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System-Level Authentication Guide

Red Hat Enterprise Linux 7

About System-Level Services for Authentication and Identity Management

Marc Muehlfeld

Red Hat Customer Content Services

Filip Hanzelka

Red Hat Customer Content Services

Lucie Maňásková

Red Hat Customer Content Services

Aneta Šteflová Petrová

Red Hat Customer Content Services

Tomáš Čapek

Red Hat Customer Content Services

Ella Deon Ballard

Red Hat Customer Content Services

Abstract

This guide covers different applications and services available to configure authentication on local systems.
In addition to this guide, you can find documentation on the features and services related to Red Hat Enterprise Linux Identity Management in the following guides:

The Linux Domain Identity, Authentication, and Policy Guide documents Red Hat Identity Management, a solution that provides a centralized and unified way to manage identity stores as well as authentication and authorization policies in a Linux-based domain.

The Windows Integration Guide documents how to integrate Linux domains with Microsoft Windows Active Directory (AD) using Identity Management. Among other topics, the guide covers various aspects of direct and indirect AD integration, using SSSD to access a Common Internet File System (CIFS), and the realmd system.

Chapter 1. Introduction to System Authentication

One of the cornerstones of establishing a secure network environment is making sure that access is restricted to people who have the right to access the network. If access is allowed, users can authenticate to the system, meaning they can verify their identities.
On any Red Hat Enterprise Linux system, there are a number of different services available to create and identify user identities. These can be local system files, services which connect to larger identity domains like Kerberos or Samba, or tools to create those domains.
This guide reviews some common system services and applications which are available to administrators to manage authentication and identities for a local system. Other guides are available which provide more detailed information on creating Linux domains and integrating a Linux system into a Windows domain.

1.1. Confirming User Identities

Authentication is the process of confirming an identity. For network interactions, authentication involves the identification of one party by another party. There are many ways to use authentication over networks: simple passwords, certificates, one-time password (OTP) tokens, biometric scans.
Authorization, on the other hand, defines what the authenticated party is allowed to do or access.
Authentication requires that a user presents some kind of credential to verify his identity. The kind of credential that is required is defined by the authentication mechanism being used. There are several kinds of authentication for local users on a system:
  • Password-based authentication. Almost all software permits the user to authenticate by providing a recognized name and password. This is also called simple authentication.
  • Certificate-based authentication. Client authentication based on certificates is part of the SSL protocol. The client digitally signs a randomly generated piece of data and sends both the certificate and the signed data across the network. The server validates the signature and confirms the validity of the certificate.
  • Kerberos authentication. Kerberos establishes a system of short-lived credentials, called ticket-granting tickets (TGTs). The user presents credentials, that is, user name and password, that identify the user and indicate to the system that the user can be issued a ticket. TGT can then be repeatedly used to request access tickets to other services, like websites and email. Authentication using TGT allows the user to undergo only a single authentication process in this way.
  • Smart card-based authentication. This is a variant of certificate-based authentication. The smart card (or token) stores user certificates; when a user inserts the token into a system, the system can read the certificates and grant access. Single sign-on using smart cards goes through three steps:
    1. A user inserts a smart card into the card reader. Pluggable authentication modules (PAMs) on Red Hat Enterprise Linux detect the inserted smart card.
    2. The system maps the certificate to the user entry and then compares the presented certificates on the smart card, which are encrypted with a private key as explained under the certificate-based authentication, to the certificates stored in the user entry.
    3. If the certificate is successfully validated against the key distribution center (KDC), then the user is allowed to log in.
    Smart card-based authentication builds on the simple authentication layer established by Kerberos by adding certificates as additional identification mechanisms as well as by adding physical access requirements.

1.2. As Part of Planning Single Sign-On

The thing about authentication as described in Section 1.1, “Confirming User Identities” is that every secure application requires at least a password to access it. Without a central identity store and every application maintaining its own set of users and credentials, a user has to enter a password for every single service or application he opens. This can require entering a password several times a day, maybe even every few minutes.
Maintaining multiple passwords, and constantly being prompted to enter them, is a hassle for users and administrators. Single sign-on is a configuration which allows administrators to create a single password store so that users can log in once, using a single password, and be authenticated to all network resources.
Red Hat Enterprise Linux supports single sign-on for several resources, including logging into workstations, unlocking screen savers, and accessing secured web pages using Mozilla Firefox. With other available system services such as PAM, NSS, and Kerberos, other system applications can be configured to use those identity sources.
Single sign-on is both a convenience to users and another layer of security for the server and the network. Single sign-on hinges on secure and effective authentication. Red Hat Enterprise Linux provides two authentication mechanisms which can be used to enable single sign-on:
  • Kerberos-based authentication, through both Kerberos realms and Active Directory domains
  • Smart card-based authentication
Both of these methods create a centralized identity store (either through a Kerberos realm or a certificate authority in a public key infrastructure), and the local system services then use those identity domains rather than maintaining multiple local stores.

