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Integrating RHEL systems directly with Windows Active Directory

Red Hat Enterprise Linux 9

Understanding and configuring RHEL systems to connect directly with Active Directory

Red Hat Customer Content Services

Abstract

This documentation collection provides instructions on how to integrate RHEL systems directly with Windows Active Directory using SSSD.

Making open source more inclusive

Red Hat is committed to replacing problematic language in our code, documentation, and web properties. We are beginning with these four terms: master, slave, blacklist, and whitelist. Because of the enormity of this endeavor, these changes will be implemented gradually over several upcoming releases. For more details, see our CTO Chris Wright’s message.

In Identity Management, planned terminology replacements include:

  • block list replaces blacklist
  • allow list replaces whitelist
  • secondary replaces slave
  • The word master is going to be replaced with more precise language, depending on the context:

    • IdM server replaces IdM master
    • CA renewal server replaces CA renewal master
    • CRL publisher server replaces CRL master
    • multi-supplier replaces multi-master

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Chapter 1. Connecting RHEL systems directly to AD using SSSD

You need two components to connect a RHEL system to Active Directory (AD). One component, SSSD, interacts with the central identity and authentication source, and the other component, realmd, detects available domains and configures the underlying RHEL system services, in this case SSSD, to connect to the domain.

This section describes using the System Security Services Daemon (SSSD) to connect a RHEL system to Active Directory (AD).

1.1. Overview of direct integration using SSSD

You use SSSD to access a user directory for authentication and authorization through a common framework with user caching to permit offline logins. SSSD is highly configurable; it provides Pluggable Authentication Modules (PAM) and Name Switch Service (NSS) integration and a database to store local users as well as extended user data retrieved from a central server. SSSD is the recommended component to connect a RHEL system with one of the following types of identity server:

  • Active Directory
  • Identity Management (IdM) in RHEL
  • Any generic LDAP or Kerberos server
Note

Direct integration with SSSD works only within a single AD forest by default.

The most convenient way to configure SSSD to directly integrate a Linux system with AD is to use the realmd service. It allows callers to configure network authentication and domain membership in a standard way. The realmd service automatically discovers information about accessible domains and realms and does not require advanced configuration to join a domain or realm.

You can use SSSD for both direct and indirect integration with AD and it allows you to switch from one integration approach to another. Direct integration is a simple way to introduce RHEL systems to an AD environment. However, as the share of RHEL systems grows, your deployments usually need a better centralized management of the identity-related policies such as host-based access control, sudo, or SELinux user mappings. Initially, you can maintain the configuration of these aspects of the RHEL systems in local configuration files. However, with a growing number of systems, distribution and management of the configuration files is easier with a provisioning system such as Red Hat Satellite. When direct integration does not scale anymore, you should consider indirect integration. For more information on moving from direct integration (RHEL clients are in the AD domain) to indirect integration (IdM with trust to AD), see Moving RHEL clients from AD domain to IdM Server.

For more information on which type of integration fits your use case, see Deciding between indirect and direct integration.

Additional resources

  • The realm(8) man page.
  • The sssd-ad(5) man page.
  • The sssd(8) man page.

1.2. Supported Windows platforms for direct integration

You can directly integrate your RHEL system with Active Directory forests that use the following forest and domain functional levels:

  • Forest functional level range: Windows Server 2008 - Windows Server 2016
  • Domain functional level range: Windows Server 2008 - Windows Server 2016

Direct integration has been tested on the following supported operating systems:

  • Windows Server 2019
  • Windows Server 2016
  • Windows Server 2012 R2
Note

Windows Server 2019 does not introduce a new functional level. The highest functional level Windows Server 2019 uses is Windows Server 2016.

1.3. Ensuring support for common encryption types in AD and RHEL

By default, SSSD supports RC4, AES-128, and AES-256 Kerberos encryption types.

RC4 encryption has been deprecated and disabled by default, as it is considered less secure than the newer AES-128 and AES-256 encryption types. In contrast, Active Directory (AD) user credentials and trusts between AD domains support RC4 encryption and they might not support AES encryption types.

Without any common encryption types, communication between RHEL hosts and AD domains might not work, or some AD accounts might not be able to authenticate. To remedy this situation, modify one of the following configurations:

Enable AES encryption support in Active Directory (recommended option)
To ensure trusts between AD domains in an AD forest support strong AES encryption types, see the following Microsoft article: AD DS: Security: Kerberos "Unsupported etype" error when accessing a resource in a trusted domain
Enable RC4 support in RHEL

On every RHEL host where authentication against AD Domain Controllers takes place:

  1. Use the update-crypto-policies command to enable the AD-SUPPORT cryptographic subpolicy in addition to the DEFAULT cryptographic policy.

    [root@host ~]# update-crypto-policies --set DEFAULT:AD-SUPPORT
    Setting system policy to DEFAULT:AD-SUPPORT
    Note: System-wide crypto policies are applied on application start-up.
    It is recommended to restart the system for the change of policies
    to fully take place.
  2. Restart the host.

Additional resources

1.4. Connecting directly to AD

The System Security Services Daemon (SSSD) is the recommended component to connect a Red Hat Enterprise Linux (RHEL) system with Active Directory (AD). This section describes how to integrate directly with AD by using either ID mapping, which is the default for SSSD, or by using POSIX attributes.

1.4.1. Discovering and joining an AD Domain using SSSD

This procedure describes how to discover an AD domain and connect a RHEL system to that domain using SSSD.

Prerequisites

  • Ensure that the following ports on the RHEL host are open and accessible to the AD domain controllers.

    Table 1.1. Ports Required for Direct Integration of Linux Systems into AD Using SSSD

    ServicePortProtocolNotes

    DNS

    53

    UDP and TCP

     

    LDAP

    389

    UDP and TCP

     

    Samba

    445

    UDP and TCP

    For AD Group Policy Objects (GPOs)

    Kerberos

    88

    UDP and TCP

     

    Kerberos

    464

    UDP and TCP

    Used by kadmin for setting and changing a password

    LDAP Global Catalog

    3268

    TCP

    If the id_provider = ad option is being used

    NTP

    123

    UDP

    Optional

  • Ensure that you are using the AD domain controller server for DNS.
  • Verify that the system time on both systems is synchronized. This ensures that Kerberos is able to work correctly.

