groups of the user.

Procedure

  • To add a group to the supplementary groups of the user, use:

    # usermod --append -G group-name username

    Replace group-name with the name of the group, and replace username with the name of the user.

Example

  • To add the user sysadmin to the group system-administrators, use:

    # usermod --append -G system-administrators sysadmin

Verification steps

  • To verify the new groups is added to the supplementary groups of the user sysadmin, use:

    # groups sysadmin

    The output returns:

    sysadmin: sysadmin system-administrators

6.4. Creating a group directory

Under the UPG system configuration, you can apply the set-group identification permission (setgid bit) to a directory. The setgid bit makes managing group projects that share a directory simpler. When you apply the setgid bit to a directory, files created within that directory are automatically assigned to a group that owns the directory. Any user that has the permission to write and execute within this group can now create, modify, and delete files in the directory.

The following section describes how to create group directories.

Procedure

  1. Create a directory:

    # mkdir directory-name

    Replace directory-name with the name of the directory.

  2. Create a group:

    # groupadd group-name

    Replace group-name with the name of the group.

  3. Add users to the group:

    # usermod --append -G group-name username

    Replace group-name with the name of the group, and replace username with the name of the user.

  4. Associate the user and group ownership of the directory with the group-name group:

    # chown :group-name directory-name

    Replace group-name with the name of the group, and replace directory-name with the name of the directory.

  5. Set the write permissions to allow the users to create and modify files and directories and set the setgid bit to make this permission be applied within the directory-name directory:

    # chmod g+rwxs directory-name

    Replace directory-name with the name of the directory.

    Now all members of the group-name group can create and edit files in the directory-name directory. Newly created files retain the group ownership of group-name group.

Verification steps

  • To verify the correctness of set permissions, use:

    # ls -ld directory-name

    Replace directory-name with the name of the directory.

    The output returns:

    drwxrwsr-x. 2 root group-name 6 Nov 25 08:45 directory-name

Chapter 7. Removing a user from a group from the command line

You can remove a user from a primary or supplementary group by overriding the groups the user belongs to with a new set of groups that does not contain the group you want to remove the user from.

7.1. Overriding the primary group of the user

This section describes how to use the usermod command to override the primary group of the user.

Procedure

  • To override the primary group of the user, use:

    # usermod -g group-name username

    Replace group-name with the name of the group, and replace username with the name of the user.

Example

  • If the user sarah belongs to the primary groups sarah1, and you want to change the primary group of the user to sarah2, use:

    # usermod -g sarah2 sarah

Verification steps

  • To verify that the primary group of the user is overridden, use:

    # groups sarah

    The output returns:

    sarah : sarah2

7.2. Overriding the supplementary groups of the user

This section describes how to use the usermod command to override the supplementary groups of the user.

Procedure

  • To override the supplementary groups of the user, use:

    # usermod -G group-name username

    Replace group-name with the name of the group, and replace username with the name of the user.

Example

  • If the user sarah belongs to the system-administrator group and to the developer group and you want to remove the user sarah from the system-administrator group, you can do that by replacing the old list of groups with a new one. To do that, use:

    # usermod -G developer sarah

Verification steps

  • To verify that the supplementary groups of the user are overridden, use:

    # groups sarah

    The output returns:

    sarah : sarah developer

Chapter 8. Managing sudo access

System administrators can grant sudo access to allow non-root users to execute administrative commands. The sudo command provides users with administrative access without using the password of the root user.

When users need to perform an administrative command, they can precede that command with sudo. The command is then executed as if they were the root user.

Be aware of the following limitations:

  • Only users listed in the /etc/sudoers configuration file can use the sudo command.
  • The command is executed in the shell of the user, not in the root shell.

8.1. Granting sudo access to a user

A non-root user requires sudo access to perform administrative commands. The following section describes how to grant sudo access to a user.

Prerequisites

  • Root access.

Procedure

  1. Open the /etc/sudoers file.

    # visudo

    The /etc/sudoers file defines the policies applied by the sudo command.

  2. In the /etc/sudoers file find the lines that grant sudo access to users in the administrative wheel group.

    ## Allows people in group wheel to run all commands
    %wheel        ALL=(ALL)       ALL
  3. Make sure the line that starts with %wheel does not have # comment character before it.
  4. Save any changes, and exit the editor.
  5. Add users you want to grant sudo access to into the administrative wheel group .

     # usermod --append -G wheel username

    Replace username with the name of the user.

Example

  • To add the user sarah to the administrative wheel group, use:

     # usermod --append -G wheel sarah

Verification steps

  • To verify the user is added to the administrative wheel group, use the id utility.

    # id sarah

    The output returns:

    uid=5000(sarah) gid=5000(sarah) groups=5000(sarah),10(wheel)

Chapter 9. Changing and resetting the root password

If the existing root password is no longer satisfactory or is forgotten, you can change or reset it both as the root user and a non-root user.

9.1. Changing the root password as the root user

This section describes how to use the passwd command to change the root password as the root user.

Prerequisites

  • Root access.

Procedure

  • To change the root password, use:

    # passwd

    You are prompted to enter your current password before you can change it.

9.2. Changing or resetting the forgotten root password as a non-root user

This section describes how to use the passwd command to change or reset the forgotten root password as a non-root user.

Prerequisites

  • You are able to log in as a non-root user.
  • You are a member of the administrative wheel group.

Procedure

  • To change or reset the root password as a non-root user that belongs to the wheel group, use:

    $ sudo passwd root

    You are prompted to enter your current non-root password before you can change the root password.

9.3. Resetting the forgotten root password on boot

If you are unable to log in as a non-root user or do not belong to the administrative wheel group, you can reset the root password on boot by switching into a specialized chroot jail environment.

Procedure

  1. Reboot the system and, on the GRUB 2 boot screen, press the e key to interrupt the boot process.

    The kernel boot parameters appear.

    load_video
    set gfx_payload=keep
    insmod gzio
    linux ($root)/vmlinuz-4.18.0-80.e18.x86_64 root=/dev/mapper/rhel-root ro crash\
    kernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv/swap rhab quiet
    initrd ($root)/initramfs-4.18.0-80.e18.x86_64.img $tuned_initrd
  2. Go to the end of the line that starts with linux.

    linux ($root)/vmlinuz-4.18.0-80.e18.x86_64 root=/dev/mapper/rhel-root ro crash\
    kernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv/swap rhab quiet

    Press Ctrl+e to jump to the end of the line.

  3. Add rd.break to the end of the line that starts with linux.

    linux ($root)/vmlinuz-4.18.0-80.e18.x86_64 root=/dev/mapper/rhel-root ro crash\
    kernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv/swap rhab quiet rd.break
  4. Press Ctrl+x to start the system with the changed parameters.

    The switch_root prompt appears.

  5. Remount the file system as writable:

    mount -o remount,rw /sysroot

    The file system is mounted as read-only in the /sysroot directory. Remounting the file system as writable allows you to change the password.

  6. Enter the chroot environment:

    chroot /sysroot

    The sh-4.4# prompt appears.

  7. Reset the root password:

    passwd

    Follow the instructions displayed by the command line to finalize the change of the root password.

  8. Enable the SELinux relabeling process on the next system boot:

    touch /.autorelabel
  9. Exit the chroot environment:

    exit
  10. Exit the switch_root prompt:

    exit
  11. Wait until the SELinux relabeling process is finished. Note that relabeling a large disk might take a long time. The system reboots automatically when the process is complete.

Chapter 10. Managing file permissions

10.1. Introduction to file permissions

Every file or directory has three levels of ownership:

  • User owner (u).
  • Group owner (g).
  • Others (o).

Each level of ownership can be assigned the following permissions:

  • Read (r).
  • Write (w).
  • Execute (x).

Note that the execute permission for a file allows you to execute that file. The execute permission for a directory allows you to access the contents of the directory, but not execute it.

When a new file or directory is created, the default set of permission is automatically assigned to it. The default permission for a file or directory is based on two factors:

  • Base permission.
  • The user file-creation mode mask (umask).

10.1.1. Base permissions

Whenever a new file or directory is created, a base permission is automatically assigned to it.

Base permissions for a file or directory can be expressed in symbolic or octal values.

Permission

Symbolic value

Octal value

No permission

---

0

Execute

--x

1

Write

-w-

2

Write and execute

-wx

3

Read

r--

4

Read and execute

r-x

5

Read and write

rw-

6

Read, write, execute

rwx

7

The base permission for a directory is 777 (drwxrwxrwx), which grants everyone the permissions to read, write, and execute. This means that the directory owner, the group, and others can list the contents of the directory, create, delete, and edit items within the directory, and descend into it.

Note that individual files within a directory can have their own permission that might prevent you from editing them, despite having unrestricted access to the directory.

The base permission for a file is 666 (-rw-rw-rw-), which grants everyone the permissions to read and write. This means that the file owner, the group, and others can read and edit the file.

Example 1

If a file has the following permissions:

$ ls -l
-rwxrw----. 1 sysadmins sysadmins 2 Mar 2 08:43 file
  • - indicates it is a file.
  • rwx indicates that the file owner has permissions to read, write, and execute the file.
  • rw- indicates that the group has permissions to read and write, but not execute the file.
  • --- indicates that other users have no permission to read, write, or execute the file.
  • . indicates that the SELinux security context is set for the file.

Example 2

If a directory has the following permissions:

$ ls -dl
drwxr-----. 1 sysadmins sysadmins 2 Mar 2 08:43 directory
  • d indicates it is a directory.
  • rwx indicates that the directory owner has the permissions to read, write, and access the contents of the directory.

    As a directory owner, you can list the items (files, subdirectories) within the directory, access the content of those items, and modify them.

  • r-- indicates that the group has permissions to read, but not write or access the contents of the directory.

    As a member of the group that owns the directory, you can list the items within the directory. You cannot access information about the items within the directory or modify them.

  • --- indicates that other users have no permission to read, write, or access the contents of the directory.

    As someone who is not an user owner, or as group owner of the directory, you cannot list the items within the directory, access information about those items, or modify them.

  • . indicates that the SELinux security context is set for the directory.
Note

The base permission that is automatically assigned to a file or directory is not the default permission the file or directory ends up with. When you create a file or directory, the base permission is altered by the umask. The combination of the base permission and the umask creates the default permission for files and directories.

10.1.2. User file-creation mode mask

The umask is variable that automatically removes permissions from the base permission value whenever a file or directory is created to increase the overall security of a linux system.

The umask can be expressed in symbolic or octal.

Permission

Symbolic value

Octal value

Read, write, and execute

rwx

0

Read and write

rw-

1

Read and execute

r-x

2

Read

r--

3

Write and execute

-wx

4

Write

-w-

5

Execute

--x

6

No permissions

---

7

The default umask for a standard user is 0002. The default umask for a root user is 0022.

The first digit of the umask represents special permissions (sticky bit, ). The last three digits of the umask represent the permissions that are removed from the user owner (u), group owner (g), and others (o) respectively.

Example

The following example illustrates how the umask with an octal value of 0137 is applied to the file with the base permission of 777, to create the file with the default permission of 640.

Figure 10.1. Applying the umask when creating a file

Users Groups Umask Example

10.1.3. Default permissions

The default permission for a new file or directory is determined by applying the umask to the base permission.

Example 1

When a standard user creates a new directory, the umask is set to 002 (rwxrwxr-x), and the base permission for a directory is set to 777 (rwxrwxrwx). This brings the default permission to 775 (drwxrwxr-x).

 

Symbolic value

Octal value

Base permission

rwxrwxrwx

777

Umask

rwxrwxr-x

002

Default permission

rwxrwxr-x

775

This means that the directory owner and the group can list the contents of the directory, create, delete, and edit items within the directory, and descend into it. Other users can only list the contents of the directory and descend into it.

Example 2

When a standard user creates a new file, the umask is set to 002 (rwxrwxr-x), and the base permission for a file is set to 666 (rw-rw-rw-). This brings the default permission to 664 (-rw-rw-r--).

 

Symbolic value

Octal value

Base permission

rw-rw-rw-

666

Umask

rwxrwxr-x

002

Default permission

rw-rw-r--

664

This means that the file owner and the group can read and edit the file, while other users can only read the file.

Example 3

When a root user creates a new directory, the umask is set to 022 (rwxr-xr-x), and the base permission for a directory is set to 777 (rwxrwxrwx). This brings the default permission to 755 (rwxr-xr-x).

 

Symbolic value

Octal value

Base permission

rwxrwxrwx

777

Umask

rwxr-xr-x

022

Default permission

rwxr-xr-x

755

This means that the directory owner can list the contents of the directory, create, delete, and edit items within the directory, and descend into it. The group and others can only list the contents of the directory and descend into it.

Example 4

When a root user creates a new file, the umask is set to 022 (rwxr-xr-x), and the base permission for a file is set to 666 (rw-rw-rw-). This brings the default permission to 644 (-rw-r—​r--).

 

Symbolic value

Octal value

Base permission

rw-rw-rw-

666

Umask

rwxr-xr-x

022

Default permission

rw-r—​r--

644

This means that the file owner can read and edit the file, while the group and others can only read the file.

Note

For security reasons, regular files cannot have execute permissions by default, even if the umask is set to 000 (rwxrwxrwx). However, directories can be created with execute permissions.

10.2. Displaying file permissions

The following section describes how to use the ls command to display the permissions for directories, files, files within directories.

Procedure

  • To see the permissions for a particular directory, use:

    $ ls -dl directory-name

    Replace directory-name with the name of the directory.

  • To see the permissions for a particular directory and all files within that directory, use:

    $ ls -l directory-name

    Replace directory-name with the name of the directory.

  • To see the permissions for a particular file, use:

    $ ls -l file-name

    Replace file-name with the name of the file.

Additional information

  • See the ls man page for more details.

10.3. Changing file permissions

The following section describes how to:

  • Change file permissions using symbolic values.
  • Change file permissions using octal values.

10.3.1. Changing file permissions using symbolic values

You can assign the following permissions:

  • Read (r).
  • Write (w).
  • Execute (x).

Permissions can be assigned to:

  • User owner (u).
  • Group owner (g).
  • Other (o).
  • All (a).

To add or take away the permissions you can use the following signs:

  • + to add the permissions on top of the existing permissions.
  • - to take away the permissions from the existing permission.
  • = to omit the existing permissions and explicitly define the new ones.

The following section describes how to set and remove file permissions using the symbolic values.

Procedure

  • To change the file permissions for an existing file or directory, use:

    $ chmod u=symbolic_value,g+symbolic_value,o-symbolic_value file-name

    Replace file-name with the name of the file or directory, and replace symbolic_value for user, groups, and others with corresponding symbolic values. See Section 10.1.1, “Base permissions” for more details.

    Example

    To change file permissions for my-file.txt from 664 (-rw-rw-r--) to 740 (-rwx-r---), use:

    $ chmod u+x,g-w,o= my-file.txt

    Note that any permission that is not specified after the equals sign (=) is automatically prohibited.

  • To set the same permissions for user, group, and others, use:

    $ chmod a=symbolic_value file-name

    Replace file-name with the name of the file or directory, and replace symbolic_value with a symbolic value. See Section 10.1.1, “Base permissions” for more details.

    Example

    To set the permission for my-file.txt to 777 (-rwxrwxrwx or drwxrwxrwx), use:

    $ chmod a=rwx my-file
  • To change the permissions for a directory and all its sub-directories, add the -R option:

    $ chmod -R symbolic_value directory-name

    Replace directory-name with the name of the directory, and replace symbolic_value with a symbolic value. See Section 10.1.1, “Base permissions” for more details.

    Example

    To change the permissions for /my-directory/ and all its sub-directories from 775 (drwxrwxr-x) to 740 (drwx-r---), use:

    $ chmod -R g-wx,o= /my-directory

10.3.2. Changing file permissions using octal values

The following section describes how to use the chmod command to change the permissions for a file or directory.

Procedure

  • To change the file permissions for an existing file or directory, use:

    $ chmod octal_value file-name

    Replace file-name with the name of the file or directory, and replace octal_value with an octal value. See Section 10.1.1, “Base permissions” for more details.

10.4. Displaying the umask

The following section describes how to:

  • Display the current octal value of the umask.
  • Display the current symbolic value of the umask.
  • Display the default bash umask.

10.4.1. Displaying the current octal value of the umask

The following section describes how to use the umask command to display the current umask.

Procedure:

  • To display the current octal value of the umask for a standard user, use:

    $ umask
  • To display the current octal value of the umask for a root user, use:

    $ sudo umask

    Or:

    # umask
Note

When displaying the umask, you may notice it displayed as a four digit number (0002 or 0022). The first digit of the umask represents a special bit (sticky bit, SGID bit, or SUID bit). If the first digit is set to 0, the special bit is not set.

10.4.2. Displaying the current symbolic value of the umask

The following section describes how to use the umask command to display the current umask.

Procedure

  • To display the current symbolic value of the umask, use:

    $ umask -S
  • To display the current symbolic value of the umask for a root user, use:

    $ sudo umask -S

    Or:

    # umask -S

10.4.3. Displaying the default bash umask

There are a number of shells you can use, such as bash, ksh, zsh and tcsh.