1.3. Available Services

All Red Hat Enterprise Linux systems have some services already available to configure authentication for local users on local systems. These include:
Authentication Setup
  • The Authentication Configuration tool (authconfig) sets up different identity back ends and means of authentication (such as passwords, fingerprints, or smart cards) for the system.
Identity Back End Setup
  • The Security System Services Daemon (SSSD) sets up multiple identity providers (primarily LDAP-based directories such as Microsoft Active Directory or Red Hat Enterprise Linux IdM) which can then be used by both the local system and applications for users. Passwords and tickets are cached, allowing both offline authentication and single sign-on by reusing credentials.
  • The realmd service is a command-line utility that allows you to configure an authentication back end, which is SSSD for IdM. The realmd service detects available IdM domains based on the DNS records, configures SSSD, and then joins the system as an account to a domain.
  • Name Service Switch (NSS) is a mechanism for low-level system calls that return information about users, groups, or hosts. NSS determines what source, that is, which modules, should be used to obtain the required information. For example, user information can be located in traditional UNIX files, such as the /etc/passwd file, or in LDAP-based directories, while host addresses can be read from files, such as the /etc/hosts file, or the DNS records; NSS locates where the information is stored.
Authentication Mechanisms
  • Pluggable Authentication Modules (PAM) provide a system to set up authentication policies. An application using PAM for authentication loads different modules that control different aspects of authentication; which PAM module an application uses is based on how the application is configured. The available PAM modules include Kerberos, Winbind, or local UNIX file-based authentication.
Other services and applications are also available, but these are common ones.

Part I. System Logins

Chapter 2. Configuring System Authentication

Authentication is the process in which a user is identified and verified to a system. It requires presenting some sort of identity and credentials, such as a user name and password. The system then compares the credentials against the configured authentication service. If the credentials match and the user account is active, then the user is authenticated.
Once a user is authenticated, the information is passed to the access control service to determine what the user is permitted to do. Those are the resources the user is authorized to access. Note that authentication and authorization are two separate processes.
The system must have a configured list of valid account databases for it to check for user authentication. The information to verify the user can be located on the local system or the local system can reference a user database on a remote system, such as LDAP or Kerberos. A local system can use a variety of different data stores for user information, including Lightweight Directory Access Protocol (LDAP), Network Information Service (NIS), and Winbind. Both LDAP and NIS data stores can use Kerberos to authenticate users.
For convenience and potentially part of single sign-on, Red Hat Enterprise Linux can use the System Security Services Daemon (SSSD) as a central daemon to authenticate the user to different identity back ends or even to ask for a ticket-granting ticket (TGT) for the user. SSSD can interact with LDAP, Kerberos, and external applications to verify user credentials.
This chapter explains what tools are available in Red Hat Enterprise Linux for configuring system authentication:

2.1. Identity Management Tools for System Authentication

You can use the ipa-client-install utility and the realmd system to automatically configure system authentication on Identity Management machines.
ipa-client-install
The ipa-client-install utility configures a system to join the Identity Management domain as a client machine. For more information about ipa-client-install, see the Linux Domain Identity, Authentication, and Policy Guide.
Note that for Identity Management systems, ipa-client-install is preferred over realmd.
realmd
The realmd system joins a machine to an identity domain, such as an Identity Management or Active Directory domain. For more information about realmd, see the Windows Integration Guide.

2.2. Using authconfig

The authconfig tool can help configure what kind of data store to use for user credentials, such as LDAP. On Red Hat Enterprise Linux, authconfig has both GUI and command-line options to configure any user data stores. The authconfig tool can configure the system to use specific services — SSSD, LDAP, NIS, or Winbind — for its user database, along with using different forms of authentication mechanisms.

Important

To configure Identity Management systems, Red Hat recommends using the ipa-client-install utility or the realmd system instead of authconfig. The authconfig utilities are limited and substantially less flexible. For more information, see Section 2.1, “Identity Management Tools for System Authentication”.
The following three authconfig utilities are available for configuring authentication settings:
  • authconfig-gtk provides a full graphical interface.
  • authconfig provides a command-line interface for manual configuration.
  • authconfig-tui provides a text-based UI. Note that this utility has been deprecated.
All of these configuration utilities must be run as root.

2.2.1. Tips for Using the authconfig CLI

The authconfig command-line tool updates all of the configuration files and services required for system authentication, according to the settings passed to the script. Along with providing even more identity and authentication configuration options than can be set through the UI, the authconfig tool can also be used to create backup and kickstart files.
For a complete list of authconfig options, check the help output and the man page.
There are some things to remember when running authconfig:
  • With every command, use either the --update or --test option. One of those options is required for the command to run successfully. Using --update writes the configuration changes. The --test option displays the changes but does not apply the changes to the configuration.
    If the --update option is not used, then the changes are not written to the system configuration files.
  • The command line can be used to update existing configuration as well as to set new configuration. Because of this, the command line does not enforce that required attributes are used with a given invocation (because the command may be updating otherwise complete settings).
    When editing the authentication configuration, be very careful that the configuration is complete and accurate. Changing the authentication settings to incomplete or wrong values can lock users out of the system. Use the --test option to confirm that the settings are proper before using the --update option to write them.
  • Each enable option has a corresponding disable option.