Procedure

  1. Install the following packages:

    # dnf install samba-common-tools realmd oddjob oddjob-mkhomedir sssd adcli krb5-workstation
  2. To display information for a specific domain, run realm discover and add the name of the domain you want to discover:

    # realm discover ad.example.com
    ad.example.com
      type: kerberos
      realm-name: AD.EXAMPLE.COM
      domain-name: ad.example.com
      configured: no
      server-software: active-directory
      client-software: sssd
      required-package: oddjob
      required-package: oddjob-mkhomedir
      required-package: sssd
      required-package: adcli
      required-package: samba-common

    The realmd system uses DNS SRV lookups to find the domain controllers in this domain automatically.

    Note

    The realmd system can discover both Active Directory and Identity Management domains. If both domains exist in your environment, you can limit the discovery results to a specific type of server using the --server-software=active-directory option.

  3. Configure the local RHEL system with the realm join command. The realmd suite edits all required configuration files automatically. For example, for a domain named ad.example.com:

    # realm join ad.example.com

Verification steps

  • Display an AD user details, such as the administrator user:

    # getent passwd administrator@ad.example.com
    administrator@ad.example.com:*:1450400500:1450400513:Administrator:/home/administrator@ad.example.com:/bin/bash

Additional resources

  • See the realm(8) man page.
  • See the nmcli(1) man page.

1.4.2. Options for integrating with AD: using ID mapping or POSIX attributes

Linux and Windows systems use different identifiers for users and groups:

Important

After connecting a RHEL system to AD, you can authenticate with your AD username and password. Do not create a Linux user with the same name as a Windows user, as duplicate names might cause a conflict and interrupt the authentication process.

To authenticate to a RHEL system as an AD user, you must have a UID and GID assigned. SSSD provides the option to integrate with AD either using ID mapping or POSIX attributes. The default is to use ID mapping.

Automatically generate new UIDs and GIDs for AD users

SSSD can use the SID of an AD user to algorithmically generate POSIX IDs in a process called ID mapping. ID mapping creates a map between SIDs in AD and IDs on Linux.

  • When SSSD detects a new AD domain, it assigns a range of available IDs to the new domain.
  • When an AD user logs in to an SSSD client machine for the first time, SSSD creates an entry for the user in the SSSD cache, including a UID based on the user’s SID and the ID range for that domain.
  • Because the IDs for an AD user are generated in a consistent way from the same SID, the user has the same UID and GID when logging in to any Red Hat Enterprise Linux system.

See Discovering and joining an AD domain using SSSD.

Note

When all client systems use SSSD to map SIDs to Linux IDs, the mapping is consistent. If some clients use different software, choose one of the following:

  • Ensure that the same mapping algorithm is used on all clients.
  • Use explicit POSIX attributes defined in AD.

Use POSIX attributes defined in AD

AD can create and store POSIX attributes, such as uidNumber, gidNumber, unixHomeDirectory, or loginShell.

When using ID mapping described above, SSSD creates new UIDs and GIDs, which overrides the values defined in AD. To keep the AD-defined values, you must disable ID mapping in SSSD.

See Connecting to AD using POSIX attributes defined in Active Directory.

1.4.3. Connecting to AD using POSIX attributes defined in Active Directory

For best performance, publish the POSIX attributes to the AD global catalog. If POSIX attributes are not present in the global catalog, SSSD connects to the individual domain controllers directly on the LDAP port.

Prerequisites

  • Ensure that the following ports on the RHEL host are open and accessible to the AD domain controllers.

    Table 1.2. Ports Required for Direct Integration of Linux Systems into AD Using SSSD

    ServicePortProtocolNotes

    DNS

    53

    UDP and TCP

     

    LDAP

    389

    UDP and TCP

     

    Kerberos

    88

    UDP and TCP

     

    Kerberos

    464

    UDP and TCP

    Used by kadmin for setting and changing a password

    LDAP Global Catalog

    3268

    TCP

    If the id_provider = ad option is being used

    NTP

    123

    UDP

    Optional

  • Ensure that you are using the AD domain controller server for DNS.
  • Verify that the system time on both systems is synchronized. This ensures that Kerberos is able to work correctly.

Procedure

  1. Install the following packages:

    # dnf install realmd oddjob oddjob-mkhomedir sssd adcli krb5-workstation
  2. Configure the local RHEL system with ID mapping disabled using the realm join command with the --automatic-id-mapping=no option. The realmd suite edits all required configuration files automatically. For example, for a domain named ad.example.com:

    # realm join --automatic-id-mapping=no ad.example.com
  3. If you already joined a domain, you can manually disable ID Mapping in SSSD:

    1. Open the /etc/sssd/sssd.conf file.
    2. In the AD domain section, add the ldap_id_mapping = false setting.
    3. Remove the SSSD caches:

      rm -f /var/lib/sss/db/*
    4. Restart SSSD:

      systemctl restart sssd

SSSD now uses POSIX attributes from AD, instead of creating them locally.

Note

You must have the relevant POSIX attributes (uidNumber, gidNumber, unixHomeDirectory, and loginShell) configured for the users in AD.

Verification steps

  • Display an AD user details, such as the administrator user:

    # getent passwd administrator@ad.example.com
    administrator@ad.example.com:*:10000:10000:Administrator:/home/Administrator:/bin/bash

Additional resources

  • For further details about ID mapping and the ldap_id_mapping parameter, see the sssd-ldap(8) man page.

1.4.4. Connecting to multiple domains in different AD forests with SSSD

You can use an Active Directory (AD) Managed Service Account (MSA) to access AD domains from different forests where there is no trust between them.

See Accessing AD with a Managed Service Account.

1.5. How the AD provider handles dynamic DNS updates

Active Directory (AD) actively maintains its DNS records by timing out (aging) and removing (scavenging) inactive records.

By default, the SSSD service refreshes a RHEL client’s DNS record at the following intervals:

  • Every time the identity provider comes online.
  • Every time the RHEL system reboots.
  • At the interval specified by the dyndns_refresh_interval option in the /etc/sssd/sssd.conf configuration file. The default value is 86400 seconds (24 hours).