Those shells can behave as login or non-login shells. The login shell is typically invoked by opening a native or a GUI terminal.

To determine whether you are executing a command in a login or a non-login shell, use the echo $0 command.

In bash shell, if the output returns bash, you are executing a command in a non-login shell.

$ echo $0
bash

The default umask for the non-login shell is set in /etc/bashrc configuration file.

If the output returns -bash, you are executing a command in a login shell.

# echo $0
-bash

The default umask for the login shell is set in /etc/profile configuration file.

Procedure

  • To display the default bash umask for the non-login shell, use:

    $ grep umask /etc/bashrc

    The output returns:

    # By default, we want umask to get set. This sets it for non-login shell.
           umask 002
           umask 022
  • To display the default bash umask for the login shell, use:

    $ grep umask /etc/profile

    The output returns:

    # By default, we want umask to get set. This sets it for login shell
           umask 002
           umask 022

10.5. Setting the umask for the current shell session

The following section describes how to set the umask for the current shell session:

  • Using symbolic values.
  • Using octal values.

Note that the umask is valid only during the current shell session and reverts to the default umask after the session is complete.

10.5.1. Setting the umask using symbolic values

The following section describes how to set the umask with symbolic values.

Procedure

  • To set or remove permissions for the current shell session, you can use minus (-), plus (+), and equals (=) signs in combination with symbolic values.

    $ umask -S u=symbolic_value,g+symbolic_value,o-symbolic_value

    Replace symbolic_value for user, group, and others with symbolic values. See Section 10.1.2, “User file-creation mode mask” for more details.

    Example

    If your current umask is set to 113 (u=rw-,g=rw-,o=r--) and you want to set it to 037 (u=rwx,g=-r-,o=---), use:

    $ umask -S u+x,g-w,o=

    Note that any permission that is not specified after the equals sign (=) is automatically prohibited.

  • To set the same permissions for user, group, and others, use:

    $ umask a=symbolic_value

    Replace symbolic_value with a symbolic value. See Section 10.1.2, “User file-creation mode mask” for more details.

    Example

    To set the umask to 000 (u=rwx,g=rwx,o=rwx), use:

    $ umask a=rwx

Note that the umask is only valid for the current shell session.

10.5.2. Setting the umask using octal values

The following section describes how to set the umask with octal values.

Procedure

Note that the umask is only valid for the current shell session.

10.6. Changing the default umask

The following section describes how to:

  • Change the default bash umask for the non-login shell.
  • Change the default bash umask for the login shell.
  • Change the default bash umask for a specific user.
  • Set default permissions for newly created home directories.

Prerequisites

  • Root access.

10.6.1. Changing the default umask for the non-login shell

The following section describes how to change the default bash umask for standard users.

Procedure

  1. As root, open the /etc/bashrc file in an editor of your choice.
  2. Modify the following sections to set a new default bash umask:

        if [ $UID -gt 199 ] && [ “id -gn” = “id -un” ]; then
           umask 002
        else
           umask 022
        fi

    Replace the default octal value of the umask (002) with another octal value. See Section 10.1.2, “User file-creation mode mask” for more details.

  3. Save the changes.

10.6.2. Changing the default umask for the login shell

The following section describes how to change the default bash umask for the root user.

Procedure

  1. As root, open the /etc/profile file in an editor of your choice.
  2. Modify the following sections to set a new default bash umask:

    if [ $UID -gt 199 ] && [ “/usr/bin/id -gn” = “/usr/bin/id -un” ]; then
        umask 002
    else
        umask 022
    fi

    Replace the default octal value of the umask (022) with another octal value. See Section 10.1.2, “User file-creation mode mask” for more details.

  3. Save the changes.

10.6.3. Changing the default umask for a specific user

The following section describes how to change the default umask for a specific user.

Procedure

  • Put the line that specifies the octal value of the umask into the .bashrc file for the particular user.

    $ echo 'umask octal_value' >> /home/username/.bashrc

    Replace octal_value with an octal value and replace username with the name of the user. See Section 10.1.2, “User file-creation mode mask” for more details.

10.6.4. Setting default UMASK for newly created home directories

The following section describes how to change the permissions that specify the UMASK for newly created user home directories.

Procedure

  1. As root, open the /etc/login.defs file in an editor of your choice.
  2. Modify the following section to set a new default UMASK:

    # The permission mask is initialized to this value. If not specified,
    # the permission mask will be initialized to 022.
    UMASK 077

    Replace the default octal value (077) with another octal value. See Section 10.1.2, “User file-creation mode mask” for more details.

  3. Save the changes.

10.7. Access control list

Traditionally, each file and directory can only have one user owner and one group owner at a time. If you want to apply a more specific set of permissions to a file or directory (allow certain users outside the group to gain access to a specific file within a directory but not to other files) without changing the ownership and permissions of a file or directory, you can use the access control lists (ACL).

The following section describes how to:

  • Display the current ACL.
  • Set the ACL.

10.7.1. Displaying the current ACL

The following section describes how to display the current ACL.

Procedure

  • To display the current ACL for a particular file or directory, use:

    $ getfacl file-name

    Replace file-name with the name of the file or directory.

10.7.2. Setting the ACL

The following section describes how to set the ACL.

Prerequisites

  • Root access

Procedure

  • To set the ACL for a file or directory, use:
# setfacl -m u:username:symbolic_value file-name

Replace username with the name of the user, symbolic_value with a symbolic value, and file-name with the name of the file or directory. For more information see the setfacl man page.

Example

The following example describes how to modify permissions for the group-project file owned by the root user that belongs to the root group so that this file is:

  • Not executable by anyone.
  • The user andrew has the rw- permission.
  • The user susan has the --- permission.
  • Other users have the r-- permission.

Procedure

# setfacl -m u:andrew:rw- group-project
# setfacl -m u:susan:--- group-project

Verification steps

  • To verify that the user andrew has the rw- permission, the user susan has the --- permission, and other users have the r-- permission, use:

    $ getfacl group-project

    The output returns:

    # file: group-project
    # owner: root
    # group: root
    user:andrew:rw-
    user:susan:---
    group::r--
    mask::rw-
    other::r--

Chapter 11. Using the Chrony suite to configure NTP

11.1. Introduction to configuring NTP with chrony

Accurate timekeeping is important for a number of reasons in IT. In networking for example, accurate time stamps in packets and logs are required. In Linux systems, the NTP protocol is implemented by a daemon running in user space.

The user space daemon updates the system clock running in the kernel. The system clock can keep time by using various clock sources. Usually, the Time Stamp Counter (TSC) is used. The TSC is a CPU register which counts the number of cycles since it was last reset. It is very fast, has a high resolution, and there are no interruptions.

In Red Hat Enterprise Linux 8, the NTP protocol is implemented by the chronyd daemon, available from the repositories in the chrony package.

These sections describe the use of the chrony suite.

11.2. Introduction to chrony suite

chrony is an implementation of the Network Time Protocol (NTP). You can use chrony:

  • To synchronize the system clock with NTP servers
  • To synchronize the system clock with a reference clock, for example a GPS receiver
  • To synchronize the system clock with a manual time input
  • As an NTPv4(RFC 5905) server or peer to provide a time service to other computers in the network

chrony performs well in a wide range of conditions, including intermittent network connections, heavily congested networks, changing temperatures (ordinary computer clocks are sensitive to temperature), and systems that do not run continuously, or run on a virtual machine.

Typical accuracy between two machines synchronized over the Internet is within a few milliseconds, and for machines on a LAN within tens of microseconds. Hardware timestamping or a hardware reference clock may improve accuracy between two machines synchronized to a sub-microsecond level.

chrony consists of chronyd, a daemon that runs in user space, and chronyc, a command line program which can be used to monitor the performance of chronyd and to change various operating parameters when it is running.

The chrony daemon, chronyd, can be monitored and controlled by the command line utility chronyc. This utility provides a command prompt which allows entering a number of commands to query the current state of chronyd and make changes to its configuration. By default, chronyd accepts only commands from a local instance of chronyc, but it can be configured to accept monitoring commands also from remote hosts. The remote access should be restricted.

11.2.1. Using chronyc to control chronyd

To make changes to the local instance of chronyd using the command line utility chronyc in interactive mode, enter the following command as root:

# chronyc

chronyc must run as root if some of the restricted commands are to be used.

The chronyc command prompt will be displayed as follows:

chronyc>

You can type help to list all of the commands.

The utility can also be invoked in non-interactive command mode if called together with a command as follows:

chronyc command
Note

Changes made using chronyc are not permanent, they will be lost after a chronyd restart. For permanent changes, modify /etc/chrony.conf.

11.3. Differences between chrony and ntp

Network Time Protocol (NTP) has two different implementations with similar basic functionality - ntp and chrony.

Both ntp and chrony can operate as an NTP client in order to synchronize the system clock with NTP servers and they can operate as an NTP server for other computers in the network. Each implementation has some unique features. For comparison of ntp and chrony, see Comparison of NTP implementations.

Configuration specific to an NTP client is identical in most cases. NTP servers are specified with the server directive. A pool of servers can be specified with the pool directive.

Configuration specific to an NTP server differs in how the client access is controlled. By default, ntpd responds to client requests from any address. The access can be restricted with the restrict directive, but it is not possible to disable the access completely if ntpd uses any servers as a client. chronyd allows no access by default and operates as an NTP client only. To make chrony operate as an NTP server, you need to specify some addresses within the allow directive.

ntpd and chronyd differ also in the default behavior with respect to corrections of the system clock. ntpd corrects the clock by step when the offset is larger than 128 milliseconds. If the offset is larger than 1000 seconds, ntpd exits unless it is the first correction of the clock and ntpd is started with the -g option. chronyd does not step the clock by default, but the default chrony.conf file provided in the chrony package allows steps in the first three updates of the clock. After that, all corrections are made slowly by speeding up or slowing down the clock. The chronyc makestep command can be issued to force chronyd to step the clock at any time.

11.4. Migrating to chrony

In Red Hat Enterprise Linux 7, users could choose between ntp and chrony to ensure accurate timekeeping. For differences between ntp and chrony, ntpd and chronyd, see Differences between ntpd and chronyd.

In Red Hat Enterprise Linux 8, ntp is no longer supported. chrony is enabled by default. For this reason, you might need to migrate from ntp to chrony.

Migrating from ntp to chrony is straightforward in most cases. The corresponding names of the programs, configuration files and services are:

Table 11.1. Corresponding names of the programs, configuration files and services when migrating from ntp to chrony

ntp namechrony name

/etc/ntp.conf

/etc/chrony.conf

/etc/ntp/keys

/etc/chrony.keys

ntpd

chronyd

ntpq

chronyc

ntpd.service

chronyd.service

ntp-wait.service

chrony-wait.service

The ntpdate and sntp utilities, which are included in the ntp distribution, can be replaced with chronyd using the -q option or the -t option. The configuration can be specified on the command line to avoid reading /etc/chrony.conf. For example, instead of running ntpdate ntp.example.com, chronyd could be started as:

# chronyd -q 'server ntp.example.com iburst'
2018-05-18T12:37:43Z chronyd version 3.3 starting (+CMDMON +NTP +REFCLOCK +RTC +PRIVDROP +SCFILTER +SIGND +ASYNCDNS +SECHASH +IPV6 +DEBUG)
2018-05-18T12:37:43Z Initial frequency -2.630 ppm
2018-05-18T12:37:48Z System clock wrong by 0.003159 seconds (step)
2018-05-18T12:37:48Z chronyd exiting

The ntpstat utility, which was previously included in the ntp package and supported only ntpd, now supports both ntpd and chronyd. It is available in the ntpstat package.

11.4.1. Migration script

A Python script called ntp2chrony.py is included in the documentation of the chrony package (/usr/share/doc/chrony). The script automatically converts an existing ntp configuration to chrony. It supports the most common directives and options in the ntp.conf file. Any lines that are ignored in the conversion are included as comments in the generated chrony.conf file for review. Keys that are specified in the ntp key file, but are not marked as trusted keys in ntp.conf are included in the generated chrony.keys file as comments.

By default, the script does not overwrite any files. If /etc/chrony.conf or /etc/chrony.keys already exist, the -b option can be used to rename the file as a backup. The script supports other options. The --help option prints all supported options.

An example of an invocation of the script with the default ntp.conf provided in the ntp package is:

# python3 /usr/share/doc/chrony/ntp2chrony.py -b -v
Reading /etc/ntp.conf
Reading /etc/ntp/crypto/pw
Reading /etc/ntp/keys
Writing /etc/chrony.conf
Writing /etc/chrony.keys

The only directive ignored in this case is disable monitor, which has a chrony equivalent in the noclientlog directive, but it was included in the default ntp.conf only to mitigate an amplification attack.

The generated chrony.conf file typically includes a number of allow directives corresponding to the restrict lines in ntp.conf. If you do not want to run chronyd as an NTP server, remove all allow directives from chrony.conf.

11.4.2. Timesync role

Note that using the timesync role on your Red Hat Enterprise Linux 7 system facilitates the migration to chrony, because you can use the same playbook on all versions of RHEL starting with RHEL 6 regardless of whether the system uses ntp or chrony to implement the NTP protocol.

Additional resources

  • For a detailed reference on timesync role variables, install the rhel-system-roles package, and see the README.md or README.html files in the /usr/share/doc/rhel-system-roles/timesync directory.
  • For more information on RHEL System Roles, see Introduction to RHEL System Roles.

11.5. Configuring chrony

The default configuration file for chronyd is /etc/chrony.conf. The -f option can be used to specify an alternate configuration file path. See the chrony.conf(5) man page for further options. For a complete list of the directives that can be used see The chronyd configuration file.

Below is a selection of chronyd configuration options:

Comments
Comments should be preceded by #, %, ; or !
allow

Optionally specify a host, subnet, or network from which to allow NTP connections to a machine acting as NTP server. The default is not to allow connections.

Examples:

allow 192.0.2.0/24

Use this command to grant access to a specific network.

allow 2001:0db8:85a3::8a2e:0370:7334

Use this this command to grant access to an IPv6.

The UDP port number 123 needs to be open in the firewall in order to allow the client access:

#  firewall-cmd --zone=public --add-port=123/udp

If you want to open port 123 permanently, use the --permanent option:

#  firewall-cmd --permanent --zone=public --add-port=123/udp
cmdallow
This is similar to the allow directive (see section allow), except that it allows control access (rather than NTP client access) to a particular subnet or host. (By "control access" is meant that chronyc can be run on those hosts and successfully connect to chronyd on this computer.) The syntax is identical. There is also a cmddeny all directive with similar behavior to the cmdallow all directive.
dumpdir
Path to the directory to save the measurement history across restarts of chronyd (assuming no changes are made to the system clock behavior whilst it is not running). If this capability is to be used (via the dumponexit command in the configuration file, or the dump command in chronyc), the dumpdir command should be used to define the directory where the measurement histories are saved.
dumponexit
If this command is present, it indicates that chronyd should save the measurement history for each of its time sources recorded whenever the program exits. (See the dumpdir command above).
hwtimestamp
The hwtimestamp directive enables hardware timestamping for extremely accurate synchronization. For more details, see the chrony.conf(5) manual page.
local

The local keyword is used to allow chronyd to appear synchronized to real time from the viewpoint of clients polling it, even if it has no current synchronization source. This option is normally used on the "master" computer in an isolated network, where several computers are required to synchronize to one another, and the "master" is kept in line with real time by manual input.

An example of the command is:

local stratum 10

A large value of 10 indicates that the clock is so many hops away from a reference clock that its time is unreliable. If the computer ever has access to another computer which is ultimately synchronized to a reference clock, it will almost certainly be at a stratum less than 10. Therefore, the choice of a high value like 10 for the local command prevents the machine’s own time from ever being confused with real time, were it ever to leak out to clients that have visibility of real servers.

log

The log command indicates that certain information is to be logged. It accepts the following options:

measurements
This option logs the raw NTP measurements and related information to a file called measurements.log.
statistics
This option logs information about the regression processing to a file called statistics.log.
tracking
This option logs changes to the estimate of the system’s gain or loss rate, and any slews made, to a file called tracking.log.
rtc
This option logs information about the system’s real-time clock.
refclocks
This option logs the raw and filtered reference clock measurements to a file called refclocks.log.
tempcomp

This option logs the temperature measurements and system rate compensations to a file called tempcomp.log.

The log files are written to the directory specified by the logdir command.

An example of the command is:

log measurements statistics tracking
logdir

This directive allows the directory where log files are written to be specified.

An example of the use of this directive is:

logdir /var/log/chrony
makestep

Normally chronyd will cause the system to gradually correct any time offset, by slowing down or speeding up the clock as required. In certain situations, the system clock may be so far adrift that this slewing process would take a very long time to correct the system clock. This directive forces chronyd to step system clock if the adjustment is larger than a threshold value, but only if there were no more clock updates since chronyd was started than a specified limit (a negative value can be used to disable the limit). This is particularly useful when using reference clock, because the initstepslew directive only works with NTP sources.