2.2.2. Installing the authconfig UI

The authconfig UI is not installed by default, but it can be useful for administrators to make quick changes to the authentication configuration.
To install the UI, install the authconfig-gtk package. This has dependencies on some common system packages, such as the authconfig command-line tool, Bash, and Python. Most of those are installed by default.
[root@server ~]# yum install authconfig-gtk
Loaded plugins: langpacks, product-id, subscription-manager
Resolving Dependencies
--> Running transaction check
---> Package authconfig-gtk.x86_64 0:6.2.8-8.el7 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

================================================================================
 Package              Arch         Version            Repository           Size
================================================================================
Installing:
 authconfig-gtk       x86_64       6.2.8-8.el7        RHEL-Server       105 k

Transaction Summary
================================================================================
Install  1 Package

... 8< ...

2.2.3. Launching the authconfig UI

  1. Open the terminal and log in as root.
  2. Run the system-config-authentication command.

Important

Any changes take effect immediately when the authconfig UI is closed.
There are three configuration tabs in the Authentication dialog box:
  • Identity & Authentication, which configures the resource used as the identity store (the data repository where the user IDs and corresponding credentials are stored).
  • Advanced Options, which configures authentication methods other than passwords or certificates, like smart cards and fingerprint.
  • Password Options, which configures password authentication methods.
authconfig Window

Figure 2.1. authconfig Window

2.2.4. Testing Authentication Settings

It is critical that authentication is fully and properly configured. Otherwise all users (even root) could be locked out of the system, or some users blocked.
The --test option prints all of the authentication configuration for the system, for every possible identity and authentication mechanism. This shows both the settings for what is enabled and what areas are disabled.
The test option can be run by itself to show the full, current configuration or it can be used with an authconfig command to show how the configuration will be changed (without actually changing it). This can be very useful in verifying that the proposed authentication settings are complete and correct.
[root@server ~]# authconfig --test
caching is disabled
nss_files is always enabled
nss_compat is disabled
nss_db is disabled
nss_hesiod is disabled
 hesiod LHS = ""
 hesiod RHS = ""
nss_ldap is disabled
 LDAP+TLS is disabled
 LDAP server = ""
 LDAP base DN = ""
nss_nis is disabled
 NIS server = ""
 NIS domain = ""
nss_nisplus is disabled
nss_winbind is disabled
 SMB workgroup = "MYGROUP"
 SMB servers = ""
 SMB security = "user"
 SMB realm = ""
 Winbind template shell = "/bin/false"
 SMB idmap range = "16777216-33554431"
nss_sss is enabled by default
nss_wins is disabled
nss_mdns4_minimal is disabled
DNS preference over NSS or WINS is disabled
pam_unix is always enabled
 shadow passwords are enabled
 password hashing algorithm is sha512
pam_krb5 is disabled
 krb5 realm = "#"
 krb5 realm via dns is disabled
 krb5 kdc = ""
 krb5 kdc via dns is disabled
 krb5 admin server = ""
pam_ldap is disabled
 LDAP+TLS is disabled
 LDAP server = ""
 LDAP base DN = ""
 LDAP schema = "rfc2307"
pam_pkcs11 is disabled
 use only smartcard for login is disabled
 smartcard module = ""
 smartcard removal action = ""
pam_fprintd is disabled
pam_ecryptfs is disabled
pam_winbind is disabled
 SMB workgroup = "MYGROUP"
 SMB servers = ""
 SMB security = "user"
 SMB realm = ""
pam_sss is disabled by default
 credential caching in SSSD is enabled
 SSSD use instead of legacy services if possible is enabled
IPAv2 is disabled
IPAv2 domain was not joined
 IPAv2 server = ""
 IPAv2 realm = ""
 IPAv2 domain = ""
pam_pwquality is enabled (try_first_pass local_users_only retry=3 authtok_type=)
pam_passwdqc is disabled ()
pam_access is disabled ()
pam_mkhomedir or pam_oddjob_mkhomedir is disabled (umask=0077)
Always authorize local users is enabled ()
Authenticate system accounts against network services is disabled

2.2.5. Saving and Restoring Configuration Using authconfig

Changing authentication settings can be problematic. Improperly changing the configuration can wrongly exclude users who should have access, can cause connections to the identity store to fail, or can even lock all access to a system.
Before editing the authentication configuration, it is strongly recommended that administrators take a backup of all configuration files. This is done with the --savebackup option.
[root@server ~]# authconfig --savebackup=/backups/authconfigbackup20190701
The authentication configuration can be restored to any previous saved version using the --restorebackup option, with the name of the backup to use.
[root@server ~]# authconfig --restorebackup=/backups/authconfigbackup20190701
The authconfig command saves an automatic backup every time the configuration is altered. It is possible to restore the last backup using the --restorelastbackup option.
[root@server ~]# authconfig --restorelastbackup

Chapter 3. Selecting the Identity Store for Authentication with authconfig

The Identity & Authentication tab in the authconfig UI sets how users should be authenticated. The default is to use local system authentication, meaning the users and their passwords are checked against local system accounts. A Red Hat Enterprise Linux machine can also use external resources which contain the users and credentials, including LDAP, NIS, and Winbind.

3.1. IPAv2

There are two different ways to configure an Identity Management server as an identity back end. For IdM version 2 (Red Hat Enterprise Linux version 6.3 and earlier), version 3 (in Red Hat Enterprise Linux 6.4 and later), and version 4 (in Red Hat Enterprise Linux 7.1 and later), these are configured as IPAv2 providers in authconfig. For previous IdM versions and for community FreeIPA servers, these are configured as LDAP providers.