    Note

    If you set the dyndns_refresh_interval option to the same interval as the DHCP lease, you can update the DNS record after the IP lease is renewed.

SSSD sends dynamic DNS updates to the AD server using Kerberos/GSSAPI for DNS (GSS-TSIG). This means that you only need to enable secure connections to AD.

Additional resources

  • The sssd-ad(5) man page.

1.6. Modifying dynamic DNS settings for the AD provider

The System Security Services Daemon (SSSD) service refreshes the DNS record of a Red Hat Enterprise Linux (RHEL) client joined to an AD environment at default intervals. The following procedure adjusts these intervals.

Prerequisites

  • You have joined a RHEL host to an Active Directory environment with the SSSD service.
  • You need root permissions to edit the /etc/sssd/sssd.conf configuration file.

Procedure

  1. Open the /etc/sssd/sssd.conf configuration file in a text editor.
  2. Add the following options to the [domain] section for your AD domain to set the DNS record refresh interval to 12 hours, disable updating PTR records, and set the DNS record Time To Live (TTL) to 1 hour.

    [domain/ad.example.com]
    id_provider = ad
    ...
    dyndns_refresh_interval = 43200
    dyndns_update_ptr = false
    dyndns_ttl = 3600
  3. Save and close the /etc/sssd/sssd.conf configuration file.
  4. Restart the SSSD service to load the configuration changes.

    [root@client ~]# systemctl restart sssd
Note

You can disable dynamic DNS updates by setting the dyndns_update option in the sssd.conf file to false:

[domain/ad.example.com]
id_provider = ad
...

dyndns_update = false

Additional resources

1.7. How the AD provider handles trusted domains

This section describes how SSSD handles trusted domains if you set the id_provider = ad option in the /etc/sssd/sssd.conf configuration file.

  • SSSD only supports domains in a single AD forest. If SSSD requires access to multiple domains from multiple forests, consider using IPA with trusts (preferred) or the winbindd service instead of SSSD.
  • By default, SSSD discovers all domains in the forest and, if a request for an object in a trusted domain arrives, SSSD tries to resolve it.

    If the trusted domains are not reachable or geographically distant, which makes them slow, you can set the ad_enabled_domains parameter in /etc/sssd/sssd.conf to limit from which trusted domains SSSD resolves objects.

  • By default, you must use fully-qualified user names to resolve users from trusted domains.

Additional resources

  • The sssd.conf(5) man page.

1.8. realm commands

The realmd system has two major task areas:

  • Managing system enrollment in a domain.
  • Controlling which domain users are allowed to access local system resources.

In realmd use the command line tool realm to run commands. Most realm commands require the user to specify the action that the utility should perform, and the entity, such as a domain or user account, for which to perform the action.

Table 1.3. realmd Commands

CommandDescription

Realm Commands

discover

Run a discovery scan for domains on the network.

join

Add the system to the specified domain.

leave

Remove the system from the specified domain.

list

List all configured domains for the system or all discovered and configured domains.

Login Commands

permit

Enable access for specific users or for all users within a configured domain to access the local system.

deny

Restrict access for specific users or for all users within a configured domain to access the local system.

Additional resources

  • The realm(8) man page.

Chapter 2. Connecting RHEL systems directly to AD using Samba Winbind

You need two components to connect a RHEL system to AD. One component, Samba Winbind, interacts with the AD identity and authentication source, and the other component, realmd, detects available domains and configures the underlying RHEL system services, in this case Samba Winbind, to connect to the AD domain.

This section describes using Samba Winbind to connect a RHEL system to Active Directory (AD).

2.1. Overview of direct integration using Samba Winbind

Samba Winbind emulates a Windows client on a Linux system and communicates with AD servers.

You can use the realmd service to configure Samba Winbind by:

  • Configuring network authentication and domain membership in a standard way.
  • Automatically discovering information about accessible domains and realms.
  • Not requiring advanced configuration to join a domain or realm.

Note that:

  • Direct integration with Winbind in a multi-forest AD setup requires bidirectional trusts.
  • Remote forests must trust the local forest to ensure that the idmap_ad plug-in handles remote forest users correctly.

Samba’s winbindd service provides an interface for the Name Service Switch (NSS) and enables domain users to authenticate to AD when logging into the local system.

Using winbindd provides the benefit that you can enhance the configuration to share directories and printers without installing additional software. For further detail, see the section about Using Samba as a server in the Deploying Different Types of Servers Guide.

Additional resources

  • See the realmd man page.
  • See the winbindd man page.

2.2. Supported Windows platforms for direct integration

You can directly integrate your RHEL system with Active Directory forests that use the following forest and domain functional levels:

  • Forest functional level range: Windows Server 2008 - Windows Server 2016
  • Domain functional level range: Windows Server 2008 - Windows Server 2016

Direct integration has been tested on the following supported operating systems:

  • Windows Server 2019
  • Windows Server 2016
  • Windows Server 2012 R2
Note

Windows Server 2019 does not introduce a new functional level. The highest functional level Windows Server 2019 uses is Windows Server 2016.

2.3. Ensuring support for common encryption types in AD and RHEL

By default, Samba Winbind supports RC4, AES-128, and AES-256 Kerberos encryption types.

RC4 encryption has been deprecated and disabled by default, as it is considered less secure than the newer AES-128 and AES-256 encryption types. In contrast, Active Directory (AD) user credentials and trusts between AD domains support RC4 encryption and they might not support AES encryption types.

Without any common encryption types, communication between RHEL hosts and AD domains might not work, or some AD accounts might not be able to authenticate. To remedy this situation, modify one of the following configurations:

Enable AES encryption support in Active Directory (recommended option)
To ensure trusts between AD domains in an AD forest support strong AES encryption types, see the following Microsoft article: AD DS: Security: Kerberos "Unsupported etype" error when accessing a resource in a trusted domain
Enable RC4 support in RHEL

On every RHEL host where authentication against AD Domain Controllers takes place:

  1. Use the update-crypto-policies command to enable the AD-SUPPORT cryptographic subpolicy in addition to the DEFAULT cryptographic policy.