An example of the use of this directive is:

makestep 1000 10

This would step the system clock if the adjustment is larger than 1000 seconds, but only in the first ten clock updates.

maxchange

This directive sets the maximum allowed offset corrected on a clock update. The check is performed only after the specified number of updates to allow a large initial adjustment of the system clock. When an offset larger than the specified maximum occurs, it will be ignored for the specified number of times and then chronyd will give up and exit (a negative value can be used to never exit). In both cases a message is sent to syslog.

An example of the use of this directive is:

maxchange 1000 1 2

After the first clock update, chronyd will check the offset on every clock update, it will ignore two adjustments larger than 1000 seconds and exit on another one.

maxupdateskew

One of chronyd's tasks is to work out how fast or slow the computer’s clock runs relative to its reference sources. In addition, it computes an estimate of the error bounds around the estimated value.

If the range of error is too large, it indicates that the measurements have not settled down yet, and that the estimated gain or loss rate is not very reliable.

The maxupdateskew parameter is the threshold for determining whether an estimate is too unreliable to be used. By default, the threshold is 1000 ppm.

The format of the syntax is:

maxupdateskew skew-in-ppm

Typical values for skew-in-ppm might be 100 for a dial-up connection to servers over a telephone line, and 5 or 10 for a computer on a LAN.

It should be noted that this is not the only means of protection against using unreliable estimates. At all times, chronyd keeps track of both the estimated gain or loss rate, and the error bound on the estimate. When a new estimate is generated following another measurement from one of the sources, a weighted combination algorithm is used to update the master estimate. So if chronyd has an existing highly-reliable master estimate and a new estimate is generated which has large error bounds, the existing master estimate will dominate in the new master estimate.

minsources

The minsources directive sets the minimum number of sources that need to be considered as selectable in the source selection algorithm before the local clock is updated.

The format of the syntax is:

minsources number-of-sources

By default, number-of-sources is 1. Setting minsources to a larger number can be used to improve the reliability, because multiple sources will need to correspond with each other.

noclientlog
This directive, which takes no arguments, specifies that client accesses are not to be logged. Normally they are logged, allowing statistics to be reported using the clients command in chronyc and enabling the clients to use interleaved mode with the xleave option in the server directive.
reselectdist

When chronyd selects synchronization source from available sources, it will prefer the one with minimum synchronization distance. However, to avoid frequent reselecting when there are sources with similar distance, a fixed distance is added to the distance for sources that are currently not selected. This can be set with the reselectdist option. By default, the distance is 100 microseconds.

The format of the syntax is:

reselectdist dist-in-seconds
stratumweight

The stratumweight directive sets how much distance should be added per stratum to the synchronization distance when chronyd selects the synchronization source from available sources.

The format of the syntax is:

stratumweight dist-in-seconds

By default, dist-in-seconds is 1 millisecond. This means that sources with lower stratum are usually preferred to sources with higher stratum even when their distance is significantly worse. Setting stratumweight to 0 makes chronyd ignore stratum when selecting the source.

rtcfile

The rtcfile directive defines the name of the file in which chronyd can save parameters associated with tracking the accuracy of the system’s real-time clock (RTC).

The format of the syntax is:

rtcfile /var/lib/chrony/rtc

chronyd saves information in this file when it exits and when the writertc command is issued in chronyc. The information saved is the RTC’s error at some epoch, that epoch (in seconds since January 1 1970), and the rate at which the RTC gains or loses time. Not all real-time clocks are supported as their code is system-specific. Note that if this directive is used then the real-time clock should not be manually adjusted as this would interfere with chrony's need to measure the rate at which the real-time clock drifts if it was adjusted at random intervals.

rtcsync
The rtcsync directive is present in the /etc/chrony.conf file by default. This will inform the kernel the system clock is kept synchronized and the kernel will update the real-time clock every 11 minutes.

11.5.1. Configuring chrony for security

chronyc can access chronyd in two ways:

  • Internet Protocol, IPv4 or IPv6.
  • Unix domain socket, which is accessible locally by the root or chrony user.

By default, chronyc connects to the Unix domain socket. The default path is /var/run/chrony/chronyd.sock. If this connection fails, which can happen for example when chronyc is running under a non-root user, chronyc tries to connect to 127.0.0.1 and then ::1.

Only the following monitoring commands, which do not affect the behavior of chronyd, are allowed from the network:

  • activity
  • manual list
  • rtcdata
  • smoothing
  • sources
  • sourcestats
  • tracking
  • waitsync

The set of hosts from which chronyd accepts these commands can be configured with the cmdallow directive in the configuration file of chronyd, or the cmdallow command in chronyc. By default, the commands are accepted only from localhost (127.0.0.1 or ::1).

All other commands are allowed only through the Unix domain socket. When sent over the network, chronyd responds with a Not authorised error, even if it is from localhost.

Accessing chronyd remotely with chronyc

  1. Allow access from both IPv4 and IPv6 addresses by adding the following to the /etc/chrony.conf file:

    bindcmdaddress 0.0.0.0

    or

    bindcmdaddress :
  2. Allow commands from the remote IP address, network, or subnet by using the cmdallow directive.

    Add the following content to the /etc/chrony.conf file:

    cmdallow 192.168.1.0/24
  3. Open port 323 in the firewall to connect from a remote system.

    #  firewall-cmd --zone=public --add-port=323/udp

    If you want to open port 323 permanently, use the --permanent.

    #  firewall-cmd --permanent --zone=public --add-port=323/udp

Note that the allow directive is for NTP access whereas the cmdallow directive is to enable receiving of remote commands. It is possible to make these changes temporarily using chronyc running locally. Edit the configuration file to make permanent changes.

11.6. Using Chrony

11.6.1. Installing chrony

The chrony suite is installed by default on Red Hat Enterprise Linux. To ensure that it is, run the following command as root:

# yum install chrony

The default location for the chrony daemon is /usr/sbin/chronyd. The command line utility will be installed to /usr/bin/chronyc.

11.6.2. Checking the status of chronyd

To check the status of chronyd, issue the following command:

systemctl status chronyd
chronyd.service - NTP client/server
   Loaded: loaded (/usr/lib/systemd/system/chronyd.service; enabled)
   Active: active (running) since Wed 2013-06-12 22:23:16 CEST; 11h ago

11.6.3. Starting chronyd

To start chronyd, issue the following command as root:

# systemctl start chronyd

To ensure chronyd starts automatically at system start, issue the following command as root:

# systemctl enable chronyd

11.6.4. Stopping chronyd

To stop chronyd, issue the following command as root:

# systemctl stop chronyd

To prevent chronyd from starting automatically at system start, issue the following command as root:

# systemctl disable chronyd

11.6.5. Checking if chrony is synchronized

To check if chrony is synchronized, make use of the tracking, sources, and sourcestats commands.

11.6.5.1. Checking chrony tracking

To check chrony tracking, issue the following command:

chronyc tracking
Reference ID    : CB00710F (foo.example.net)
Stratum         : 3
Ref time (UTC)  : Fri Jan 27 09:49:17 2017
System time     :  0.000006523 seconds slow of NTP time
Last offset     : -0.000006747 seconds
RMS offset      : 0.000035822 seconds
Frequency       : 3.225 ppm slow
Residual freq   : 0.000 ppm
Skew            : 0.129 ppm
Root delay      : 0.013639022 seconds
Root dispersion : 0.001100737 seconds
Update interval : 64.2 seconds
Leap status     : Normal

The fields are as follows:

Reference ID
This is the reference ID and name (or IP address) if available, of the server to which the computer is currently synchronized. Reference ID is a hexadecimal number to avoid confusion with IPv4 addresses.
Stratum
The stratum indicates how many hops away from a computer with an attached reference clock we are. Such a computer is a stratum-1 computer, so the computer in the example is two hops away (that is to say, a.b.c is a stratum-2 and is synchronized from a stratum-1).
Ref time
This is the time (UTC) at which the last measurement from the reference source was processed.
System time
In normal operation, chronyd never steps the system clock, because any jump in the timescale can have adverse consequences for certain application programs. Instead, any error in the system clock is corrected by slightly speeding up or slowing down the system clock until the error has been removed, and then returning to the system clock’s normal speed. A consequence of this is that there will be a period when the system clock (as read by other programs using the gettimeofday() system call, or by the date command in the shell) will be different from chronyd's estimate of the current true time (which it reports to NTP clients when it is operating in server mode). The value reported on this line is the difference due to this effect.
Last offset
This is the estimated local offset on the last clock update.
RMS offset
This is a long-term average of the offset value.
Frequency
The "frequency" is the rate by which the system’s clock would be wrong if chronyd was not correcting it. It is expressed in ppm (parts per million). For example, a value of 1 ppm would mean that when the system’s clock thinks it has advanced 1 second, it has actually advanced by 1.000001 seconds relative to true time.
Residual freq

This shows the "residual frequency" for the currently selected reference source. This reflects any difference between what the measurements from the reference source indicate the frequency should be and the frequency currently being used.

The reason this is not always zero is that a smoothing procedure is applied to the frequency. Each time a measurement from the reference source is obtained and a new residual frequency computed, the estimated accuracy of this residual is compared with the estimated accuracy (see skew) of the existing frequency value. A weighted average is computed for the new frequency, with weights depending on these accuracies. If the measurements from the reference source follow a consistent trend, the residual will be driven to zero over time.

Skew
This is the estimated error bound on the frequency.
Root delay
This is the total of the network path delays to the stratum-1 computer from which the computer is ultimately synchronized. Root delay values are printed in nanosecond resolution. In certain extreme situations, this value can be negative. (This can arise in a symmetric peer arrangement where the computers’ frequencies are not tracking each other and the network delay is very short relative to the turn-around time at each computer.)
Root dispersion
This is the total dispersion accumulated through all the computers back to the stratum-1 computer from which the computer is ultimately synchronized. Dispersion is due to system clock resolution, statistical measurement variations etc. Root dispersion values are printed in nanosecond resolution.
Leap status
This is the leap status, which can be Normal, Insert second, Delete second or Not synchronized.

11.6.5.2. Checking chrony sources

The sources command displays information about the current time sources that chronyd is accessing.

The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns.

$ chronyc sources
	210 Number of sources = 3
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#* GPS0                          0   4   377    11   -479ns[ -621ns] /-  134ns
^? a.b.c                         2   6   377    23   -923us[ -924us] +/-   43ms
^ d.e.f                         1   6   377    21  -2629us[-2619us] +/-   86ms

The columns are as follows:

M
This indicates the mode of the source. ^ means a server, = means a peer and # indicates a locally connected reference clock.
S
This column indicates the state of the sources. "*" indicates the source to which chronyd is currently synchronized. "+" indicates acceptable sources which are combined with the selected source. "-" indicates acceptable sources which are excluded by the combining algorithm. "?" indicates sources to which connectivity has been lost or whose packets do not pass all tests. "x" indicates a clock which chronyd thinks is a falseticker (its time is inconsistent with a majority of other sources). "~" indicates a source whose time appears to have too much variability. The "?" condition is also shown at start-up, until at least 3 samples have been gathered from it.
Name/IP address
This shows the name or the IP address of the source, or reference ID for reference clock.
Stratum
This shows the stratum of the source, as reported in its most recently received sample. Stratum 1 indicates a computer with a locally attached reference clock. A computer that is synchronized to a stratum 1 computer is at stratum 2. A computer that is synchronized to a stratum 2 computer is at stratum 3, and so on.
Poll

This shows the rate at which the source is being polled, as a base-2 logarithm of the interval in seconds. Thus, a value of 6 would indicate that a measurement is being made every 64 seconds.

chronyd automatically varies the polling rate in response to prevailing conditions.

Reach
This shows the source’s reach register printed as an octal number. The register has 8 bits and is updated on every received or missed packet from the source. A value of 377 indicates that a valid reply was received for all of the last eight transmissions.
LastRx
This column shows how long ago the last sample was received from the source. This is normally in seconds. The letters m, h, d or y indicate minutes, hours, days or years. A value of 10 years indicates there were no samples received from this source yet.
Last sample
This column shows the offset between the local clock and the source at the last measurement. The number in the square brackets shows the actual measured offset. This may be suffixed by ns (indicating nanoseconds), us (indicating microseconds), ms (indicating milliseconds), or s (indicating seconds). The number to the left of the square brackets shows the original measurement, adjusted to allow for any slews applied to the local clock since. The number following the +/- indicator shows the margin of error in the measurement. Positive offsets indicate that the local clock is ahead of the source.

11.6.5.3. Checking chrony source statistics

The sourcestats command displays information about the drift rate and offset estimation process for each of the sources currently being examined by chronyd.

The optional argument -v can be specified, meaning verbose. In this case, extra caption lines are shown as a reminder of the meanings of the columns.

chronyc sourcestats
210 Number of sources = 1
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
===============================================================================
abc.def.ghi                11   5   46m     -0.001      0.045      1us    25us

The columns are as follows:

Name/IP address
This is the name or IP address of the NTP server (or peer) or reference ID of the reference clock to which the rest of the line relates.
NP
This is the number of sample points currently being retained for the server. The drift rate and current offset are estimated by performing a linear regression through these points.
NR
This is the number of runs of residuals having the same sign following the last regression. If this number starts to become too small relative to the number of samples, it indicates that a straight line is no longer a good fit to the data. If the number of runs is too low, chronyd discards older samples and re-runs the regression until the number of runs becomes acceptable.
Span
This is the interval between the oldest and newest samples. If no unit is shown the value is in seconds. In the example, the interval is 46 minutes.
Frequency
This is the estimated residual frequency for the server, in parts per million. In this case, the computer’s clock is estimated to be running 1 part in 109 slow relative to the server.
Freq Skew
This is the estimated error bounds on Freq (again in parts per million).
Offset
This is the estimated offset of the source.
Std Dev
This is the estimated sample standard deviation.

11.6.6. Manually Adjusting the System Clock

To step the system clock immediately, bypassing any adjustments in progress by slewing, issue the following command as root:

# chronyc makestep

If the rtcfile directive is used, the real-time clock should not be manually adjusted. Random adjustments would interfere with chrony's need to measure the rate at which the real-time clock drifts.

11.7. Setting up chrony for different environments

11.7.1. Setting up chrony for a system in an isolated network

For a network that is never connected to the Internet, one computer is selected to be the master timeserver. The other computers are either direct clients of the master, or clients of clients. On the master, the drift file must be manually set with the average rate of drift of the system clock. If the master is rebooted, it will obtain the time from surrounding systems and calculate an average to set its system clock. Thereafter it resumes applying adjustments based on the drift file. The drift file will be updated automatically when the settime command is used.

On the system selected to be the master, using a text editor running as root, edit /etc/chrony.conf as follows:

driftfile /var/lib/chrony/drift
commandkey 1
keyfile /etc/chrony.keys
initstepslew 10 client1 client3 client6
local stratum 8
manual
allow 192.0.2.0

Where 192.0.2.0 is the network or subnet address from which the clients are allowed to connect.

On the systems selected to be direct clients of the master, using a text editor running as root, edit the /etc/chrony.conf as follows:

server master
driftfile /var/lib/chrony/drift
logdir /var/log/chrony
log measurements statistics tracking
keyfile /etc/chrony.keys
commandkey 24
local stratum 10
initstepslew 20 master
allow 192.0.2.123

Where 192.0.2.123 is the address of the master, and master is the host name of the master. Clients with this configuration will resynchronize the master if it restarts.

On the client systems which are not to be direct clients of the master, the /etc/chrony.conf file should be the same except that the local and allow directives should be omitted.

In an isolated network, you can also use the local directive that enables a local reference mode, which allows chronyd operating as an NTP server to appear synchronized to real time, even when it was never synchronized or the last update of the clock happened a long time ago.

To allow multiple servers in the network to use the same local configuration and to be synchronized to one another, without confusing clients that poll more than one server, use the orphan option of the local directive which enables the orphan mode. Each server needs to be configured to poll all other servers with local. This ensures that only the server with the smallest reference ID has the local reference active and other servers are synchronized to it. When the server fails, another one will take over.

11.8. Chrony with HW timestamping

11.8.1. Understanding hardware timestamping

Hardware timestamping is a feature supported in some Network Interface Controller (NICs) which provides accurate timestamping of incoming and outgoing packets. NTP timestamps are usually created by the kernel and chronyd with the use of the system clock. However, when HW timestamping is enabled, the NIC uses its own clock to generate the timestamps when packets are entering or leaving the link layer or the physical layer. When used with NTP, hardware timestamping can significantly improve the accuracy of synchronization. For best accuracy, both NTP servers and NTP clients need to use hardware timestamping. Under ideal conditions, a sub-microsecond accuracy may be possible.

Another protocol for time synchronization that uses hardware timestamping is PTP.

Unlike NTP, PTP relies on assistance in network switches and routers. If you want to reach the best accuracy of synchronization, use PTP on networks that have switches and routers with PTP support, and prefer NTP on networks that do not have such switches and routers.

11.8.2. Verifying support for hardware timestamping

To verify that hardware timestamping with NTP is supported by an interface, use the ethtool -T command. An interface can be used for hardware timestamping with NTP if ethtool lists the SOF_TIMESTAMPING_TX_HARDWARE and SOF_TIMESTAMPING_TX_SOFTWARE capabilities and also the HWTSTAMP_FILTER_ALL filter mode.