3.1.1. Configuring IdM from the UI

  1. Open the authconfig UI.
  2. Select IPAv2 in the User Account Database drop-down menu.
    Authentication Configuration

    Figure 3.1. Authentication Configuration

  3. Set the information that is required to connect to the IdM server.
    • IPA Domain gives the DNS domain of the IdM domain.
    • IPA Realm gives the Kerberos domain of the IdM domain.
    • IPA Server gives the host name of any IdM server within the IdM domain topology.
    • Do not configure NTP optionally disables NTP services when the client is configured. This is usually not recommended, because the IdM server and all clients need to have synchronized clocks for Kerberos authentication and certificates to work properly. This could be disabled if the IdM servers are using a different NTP server rather than hosting it within the domain.
  4. Click the Join the domain button.
    This runs the ipa-client-install command and, if necessary, installs the ipa-client packages. The installation script automatically configures all system files that are required for the local system and contacts the domain servers to update the domain information.

3.1.2. Configuring IdM from the Command Line

An IdM domain centralizes several common and critical services in a single hierarchy, most notably DNS and Kerberos.
authconfig (much like realmd in Chapter 8, Using realmd to Connect to an Identity Domain) can be used to enroll a system in the IdM domain. That runs the ipa-client-install command and, if necessary, installs the ipa-client packages. The installation script automatically configures all system files that are required for the local system and contacts the domain servers to update the domain information.
Joining a domain requires three pieces of information to identify the domain: the DNS domain name (--ipav2domain), the Kerberos realm name (--ipav2realm), and the IdM server to contact (--ipav2server). The --ipav2join option gives the administrator user name to use to connect to the IdM server; this is typically admin.
[root@server ~]# authconfig --enableipav2 --ipav2domain=IPAEXAMPLE --ipav2realm=IPAEXAMPLE --ipav2server=ipaexample.com --ipav2join=admin
If the IdM domain is not running its own NTP services, then it is possible to use the --disableipav2nontp option to prevent the setup script to use the IdM server as the NTP server. This is generally not recommended, because the IdM server and all clients need to have synchronized clocks for Kerberos authentication and certificates to work properly.

3.2. LDAP and IdM

Both standard LDAP directories (such as OpenLDAP and Red Hat Directory Server) can be used as LDAP identity providers. Additionally, older IdM versions and FreeIPA can be configured as identity providers by configuring them as LDAP providers with a related Kerberos server.
Either the openldap-clients package or the sssd package is used to configure an LDAP server for the user database. Both packages are installed by default.

3.2.1. Configuring LDAP Authentication from the UI

  1. Open the authconfig UI, as in Section 2.2.3, “Launching the authconfig UI”.
  2. Select LDAP in the User Account Database drop-down menu.
  3. Set the information that is required to connect to the LDAP server.
    • LDAP Search Base DN gives the root suffix or distinguished name (DN) for the user directory. All of the user entries used for identity or authentication exist below this parent entry. For example, ou=people,dc=example,dc=com.
      This field is optional. If it is not specified, the System Security Services Daemon (SSSD) attempts to detect the search base using the namingContexts and defaultNamingContext attributes in the LDAP server's configuration entry.
    • LDAP Server gives the URL of the LDAP server. This usually requires both the host name and port number of the LDAP server, such as ldap://ldap.example.com:389.
      Entering the secure protocol by using a URL starting with ldaps:// enables the Download CA Certificate button, which retrieves the issuing CA certificate for the LDAP server from whatever certificate authority issued it. The CA certificate must be in the privacy enhanced mail (PEM) format.
    • If you use a insecure standard port connection (URL starting with ldap://), you can use the Use TLS to encrypt connections check box to encrypt communication with the LDAP server using STARTTLS. Selecting this check box also enables the Download CA Certificate button.

      Note

      You do not need to select the Use TLS to encrypt connections check box if the server URL uses the LDAPS (LDAP over SSL) secure protocol as the communication is already encrypted.
  4. Select the authentication method. LDAP allows simple password authentication or Kerberos authentication.
    The LDAP password option uses PAM applications to use LDAP authentication. This option requires a secure connection to be set either by using LDAPS or TLS to connect to the LDAP server.

3.2.2. Configuring LDAP User Stores from the Command Line

To use an LDAP identity store, use the --enableldap. To use LDAP as the authentication source, use --enableldapauth and then the requisite connection information, like the LDAP server name, base DN for the user suffix, and (optionally) whether to use TLS. The authconfig command also has options to enable or disable RFC 2307bis schema for user entries, which is not possible through the authconfig UI.
Be sure to use the full LDAP URL, including the protocol (ldap or ldaps) and the port number. Do not use a secure LDAP URL (ldaps) with the --enableldaptls option.
authconfig --enableldap --enableldapauth --ldapserver=ldap://ldap.example.com:389,ldap://ldap2.example.com:389 --ldapbasedn="ou=people,dc=example,dc=com" --enableldaptls --ldaploadcacert=https://ca.server.example.com/caCert.crt --update
Instead of using --ldapauth for LDAP password authentication, it is possible to use Kerberos with the LDAP user store. These options are described in Section 4.3.2, “Configuring Kerberos Authentication from the Command Line”.