    [root@host ~]# update-crypto-policies --set DEFAULT:AD-SUPPORT
    Setting system policy to DEFAULT:AD-SUPPORT
    Note: System-wide crypto policies are applied on application start-up.
    It is recommended to restart the system for the change of policies
    to fully take place.
  2. Restart the host.

Additional resources

2.4. Joining a RHEL system to an AD domain

Samba Winbind is an alternative to the System Security Services Daemon (SSSD) for connecting a Red Hat Enterprise Linux (RHEL) system with Active Directory (AD). This section describes how to join a RHEL system to an AD domain by using realmd to configure Samba Winbind.

Procedure

  1. If your AD requires the deprecated RC4 encryption type for Kerberos authentication, enable support for these ciphers in RHEL:

    # update-crypto-policies --set DEFAULT:AD-SUPPORT
  2. Install the following packages:

    # dnf install realmd oddjob-mkhomedir oddjob samba-winbind-clients \
           samba-winbind samba-common-tools samba-winbind-krb5-locator
  3. To share directories or printers on the domain member, install the samba package:

    # dnf install samba
  4. Backup the existing /etc/samba/smb.conf Samba configuration file:

    # mv /etc/samba/smb.conf /etc/samba/smb.conf.bak
  5. Join the domain. For example, to join a domain named ad.example.com:

    # realm join --membership-software=samba --client-software=winbind ad.example.com

    Using the previous command, the realm utility automatically:

    • Creates a /etc/samba/smb.conf file for a membership in the ad.example.com domain
    • Adds the winbind module for user and group lookups to the /etc/nsswitch.conf file
    • Updates the Pluggable Authentication Module (PAM) configuration files in the /etc/pam.d/ directory
    • Starts the winbind service and enables the service to start when the system boots
  6. Optionally, set an alternative ID mapping back end or customized ID mapping settings in the /etc/samba/smb.conf file.

For details, see the Understanding and configuring Samba ID mapping

  1. Edit the /etc/krb5.conf file and add the following section:

    [plugins]
        localauth = {
            module = winbind:/usr/lib64/samba/krb5/winbind_krb5_localauth.so
            enable_only = winbind
        }
  2. Verify that the winbind service is running:

    # systemctl status winbind
    ...
       Active: active (running) since Tue 2018-11-06 19:10:40 CET; 15s ago
    Important

    To enable Samba to query domain user and group information, the winbind service must be running before you start smb.

  3. If you installed the samba package to share directories and printers, enable and start the smb service:

    # systemctl enable --now smb

Verification steps

  1. Display an AD user’s details, such as the AD administrator account in the AD domain:

    # getent passwd "AD\administrator"
    AD\administrator:*:10000:10000::/home/administrator@AD:/bin/bash
  2. Query the members of the domain users group in the AD domain:

    # getent group "AD\Domain Users"
        AD\domain users:x:10000:user1,user2
  3. Optionally, verify that you can use domain users and groups when you set permissions on files and directories. For example, to set the owner of the /srv/samba/example.txt file to AD\administrator and the group to AD\Domain Users:

    # chown "AD\administrator":"AD\Domain Users" /srv/samba/example.txt
  4. Verify that Kerberos authentication works as expected:

    1. On the AD domain member, obtain a ticket for the administrator@AD.EXAMPLE.COM principal:

      # kinit administrator@AD.EXAMPLE.COM
    2. Display the cached Kerberos ticket:

      # klist
      Ticket cache: KCM:0
      Default principal: administrator@AD.EXAMPLE.COM
      
      Valid starting       Expires              Service principal
      01.11.2018 10:00:00  01.11.2018 20:00:00  krbtgt/AD.EXAMPLE.COM@AD.EXAMPLE.COM
              renew until 08.11.2018 05:00:00
  5. Display the available domains:

    # wbinfo --all-domains
    BUILTIN
    SAMBA-SERVER
    AD

Additional resources

2.5. realm commands

The realmd system has two major task areas:

  • Managing system enrollment in a domain.
  • Controlling which domain users are allowed to access local system resources.

In realmd use the command line tool realm to run commands. Most realm commands require the user to specify the action that the utility should perform, and the entity, such as a domain or user account, for which to perform the action.

Table 2.1. realmd Commands

CommandDescription

Realm Commands

discover

Run a discovery scan for domains on the network.

join

Add the system to the specified domain.

leave

Remove the system from the specified domain.

list

List all configured domains for the system or all discovered and configured domains.

Login Commands

permit

Enable access for specific users or for all users within a configured domain to access the local system.

deny

Restrict access for specific users or for all users within a configured domain to access the local system.

Additional resources

  • The realm(8) man page.

Chapter 3. Managing direct connections to AD

You can use the System Security Services Daemon (SSSD) or Samba Winbind to connect your Red Hat Enterprise Linux (RHEL) system to Active Directory (AD). This section describes how to modify and manage your connection to AD when your RHEL system is already configured as an AD client.

Prerequisites

  • You have connected your RHEL system to the Active Directory domain, either with SSSD or Samba Winbind.

3.1. Modifying the default Kerberos host keytab renewal interval

SSSD automatically renews the Kerberos host keytab file in an AD environment if the adcli package is installed. The daemon checks daily if the machine account password is older than the configured value and renews it if necessary.

The default renewal interval is 30 days. To change the default, follow the steps in this procedure.

Procedure

  1. Add the following parameter to the AD provider in your /etc/sssd/sssd.conf file:

    ad_maximum_machine_account_password_age = value_in_days
  2. Restart SSSD:

    # systemctl restart sssd
  3. To disable the automatic Kerberos host keytab renewal, set ad_maximum_machine_account_password_age = 0.

Additional resources

  • The adcli(8) man page.
  • The sssd.conf(5) man page.

3.2. Removing a RHEL system from an AD domain

This procedure describes how to remove a Red Hat Enterprise Linux (RHEL) system that is integrated into Active Directory (AD) directly from the AD domain.

Prerequisites

  • You have used the System Security Services Daemon (SSSD) or Samba Winbind to connect your RHEL system to AD.