Example 11.1. Verifying support for hardware timestamping on a specific interface

# ethtool -T eth0

Output:

Timestamping parameters for eth0:
Capabilities:
        hardware-transmit     (SOF_TIMESTAMPING_TX_HARDWARE)
        software-transmit     (SOF_TIMESTAMPING_TX_SOFTWARE)
        hardware-receive      (SOF_TIMESTAMPING_RX_HARDWARE)
        software-receive      (SOF_TIMESTAMPING_RX_SOFTWARE)
        software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
        hardware-raw-clock    (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 0
Hardware Transmit Timestamp Modes:
        off                   (HWTSTAMP_TX_OFF)
        on                    (HWTSTAMP_TX_ON)
Hardware Receive Filter Modes:
        none                  (HWTSTAMP_FILTER_NONE)
        all                   (HWTSTAMP_FILTER_ALL)
        ptpv1-l4-sync         (HWTSTAMP_FILTER_PTP_V1_L4_SYNC)
        ptpv1-l4-delay-req    (HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ)
        ptpv2-l4-sync         (HWTSTAMP_FILTER_PTP_V2_L4_SYNC)
        ptpv2-l4-delay-req    (HWTSTAMP_FILTER_PTP_V2_L4_DELAY_REQ)
        ptpv2-l2-sync         (HWTSTAMP_FILTER_PTP_V2_L2_SYNC)
        ptpv2-l2-delay-req    (HWTSTAMP_FILTER_PTP_V2_L2_DELAY_REQ)
        ptpv2-event           (HWTSTAMP_FILTER_PTP_V2_EVENT)
        ptpv2-sync            (HWTSTAMP_FILTER_PTP_V2_SYNC)
        ptpv2-delay-req       (HWTSTAMP_FILTER_PTP_V2_DELAY_REQ)

11.8.3. Enabling hardware timestamping

To enable hardware timestamping, use the hwtimestamp directive in the /etc/chrony.conf file. The directive can either specify a single interface, or a wildcard character can be used to enable hardware timestamping on all interfaces that support it. Use the wildcard specification in case that no other application, like ptp4l from the linuxptp package, is using hardware timestamping on an interface. Multiple hwtimestamp directives are allowed in the chrony configuration file.

Example 11.2. Enabling hardware timestamping by using the hwtimestamp directive

hwtimestamp eth0
hwtimestamp eth1
hwtimestamp *

11.8.4. Configuring client polling interval

The default range of a polling interval (64-1024 seconds) is recommended for servers on the Internet. For local servers and hardware timestamping, a shorter polling interval needs to be configured in order to minimize offset of the system clock.

The following directive in /etc/chrony.conf specifies a local NTP server using one second polling interval:

server ntp.local minpoll 0 maxpoll 0

11.8.5. Enabling interleaved mode

NTP servers that are not hardware NTP appliances, but rather general purpose computers running a software NTP implementation, like chrony, will get a hardware transmit timestamp only after sending a packet. This behavior prevents the server from saving the timestamp in the packet to which it corresponds. In order to enable NTP clients receiving transmit timestamps that were generated after the transmission, configure the clients to use the NTP interleaved mode by adding the xleave option to the server directive in /etc/chrony.conf:

server ntp.local minpoll 0 maxpoll 0 xleave

11.8.6. Configuring server for large number of clients

The default server configuration allows a few thousands of clients at most to use the interleaved mode concurrently. To configure the server for a larger number of clients, increase the clientloglimit directive in /etc/chrony.conf. This directive specifies the maximum size of memory allocated for logging of clients' access on the server:

clientloglimit 100000000

11.8.7. Verifying hardware timestamping

To verify that the interface has successfully enabled hardware timestamping, check the system log. The log should contain a message from chronyd for each interface with successfully enabled hardware timestamping.

Example 11.3. Log messages for interfaces with enabled hardware timestamping

chronyd[4081]: Enabled HW timestamping on eth0
chronyd[4081]: Enabled HW timestamping on eth1

When chronyd is configured as an NTP client or peer, you can have the transmit and receive timestamping modes and the interleaved mode reported for each NTP source by the chronyc ntpdata command:

Example 11.4. Reporting the transmit, receive timestamping and interleaved mode for each NTP source

# chronyc ntpdata

Output:

Remote address  : 203.0.113.15 (CB00710F)
Remote port     : 123
Local address   : 203.0.113.74 (CB00714A)
Leap status     : Normal
Version         : 4
Mode            : Server
Stratum         : 1
Poll interval   : 0 (1 seconds)
Precision       : -24 (0.000000060 seconds)
Root delay      : 0.000015 seconds
Root dispersion : 0.000015 seconds
Reference ID    : 47505300 (GPS)
Reference time  : Wed May 03 13:47:45 2017
Offset          : -0.000000134 seconds
Peer delay      : 0.000005396 seconds
Peer dispersion : 0.000002329 seconds
Response time   : 0.000152073 seconds
Jitter asymmetry: +0.00
NTP tests       : 111 111 1111
Interleaved     : Yes
Authenticated   : No
TX timestamping : Hardware
RX timestamping : Hardware
Total TX        : 27
Total RX        : 27
Total valid RX  : 27

Example 11.5. Reporting the stability of NTP measurements

# chronyc sourcestats

With hardware timestamping enabled, stability of NTP measurements should be in tens or hundreds of nanoseconds, under normal load. This stability is reported in the Std Dev column of the output of the chronyc sourcestats command:

Output:

210 Number of sources = 1
Name/IP Address            NP  NR  Span  Frequency  Freq Skew  Offset  Std Dev
ntp.local                  12   7    11     +0.000      0.019     +0ns    49ns

11.8.8. Configuring PTP-NTP bridge

If a highly accurate Precision Time Protocol (PTP) grandmaster is available in a network that does not have switches or routers with PTP support, a computer may be dedicated to operate as a PTP slave and a stratum-1 NTP server. Such a computer needs to have two or more network interfaces, and be close to the grandmaster or have a direct connection to it. This will ensure highly accurate synchronization in the network.

Configure the ptp4l and phc2sys programs from the linuxptp packages to use one interface to synchronize the system clock using PTP.

Configure chronyd to provide the system time using the other interface:

Example 11.6. Configuring chronyd to provide the system time using the other interface

bindaddress 203.0.113.74
hwtimestamp eth1
local stratum 1

11.9. Achieving some settings previously supported by ntp in chrony

Some settings that were in previous major version of Red Hat Enterprise Linux supported by ntp, are not supported by chrony. This section lists such settings, and describes ways to achieve them on a system with chrony.

11.9.1. Monitoring by ntpq and ntpdc

chronyd cannot be monitored by the ntpq and ntpdc utilities from the ntp distribution, because chrony does not support the NTP modes 6 and 7. It supports a different protocol and chronyc is the client implementation. For more information, see the chronyc(1) man page.

To monitor the status of the system clock sychronized by chronyd, you can:

  • Use the tracking command
  • Use the ntpstat utility, which supports chrony and provides a similar output as it used to with ntpd

Example 11.7. Using the tracking command

$ chronyc -n tracking
Reference ID    : 0A051B0A (10.5.27.10)
Stratum         : 2
Ref time (UTC)  : Thu Mar 08 15:46:20 2018
System time     : 0.000000338 seconds slow of NTP time
Last offset     : +0.000339408 seconds
RMS offset      : 0.000339408 seconds
Frequency       : 2.968 ppm slow
Residual freq   : +0.001 ppm
Skew            : 3.336 ppm
Root delay      : 0.157559142 seconds
Root dispersion : 0.001339232 seconds
Update interval : 64.5 seconds
Leap status     : Normal

Example 11.8. Using the ntpstat utility

$ ntpstat
synchronised to NTP server (10.5.27.10) at stratum 2
   time correct to within 80 ms
   polling server every 64 s

11.9.2. Using authentication mechanism based on public key cryptography

In Red Hat Enterprise Linux 7, ntp supported Autokey, which is an authentication mechanism based on public key cryptography. Autokey is not supported in chronyd.

On a Red Hat Enterprise Linux 8 system, it is recommended to use symmetric keys instead. Generate the keys with the chronyc keygen command. A client and server need to share a key specified in /etc/chrony.keys. The client can enable authentication using the key option in the server, pool, or peer directive.

11.9.3. Using ephemeral symmetric associations

In Red Hat Enterprise Linux 7, ntpd supported ephemeral symmetric associations, which can be mobilized by packets from peers which are not specified in the ntp.conf configuration file. In Red Hat Enterprise Linux 8, chronyd needs all peers to be specified in chrony.conf. Ephemeral symmetric associations are not supported.

Note that using the client/server mode enabled by the server or pool directive is more secure compared to the symmetric mode enabled by the peer directive.

11.9.4. multicast/broadcast client

Red Hat Enterprise Linux 7 supported the broadcast/multicast NTP mode, which simplifies configuration of clients. With this mode, clients can be configured to just listen for packets sent to a multicast/broadcast address instead of listening for specific names or addresses of individual servers, which may change over time.

In Red Hat Enterprise Linux 8, chronyd does not support the broadcast/multicast mode. The main reason is that it is less accurate and less secure than the ordinary client/server and symmetric modes.

There are several options of migration from an NTP broadcast/multicast setup:

  • Configure DNS to translate a single name, such as ntp.example.com, to multiple addresses of different servers

    Clients can have a static configuration using only a single pool directive to synchronize with multiple servers. If a server from the pool becomes unreacheable, or otherwise unsuitable for synchronization, the clients automatically replace it with another server from the pool.

  • Distribute the list of NTP servers over DHCP

    When NetworkManager gets a list of NTP servers from the DHCP server, chronyd is automatically configured to use them. This feature can be disabled by adding PEERNTP=no to the /etc/sysconfig/network file.

  • Use the Precision Time Protocol (PTP)

    This option is suitable mainly for environments where servers change frequently, or if a larger group of clients needs to be able to synchronize to each other without having a designated server.

    PTP was designed for multicast messaging and works similarly to the NTP broadcast mode. A PTP implementation is available in the linuxptp package.

    PTP normally requires hardware timestamping and support in network switches to perform well. However, PTP is expected to work better than NTP in the broadcast mode even with software timestamping and no support in network switches.

    In networks with very large number of PTP slaves in one communication path, it is recommended to configure the PTP slaves with the hybrid_e2e option in order to reduce the amount of network traffic generated by the slaves. You can configure a computer running chronyd as an NTP client, and possibly NTP server, to operate also as a PTP grandmaster to distribute synchronized time to a large number of computers using multicast messaging.

11.10. Additional resources

The following sources of information provide additional resources regarding chrony.

11.10.1. Installed Documentation

  • chronyc(1) man page — Describes the chronyc command-line interface tool including commands and command options.
  • chronyd(8) man page — Describes the chronyd daemon including commands and command options.
  • chrony.conf(5) man page — Describes the chrony configuration file.

11.10.2. Online Documentation

For answers to FAQs, see https://chrony.tuxfamily.org/faq.html

11.11. Managing time synchronization using RHEL System Roles

You can manage time synchronization on multiple target machines using the timesync role.

The timesync role installs and configures an NTP or PTP implementation to operate as an NTP client or PTP slave in order to synchronize the system clock with NTP servers or grandmasters in PTP domains.

Note that using the timesync role also facilitates migration to chrony, because you can use the same playbook on all versions of Red Hat Enterprise Linux starting with RHEL 6 regardless of whether the system uses ntp or chrony to implement the NTP protocol.

Warning

The timesync role replaces the configuration of the given or detected provider service on the managed host. Previous settings are lost, even if they are not specified in the role variables. The only preserved setting is the choice of provider if the timesync_ntp_provider variable is not defined.

The following example shows how to apply the timesync role in a situation with just one pool of servers.

Example 11.9. An example playbook applying the timesync role for a single pool of servers

---
- hosts: timesync-test
  vars:
    timesync_ntp_servers:
      - hostname: 2.rhel.pool.ntp.org
        pool: yes
        iburst: yes
  roles:
    - rhel-system-roles.timesync

Additional resources

  • For a detailed reference on timesync role variables, install the rhel-system-roles package, and see the README.md or README.html files in the /usr/share/doc/rhel-system-roles/timesync directory.
  • For more information on RHEL System Roles, see Introduction to RHEL System Roles.

Chapter 12. Using secure communications between two systems with OpenSSH

SSH (Secure Shell) is a protocol which provides secure communications between two systems using a client-server architecture and allows users to log in to server host systems remotely. Unlike other remote communication protocols, such as FTP or Telnet, SSH encrypts the login session, which prevents intruders to collect unencrypted passwords from the connection.

Red Hat Enterprise Linux includes the basic OpenSSH packages: the general openssh package, the openssh-server package and the openssh-clients package. Note that the OpenSSH packages require the OpenSSL package openssl-libs, which installs several important cryptographic libraries that enable OpenSSH to provide encrypted communications.

12.1. SSH and OpenSSH

SSH (Secure Shell) is a program for logging into a remote machine and executing commands on that machine. The SSH protocol provides secure encrypted communications between two untrusted hosts over an insecure network. You can also forward X11 connections and arbitrary TCP/IP ports over the secure channel.

The SSH protocol mitigates security threats, such as interception of communication between two systems and impersonation of a particular host, when you use it for remote shell login or file copying. This is because the SSH client and server use digital signatures to verify their identities. Additionally, all communication between the client and server systems is encrypted.

OpenSSH is an implementation of the SSH protocol supported by a number of Linux, UNIX, and similar operating systems. It includes the core files necessary for both the OpenSSH client and server. The OpenSSH suite consists of the following user-space tools:

  • ssh is a remote login program (SSH client)
  • sshd is an OpenSSH SSH daemon
  • scp is a secure remote file copy program
  • sftp is a secure file transfer program
  • ssh-agent is an authentication agent for caching private keys
  • ssh-add adds private key identities to ssh-agent
  • ssh-keygen generates, manages, and converts authentication keys for ssh
  • ssh-copy-id is a script that adds local public keys to the authorized_keys file on a remote SSH server
  • ssh-keyscan - gathers SSH public host keys

Two versions of SSH currently exist: version 1, and the newer version 2. The OpenSSH suite in Red Hat Enterprise Linux 8 supports only SSH version 2, which has an enhanced key-exchange algorithm not vulnerable to known exploits in version 1.

OpenSSH, as one of the RHEL core cryptographic subsystems uses system-wide crypto policies. This ensures that weak cipher suites and cryptographic algorithms are disabled in the default configuration. To adjust the policy, the administrator must either use the update-crypto-policies command to make settings stricter or looser or manually opt-out of the system-wide crypto policies.

The OpenSSH suite uses two different sets of configuration files: those for client programs (that is, ssh, scp, and sftp), and those for the server (the sshd daemon). System-wide SSH configuration information is stored in the /etc/ssh/ directory. User-specific SSH configuration information is stored in ~/.ssh/ in the user’s home directory. For a detailed list of OpenSSH configuration files, see the FILES section in the sshd(8) man page.

Additional resources

12.2. Configuring and starting an OpenSSH server

Use the following procedure for a basic configuration that might be required for your environment and for starting an OpenSSH server. Note that after the default RHEL installation, the sshd daemon is already started and server host keys are automatically created.

Prerequisites

  • The openssh-server package is installed.

Procedure

  1. Start the sshd daemon in the current session and set it to start automatically at boot time:

    # systemctl start sshd
    # systemctl enable sshd
  2. To specify different addresses than the default 0.0.0.0 (IPv4) or :: (IPv6) for the ListenAddress directive in the /etc/ssh/sshd_config configuration file and to use a slower dynamic network configuration, add the dependency on the network-online.target target unit to the sshd.service unit file. To achieve this, create the /etc/systemd/system/sshd.service.d/local.conf file with the following content:

    [Unit]
    Wants=network-online.target
    After=network-online.target
  3. Review if OpenSSH server settings in the /etc/ssh/sshd_config configuration file meet the requirements of your scenario.
  4. Optionally, change the welcome message that your OpenSSH server displays before a client authenticates by editing the /etc/issue file, for example:

    Welcome to ssh-server.example.com
    Warning: By accessing this server, you agree to the referenced terms and conditions.

    Note that to change the message displayed after a successful login you have to edit the /etc/motd file on the server. See the pam_motd man page for more information.

  5. Reload the systemd configuration to apply the changes:

    # systemctl daemon-reload

Verification steps

  1. Check that the sshd daemon is running:

    # systemctl status sshd
    ● sshd.service - OpenSSH server daemon
       Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
       Active: active (running) since Mon 2019-11-18 14:59:58 CET; 6min ago
         Docs: man:sshd(8)
               man:sshd_config(5)
     Main PID: 1149 (sshd)
        Tasks: 1 (limit: 11491)
       Memory: 1.9M
       CGroup: /system.slice/sshd.service
               └─1149 /usr/sbin/sshd -D -oCiphers=aes128-ctr,aes256-ctr,aes128-cbc,aes256-cbc -oMACs=hmac-sha2-256,>
    
    Nov 18 14:59:58 ssh-server-example.com systemd[1]: Starting OpenSSH server daemon...
    Nov 18 14:59:58 ssh-server-example.com sshd[1149]: Server listening on 0.0.0.0 port 22.
    Nov 18 14:59:58 ssh-server-example.com sshd[1149]: Server listening on :: port 22.
    Nov 18 14:59:58 ssh-server-example.com systemd[1]: Started OpenSSH server daemon.
  2. Connect to the SSH server with an SSH client.