3.3. NIS

Important

Before NIS can be configured as an identity store, NIS itself must be configured for the environment:
  • A NIS server must be fully configured with user accounts set up.
  • The ypbind package must be installed on the local system. This is required for NIS services, but is not installed by default.
  • The portmap and ypbind services are started and enabled to start at boot time. This should be configured as part of the ypbind package installation.

3.3.1. Configuring NIS Authentication from the UI

  1. Open the authconfig UI, as in Section 2.2.3, “Launching the authconfig UI”.
  2. Select NIS in the User Account Database drop-down menu.
  3. Set the information to connect to the NIS server, meaning the NIS domain name and the server host name. If the NIS server is not specified, the authconfig daemon scans for the NIS server.
  4. Select the authentication method. NIS allows simple password authentication or Kerberos authentication.

3.3.2. Configuring NIS from the Command Line

To use a NIS identity store, use the --enablenis. This automatically uses NIS authentication, unless the Kerberos parameters are explicitly set (Section 4.3.2, “Configuring Kerberos Authentication from the Command Line”). The only parameters are to identify the NIS server and NIS domain; if these are not used, then the authconfig service scans the network for NIS servers.
[root@server ~]# authconfig --enablenis --nisdomain=EXAMPLE --nisserver=nis.example.com --update

3.4. Winbind

Samba must be configured before Winbind can be configured as an identity store for a system. A Samba server must be set up and used for user accounts, or Samba must be configured to use Active Directory as a back end identity store.
Configuring Samba is covered in the Samba project documentation. Specifically configuring Samba as an integration point with Active Directory is also covered in the Red Hat Enterprise Linux Windows Integration Guide.

3.4.1. Enabling Winbind in the authconfig GUI

  1. Install the samba-winbind package. This is required for Windows integration features in Samba services, but is not installed by default.
    [root@server ~]# yum install samba-winbind
  2. Open the authconfig UI.
    [root2server ~]# authconfig-gtk
  3. In the Identity & Authentication tab, select Winbind in the User Account Database drop-down menu.
  4. Set the information that is required to connect to the Microsoft Active Directory domain controller.
    • Winbind Domain gives the Windows domain to connect to.
      This should be in the Windows 2000 format, such as DOMAIN.
    • Security Model sets the security model to use for Samba clients. authconfig supports four types of security models:
      • ads configures Samba to act as a domain member in an Active Directory Server realm. To operate in this mode, the krb5-server package must be installed and Kerberos must be configured properly.
      • domain has Samba validate the user name and password by authenticating it through a Windows primary or backup domain controller, much like a Windows server.
      • server has a local Samba server validate the user name and password by authenticating it through another server, such as a Windows server. If the server authentication attempt fails, the system then attempts to authenticate using user mode.
      • user requires a client to log in with a valid user name and password. This mode does support encrypted passwords.
        The user name format must be domain\user, such as EXAMPLE\jsmith.

        Note

        When verifying that a given user exists in the Windows domain, always use the domain\user_name format and escape the backslash (\) character. For example:
        [root@server ~]# getent passwd domain\\user DOMAIN\user:*:16777216:16777216:Name Surname:/home/DOMAIN/user:/bin/bash
        This is the default option.
    • Winbind ADS Realm gives the Active Directory realm that the Samba server will join. This is only used with the ads security model.
    • Winbind Domain Controllers gives the host name or IP address of the domain controller to use to enroll the system.
    • Template Shell sets which login shell to use for Windows user account settings.
    • Allow offline login allows authentication information to be stored in a local cache. The cache is referenced when a user attempts to authenticate to system resources while the system is offline.

3.4.2. Enabling Winbind in the Command Line

Windows domains have several different security models, and the security model used in the domain determines the authentication configuration for the local system. For user and server security models, the Winbind configuration requires only the domain (or workgroup) name and the domain controller host names.
The --winbindjoin parameter sets the user to use to connect to the Active Directory domain, and --enablelocalauthorize sets local authorization operations to check the /etc/passwd file.
After running the authconfig command, join the Active Directory domain.
[root@server ~]# authconfig --enablewinbind --enablewinbindauth --smbsecurity=user|server  --enablewinbindoffline --smbservers=ad.example.com --smbworkgroup=EXAMPLE --update --enablelocauthorize --winbindjoin=admin
[root@server ~]# net join ads

Note

The user name format must be domain\user, such as EXAMPLE\jsmith.
When verifying that a given user exists in the Windows domain, always use the domain\user formats and escape the backslash (\) character. For example:
[root@server ~]# getent passwd domain\\user DOMAIN\user:*:16777216:16777216:Name Surname:/home/DOMAIN/user:/bin/bash
For ads and domain security models, the Winbind configuration allows additional configuration for the template shell and realm (ads only). For example:
[root@server ~]# authconfig --enablewinbind --enablewinbindauth --smbsecurity ads  --enablewinbindoffline --smbservers=ad.example.com --smbworkgroup=EXAMPLE --smbrealm EXAMPLE.COM --winbindtemplateshell=/bin/sh --update
There are a lot of other options for configuring Windows-based authentication and the information for Windows user accounts, such as name formats, whether to require the domain name with the user name, and UID ranges. These options are listed in the authconfig help.