Procedure

  1. Remove a system from an identity domain using the realm leave command. The command removes the domain configuration from SSSD and the local system.

    # realm leave ad.example.com
    Note

    When a client leaves a domain, the account is not deleted from AD; the local client configuration is only removed. If you want to delete the AD account, run the command with the --remove option. You are prompted for your user password and you must have the rights to remove an account from Active Directory.

  2. Use the -U option with the realm leave command to specify a different user to remove a system from an identity domain.

    By default, the realm leave command is executed as the default administrator. For AD, the administrator account is called Administrator. If a different user was used to join to the domain, it might be required to perform the removal as that user.

    # realm leave [ad.example.com] -U [AD.EXAMPLE.COM\user]'

The command first attempts to connect without credentials, but it prompts for a password if required.

Verification steps

  • Verify the domain is no longer configured:

    # realm discover [ad.example.com]
    ad.example.com
        type: kerberos
        realm-name: EXAMPLE.COM
        domain-name: example.com
        configured: no
        server-software: active-directory
        client-software: sssd
        required-package: oddjob
        required-package: oddjob-mkhomedir
        required-package: sssd
        required-package: adcli
        required-package: samba-common-tools

Additional resources

  • See the realm(8) man page.

3.3. Setting the domain resolution order in SSSD to resolve short AD user names

By default, you must specify fully qualified usernames, like ad_username@ad.example.com and group@ad.example.com, to resolve Active Directory (AD) users and groups on a RHEL host connected to AD with the SSSD service.

This procedure sets the domain resolution order in the SSSD configuration so you can resolve AD users and groups using short names, like ad_username. This example configuration searches for users and groups in the following order:

  1. Active Directory (AD) child domain subdomain2.ad.example.com
  2. AD child domain subdomain1.ad.example.com
  3. AD root domain ad.example.com

Prerequisites

  • You have used the SSSD service to connect the RHEL host directly to AD.

Procedure

  1. Open the /etc/sssd/sssd.conf file in a text editor.
  2. Set the domain_resolution_order option in the [sssd] section of the file.

    domain_resolution_order = subdomain2.ad.example.com, subdomain1.ad.example.com, ad.example.com
  3. Save and close the file.
  4. Restart the SSSD service to load the new configuration settings.

    [root@ad-client ~]# systemctl restart sssd

Verification Steps

  • Verify you can retrieve user information for a user from the first domain using only a short name.

    [root@ad-client ~]# id <user_from_subdomain2>
    uid=1916901142(user_from_subdomain2) gid=1916900513(domain users) groups=1916900513(domain users)

3.4. Managing login permissions for domain users

By default, domain-side access control is applied, which means that login policies for Active Directory (AD) users are defined in the AD domain itself. This default behavior can be overridden so that client-side access control is used. With client-side access control, login permission is defined by local policies only.

If a domain applies client-side access control, you can use the realmd to configure basic allow or deny access rules for users from that domain.

Note

Access rules either allow or deny access to all services on the system. More specific access rules must be set on a specific system resource or in the domain.

3.4.1. Enabling access to users within a domain

By default, login policies for Active Directory (AD) users are defined in the AD domain itself. This section describes how to override this default behavior and configure a RHEL host to enable access for users within an AD domain.

Important

It is not recommended to allow access to all by default while only denying it to specific users with realm permit -x. Instead, Red Hat recommends maintaining a default no access policy for all users and only grant access to selected users using realm permit.

Prerequisites

  • Your RHEL system is a member of the Active Directory domain.

Procedure

  1. Grant access to all users:

    # realm permit --all
  2. Grant access to specific users:

    $ realm permit aduser01@example.com
    $ realm permit 'AD.EXAMPLE.COM\aduser01'

Currently, you can only allow access to users in primary domains and not to users in trusted domains. This is due to the fact that user login must contain the domain name and SSSD cannot currently provide realmd with information about available child domains.

Verification steps

  1. Use SSH to log in to the server as the aduser01@example.com user:

    $ ssh aduser01@example.com@server_name
    [aduser01@example.com@server_name ~]$
  2. Use the ssh command a second time to access the same server, this time as the aduser02@example.com user:

    $ ssh aduser02@example.com@server_name
    Authentication failed.

Notice how the aduser02@example.com user is denied access to the system. You have granted the permission to log in to the system to the aduser01@example.com user only. All other users from that Active Directory domain are rejected because of the specified login policy.

Note

If you set use_fully_qualified_names to true in the sssd.conf file, all requests must use the fully qualified domain name. However, if you set use_fully_qualified_names to false, it is possible to use the fully-qualified name in the requests, but only the simplified version is displayed in the output.

Additional resources

  • See the realm(8) man page.

3.4.2. Denying access to users within a domain

By default, login policies for Active Directory (AD) users are defined in the AD domain itself. This section describes how to override this default behavior and configure a RHEL host to deny access to users within an AD domain.

Important

It is safer to only allow access to specific users or groups than to deny access to some, while enabling it to everyone else. Therefore, it is not recommended to allow access to all by default while only denying it to specific users with realm permit -x. Instead, Red Hat recommends maintaining a default no access policy for all users and only grant access to selected users using realm permit.

Prerequisites

  • Your RHEL system is a member of the Active Directory domain.

Procedure

  1. Deny access to all users within the domain:

    # realm deny --all

    This command prevents realm accounts from logging into the local machine. Use realm permit to restrict login to specific accounts.

  2. Verify that the domain user’s login-policy is set to deny-any-login:

    [root@replica1 ~]# realm list
    example.net
      type: kerberos
      realm-name: EXAMPLE.NET
      domain-name: example.net
      configured: kerberos-member
      server-software: active-directory
      client-software: sssd
      required-package: oddjob
      required-package: oddjob-mkhomedir
      required-package: sssd
      required-package: adcli
      required-package: samba-common-tools
      login-formats: %U@example.net
      login-policy: deny-any-login
  3. Deny access to specific users by using the -x option:

    $ realm permit -x 'AD.EXAMPLE.COM\aduser02'

Verification steps

  • Use SSH to log in to the server as the aduser01@example.net user.