    # ssh user@ssh-server-example.com
    ECDSA key fingerprint is SHA256:dXbaS0RG/UzlTTku8GtXSz0S1++lPegSy31v3L/FAEc.
    Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
    Warning: Permanently added 'ssh-server-example.com' (ECDSA) to the list of known hosts.
    
    user@ssh-server-example.com's password:

Additional resources

  • sshd(8) and sshd_config(5) man pages

12.3. Using key pairs instead of passwords for SSH authentication

To improve system security even further, generate SSH key pairs and then enforce key-based authentication by disabling password authentication.

12.3.1. Setting an OpenSSH server for key-based authentication

Follow these steps to configure your OpenSSH server for enforcing key-based authentication.

Prerequisites

  • The openssh-server package is installed.
  • The sshd daemon is running on the server.

Procedure

  1. Open the /etc/ssh/sshd_config configuration in a text editor, for example:

    # vi /etc/ssh/sshd_config
  2. Change the PasswordAuthentication option to no:

    PasswordAuthentication no

    On a system other than a new default installation, check that PubkeyAuthentication no has not been set and the ChallengeResponseAuthentication directive is set to no. If you are connected remotely, not using console or out-of-band access, test the key-based login process before disabling password authentication.

  3. To use key-based authentication with NFS-mounted home directories, enable the use_nfs_home_dirs SELinux boolean:

    # setsebool -P use_nfs_home_dirs 1
  4. Reload the sshd daemon to apply the changes:

    # systemctl reload sshd

Additional resources

  • sshd(8), sshd_config(5), and setsebool(8) man pages

12.3.2. Generating SSH key pairs

Use this procedure to generate an SSH key pair on a local system and to copy the generated public key to an OpenSSH server. If the server is configured accordingly, you can log in to the OpenSSH server without providing any password.

Important

If you complete the following steps as root, only root is able to use the keys.

Procedure

  1. To generate an ECDSA key pair for version 2 of the SSH protocol:

    $ ssh-keygen -t ecdsa
    Generating public/private ecdsa key pair.
    Enter file in which to save the key (/home/joesec/.ssh/id_ecdsa):
    Enter passphrase (empty for no passphrase):
    Enter same passphrase again:
    Your identification has been saved in /home/joesec/.ssh/id_ecdsa.
    Your public key has been saved in /home/joesec/.ssh/id_ecdsa.pub.
    The key fingerprint is:
    SHA256:Q/x+qms4j7PCQ0qFd09iZEFHA+SqwBKRNaU72oZfaCI joesec@localhost.example.com
    The key's randomart image is:
    +---[ECDSA 256]---+
    |.oo..o=++        |
    |.. o .oo .       |
    |. .. o. o        |
    |....o.+...       |
    |o.oo.o +S .      |
    |.=.+.   .o       |
    |E.*+.  .  . .    |
    |.=..+ +..  o     |
    |  .  oo*+o.      |
    +----[SHA256]-----+

    You can also generate an RSA key pair by using the -t rsa option with the ssh-keygen command or an Ed25519 key pair by entering the ssh-keygen -t ed25519 command.

  2. To copy the public key to a remote machine:

    $ ssh-copy-id joesec@ssh-server-example.com
    /usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
    ...
    Number of key(s) added: 1
    
    Now try logging into the machine, with: "ssh 'joesec@ssh-server-example.com'" and check to make sure that only the key(s) you wanted were added.

    If you do not use the ssh-agent program in your session, the previous command copies the most recently modified ~/.ssh/id*.pub public key if it is not yet installed. To specify another public-key file or to prioritize keys in files over keys cached in memory by ssh-agent, use the ssh-copy-id command with the -i option.

Note

If you reinstall your system and want to keep previously generated key pairs, back up the ~/.ssh/ directory. After reinstalling, copy it back to your home directory. You can do this for all users on your system, including root.

Verification steps

  1. Log in to the OpenSSH server without providing any password:

    $ ssh joesec@ssh-server-example.com
    Welcome message.
    ...
    Last login: Mon Nov 18 18:28:42 2019 from ::1

Additional resources

  • ssh-keygen(1) and ssh-copy-id(1) man pages

12.4. Using SSH keys stored on a smart card

Red Hat Enterprise Linux 8 enables you to use RSA and ECDSA keys stored on a smart card on OpenSSH clients. Use this procedure to enable authentication using a smart card instead of using a password.

Prerequisites

  • On the client side, the opensc package is installed and the pcscd service is running.

Procedure

  1. List all keys provided by the OpenSC PKCS #11 module including their PKCS #11 URIs and save the output to the keys.pub file:

    $ ssh-keygen -D pkcs11: > keys.pub
    $ ssh-keygen -D pkcs11:
    ssh-rsa AAAAB3NzaC1yc2E...KKZMzcQZzx pkcs11:id=%02;object=SIGN%20pubkey;token=SSH%20key;manufacturer=piv_II?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so
    ecdsa-sha2-nistp256 AAA...J0hkYnnsM= pkcs11:id=%01;object=PIV%20AUTH%20pubkey;token=SSH%20key;manufacturer=piv_II?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so
  2. To enable authentication using a smart card on a remote server (example.com), transfer the public key to the remote server. Use the ssh-copy-id command with keys.pub created in the previous step:

    $ ssh-copy-id -f -i keys.pub username@example.com
  3. To connect to example.com using the ECDSA key from the output of the ssh-keygen -D command in step 1, you can use just a subset of the URI, which uniquely references your key, for example:

    $ ssh -i "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so" example.com
    Enter PIN for 'SSH key':
    [example.com] $
  4. You can use the same URI string in the ~/.ssh/config file to make the configuration permanent:

    $ cat ~/.ssh/config
    IdentityFile "pkcs11:id=%01?module-path=/usr/lib64/pkcs11/opensc-pkcs11.so"
    $ ssh example.com
    Enter PIN for 'SSH key':
    [example.com] $

    Because OpenSSH uses the p11-kit-proxy wrapper and the OpenSC PKCS #11 module is registered to PKCS#11 Kit, you can simplify the previous commands:

    $ ssh -i "pkcs11:id=%01" example.com
    Enter PIN for 'SSH key':
    [example.com] $

If you skip the id= part of a PKCS #11 URI, OpenSSH loads all keys that are available in the proxy module. This can reduce the amount of typing required:

$ ssh -i pkcs11: example.com
Enter PIN for 'SSH key':
[example.com] $

Additional resources

12.5. Making OpenSSH more secure

The following tips help you to increase security when using OpenSSH. Note that changes in the /etc/ssh/sshd_config OpenSSH configuration file require reloading the sshd daemon to take effect:

# systemctl reload sshd
Important

The majority of security hardening configuration changes reduce compatibility with clients that do not support up-to-date algorithms or cipher suites.

Disabling insecure connection protocols

  • To make SSH truly effective, prevent the use of insecure connection protocols that are replaced by the OpenSSH suite. Otherwise, a user’s password might be protected using SSH for one session only to be captured later when logging in using Telnet. For this reason, consider disabling insecure protocols, such as telnet, rsh, rlogin, and ftp.

Enabling key-based authentication and disabling password-based authentication

  • Disabling passwords for authentication and allowing only key pairs reduces the attack surface and it also might save users’ time. On clients, generate key pairs using the ssh-keygen tool and use the ssh-copy-id utility to copy public keys from clients on the OpenSSH server. To disable password-based authentication on your OpenSSH server, edit /etc/ssh/sshd_config and change the PasswordAuthentication option to no:

    PasswordAuthentication no

Key types

  • Although the ssh-keygen command generates a pair of RSA keys by default, you can instruct it to generate ECDSA or Ed25519 keys by using the -t option. The ECDSA (Elliptic Curve Digital Signature Algorithm) offers better performance than RSA at the equivalent symmetric key strength. It also generates shorter keys. The Ed25519 public-key algorithm is an implementation of twisted Edwards curves that is more secure and also faster than RSA, DSA, and ECDSA.

    OpenSSH creates RSA, ECDSA, and Ed25519 server host keys automatically if they are missing. To configure the host key creation in RHEL 8, use the sshd-keygen@.service instantiated service. For example, to disable the automatic creation of the RSA key type:

    # systemctl mask sshd-keygen@rsa.service
  • To exclude particular key types for SSH connections, comment out the relevant lines in /etc/ssh/sshd_config, and reload the sshd service. For example, to allow only Ed25519 host keys:
# HostKey /etc/ssh/ssh_host_rsa_key
# HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key

Non-default port

  • By default, the sshd daemon listens on TCP port 22. Changing the port reduces the exposure of the system to attacks based on automated network scanning and thus increase security through obscurity. You can specify the port using the Port directive in the /etc/ssh/sshd_config configuration file.

    You also have to update the default SELinux policy to allow the use of a non-default port. To do so, use the semanage tool from the policycoreutils-python-utils package:

    # semanage port -a -t ssh_port_t -p tcp port_number

    Furthermore, update firewalld configuration:

    # firewall-cmd --add-port port_number/tcp
    # firewall-cmd --runtime-to-permanent

    In the previous commands, replace port_number with the new port number specified using the Port directive.

No root login

  • If your particular use case does not require the possibility of logging in as the root user, you should consider setting the PermitRootLogin configuration directive to no in the /etc/ssh/sshd_config file. By disabling the possibility of logging in as the root user, the administrator can audit which users run what privileged commands after they log in as regular users and then gain root rights.

    Alternatively, set PermitRootLogin to prohibit-password:

    PermitRootLogin prohibit-password

    This enforces the use of key-based authentication instead of the use of passwords for logging in as root and reduces risks by preventing brute-force attacks.

Using the ⁠X Security extension

  • The X server in Red Hat Enterprise Linux clients does not provide the X Security extension. Therefore, clients cannot request another security layer when connecting to untrusted SSH servers with X11 forwarding. Most applications are not able to run with this extension enabled anyway.

    By default, the ForwardX11Trusted option in the /etc/ssh/ssh_config.d/05-redhat.conf file is set to yes, and there is no difference between the ssh -X remote_machine (untrusted host) and ssh -Y remote_machine (trusted host) command.

    If your scenario does not require the X11 forwarding feature at all, set the X11Forwarding directive in the /etc/ssh/sshd_config configuration file to no.

Restricting access to specific users, groups, or domains

  • The AllowUsers and AllowGroups directives in the /etc/ssh/sshd_config configuration file server enable you to permit only certain users, domains, or groups to connect to your OpenSSH server. You can combine AllowUsers and AllowGroups to restrict access more precisely, for example:

    AllowUsers *@192.168.1.*,*@10.0.0.*,!*@192.168.1.2
    AllowGroups example-group

    The previous configuration lines accept connections from all users from systems in 192.168.1.* and 10.0.0.* subnets except from the system with the 192.168.1.2 address. All users must be in the example-group group. The OpenSSH server denies all other connections.

    Note that using whitelists (directives starting with Allow) is more secure than using blacklists (options starting with Deny) because whitelists block also new unauthorized users or groups.

Changing system-wide cryptographic policies

  • OpenSSH uses RHEL system-wide cryptographic policies, and the default system-wide cryptographic policy level offers secure settings for current threat models. To make your cryptographic settings more strict, change the current policy level:

    # update-crypto-policies --set FUTURE
    Setting system policy to FUTURE
  • To opt-out of the system-wide crypto policies for your OpenSSH server, uncomment the line with the CRYPTO_POLICY= variable in the /etc/sysconfig/sshd file. After this change, values that you specify in the Ciphers, MACs, KexAlgoritms, and GSSAPIKexAlgorithms sections in the /etc/ssh/sshd_config file are not overridden. Note that this task requires deep expertise in configuring cryptographic options.
  • See Using system-wide cryptographic policies in the RHEL 8 Security hardening title for more information.

Additional resources

  • sshd_config(5), ssh-keygen(1), crypto-policies(7), and update-crypto-policies(8) man pages

12.6. Connecting to a remote server using an SSH jump host

Use this procedure for connecting to a remote server through an intermediary server, also called jump host.

Prerequisites

  • A jump host accepts SSH connections from your system.
  • A remote server accepts SSH connections only from the jump host.

Procedure

  1. Define the jump host by editing the ~/.ssh/config file, for example:

    Host jump-server1
      HostName jump1.example.com
  2. Add the remote server jump configuration with the ProxyJump directive to ~/.ssh/config, for example:

    Host remote-server
      HostName remote1.example.com
      ProxyJump jump-server1
  3. Connect to the remote server through the jump server:

    $ ssh remote-server

    The previous command is equivalent to the ssh -J jump-server1 remote-server command if you omit the configuration steps 1 and 2.

Note

You can specify more jump servers and you can also skip adding host definitions to the configurations file when you provide their complete host names, for example:

$ ssh -J jump1.example.com,jump2.example.com,jump3.example.com remote1.example.com

Change the host name-only notation in the previous command if the user names or SSH ports on the jump servers differ from the names and ports on the remote server, for example:

$ ssh -J johndoe@jump1.example.com:75,johndoe@jump2.example.com:75,johndoe@jump3.example.com:75 joesec@remote1.example.com:220

Additional resources

  • ssh_config(5) and ssh(1) man pages

12.7. Connecting to remote machines with SSH keys using ssh-agent

To avoid entering a passphrase each time you initiate an SSH connection, you can use the ssh-agent utility to cache the private SSH key. The private key and the passphrase remain secure.

Prerequisites

  • You have a remote host with SSH daemon running and reachable through the network.
  • You know the IP address or hostname and credentials to log in to the remote host.
  • You have generated an SSH key pair with a passphrase and transferred the public key to the remote machine. For more information, see Generating SSH key pairs.

Procedure

  1. Optional: Verify you can use the key to authenticate to the remote host:

    1. Connect to the remote host using SSH:

      $ ssh example.user1@198.51.100.1 hostname
    2. Enter the passphrase you set while creating the key to grant access to the private key.

      $ ssh example.user1@198.51.100.1 hostname
       host.example.com
  2. Start the ssh-agent.

    $ eval $(ssh-agent)
    Agent pid 20062
  3. Add the key to ssh-agent.

    $ ssh-add ~/.ssh/id_rsa
    Enter passphrase for ~/.ssh/id_rsa:
    Identity added: ~/.ssh/id_rsa (example.user0@198.51.100.12)

Verification steps

  • Optional: Log in to the host machine using SSH.

    $ ssh example.user1@198.51.100.1
    
    Last login: Mon Sep 14 12:56:37 2020

    Note that you did not have to enter the passphrase.

12.8. Additional resources

For more information on configuring and connecting to OpenSSH servers and clients on Red Hat Enterprise Linux, see the resources listed below.

Installed documentation

  • sshd(8) man page documents available command-line options and provides a complete list of supported configuration files and directories.
  • ssh(1) man page provides a complete list of available command-line options and supported configuration files and directories.
  • scp(1) man page provides a more detailed description of the scp utility and its usage.
  • sftp(1) man page provides a more detailed description of the sftp utility and its usage.
  • ssh-keygen(1) man page documents in detail the use of the ssh-keygen utility to generate, manage, and convert authentication keys used by ssh.
  • ssh-copy-id(1) man page describes the use of the ssh-copy-id script.
  • ssh_config(5) man page documents available SSH client configuration options.
  • sshd_config(5) man page provides a full description of available SSH daemon configuration options.
  • update-crypto-policies(8) man page provides guidance on managing system-wide cryptographic policies
  • crypto-policies(7) man page provides an overview of system-wide cryptographic policy levels

Online documentation

Chapter 13. Configuring a remote logging solution

To ensure that logs from various machines in your environment are recorded centrally on a logging server, you can configure the Rsyslog application to record logs that fit specific criteria from the client system to the server.

13.1. The Rsyslog logging service

The Rsyslog application, in combination with the systemd-journald service, provides local and remote logging support in Red Hat Enterprise Linux. The rsyslogd daemon continuously reads syslog messages received by the systemd-journald service from the journal. rsyslogd then filters and processes these syslog events and records them to rsyslog log files or forwards them to other services according to its configuration.

The rsyslogd daemon also provides extended filtering, encryption protected relaying of messages, input and output modules, and support for transportation using the TCP and UDP protocols.

In /etc/rsyslog.conf, which is the main configuration file for rsyslog, you can specify the rules according to which rsyslogd handles the messages. Generally, you can classify messages by their source and topic (facility) and urgency (priority), and then assign an action that should be performed when a message fits these criteria.

In /etc/rsyslog.conf, you can also see a list of log files maintained by rsyslogd. Most log files are located in the /var/log/ directory. Some applications, such as httpd and samba, store their log files in a subdirectory within /var/log/.