Chapter 4. Configuring Authentication Mechanisms

Red Hat Enterprise Linux supports several different authentication methods. They can be configured using the authconfig tool or, in some cases, also using Identity Management tools.

4.1. Configuring Local Authentication Using authconfig

The Local Authentication Options area defines settings for local system accounts, not the users stored on the back end. These settings define user-based authorization to system services (as defined in /etc/security/access.conf). Otherwise, authorization policies can be defined within the identity provider or the services themselves.

4.1.1. Enabling Local Access Control in the UI

Enable local access control sets the system to check the /etc/security/access.conf file for local user authorization rules. This is PAM authorization.
Local Accounts Fields

Figure 4.1. Local Accounts Fields

4.1.2. Configuring Local Access Control in the Command Line

There are two options for authconfig to enable local authorization controls. --enablelocauthorize skips network authentication and only checks local files for system users. --enablepamaccess configures the system to look for system authorization policies in /etc/security/access.conf.
[root@server ~]# authconfig --enablelocauthorize --enablepamaccess --update

4.2. Configuring System Passwords Using authconfig

4.2.1. Password Security

If passwords are stored in plain text format, they are vulnerable to cracking, unauthorized access, or tampering. To prevent this, cryptographic hashing algorithms can be used to securely store password hash digests. The recommended (and also default) hashing algorithm supported in IdM is SHA-512, which uses 64-bit words and also salt and stretching for extra security. To ensure backward compatibility, the SHA-256, DES, BigCrypt, and MD5 hashing algorithms are also supported.

Important

If you do not need backward compatibility, only use SHA-512 as it is more secure.

4.2.1.1. Configuring Password Hashing in the UI

The Local Authentication Options tab sets how local passwords are stored on the system. The Password Hashing Algorithm drop-down menu sets the algorithm to securely store passwords hashes.
  1. Open the authconfig UI, as in Section 2.2.3, “Launching the authconfig UI”.
  2. Open the Advanced Options tab.
  3. Select the algorithm to use in the Password Hashing Algorithm drop-down menu.
  4. Click the Apply button.

4.2.1.2. Configuring Password Hashing on the Command Line

To set or change the hashing algorithm used to securely store user passwords digests, use the --passalgo option and the short name for the algorithm. The following example uses the SHA-512 algorithm:
[root@server ~]# authconfig --passalgo=sha512 --update

4.2.2. Password Complexity

Password complexity sets how strong a password must be for it to be allowed to be set for a local user account. Complexity is a combination of length and a variation of character classes. One way to look at it is that there are two parts to setting policy for complex passwords: identifying what types of characters can be used in a password (such as upper and lower case letters and special characters) and how those characters can be used within the password (how long must it be and how often can those characters be repeated).

4.2.2.1. Configuring Password Complexity in the UI

  1. Open the authconfig UI, as in Section 2.2.3, “Launching the authconfig UI”.
  2. Open the Password Options tab.
  3. Set the minimum requirements for the password:
    • The minimum length of the password
    • The minimum number of character classes which must be used in the password.
  4. Enable characters classes which must be used for passwords. For example, an uppercase letter can be used with any password, but if the Uppercase check box is selected, then an uppercase letter must be used in every password.
  5. Set the number of times that a character or character class can be repeated consecutively. (If this is set to zero, then there is no repeat limit.)
    For the Same Character field, this sets how often a single letter or character can be repeated. If this is set to 2, for example, then ssecret is allowed but sssecret is rejected.
    Likewise, Same Class sets a limit on how many times any character from a character class (uppercase, number, special character) can be repeated. If this is set to 3, for example, secret!! is allowed but secret!!@ or secret1234 would be rejected.
  6. Click the Apply button.

4.2.2.2. Configuring Password Complexity in the Command Line

When defining password complexity in the comment line, there are two halves to setting the requirements. The first is setting the requirements on how a password is constructed — its length, can characters be repeated, and how many different types of characters must be used:
  • The minimum length (--passminlen).
  • The minimum number of different types of characters which must be used (--passminclass).
  • The number of times a character can be repeated consecutively (--passmaxrepeat). Setting this to zero means there is no repeat limit.
  • The number of time the same type of character (such as a number) can be used in a row (--passmaxclassrepeat). Setting this to zero means there is no repeat limit.
The second half is defining what types or classes of characters are allowed to be used for passwords. All character types are implicitly allowed; using the --enablereqType option means that a given class is absolutely required or the password is rejected. (Conversely, types can be explicitly denied, as well.)
  • Uppercase letters (--enablerequpper)
  • Lowercase letters (--enablereqlower)
  • Numbers (--enablereqdigit)
  • Special characters (--enablereqother)
For example, this sets a minimum length of nine characters, does not allow characters or classes to be repeated more than twice, and requires both uppercase and special characters.
[root@server ~]# authconfig --passminlen=9 --passminclass=3 --passmaxrepeat=2 -passmaxclassrepeat=2 --enablerequpper --enablereqother --update

4.3. Configuring Kerberos (with LDAP or NIS) Using authconfig

Both LDAP and NIS authentication stores support Kerberos authentication methods. Using Kerberos has a couple of benefits:
  • It uses a security layer for communication while still allowing connections over standard ports.
  • It automatically uses credentials caching with SSSD, which allows offline logins.