    $ ssh aduser01@example.net@server_name
    Authentication failed.
Note

If you set use_fully_qualified_names to true in the sssd.conf file, all requests must use the fully qualified domain name. However, if you set use_fully_qualified_names to false, it is possible to use the fully-qualified name in the requests, but only the simplified version is displayed in the output.

Additional resources

  • See the realm(8) man page.

3.5. Applying Group Policy Object access control in RHEL

A Group Policy Object (GPO) is a collection of access control settings stored in Microsoft Active Directory (AD) that can apply to computers and users in an AD environment. By specifying GPOs in AD, administrators can define login policies honored by both Windows clients and Red Hat Enterprise Linux (RHEL) hosts joined to AD.

The following sections describe how you can manage GPOs in your environment:

3.5.1. How SSSD interprets GPO access control rules

By default, SSSD retrieves Group Policy Objects (GPOs) from Active Directory (AD) domain controllers and evaluates them to determine if a user is allowed to log in to a particular RHEL host joined to AD.

SSSD maps AD Windows Logon Rights to Pluggable Authentication Module (PAM) service names to enforce those permissions in a GNU/Linux environment.

As an AD Administrator, you can limit the scope of GPO rules to specific users, groups, or hosts by listing them in a security filter.

Limitations on filtering by hosts

Older versions of SSSD do not evaluate hosts in AD GPO security filters.

  • RHEL 8.3.0 and newer: SSSD supports users, groups, and hosts in security filters.
  • RHEL versions older than 8.3.0: SSSD ignores host entries and only supports users and groups in security filters.
    To ensure that SSSD applies GPO-based access control to a specific host, create a new Organizational Unit (OU) in the AD domain, move the system to the new OU, and then link the GPO to this OU.

Limitations on filtering by groups

SSSD currently does not support Active Directory’s built-in groups, such as Administrators with Security Identifier (SID) S-1-5-32-544. Red Hat recommends against using AD built-in groups in AD GPOs targeting RHEL hosts.

Additional resources

3.5.2. List of GPO settings that SSSD supports

The following table shows the SSSD options that correspond to Active Directory GPO options as specified in the Group Policy Management Editor on Windows.

Table 3.1. GPO access control options retrieved by SSSD

GPO optionCorresponding sssd.conf option

Allow log on locally
Deny log on locally

ad_gpo_map_interactive

Allow log on through Remote Desktop Services
Deny log on through Remote Desktop Services

ad_gpo_map_remote_interactive

Access this computer from the network
Deny access to this computer from the network

ad_gpo_map_network

Allow log on as a batch job
Deny log on as a batch job

ad_gpo_map_batch

Allow log on as a service
Deny log on as a service

ad_gpo_map_service

Additional resources

  • For more information on these sssd.conf settings, such as the Pluggable Authentication Module (PAM) services that map to GPO options, see the sssd-ad(5) Manual page entry.

3.5.3. List of SSSD options to control GPO enforcement

You can set the following SSSD options to limit the scope of GPO rules.

The ad_gpo_access_control option

You can set the ad_gpo_access_control option in the /etc/sssd/sssd.conf file to choose between three different modes in which GPO-based access control operates.

Table 3.2. Table of ad_gpo_access_control values

Value of
ad_gpo_access_control
Behavior

enforcing

GPO-based access control rules are evaluated and enforced.
This is the default setting in RHEL 8.

permissive

GPO-based access control rules are evaluated but not enforced; a syslog message is recorded every time access would be denied. This is the default setting in RHEL 7.
This mode is ideal for testing policy adjustments while allowing users to continue logging in.

disabled

GPO-based access control rules are neither evaluated nor enforced.

The ad_gpo_implicit_deny option

The ad_gpo_implicit_deny option is set to False by default. In this default state, users are allowed access if applicable GPOs are not found. If you set this option to True, you must explicitly allow users access with a GPO rule.

You can use this feature to harden security, but be careful not to deny access unintentionally. Red Hat recommends testing this feature while ad_gpo_access_control is set to permissive.

The following two tables illustrate when a user is allowed or rejected access based on the allow and deny login rights defined on the AD server-side and the value of ad_gpo_implicit_deny.

Table 3.3. Login behavior with ad_gpo_implicit_deny set to False (default)

allow-rulesdeny-rulesresult

missing

missing

all users are allowed

missing

present

only users not in deny-rules are allowed

present

missing

only users in allow-rules are allowed

present

present

only users in allow-rules and not in deny-rules are allowed

Table 3.4. Login behavior with ad_gpo_implicit_deny set to True

allow-rulesdeny-rulesresult

missing

missing

no users are allowed

missing

present

no users are allowed

present

missing

only users in allow-rules are allowed

present

present

only users in allow-rules and not in deny-rules are allowed

Additional resources

  • For the procedure to change the GPO enforcement mode in SSSD, see Changing the GPO access control mode.
  • For more details on each of the different GPO modes of operation, see the ad_gpo_access_control entry in the sssd-ad(5) Manual page.

3.5.4. Changing the GPO access control mode

This procedure changes how GPO-based access control rules are evaluated and enforced on a RHEL host joined to an Active Directory (AD) environment.

In this example, you will change the GPO operation mode from enforcing (the default) to permissive for testing purposes.

Important

If you see the following errors, Active Directory users are unable to log in due to GPO-based access controls:

  • In /var/log/secure:

    Oct 31 03:00:13 client1 sshd[124914]: pam_sss(sshd:account): Access denied for user aduser1: 6 (Permission denied)
    Oct 31 03:00:13 client1 sshd[124914]: Failed password for aduser1 from 127.0.0.1 port 60509 ssh2
    Oct 31 03:00:13 client1 sshd[124914]: fatal: Access denied for user aduser1 by PAM account configuration [preauth]
  • In /var/log/sssd/sssd__example.com_.log:

    (Sat Oct 31 03:00:13 2020) [sssd[be[example.com]]] [ad_gpo_perform_hbac_processing] (0x0040): GPO access check failed: [1432158236](Host Access Denied)
    (Sat Oct 31 03:00:13 2020) [sssd[be[example.com]]] [ad_gpo_cse_done] (0x0040): HBAC processing failed: [1432158236](Host Access Denied}
    (Sat Oct 31 03:00:13 2020) [sssd[be[example.com]]] [ad_gpo_access_done] (0x0040): GPO-based access control failed.