Additional resources

13.2. Installing Rsyslog documentation

The Rsyslog application has extensive documentation that is available at https://www.rsyslog.com/doc/, but you can also install the rsyslog-doc documentation package locally by following this procedure.

Prerequisites

  • You have activated the AppStream repository on your system
  • You are authorized to install new packages using sudo

Procedure

  • Install the rsyslog-doc package:

    $ sudo yum install rsyslog-doc

Verification

13.3. Configuring remote logging over TCP

The Rsyslog application enables you to both run a logging server and configure individual systems to send their log files to the logging server. To use remote logging through TCP, configure both the server and the client. The server collects and analyzes the logs sent by one or more client systems.

With the Rsyslog application, you can maintain a centralized logging system where log messages are forwarded to a server over the network. To avoid message loss when the server is not available, you can configure an action queue for the forwarding action. This way, messages that failed to be sent are stored locally until the server is reachable again. Note that such queues cannot be configured for connections using the UDP protocol.

The omfwd plug-in provides forwarding over UDP or TCP. The default protocol is UDP. Because the plug-in is built in, it does not have to be loaded.

13.3.1. Configuring a server for remote logging over TCP

Follow this procedure to configure a server for collecting and analyzing logs sent by one or more client systems.

By default, rsyslog uses TCP on port 514.

Prerequisites

  • rsyslog is installed on the server system
  • You are logged in as root on the server

Procedure

  1. Optional: To use a different port for rsyslog traffic, add the syslogd_port_t SELinux type to port. For example, enable port 30514:

    # semanage port -a -t syslogd_port_t -p tcp 30514
  2. Optional: To use a different port for rsyslog traffic, configure firewalld to allow incoming rsyslog traffic on that port. For example, allow TCP traffic on port 30514 in zone zone:

    # firewall-cmd --zone=zone --permanent --add-port=30514/tcp
    success
  3. Create a new file in the /etc/rsyslog.d/ directory named, for example, remotelog.conf, and insert the following content:

    # Define templates before the rules that use them
    ### Per-Host Templates for Remote Systems ###
    template(name="TmplAuthpriv" type="list") {
        constant(value="/var/log/remote/auth/")
        property(name="hostname")
        constant(value="/")
        property(name="programname" SecurePath="replace")
        constant(value=".log")
        }
    
    template(name="TmplMsg" type="list") {
        constant(value="/var/log/remote/msg/")
        property(name="hostname")
        constant(value="/")
        property(name="programname" SecurePath="replace")
        constant(value=".log")
        }
    
    # Provides TCP syslog reception
    module(load="imtcp")
    # Adding this ruleset to process remote messages
    ruleset(name="remote1"){
         authpriv.*   action(type="omfile" DynaFile="TmplAuthpriv")
          *.info;mail.none;authpriv.none;cron.none
    action(type="omfile" DynaFile="TmplMsg")
    }
    
    input(type="imtcp" port="30514" ruleset="remote1")
  4. Save the changes to the /etc/rsyslog.d/remotelog.conf file.
  5. Make sure the rsyslog service is running and enabled on the logging server:

    # systemctl status rsyslog
  6. Restart the rsyslog service.

    # systemctl restart rsyslog
  7. Optional: If rsyslog is not enabled, ensure the rsyslog service starts automatically after reboot:

    # systemctl enable rsyslog

Your log server is now configured to receive and store log files from the other systems in your environment.

Verification

  • Test the syntax of the /etc/rsyslog.conf file:

    # rsyslogd -N 1
    rsyslogd: version 8.1911.0-2.el8, config validation run (level 1), master config /etc/rsyslog.conf
    rsyslogd: End of config validation run. Bye.

Additional resources

13.3.2. Configuring remote logging to a server over TCP

Follow this procedure to configure a system for forwarding log messages to a server over the TCP protocol. The omfwd plug-in provides forwarding over UDP or TCP. The default protocol is UDP. Because the plug-in is built in, you do not have to load it.

Prerequisites

  • The rsyslog package is installed on the client systems that should report to the server.
  • You have configured the server for remote logging.
  • The specified port is permitted in SELinux and open in firewall.

Procedure

  1. Create a new file in the /etc/rsyslog.d/ directory named, for example, remotelog.conf, and insert the following content:

    *.* action(type="omfwd"
          queue.type="linkedlist"
          queue.filename="example_fwd"
          action.resumeRetryCount="-1"
          queue.saveOnShutdown="on"
          target="example.com" port="30514" protocol="tcp"
         )

    Where:

    • queue.type="linkedlist" enables a LinkedList in-memory queue,
    • queue.filename defines a disk storage. The backup files are created with the example_fwd prefix in the working directory specified by the preceding global workDirectory directive,
    • the action.resumeRetryCount -1 setting prevents rsyslog from dropping messages when retrying to connect if server is not responding,
    • enabled queue.saveOnShutdown="on" saves in-memory data if rsyslog shuts down,
    • the last line forwards all received messages to the logging server, port specification is optional.

    With this configuration, rsyslog sends messages to the server but keeps messages in memory if the remote server is not reachable. A file on disk is created only if rsyslog runs out of the configured memory queue space or needs to shut down, which benefits the system performance.

  2. Restart the rsyslog service.

    # systemctl restart rsyslog

Verification

To verify that the client system sends messages to the server, follow these steps:

  1. On the client system, send a test message:

    # logger test
  2. On the server system, view the /var/log/messages log, for example:

    # cat /var/log/remote/msg/hostname/root.log
    Feb 25 03:53:17 hostname root[6064]: test

    Where hostname is the host name of the client system. Note that the log contains the user name of the user that entered the logger command, in this case root.

Additional resources

13.4. Configuring remote logging over UDP

The Rsyslog application enables you to configure a system to receive logging information from remote systems. To use remote logging through UDP, configure both the server and the client. The receiving server collects and analyzes the logs sent by one or more client systems. By default, rsyslog uses UDP on port 514 to receive log information from remote systems.

13.4.1. Configuring a server for receiving remote logging information over UDP

Follow this procedure to configure a server for collecting and analyzing logs sent by one or more client systems over the UDP protocol.

Prerequisites

  • The rsyslog utility is installed.

Procedure

  1. Optional: To use a different port for rsyslog traffic than the default port 514:

    1. Add the syslogd_port_t SELinux type to the SELinux policy configuration, replacing portno with the port number you want rsyslog to use:

      # semanage port -a -t syslogd_port_t -p udp portno
    2. Configure firewalld to allow incoming rsyslog traffic, replacing portno with the port number and zone with the zone you want rsyslog to use:

      # firewall-cmd --zone=zone --permanent --add-port=portno/udp
      success
    3. Reload the firewall rules:

      # firewall-cmd --reload
  2. Create a new .conf file in the /etc/rsyslog.d/ directory, for example, remotelogserv.conf, and insert the following content:

    # Define templates before the rules that use them
    ### Per-Host Templates for Remote Systems ###
    template(name="TmplAuthpriv" type="list") {
        constant(value="/var/log/remote/auth/")
        property(name="hostname")
        constant(value="/")
        property(name="programname" SecurePath="replace")
        constant(value=".log")
        }
    
    template(name="TmplMsg" type="list") {
        constant(value="/var/log/remote/msg/")
        property(name="hostname")
        constant(value="/")
        property(name="programname" SecurePath="replace")
        constant(value=".log")
        }
    
    # Provides UDP syslog reception
    module(load="imudp")
    
    # This ruleset processes remote messages
    ruleset(name="remote1"){
         authpriv.*   action(type="omfile" DynaFile="TmplAuthpriv")
          *.info;mail.none;authpriv.none;cron.none
    action(type="omfile" DynaFile="TmplMsg")
    }
    
    input(type="imudp" port="514" ruleset="remote1")

    Where 514 is the port number rsyslog uses by default. You can specify a different port instead.

  3. Restart the rsyslog service.

    # systemctl restart rsyslog
  4. Optional: If rsyslog is not enabled, ensure the rsyslog service starts automatically after reboot:

    # systemctl enable rsyslog

Verification

  1. Verify the syntax of the /etc/rsyslog.conf file and all .conf files in the /etc/rsyslog.d/ directory:

    # rsyslogd -N 1
    rsyslogd: version 8.1911.0-2.el8, config validation run (level 1), master config /etc/rsyslog.conf
    rsyslogd: End of config validation run. Bye.

Additional resources

13.4.2. Configuring remote logging to a server over UDP

Follow this procedure to configure a system for forwarding log messages to a server over the UDP protocol. The omfwd plug-in provides forwarding over UDP or TCP. The default protocol is UDP. Because the plug-in is built in, you do not have to load it.

Prerequisites

Procedure

  1. Create a new .conf file in the /etc/rsyslog.d/ directory, for example, remotelogcli.conf, and insert the following content:

    *.* action(type="omfwd"
          queue.type="linkedlist"
          queue.filename="example_fwd"
          action.resumeRetryCount="-1"
          queue.saveOnShutdown="on"
          target="example.com" port="portno" protocol="udp"
         )

    Where:

    • queue.type="linkedlist" enables a LinkedList in-memory queue.
    • queue.filename defines a disk storage. The backup files are created with the example_fwd prefix in the working directory specified by the preceding global workDirectory directive.
    • The action.resumeRetryCount -1 setting prevents rsyslog from dropping messages when retrying to connect if the server is not responding.
    • enabled queue.saveOnShutdown="on" saves in-memory data if rsyslog shuts down.
    • portno is the port number you want rsyslog to use. The default value is 514.
    • The last line forwards all received messages to the logging server, port specification is optional.

      With this configuration, rsyslog sends messages to the server but keeps messages in memory if the remote server is not reachable. A file on disk is created only if rsyslog runs out of the configured memory queue space or needs to shut down, which benefits the system performance.

  2. Restart the rsyslog service.

    # systemctl restart rsyslog
  3. Optional: If rsyslog is not enabled, ensure the rsyslog service starts automatically after reboot:

    # systemctl enable rsyslog

Verification

To verify that the client system sends messages to the server, follow these steps:

  1. On the client system, send a test message:

    # logger test
  2. On the server system, view the /var/log/remote/msg/hostname/root.log log, for example:

    # cat /var/log/remote/msg/hostname/root.log
    Feb 25 03:53:17 hostname root[6064]: test

    Where hostname is the host name of the client system. Note that the log contains the user name of the user that entered the logger command, in this case root.

Additional resources

13.5. Configuring reliable remote logging

With the Reliable Event Logging Protocol (RELP), you can send and receive syslog messages over TCP with a much reduced risk of message loss. RELP provides reliable delivery of event messages, which makes it useful in environments where message loss is not acceptable. To use RELP, configure the imrelp input module, which runs on the server and receives the logs, and the omrelp output module, which runs on the client and sends logs to the logging server.

Prerequisites

  • You have installed the rsyslog, librelp, and rsyslog-relp packages on the server and the client systems.
  • The specified port is permitted in SELinux and open in the firewall.

Procedure

  1. Configure the client system for reliable remote logging:

    1. On the client system, create a new .conf file in the /etc/rsyslog.d/ directory named, for example, relpcli.conf, and insert the following content:

      module(load="omrelp")
      *.* action(type="omrelp" target="target_IP" port="target_port")

      Where:

      • target_IP is the IP address of the logging server.
      • target_port is the port of the logging server.
    2. Save the changes to the /etc/rsyslog.d/relpserv.conf file.
    3. Restart the rsyslog service.

      # systemctl restart rsyslog
    4. Optional: If rsyslog is not enabled, ensure the rsyslog service starts automatically after reboot:

      # systemctl enable rsyslog
  2. Configure the server system for reliable remote logging:

    1. On the server system, create a new .conf file in the /etc/rsyslog.d/ directory named, for example, relpserv.conf, and insert the following content:

      ruleset(name="relp"){
      *.* action(type="omfile" file="log_path")
      }
      
      
      module(load="imrelp")
      input(type="imrelp" port="target_port" ruleset="relp")

      Where:

      • log_path specifies the path for storing messages.
      • target_port is the port of the logging server. Use the same value as in the client configuration file.
    2. Save the changes to the /etc/rsyslog.d/relpserv.conf file.
    3. Restart the rsyslog service.

      # systemctl restart rsyslog
    4. Optional: If rsyslog is not enabled, ensure the rsyslog service starts automatically after reboot:

      # systemctl enable rsyslog

Verification

To verify that the client system sends messages to the server, follow these steps:

  1. On the client system, send a test message:

    # logger test
  2. On the server system, view the log at the specified log_path, for example:

    # cat /var/log/remote/msg/hostname/root.log
    Feb 25 03:53:17 hostname root[6064]: test

    Where hostname is the host name of the client system. Note that the log contains the user name of the user that entered the logger command, in this case root.

Additional resources

13.6. Supported Rsyslog modules

To expand the functionality of the Rsyslog utility, you can use specific additional modules. Modules provide additional inputs (Input Modules), outputs (Output Modules), and other specific functionalities. A module may also provide additional configuration directives that become available after you load that module.

List the input and output modules installed on your system with the following command:

# ls /usr/lib64/rsyslog/{i,o}m*

To view the list of all available rsyslog modules, open the following page from documentation installed from the rsyslog-doc package.

$ firefox file:///usr/share/doc/rsyslog/html/configuration/modules/idx_output.html

13.7. Additional resources

Chapter 14. Using the Logging System Role

As a system administrator, you can use the Logging System Role to configure a RHEL host as a logging server to collect logs from many client systems.

14.1. The Logging System Role

With the Logging System Role, you can deploy logging configurations on local and remote hosts.

To apply a Logging System Role on one or more systems, you define the logging configuration in a playbook. A playbook is a list of one or more plays. Playbooks are human-readable, and they are written in the YAML format. For more information about playbooks, see Working with playbooks in Ansible documentation.

The set of systems that you want Ansible to configure according to the playbook is defined in an inventory file. For more information on creating and using inventories, see How to build your inventory in Ansible documentation.

Logging solutions provide multiple ways of reading logs and multiple logging outputs.

For example, a logging system can receive the following inputs:

  • local files,
  • systemd/journal,
  • another logging system over the network.

In addition, a logging system can have the following outputs:

  • logs are stored in the local files in the /var/log directory,
  • logs are sent to Elasticsearch,
  • logs are forwarded to another logging system.

With the logging system role, you can combine the inputs and outputs to fit your needs. For example, you can configure a logging solution that stores inputs from journal in a local file, whereas inputs read from files are both forwarded to another logging system and stored in the local log files.

14.2. Logging System Role parameters

In a Logging System Role playbook, you define the inputs in the logging_inputs parameter, outputs in the logging_outputs parameter, and the relationships between the inputs and outputs in the logging_flows parameter. The Logging System Role processes these variables with additional options to configure the logging system. You can also enable encryption.

Note

Currently, the only available logging system in the Logging System Role is Rsyslog.

  • logging_inputs - List of inputs for the logging solution.

    • name - Unique name of the input. Used in the logging_flows inputs list and a part of the generated config file name.
    • type - Type of the input element. The type specifies a task type which corresponds to a directory name in roles/rsyslog/{tasks,vars}/inputs/.

      • basics - Inputs configuring inputs from systemd journal or unix socket.

        • kernel_message - Load imklog if set to true. Default to false.
        • use_imuxsock - Use imuxsock instead of imjournal. Default to false.
        • ratelimit_burst - Maximum number of messages that can be emitted within ratelimit_interval. Default to 20000 if use_imuxsock is false. Default to 200 if use_imuxsock is true.
        • ratelimit_interval - Interval to evaluate ratelimit_burst. Default to 600 seconds if use_imuxsock is false. Default to 0 if use_imuxsock is true. 0 indicates rate limiting is turned off.
        • persist_state_interval - Journal state is persisted every value messages. Default to 10. Effective only when use_imuxsock is false.
      • files - Inputs configuring inputs from local files.
      • remote - Inputs configuring inputs from the other logging system over network.
    • state - State of the configuration file. present or absent. Default to present.
  • logging_outputs - List of outputs for the logging solution.

    • files - Outputs configuring outputs to local files.
    • forwards - Outputs configuring outputs to another logging system.
    • remote_files - Outputs configuring outputs from another logging system to local files.
  • logging_flows - List of flows that define relationships between logging_inputs and logging_outputs. The logging_flows variable has the following keys:

    • name - Unique name of the flow
    • inputs - List of logging_inputs name values
    • outputs - List of logging_outputs name values.

Additional resources

  • Documentation installed with the rhel-system-roles package in /usr/share/ansible/roles/rhel-system-roles.logging/README.html

14.3. Applying a local Logging System Role

Follow these steps to prepare and apply a Red Hat Ansible Engine playbook to configure a logging solution on a set of separate machines. Each machine will record logs locally.

Prerequisites

  • You have Red Hat Ansible Engine installed on the system from which you want to run the playbook.

    Note

    You do not have to have Red Hat Ansible Engine installed on the systems on which you want to deploy the logging solution.

  • You have the rhel-system-roles package on the system from which you want to run the playbook.

    Note

    You do not have to have rsyslog installed, because the system role installs rsyslog when deployed.

  • You have an inventory file listing the systems on which you want to configure the logging solution.