Note

Using Kerberos authentication requires the krb5-libs and krb5-workstation packages.

4.3.1. Configuring Kerberos Authentication from the UI

The Kerberos password option from the Authentication Method drop-down menu automatically opens the fields required to connect to the Kerberos realm.
Kerberos Fields

Figure 4.2. Kerberos Fields

  • Realm gives the name for the realm for the Kerberos server. The realm is the network that uses Kerberos, composed of one or more key distribution centers (KDC) and a potentially large number of clients.
  • KDCs gives a comma-separated list of servers that issue Kerberos tickets.
  • Admin Servers gives a list of administration servers running the kadmind process in the realm.
  • Optionally, use DNS to resolve server host name and to find additional KDCs within the realm.

4.3.2. Configuring Kerberos Authentication from the Command Line

Both LDAP and NIS allow Kerberos authentication to be used in place of their native authentication mechanisms. At a minimum, using Kerberos authentication requires specifying the realm, the KDC, and the administrative server. There are also options to use DNS to resolve client names and to find additional admin servers.
[root@server ~]# authconfig NIS or LDAP options --enablekrb5 --krb5realm EXAMPLE --krb5kdc kdc.example.com:88,server.example.com:88 --krb5adminserver server.example.com:749 --enablekrb5kdcdns --enablekrb5realmdns --update

4.4. Smart Cards

Authentication based on smart cards is an alternative to password-based authentication. User credentials are stored on the smart card, and special software and hardware is then used to access them. In order to authenticate using a smart card, the user must place the smart card into a smart card reader and then supply the PIN code for the smart card.

Important

The following sections describe how to configure a single system for smart card authentication with local users by using the pam_pkcs11 and pam_krb5 packages. Note that these packages are now deprecated, as described in Deprecated Functionality in the 7.4 Release Notes.
To configure smart card authentication centrally, use the enhanced smart card functionality provided by the System Security Services Daemon (SSSD). For details, see Smart-card Authentication in Identity Management in the Linux Domain Identity, Authentication, and Policy Guide for Red Hat Identity Management.

4.4.1. Configuring Smart Cards Using authconfig

Once the Enable smart card support option is selected, additional controls for configuring behavior of smart cards appear.
Smart Card Options

Figure 4.3. Smart Card Options

Note that smart card login for Red Hat Enterprise Linux servers and workstations is not enabled by default and must be enabled in the system settings.

Note

Using single sign-on when logging into Red Hat Enterprise Linux requires these packages:
  • nss-tools
  • nss-pam-ldapd
  • esc
  • pam_pkcs11
  • pam_krb5
  • opensc
  • pcsc-lite-ccid
  • gdm
  • authconfig
  • authconfig-gtk
  • krb5-libs
  • krb5-workstation
  • krb5-pkinit
  • pcsc-lite
  • pcsc-lite-libs

4.4.1.1. Enabling Smart Card Authentication from the UI

  1. Log into the system as root.
  2. Download the root CA certificates for the network in base 64 format, and install them on the server. The certificates are installed in the appropriate system database using the certutil command. For example:
    [root@server ~]# certutil -A -d /etc/pki/nssdb -n "root CA cert" -t "CT,C,C" -i /tmp/ca_cert.crt

    Note

    Do not be concerned that the imported certificate is not displayed in the authconfig UI later during the process. You cannot see the certificate in the UI; it is obtained from the /etc/pki/nssdb/ directory during authentication.
  3. In the top menu, select the Application menu, select Sundry, and then click Authentication.
  4. Open the Advanced Options tab.
  5. Click the Enable Smart Card Support check box.
  6. There are two behaviors that can be configured for smart cards:
    • The Card removal action menu sets the response that the system takes if the smart card is removed during an active session. The Ignore option means that the system continues functioning as normal if the smart card is removed, while Lock immediately locks the screen.
    • The Require smart card for login check box sets whether a smart card is required for logins. When this option is selected, all other methods of authentication are blocked.

      Warning

      Do not select this until after you have successfully logged in using a smart card.
  7. By default, the mechanisms to check whether a certificate has been revoked (Online Certificate Status Protocol, or OCSP, responses) are disabled. To validate whether a certificate has been revoked before its expiration period, enable OCSP checking by adding the ocsp_on option to the cert_policy directive.
    1. Open the pam_pkcs11.conf file.
      vim /etc/pam_pkcs11/pam_pkcs11.conf
    2. Change every cert_policy line so that it contains the ocsp_on option.
      cert_policy = ca, ocsp_on, signature;