If this is undesired behavior, you can temporarily set ad_gpo_access_control to permissive as described in this procedure while you troubleshoot proper GPO settings in AD.

Prerequisites

  • You have joined a RHEL host to an AD environment using SSSD.
  • Editing the /etc/sssd/sssd.conf configuration file requires root permissions.

Procedure

  1. Stop the SSSD service.

    [root@server ~]# systemctl stop sssd
  2. Open the /etc/sssd/sssd.conf file in a text editor.
  3. Set ad_gpo_access_control to permissive in the domain section for the AD domain.

    [domain/example.com]
    ad_gpo_access_control=permissive
    ...
  4. Save the /etc/sssd/sssd.conf file.
  5. Restart the SSSD service to load configuration changes.

    [root@server ~]# systemctl restart sssd

Additional resources

3.5.5. Creating and configuring a GPO for a RHEL host in the AD GUI

A Group Policy Object (GPO) is a collection of access control settings stored in Microsoft Active Directory (AD) that can apply to computers and users in an AD environment. The following procedure creates a GPO in the AD graphical user interface (GUI) to control logon access to a RHEL host that is integrated directly to the AD domain.

Prerequisites

  • You have joined a RHEL host to an AD environment using SSSD.
  • You have AD Administrator privileges to make changes in AD using the GUI.

Procedure

  1. Within Active Directory Users and Computers, create an Organizational Unit (OU) to associate with the new GPO:

    1. Right-click on the domain.
    2. Choose New.
    3. Choose Organizational Unit.
  2. Click on the name of the Computer Object that represents the RHEL host (created when it joined Active Directory) and drag it into the new OU. By having the RHEL host in its own OU, the GPO targets this host.
  3. Within the Group Policy Management Editor, create a new GPO for the OU you created:

    1. Expand Forest.
    2. Expand Domains.
    3. Expand your domain.
    4. Right-click on the new OU.
    5. Choose Create a GPO in this domain.
  4. Specify a name for the new GPO, such as Allow SSH access or Allow Console/GUI access and click OK.
  5. Edit the new GPO:

    1. Select the OU within the Group Policy Management editor.
    2. Right-click and choose Edit.
    3. Select User Rights Assignment.
    4. Select Computer Configuration
    5. Select Policies.
    6. Select Windows Settings.
    7. Select Security Settings.
    8. Select Local Policies.
    9. Select User Rights Assignment.
  6. Assign login permissions:

    1. Double-Click on Allow log on locally to grant local console/GUI access.
    2. Double-click on Allow log on through Remote Desktop Services to grant SSH access.
  7. Add the user(s) you would like to access either of these policies to the policies themselves:

    1. Click Add User or Group.
    2. Enter the username within the blank field.
    3. Click OK.

Additional resources

3.5.6. Additional resources

Chapter 4. Accessing AD with a Managed Service Account

Active Directory (AD) Managed Service Accounts (MSAs) allow you to create an account in AD that corresponds to a specific computer. You can use an MSA to connect to AD resources as a specific user principal, without joining the RHEL host to the AD domain.

This section discusses the following topics:

4.1. The benefits of a Managed Service Account

If you want to allow a RHEL host to access an Active Directory (AD) domain without joining it, you can use a Managed Service Account (MSA) to access that domain. An MSA is an account in AD that corresponds to a specific computer, which you can use to connect to AD resources as a specific user principal.

For example, if the AD domain production.example.com has a one-way trust relationship with the lab.example.com AD domain, the following conditions apply:

  • The lab domain trusts users and hosts from the production domain.
  • The production domain does not trust users and hosts from the lab domain.

This means that a host joined to the lab domain, such as client.lab.example.com, cannot access resources from the production domain through the trust.

If you want to create an exception for the client.lab.example.com host, you can use the adcli utility to create a MSA for the client host in the production.example.com domain. By authenticating with the Kerberos principal of the MSA, you can perform secure LDAP searches in the production domain from the client host.

4.2. Configuring a Managed Service Account for a RHEL host

This procedure creates a Managed Service Account (MSA) for a host from the lab.example.com Active Directory (AD) domain, and configures SSSD so you can access and authenticate to the production.example.com AD domain.

Note

If you need to access AD resources from a RHEL host, Red Hat recommends that you join the RHEL host to the AD domain with the realm command. See Connecting RHEL systems directly to AD using SSSD.

Only perform this procedure if one of the following conditions applies:

  • You cannot join the RHEL host to the AD domain, and you want to create an account for that host in AD.
  • You have joined the RHEL host to an AD domain, and you need to access another AD domain where the host credentials from the domain you have joined are not valid, such as with a one-way trust.

Prerequisites

  • Ensure that the following ports on the RHEL host are open and accessible to the AD domain controllers.

    ServicePortProtocols

    DNS

    53

    TCP, UDP

    LDAP

    389

    TCP, UDP

    LDAPS (optional)

    636

    TCP, UDP

    Kerberos

    88

    TCP, UDP

  • You have the password for an AD Administrator that has rights to create MSAs in the production.example.com domain.
  • You have root permissions that are required to run the adcli command, and to modify the /etc/sssd/sssd.conf configuration file..
  • (Optional) You have the krb5-workstation package installed, which includes the klist diagnostic utility.