Procedure

  1. Create a playbook that defines the required role:

    1. Create a new YAML file and open it in a text editor, for example:

      # vi logging-playbook.yml
    2. Insert the following content:

      ---
      - name: Deploying basics input and implicit files output
        hosts: all
        roles:
          - linux-system-roles.logging
        vars:
          logging_inputs:
            - name: system_input
              type: basics
          logging_outputs:
            - name: files_output
              type: files
          logging_flows:
            - name: flow1
              inputs: [system_input]
              outputs: [files_output]
  2. Execute the playbook on a specific inventory:

    # ansible-playbook -i inventory-file /path/to/file/logging-playbook.yml

    Where:

    • inventory-file is the inventory file.
    • logging-playbook.yml is the playbook you use.

Verification

  1. Test the syntax of the /etc/rsyslog.conf file:

    # rsyslogd -N 1
    rsyslogd: version 8.1911.0-6.el8, config validation run (level 1), master config /etc/rsyslog.conf
    rsyslogd: End of config validation run. Bye.
  2. Verify that the system sends messages to the log:

    1. Send a test message:

      # logger test
    2. View the /var/log/messages log, for example:

      # cat /var/log/messages
      Aug  5 13:48:31 hostname root[6778]: test

      Where `hostname` is the host name of the client system. Note that the log contains the user name of the user that entered the logger command, in this case root.

14.4. Applying a remote logging solution using the Logging System Role

Follow these steps to prepare and apply a Red Hat Ansible Engine playbook to configure a remote logging solution. In this playbook, one or more clients take logs from systemd-journal and forward them to a remote server. The server receives remote input from remote_rsyslog and remote_files and outputs the logs to local files in directories named by remote host names.

Prerequisites

  • You have Red Hat Ansible Engine installed on the system from which you want to run the playbook.

    Note

    You do not have to have Red Hat Ansible Engine installed on the systems on which you want to deploy the logging solution.

  • You have the rhel-system-roles package on the system from which you want to run the playbook.

    Note

    You do not have to have rsyslog installed, because the system role installs rsyslog when deployed.

  • You have at least two systems:

    • At least one will be the logging server.
    • At least one will be the logging client.

Procedure

  1. Create a playbook that defines the required role:

    1. Create a new YAML file and open it in a text editor, for example:

      # vi logging-playbook.yml
    2. Insert the following content into the file:

      ---
      - name: Deploying remote input and remote_files output
        hosts: server
        roles:
          - linux-system-roles.logging
        vars:
          logging_inputs:
            - name: remote_udp_input
              type: remote
              udp_ports: [ 601 ]
            - name: remote_tcp_input
              type: remote
              tcp_ports: [ 601 ]
          logging_outputs:
            - name: remote_files_output
              type: remote_files
          logging_flows:
            - name: flow_0
              inputs: [remote_udp_input, remote_tcp_input]
              outputs: [remote_files_output]
      
      - name: Deploying basics input and forwards output
        hosts: clients
        roles:
          - linux-system-roles.logging
        vars:
          logging_inputs:
            - name: basic_input
              type: basics
          logging_outputs:
            - name: forward_output0
              type: forwards
              severity: info
              target: host1.example.com
              udp_port: 601
            - name: forward_output1
              type: forwards
              facility: mail
              target: host1.example.com
              tcp_port: 601
          logging_flows:
            - name: flows0
              inputs: [basic_input]
              outputs: [forward_output0, forward_output1]
      
      [basic_input]
      [forward_output0, forward_output1]

      Where host1.example.com is the logging server.

      Note

      You can modify the parameters in the playbook to fit your needs.

      Warning

      The logging solution works only with the ports defined in the SELinux policy of the server or client system and open in the firewall. The default SELinux policy includes ports 601, 514, 6514, 10514, and 20514. To use a different port, modify the SELinux policy on the client and server systems . Configuring the firewall through system roles is not yet supported.

  2. Create an inventory file that lists your servers and clients:

    1. Create a new file and open it in a text editor, for example:

      # vi inventory.ini
    2. Insert the following content into the inventory file:

      [servers]
      server ansible_host=host1.example.com
      [clients]
      client ansible_host=host2.example.com

      Where: * host1.example.com is the logging server. * host2.example.com is the logging client.

  3. Execute the playbook on your inventory.

    # ansible-playbook -i /path/to/file/inventory.ini /path/to/file/_logging-playbook.yml

    Where:

    • inventory.ini is the inventory file.
    • logging-playbook.yml is the playbook you created.

Verification steps

  1. On both the client and the server system, test the syntax of the /etc/rsyslog.conf file:

    # rsyslogd -N 1
    rsyslogd: version 8.1911.0-6.el8, config validation run (level 1), master config /etc/rsyslog.conf
    rsyslogd: End of config validation run. Bye.
  2. Verify that the client system sends messages to the server:

    1. On the client system, send a test message:

      # logger test
    2. On the server system, view the /var/log/messages log, for example:

      # cat /var/log/messages
      Aug  5 13:48:31 host2.example.com root[6778]: test

      Where host2.example.com is the host name of the client system. Note that the log contains the user name of the user that entered the logger command, in this case root.

Additional resources

14.5. Additional resources

Chapter 15. Using Python

15.1. Introduction to Python

Python is a high-level programming language that supports multiple programming paradigms, such as object-oriented, imperative, functional, and procedural. Python has dynamic semantics and can be used for general-purpose programming.

With Red Hat Enterprise Linux, many packages that are installed on the system, such as packages providing system tools, tools for data analysis or web applications are written in Python. To be able to use these packages, you need to have the python packages installed.

15.1.1. Python versions

Two incompatible versions of Python are widely used, Python 2.x and Python 3.x.

RHEL 8 provides the following versions of Python.

VersionPackage to installCommand examplesAvailable sinceLife cycle

Python 3.6

python3

python3, pip3

RHEL 8.0

full RHEL 8

Python 2.7

python2

python2, pip2

RHEL 8.0

shorter

Python 3.8

python38

python3.8, pip3.8

RHEL 8.2

shorter

See Red Hat Enterprise Linux Life Cycle and Red Hat Enterprise Linux 8 Application Streams Life Cycle for details about the length of support.

Each of the Python versions is distributed in a separate module, and by design, you can install multiple modules in parallel on the same system.

The python38 module does not include the same bindings to system tools (RPM, DNF, SELinux, and others) that are provided for the python36 module.

Important

Always specify the version of Python when installing it, invoking it, or otherwise interacting with it. For example, use python3 instead of python in package and command names. All Python-related commands should also include the version, for example, pip3, pip2, or pip3.8.

The unversioned python command (/usr/bin/python) is not available by default in RHEL 8. You can configure it using the alternatives command; for instructions, see Configuring the unversioned Python. Any manual changes to /usr/bin/python, except changes made using the alternatives command, may be overwritten upon an update.

As a system administrator, you are recommended to use preferably Python 3 for the following reasons:

  • Python 3 represents the main development direction of the Python project.
  • Support for Python 2 in the upstream community ends in 2020.
  • Popular Python libraries are dropping Python 2 support in upstream.
  • Python 2 in Red Hat Enterprise Linux 8 will have a shorter life cycle and its aim is to facilitate smoother transition to Python 3 for customers.

For developers, Python 3 has the following advantages over Python 2:

  • Python 3 allows writing expressive, maintainable, and correct code more easily.
  • Code written in Python 3 will have greater longevity.
  • Python 3 has new features, including asyncio, f-strings, advanced unpacking, keyword only arguments, chained exceptions and more.

However, existing software tends to require /usr/bin/python to be Python 2. For this reason, no default python package is distributed with Red Hat Enterprise Linux 8, and you can choose between using Python 2 and 3 as /usr/bin/python, as described in Section 15.2.5, “Configuring the unversioned Python”.

15.1.2. The internal platform-python package

System tools in Red Hat Enterprise Linux 8 use a Python version 3.6 provided by the internal platform-python package. Red Hat advises customers to use the python36 package instead.

15.2. Installing and using Python

Warning

Using the unversioned python command to install or run Python does not work by default due to ambiguity. Always specify the version of Python, or configure the system default version by using the alternatives command.

15.2.1. Installing Python 3

In Red Hat Enterprise Linux 8, Python 3 is distributed in versions 3.6 and 3.8, provided by the python36 and python38 modules in the AppStream repository.

Procedure

  • To install Python 3.6 from the python36 module, execute the following command:

    # yum install python3

    The python36:3.6 module stream is enabled automatically.

  • To install Python 3.8 from the python38 module, use:

    # yum install python38

    The python38:3.8 module stream is enabled automatically.

For details regarding modules in RHEL 8, see Installing, managing, and removing user-space components.

Note

By design, RHEL 8 modules can be installed in parallel, including the python27, python36, and python38 modules. Note that parallel installation is not supported for multiple streams within a single module.

Python 3.8 and packages built for it can be installed in parallel with Python 3.6 on the same system, with the exception of the mod_wsgi module. Due to a limitation of the Apache HTTP Server, only one of the python3-mod_wsgi and python38-mod_wsgi packages can be installed on a system.

Packages with add-on modules for Python 3.6 generally use the python3- prefix; packages for Python 3.8 include the python38- prefix. Always include the prefix when installing additional Python packages, as shown in the examples below.

Procedure

  • To install the Requests module for Python 3.6, execute this command:

    # yum install python3-requests
  • To install the Cython extension to Python 3.8, use:

    # yum install python38-Cython

15.2.1.1. Installing additional Python 3 packages for developers

Additional Python 3.8 packages for developers are distributed through the CodeReady Linux Builder repository in the python38-devel module. This module contains the python38-pytest package and its dependencies: the pyparsing, atomicwrites, attrs, packaging, py, more-itertools, pluggy, and wcwidth packages.

Important

The CodeReady Linux Builder repository and its content is unsupported by Red Hat.

To install packages from the python38-devel module, follow the procedure below.

Procedure

  • Enable the unsupported CodeReady Linux Builder repository:

    # subscription-manager repos --enable codeready-builder-for-rhel-8-x86_64-rpms
  • Enable the python38-devel module:

    # yum module enable python38-devel
  • Install the python38-pytest package:

    # yum install python38-pytest

For more information about the CodeReady Linux Builder repository, see How to enable and make use of content within CodeReady Linux Builder.

15.2.2. Installing Python 2

Some software has not yet been fully ported to Python 3, and needs Python 2 to operate. Red Hat Enterprise Linux 8 allows parallel installation of Python 3 and Python 2. If you need the Python 2 functionality, install the python27 module, which is available in the AppStream repository.

Warning

Note that Python 3 is the main development direction of the Python project. The support for Python 2 is being phased out. The python27 module has a shorter support period than Red Hat Enterprise Linux 8.

Procedure

  • To install Python 2.7 from the python27 module, execute this command:

    # yum install python2

    The python27:2.7 module stream is enabled automatically.

Note

By design, RHEL 8 modules can be installed in parallel, including the python27, python36, and python38 modules.

For details regarding modules, see Installing, managing, and removing user-space components.

Packages with add-on modules for Python 2 generally use the python2- prefix. Always include the prefix when installing additional Python packages, as shown in the examples below.

Procedure

  • To install the Requests module for Python 2, execute this command:

    # yum install python2-requests
  • To install the Cython extension to Python 2, use:

    # yum install python2-Cython

15.2.3. Using Python 3

When running the Python interpreter or Python-related commands, always specify the version.

Procedure

  • To run the Python 3.6 interpreter or related commands, use, for example:

    $ python3
    $ python3 -m cython --help
    $ pip3 install <package>
  • To run the Python 3.8 interpreter or related commands, use, for example:

    $ python3.8
    $ python3.8 -m cython --help
    $ pip3.8 install <package>

15.2.4. Using Python 2

When running the Python 2 interpreter or Python2-related commands, always specify the version.

Procedure

  • To run the Python 2 interpreter or related commands, use, for example:

    $ python2
    $ python2 -m cython --help
    $ pip2 install <package>

15.2.5. Configuring the unversioned Python

System administrators can configure the unversioned python command, located at /usr/bin/python, using the alternatives command. Note that the required package, python3, python38, or python2, needs to be installed before configuring the unversioned command to the respective version.

Important

The /usr/bin/python executable is controlled by the alternatives system. Any manual changes may be overwritten upon an update.

Additional Python-related commands, such as pip3, do not have configurable unversioned variants.

15.2.5.1. Configuring the unversioned python command directly

To configure the unversioned python command directly to a selected version of Python, use this procedure.

Procedure

  • To configure the unversioned python command to Python 3.6, execute this command:

    # alternatives --set python /usr/bin/python3
  • To configure the unversioned python command to Python 3.8, use the following command:

    # alternatives --set python /usr/bin/python3.8
  • To configure the unversioned python command to Python 2, use:

    # alternatives --set python /usr/bin/python2

15.2.5.2. Configuring the unversioned python command to the required Python version interactively

You can also configure the unversioned python command to the required Python version interactively.

To configure the unversioned python command interactively, use this procedure.

Procedure

  1. Execute the following command:

    # alternatives --config python
  2. Select the required version from the provided list.
  3. To reset this configuration and remove the unversioned python command, run:

    # alternatives --auto python

15.3. Migration from Python 2 to Python 3

As a developer, you may want to migrate your former code that is written in Python 2 to Python 3. For more information on how to migrate large code bases to Python 3, see The Conservative Python 3 Porting Guide.

Note that after this migration, the original Python 2 code becomes interpretable by the Python 3 interpreter and stays interpretable for the Python 2 interpreter as well.

15.4. Packaging of Python 3 RPMs

Most Python projects use Setuptools for packaging, and define package information in the setup.py file. For more information on Setuptools packaging, see Setuptools documentation.

You can also package your Python project into an RPM package, which provides the following advantages compared to Setuptools packaging:

  • Specification of dependencies of a package on other RPMs (even non-Python)
  • Cryptographic signing

    With cryptographic signing, content of RPM packages can be verified, integrated, and tested with the rest of the operating system.

15.4.1. SPEC file description for a Python package

A SPEC file contains instructions that the rpmbuild utility uses to build an RPM. The instructions are included in a series of sections. A SPEC file has two main parts in which the sections are defined:

  • Preamble (contains a series of metadata items that are used in the Body)
  • Body (contains the main part of the instructions)

For further information about SPEC files, see Packaging and distributing software.

An RPM SPEC file for Python projects has some specifics compared to non-Python RPM SPEC files. Most notably, a name of any RPM package of a Python library must always include the prefix determining the version, for example, python3 for Python 3.6 or python38 for Python 3.8.

Other specifics are shown in the following SPEC file example for the python3-detox package. For description of such specifics, see the notes below the example.

%global modname detox                                                           1

Name:           python3-detox                                                   2
Version:        0.12
Release:        4%{?dist}
Summary:        Distributing activities of the tox tool
License:        MIT
URL:            https://pypi.io/project/detox
Source0:        https://pypi.io/packages/source/d/%{modname}/%{modname}-%{version}.tar.gz

BuildArch:      noarch

BuildRequires:  python36-devel                                                  3
BuildRequires:  python3-setuptools
BuildRequires:  python36-rpm-macros
BuildRequires:  python3-six
BuildRequires:  python3-tox
BuildRequires:  python3-py
BuildRequires:  python3-eventlet

%?python_enable_dependency_generator                                            4

%description

Detox is the distributed version of the tox python testing tool. It makes efficient use of multiple CPUs by running all possible activities in parallel.
Detox has the same options and configuration that tox has, so after installation you can run it in the same way and with the same options that you use for tox.

    $ detox

%prep
%autosetup -n %{modname}-%{version}

%build
%py3_build                                                                      5

%install
%py3_install

%check
%{__python3} setup.py test                                                      6

%files -n python3-%{modname}
%doc CHANGELOG
%license LICENSE
%{_bindir}/detox
%{python3_sitelib}/%{modname}/
%{python3_sitelib}/%{modname}-%{version}*

%changelog
...
1
The modname macro contains the name of the Python project. In this example it is detox.
2
When packaging a Python project into RPM, the python3 prefix always needs to be added to the original name of the project. The original name here is detox and the name of the RPM is python3-detox.
3
BuildRequires specifies what packages are required to build and test this package. In BuildRequires, always include items providing tools necessary for building Python packages: python36-devel and python3-setuptools. The python36-rpm-macros package is required so that files with /usr/bin/python3 shebangs are automatically changed to /usr/bin/python3.6. For more information, see Section 15.4.4, “Handling hashbangs in Python scripts”.
4
Every Python package requires some other packages to work correctly. Such packages need to be specified in the SPEC file as well. To specify the dependencies, you can use the %python_enable_dependency_generator macro to automatically use dependencies defined in the setup.py file. If a package has dependencies that are not specified using Setuptools, specify them within additional Requires directives.
5
The %py3_build and %py3_install macros run the setup.py build and setup.py install commands, respectively, with additional arguments to specify installation locations, the interpreter to use, and other details.
6
The check section provides a macro that runs the correct version of Python. The %{__python3} macro contains a path for the Python 3 interpreter, for example /usr/bin/python3. We recommend to always use the macro rather than a literal path.