Procedure

  1. Create an MSA for the host in the production.example.com AD domain.

    [root@client ~]# adcli create-msa --domain=production.example.com
  2. Display information about the MSA from the Kerberos keytab that was created. Make note of the MSA name:

    [root@client ~]# klist -k /etc/krb5.keytab.production.example.com
    Keytab name: FILE:/etc/krb5.keytab.production.example.com
    KVNO Principal
    ---- ------------------------------------------------------------
       2 CLIENT!S3A$@PRODUCTION.EXAMPLE.COM (aes256-cts-hmac-sha1-96)
       2 CLIENT!S3A$@PRODUCTION.EXAMPLE.COM (aes128-cts-hmac-sha1-96)
  3. Open the /etc/sssd/sssd.conf file and choose the appropriate SSSD domain configuration to add:

    • If the MSA corresponds to an AD domain from a different forest, create a new domain section named [domain/<name_of_domain>], and enter information about the MSA and the keytab. The most important options are ldap_sasl_authid, ldap_krb5_keytab, and krb5_keytab:

      [domain/production.example.com]
      ldap_sasl_authid = CLIENT!S3A$@PRODUCTION.EXAMPLE.COM
      ldap_krb5_keytab = /etc/krb5.keytab.production.example.com
      krb5_keytab = /etc/krb5.keytab.production.example.com
      ad_domain = production.example.com
      krb5_realm = PRODUCTION.EXAMPLE.COM
      access_provider = ad
      ...
    • If the MSA corresponds to an AD domain from the local forest, create a new sub-domain section in the format [domain/root.example.com/sub-domain.example.com], and enter information about the MSA and the keytab. The most important options are ldap_sasl_authid, ldap_krb5_keytab, and krb5_keytab:

      [domain/ad.example.com/production.example.com]
      ldap_sasl_authid = CLIENT!S3A$@PRODUCTION.EXAMPLE.COM
      ldap_krb5_keytab = /etc/krb5.keytab.production.example.com
      krb5_keytab = /etc/krb5.keytab.production.example.com
      ad_domain = production.example.com
      krb5_realm = PRODUCTION.EXAMPLE.COM
      access_provider = ad
      ...

Verification steps

  • Verify you can retrieve a Kerberos ticket-granting ticket (TGT) as the MSA:

    [root@client ~]# kinit -k -t /etc/krb5.keytab.production.example.com 'CLIENT!S3A$'
    [root@client ~]# klist
    Ticket cache: KCM:0:54655
    Default principal: CLIENT!S3A$@PRODUCTION.EXAMPLE.COM
    
    Valid starting       Expires              Service principal
    11/22/2021 15:48:03  11/23/2021 15:48:03  krbtgt/PRODUCTION.EXAMPLE.COM@PRODUCTION.EXAMPLE.COM
  • In AD, verify you have an MSA for the host in the Managed Service Accounts Organizational Unit (OU).

4.3. Updating the password for a Managed Service Account

Managed Service Accounts (MSAs) have a complex password that is maintained automatically by Active Directory (AD). By default, the System Services Security Daemon (SSSD) automatically updates the MSA password in the Kerberos keytab if it is older than 30 days, which keeps it up to date with the password in AD. This procedure explains how to manually update the password for your MSA.

Prerequisites

  • You have previously created an MSA for a host in the production.example.com AD domain.
  • (Optional) You have the krb5-workstation package installed, which includes the klist diagnostic utility.

Procedure

  1. (Optional) Display the current Key Version Number (KVNO) for the MSA in the Kerberos keytab. The current KVNO is 2.

    [root@client ~]# klist -k /etc/krb5.keytab.production.example.com
    Keytab name: FILE:/etc/krb5.keytab.production.example.com
    KVNO Principal
    ---- ------------------------------------------------------------
       2 CLIENT!S3A$@PRODUCTION.EXAMPLE.COM (aes256-cts-hmac-sha1-96)
       2 CLIENT!S3A$@PRODUCTION.EXAMPLE.COM (aes128-cts-hmac-sha1-96)
  2. Update the password for the MSA in the production.example.com AD domain.

    [root@client ~]# adcli update --domain=production.example.com --host-keytab=/etc/krb5.keytab.production.example.com --computer-password-lifetime=0

Verification steps

  • Verify that you have incremented the KVNO in the Kerberos keytab:

    [root@client ~]# klist -k /etc/krb5.keytab.production.example.com
    Keytab name: FILE:/etc/krb5.keytab.production.example.com
    KVNO Principal
    ---- ------------------------------------------------------------
       3 CLIENT!S3A$@PRODUCTION.EXAMPLE.COM (aes256-cts-hmac-sha1-96)
       3 CLIENT!S3A$@PRODUCTION.EXAMPLE.COM (aes128-cts-hmac-sha1-96)

4.4. Managed Service Account specifications

The Managed Service Accounts (MSAs) that the adcli utility creates have the following specifications:

  • They cannot have additional service principal names (SPNs).
  • By default, the Kerberos principal for the MSA is stored in a Kerberos keytab named <default_keytab_location>.<Active_Directory_domain>, like /etc/krb5.keytab.production.example.com.
  • MSA names are limited to 20 characters or fewer. The last 4 characters are a suffix of 3 random characters from number and upper- and lowercase ASCII ranges appended to the short host name you provide, using a ! character as a separator. For example, a host with the short name myhost receives an MSA with the following specifications:

    SpecificationValue

    Common name (CN) attribute

    myhost!A2c

    NetBIOS name

    myhost!A2c$

    sAMAccountName

    myhost!A2c$

    Kerberos principal in the production.example.com AD domain

    myhost!A2c$@PRODUCTION.EXAMPLE.COM

4.5. Options for the adcli create-msa command

In addition to the global options you can pass to the adcli utility, you can specify the following options to specifically control how it handles Managed Service Accounts (MSAs).

-N, --computer-name
The short non-dotted name of the MSA that will be created in the Active Directory (AD) domain. If you do not specify a name, the first portion of the --host-fqdn or its default is used with a random suffix.
-O, --domain-ou=OU=<path_to_OU>
The full distinguished name of the Organizational Unit (OU) in which to create the MSA. If you do not specify this value, the MSA is created in the default location OU=CN=Managed Service Accounts,DC=EXAMPLE,DC=COM.
-H, --host-fqdn=host
Override the local machine’s fully qualified DNS domain name. If you do not specify this option, the host name of the local machine is used.
-K, --host-keytab=<path_to_keytab>
The path to the host keytab to store MSA credentials. If you do not specify this value, the default location /etc/krb5.keytab is used with the lower-cased Active Directory domain name added as a suffix, such as /etc/krb5.keytab.domain.example.com.
--use-ldaps
Create the MSA over a Secure LDAP (LDAPS) channel.
--verbose
Print out detailed information while creating the MSA.
--show-details
Print out information about the MSA after creating it.
--show-password
Print out the MSA password after creating the MSA.

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