15.4.2. Common macros for Python 3 RPMs

In a SPEC file, always use the macros below rather than hardcoding their values.

In macro names, always use python3 or python2 instead of unversioned python. Configure the particular Python 3 version in the BuildRequires of the SPEC file to either python36-rpm-macros or python38-rpm-macros.

MacroNormal DefinitionDescription

%{__python3}

/usr/bin/python3

Python 3 interpreter

%{python3_version}

3.6

The full version of the Python 3 interpreter.

%{python3_sitelib}

/usr/lib/python3.6/site-packages

Where pure-Python modules are installed.

%{python3_sitearch}

/usr/lib64/python3.6/site-packages

Where modules containing architecture-specific extensions are installed.

%py3_build

 

Runs the setup.py build command with arguments suitable for a system package.

%py3_install

 

Runs the setup.py install command with arguments suitable for a system package.

15.4.3. Automatic provides for Python RPMs

When packaging a Python project, make sure that, if present, the following directories are included in the resulting RPM:

  • .dist-info
  • .egg-info
  • .egg-link

From these directories, the RPM build process automatically generates virtual pythonX.Ydist provides, for example, python3.6dist(detox). These virtual provides are used by packages that are specified by the %python_enable_dependency_generator macro.

15.4.4. Handling hashbangs in Python scripts

In Red Hat Enterprise Linux 8, executable Python scripts are expected to use hashbangs (shebangs) specifying explicitly at least the major Python version.

The /usr/lib/rpm/redhat/brp-mangle-shebangs buildroot policy (BRP) script is run automatically when building any RPM package, and attempts to correct hashbangs in all executable files.

Note

The BRP script generates errors when encountering a Python script with an ambiguous hashbang, such as:

#! /usr/bin/python

or

#! /usr/bin/env python

15.4.4.1. Modifying hashbangs in Python scripts

To modify hashbangs in the Python scripts that cause the build errors at RPM build time, use this procedure.

Procedure

  • Apply the pathfix.py script from the platform-python-devel package:

    # pathfix.py -pn -i %{__python3} PATH …​

    Note that multiple PATHs can be specified. If a PATH is a directory, pathfix.py recursively scans for any Python scripts matching the pattern ^[a-zA-Z0-9_]+\.py$, not only those with an ambiguous hashbang. Add this command to the %prep section or at the end of the %install section.

Alternatively, modify the packaged Python scripts so that they conform to the expected format. For this purpose, pathfix.py can be used outside the RPM build process, too. When running pathfix.py outside a RPM build, replace __python3 from the example above with a path for the hashbang, such as /usr/bin/python3.

If the packaged Python scripts require other version than Python 3.6, adjust the commands above to include the respective version.

15.4.4.2. Changing /usr/bin/python3 hashbangs in their custom packages

Additionally, hashbangs in the form /usr/bin/python3 are by default replaced with hashbangs pointing to Python from the platform-python package used for system tools with Red Hat Enterprise Linux.

To change the /usr/bin/python3 hashbangs in their custom packages to point to a version of Python installed from Application Stream, in the form /usr/bin/python3.6, use the following procedure.

Procedure

  • Add the python36-rpm-macros package into the BuildRequires section of the SPEC file by including the following line:

    BuildRequires:  python36-rpm-macros
Note

To prevent hashbang check and modification by the BRP script, use the following RPM directive:

%undefine %brp_mangle_shebangs

If you are using other version than Python 3.6, adjust the commands above to include the respective version.

15.4.5. Additional resources

Chapter 16. Using the PHP scripting language

Hypertext Preprocessor (PHP) is a general-purpose scripting language mainly used for server-side scripting, which enables you to run the PHP code using a web server.

In RHEL 8, the PHP scripting language is provided by the php module, which is available in multiple streams (versions).

Depending on your use case, you can install a specific profile of the selected module stream:

  • common - The default profile for server-side scripting using a web server. It includes several widely used extensions.
  • minimal - This profile installs only the command-line interface for scripting with PHP without using a web server.
  • devel - This profile includes packages from the common profile and additional packages for development purposes.

16.1. Installing the PHP scripting language

This section describes how to install a selected version of the php module.

Procedure

  • To install a php module stream with the default profile, use:

    # yum module install php:stream

    Replace stream with the version of PHP you wish to install.

    For example, to install PHP 7.4:

    # yum module install php:7.4

    The default common profile installs also the php-fpm package, and preconfigures PHP for use with the Apache HTTP Server or nginx.

  • To install a specific profile of a php module stream, use:

    # yum module install php:stream/profile

    Replace stream with the desired version and profile with the name of the profile you wish to install.

    For example, to install PHP 7.4 for use without a web server:

    # yum module install php:7.4/minimal

Additional resources

16.2. Using the PHP scripting language with a web server

16.2.1. Using PHP with the Apache HTTP Server

In RHEL 8, the Apache HTTP Server enables you to run PHP as a FastCGI process server. FastCGI Process Manager (FPM) is an alternative PHP FastCGI daemon that allows a website to manage high loads. PHP uses FastCGI Process Manager by default in RHEL 8.

This section describes how to run the PHP code using the FastCGI process server.

Prerequisites

Procedure

  1. Install the httpd module:

    # yum module install httpd:2.4
  2. Start the Apache HTTP Server:

    # systemctl start httpd

    Or, if the Apache HTTP Server is already running on your system, restart the httpd service after installing PHP:

    # systemctl restart httpd
  3. Start the php-fpm service:

    # systemctl start php-fpm
  4. Optional: Enable both services to start at boot time:

    # systemctl enable php-fpm httpd
  5. To obtain information about your PHP settings, create the index.php file with the following content in the /var/www/html/ directory:

    echo '<?php phpinfo(); ?>' > /var/www/html/index.php
  6. To run the index.php file, point the browser to:

    http://<hostname>/
  7. Optional: Adjust configuration if you have specific requirements:

    • /etc/httpd/conf/httpd.conf - generic httpd configuration
    • /etc/httpd/conf.d/php.conf - PHP-specific configuration for httpd
    • /usr/lib/systemd/system/httpd.service.d/php-fpm.conf - by default, the php-fpm service is started with httpd
    • /etc/php-fpm.conf - FPM main configuration
    • /etc/php-fpm.d/www.conf - default www pool configuration

Example 16.1. Running a "Hello, World!" PHP script using the Apache HTTP Server

  1. Create a hello directory for your project in the /var/www/html/ directory:

    # mkdir hello
  2. Create a hello.php file in the /var/www/html/hello/ directory with the following content:

    # <!DOCTYPE html>
    <html>
    <head>
    <title>Hello, World! Page</title>
    </head>
    <body>
    <?php
        echo 'Hello, World!';
    ?>
    </body>
    </html>
  3. Start the Apache HTTP Server:

    # systemctl start httpd
  4. To run the hello.php file, point the browser to:

    http://<hostname>/hello/hello.php

    As a result, a web page with the “Hello, World!” text is displayed.

16.2.2. Using PHP with the nginx web server

This section describes how to run PHP code through the nginx web server.

Prerequisites

Procedure

  1. Install an nginx module stream:

    # yum module install nginx:stream

    Replace stream with the version of nginx you wish to install.

    For example, to install nginx version 1.18:

    # yum module install nginx:1.18
  2. Start the nginx server:

    # systemctl start nginx

    Or, if the nginx server is already running on your system, restart the nginx service after installing PHP:

    # systemctl restart nginx
  3. Start the php-fpm service:

    # systemctl start php-fpm
  4. Optional: Enable both services to start at boot time:

    # systemctl enable php-fpm nginx
  5. To obtain information about your PHP settings, create the index.php file with the following content in the /usr/share/nginx/html/ directory:

    echo '<?php phpinfo(); ?>' > /usr/share/nginx/html/index.php
  6. To run the index.php file, point the browser to:

    http://<hostname>/
  7. Optional: Adjust configuration if you have specific requirements:

    • /etc/nginx/nginx.conf - nginx main configuration
    • /etc/nginx/conf.d/php-fpm.conf - FPM configuration for nginx
    • /etc/php-fpm.conf - FPM main configuration
    • /etc/php-fpm.d/www.conf - default www pool configuration

Example 16.2. Running a "Hello, World!" PHP script using the nginx server

  1. Create a hello directory for your project in the /usr/share/nginx/html/ directory:

    # mkdir hello
  2. Create a hello.php file in the /usr/share/nginx/html/hello/ directory with the following content:

    # <!DOCTYPE html>
    <html>
    <head>
    <title>Hello, World! Page</title>
    </head>
    <body>
    <?php
        echo 'Hello, World!';
    ?>
    </body>
    </html>
  3. Start the nginx server:

    # systemctl start nginx
  4. To run the hello.php file, point the browser to:

    http://<hostname>/hello/hello.php

    As a result, a web page with the “Hello, World!” text is displayed.

16.3. Running a PHP script using the command-line interface

A PHP script is usually run using a web server, but also can be run using the command-line interface.

If you want to run php scripts using only command-line, install the minimal profile of a php module stream.

See Section 16.1, “Installing the PHP scripting language” for details.

Prerequisites

Procedure

  1. In a text editor, create a filename.php file

    Replace filename with the a name of your file.

  2. Execute the created filename.php file from the command line:

    # php filename.php

Example 16.3. Running a "Hello, World!" PHP script using the command-line interface

  1. Create a hello.php file with the following content using a text editor:

    <?php
        echo 'Hello, World!';
    ?>
  2. Execute the hello.php file from the command line:

    # php hello.php

    As a result, “Hello, World!” is printed.

16.4. Additional resources

  • httpd(8) — The manual page for the httpd service containing the complete list of its command-line options.
  • httpd.conf(5) — The manual page for httpd configuration, describing the structure and location of the httpd configuration files.
  • nginx(8) — The manual page for the nginx web server containing the complete list of its command-line options and list of signals.
  • php-fpm(8) — The manual page for PHP FPM describing the complete list of its command-line options and configuration files.

Chapter 17. Using langpacks

Langpacks are meta-packages which install extra add-on packages containing translations, dictionaries and locales for every package installed on the system.

On a Red Hat Enterprise Linux 8 system, langpacks installation is based on the langpacks-<langcode> language meta-packages and RPM weak dependencies (Supplements tag).

There are two prerequisites to be able to use langpacks for a selected language. If these prerequisites are fulfilled, the language meta-packages pull their langpack for the selected language automatically in the transaction set.

Prerequisites

  • The langpacks-<langcode> language meta-package for the selected language has been installed on the system.

    On Red Hat Enterprise Linux 8, the langpacks meta packages are installed automatically with the initial installation of the operating system using the Anaconda installer, because these packages are available in the in Application Stream repository.

    For more information, see Section 17.1, “Checking languages that provide langpacks”

  • The base package, for which you want to search the local packages, has already been installed on the system.

17.1. Checking languages that provide langpacks

Folow this procedure to check which languages provide langpacks.

Procedure

  • Execute the following command:

    # yum list langpacks-*

17.2. Working with RPM weak dependency-based langpacks

This section describes multiple actions that you may want to perform when querying RPM weak dependency-based langpacks, installing or removing language support.

17.2.1. Listing already installed language support

To list the already installed language support, use this procedure.

Procedure

  • Execute the following command:

    # yum list installed langpacks*

17.2.2. Checking the availability of language support

To check if language support is available for any language, use the following procedure.

Procedure

  • Execute the following command:
# yum list available langpacks*

17.2.3. Listing packages installed for a language

To list what packages get installed for any language, use the following procedure:

Procedure

  • Execute the following command:

    # yum repoquery --whatsupplements langpacks-<locale_code>

17.2.4. Installing language support

To add new a language support, use the following procedure.

Procedure

  • Execute the following command:

    # yum install langpacks-<locale_code>

17.2.5. Removing language support

To remove any installed language support, use the following procedure.

Procedure

  • Execute the following command:

    # yum remove langpacks-<locale_code>

17.3. Saving disk space by using glibc-langpack-<locale_code>

Currently, all locales are stored in the /usr/lib/locale/locale-archive file, which requires a lot of disk space.

On systems where disk space is a critical issue, such as containers and cloud images, or only a few locales are needed, you can use the glibc locale langpack packages (glibc-langpack-<locale_code>).

To install locales individually, and thus gain a smaller package installation footprint, use the following procedure.

Procedure

  • Execute the following command:

    # yum install glibc-langpack-<locale_code>

When installing the operating system with Anaconda, glibc-langpack-<locale_code> is installed for the language you used during the installation and also for the languages you selected as additional languages. Note that glibc-all-langpacks, which contains all locales, is installed by default, so some locales are duplicated. If you installed glibc-langpack-<locale_code> for one or more selected languages, you can delete glibc-all-langpacks after the installation to save the disk space.

Note that installing only selected glibc-langpack-<locale_code> packages instead of glibc-all-langpacks has impact on run time performance.

Note

If disk space is not an issue, keep all locales installed by using the glibc-all-langpacks package.

Chapter 18. Getting started with Tcl/Tk

18.1. Introduction to Tcl/Tk

Tool command language (Tcl) is a dynamic programming language. The interpreter for this language, together with the C library, is provided by the tcl package.

Using Tcl paired with Tk (Tcl/Tk) enables creating cross-platform GUI applications. Tk is provided by the tk package.

Note that Tk can refer to any of the the following:

  • A programming toolkit for multiple languages
  • A Tk C library bindings available for multiple languages, such as C, Ruby, Perl and Python
  • A wish interpreter that instantiates a Tk console
  • A Tk extension that adds a number of new commands to a particular Tcl interpreter

For more information about Tcl/Tk, see the Tcl/Tk manual or Tcl/Tk documentation web page.

18.2. Notable changes in Tcl/Tk 8.6

Red Hat Enterprise Linux 7 used Tcl/Tk 8.5. With Red Hat Enterprise Linux 8, Tcl/Tk version 8.6 is provided in the Base OS repository.

Major changes in Tcl/Tk 8.6 compared to Tcl/Tk 8.5 are:

  • Object-oriented programming support
  • Stackless evaluation implementation
  • Enhanced exceptions handling
  • Collection of third-party packages built and installed with Tcl
  • Multi-thread operations enabled
  • SQL database-powered scripts support
  • IPv6 networking support
  • Built-in Zlib compression
  • List processing

    Two new commands, lmap and dict map are available, which allow the expression of transformations over Tcl containers.

  • Stacked channels by script

    Two new commands, chan push and chan pop are available, which allow to add or remove transformations to or from I/O channels.

Major changes in Tk include:

  • Built-in PNG image support
  • Busy windows

    A new command, tk busy is available, which disables user interaction for a window or a widget and shows the busy cursor.

  • New font selection dialog interface
  • Angled text support
  • Moving things on a canvas support

For the detailed list of changes between Tcl 8.5 and Tcl 8.6, see Changes in Tcl/Tk 8.6.

18.3. Migrating to Tcl/Tk 8.6

Red Hat Enterprise Linux 7 used Tcl/Tk 8.5. With Red Hat Enterprise Linux 8, Tcl/Tk version 8.6 is provided in the Base OS repository.

This section describes migration path to Tcl/Tk 8.6 for:

  • Developers writing Tcl extensions or embedding Tcl interpreter into their applications
  • Users scripting tasks with Tcl/Tk

18.3.1. Migration path for developers of Tcl extensions

To make your code compatible with Tcl 8.6, use the following procedure.

Procedure

  1. Rewrite the code to use the interp structure. For example, if your code reads interp→errorLine, rewrite it to use the following function:

    Tcl_GetErrorLine(interp)

    This is necessary because Tcl 8.6 limits direct access to members of the interp structure.

  2. To make your code compatible with both Tcl 8.5 and Tcl 8.6, use the following code snippet in a header file of your C or C++ application or extension that includes the Tcl library:

    # include <tcl.h>
    # if !defined(Tcl_GetErrorLine)
    # define Tcl_GetErrorLine(interp) (interp→errorLine)
    # endif

18.3.2. Migration path for users scripting their tasks with Tcl/Tk

In Tcl 8.6, most scripts work the same way as with the previous version of Tcl.

To migrate you code into Tcl 8.6, use this procedure.

Procedure

  • When writing a portable code, make sure to not use the commands that are no longer supported in Tk 8.6:

    tkIconList_Arrange
    tkIconList_AutoScan
    tkIconList_Btn1
    tkIconList_Config
    tkIconList_Create
    tkIconList_CtrlBtn1
    tkIconList_Curselection
    tkIconList_DeleteAll
    tkIconList_Double1
    tkIconList_DrawSelection
    tkIconList_FocusIn
    tkIconList_FocusOut
    tkIconList_Get
    tkIconList_Goto
    tkIconList_Index
    tkIconList_Invoke
    tkIconList_KeyPress
    tkIconList_Leave1
    tkIconList_LeftRight
    tkIconList_Motion1
    tkIconList_Reset
    tkIconList_ReturnKey
    tkIconList_See
    tkIconList_Select
    tkIconList_Selection
    tkIconList_ShiftBtn1
    tkIconList_UpDown

    Note that you can check the list of unsupported commands also in the /usr/share/tk8.6/unsupported.tcl file.

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