      Note

      Because of the way the file is parsed, there must be a space between cert_policy and the equals sign. Otherwise, parsing the parameter fails.
  8. If the smart card has not yet been enrolled (set up with personal certificates and keys), enroll the smart card.
  9. If the smart card is a CAC card, create the .k5login file in the CAC user's home directory. The .k5login file is required to have the Microsoft Principal Name on the CAC card.
  10. Add the following line to the /etc/pam.d/smartcard-auth and /etc/pam.d/system-auth files:
    auth    optional    pam_krb5.so use_first_pass no_subsequent_prompt preauth_options=X509_user_identity=PKCS11:/usr/lib64/pkcs11/opensc-pkcs11.so
    If the OpenSC module does not work as expected, use the module from the coolkey package: /usr/lib64/pkcs11/libcoolkeypk11.so. In this case, consider contacting Red Hat Technical Support or filing a Bugzilla report about the problem.
  11. Configure the /etc/krb5.conf file. The settings vary depending on whether you are using a CAC card or a Gemalto 64K card.
    • With CAC cards, specify all the root certificates related to the CAC card usage in pkinit_anchors. In the following example /etc/krb5.conf file for configuring a CAC card, EXAMPLE.COM is the realm name for the CAC cards, and kdc.server.hostname.com is the KDC server host name.
      [logging]
        default = FILE:/var/log/krb5libs.log
        kdc = FILE:/var/log/krb5kdc.log
        admin_server = FILE:/var/log/kadmind.log
      
      [libdefaults]
        dns_lookup_realm = false
        dns_lookup_kdc = false
        ticket_lifetime = 1h
        renew_lifetime = 6h
        forwardable = true
      
        default_realm = EXAMPLE.COM
      [realms]
        EXAMPLE.COM = {
          kdc = kdc.server.hostname.com
          admin_server = kdc.server.hostname.com
          pkinit_anchors = FILE:/etc/pki/nssdb/ca_cert.pem
          pkinit_anchors = FILE:/etc/pki/nssdb/CAC_CA_cert.pem
          pkinit_anchors = FILE:/etc/pki/nssdb/CAC_CA_email_cert.pem
          pkinit_anchors = FILE:/etc/pki/nssdb/CAC_root_ca_cert.pem
          pkinit_cert_match = CAC card specific information
        }
      
      [domain_realm]
        EXAMPLE.COM = EXAMPLE.COM
        .EXAMPLE.COM = EXAMPLE.COM
      
        .kdc.server.hostname.com = EXAMPLE.COM
        kdc.server.hostname.com = EXAMPLE.COM
      
      [appdefaults]
          pam = {
            debug = true
            ticket_lifetime = 1h
            renew_lifetime = 3h
            forwardable = true
            krb4_convert = false
            mappings = username on the CAC card     Principal name on the card
          }
      
    • In the following example /etc/krb5.conf file for configuring a Gemalto 64K card, EXAMPLE.COM is the realm created on the KDC server, kdc-ca.pem is the CA certificate, and kdc.server.hostname.com is the KDC server host name.
      [logging]
        default = FILE:/var/log/krb5libs.log
        kdc = FILE:/var/log/krb5kdc.log
        admin_server = FILE:/var/log/kadmind.log
      
      [libdefaults]
        dns_lookup_realm = false
        dns_lookup_kdc = false
        ticket_lifetime = 15m
        renew_lifetime = 6h
        forwardable = true
      
        default_realm = EXAMPLE.COM
      [realms]
        EXAMPLE.COM = {
          kdc = kdc.server.hostname.com
          admin_server = kdc.server.hostname.com
          pkinit_anchors = FILE:/etc/pki/nssdb/kdc-ca.pem
          pkinit_cert_match = <KU>digitalSignature
          pkinit_kdc_hostname = kdc.server.hostname.com
        }
      
      [domain_realm]
        EXAMPLE.COM = EXAMPLE.COM
        .EXAMPLE.COM = EXAMPLE.COM
      
        .kdc.server.hostname.com = EXAMPLE.COM
        kdc.server.hostname.com = EXAMPLE.COM
      
      [appdefaults]
          pam = {
            debug = true
            ticket_lifetime = 1h
            renew_lifetime = 3h
            forwardable = true
            krb4_convert = false
          }
      

Note

When a smart card is inserted, the pklogin_finder utility, when run in debug mode, first maps the login ID to the certificates on the card and then attempts to output information about the validity of certificates:
pklogin_finder debug
The command is useful for diagnosing problems with using a smart card to log into the system.

4.4.1.2. Configuring Smart Card Authentication from the Command Line

All that is required to use smart cards with a system is to set the --enablesmartcard option:
[root@server ~]# authconfig --enablesmartcard --update
There are other configuration options for smart cards, such as changing the default smart card module, setting the behavior of the system when the smart card is removed, and requiring smart cards for login.
A value of 0 instructs the system to lock out a user immediately if the smart card is removed; a setting of 1 ignores it if the smart card is removed:
[root@server ~]# authconfig --enablesmartcard --smartcardaction=0 --update
Once smart card authentication has been successfully configured and tested, then the system can be configured to require smart card authentication for users rather than simple password-based authentication.
[root@server ~]# authconfig --enablerequiresmartcard --update

Warning

Do not use the --enablerequiresmartcard option until you have successfully authenticated to the system using a smart card. Otherwise, users may be unable to log into the system.

4.4.2. Smart Card Authentication in Identity Management

Red Hat Identity Management supports smart card authentication for IdM users. For more information, see the Linux Domain Identity, Authentication, and Policy Guide.

4.4.3. Supported Smart Cards

The following smart cards and readers are supported on Red Hat Enterprise Linux.

Smart Cards

  • Athena ASECard Crypto smart, pkcs15-unit
  • ATOS (Siemens) CardOS 5.0
  • Gemalto ID Classic 230 / TOP IM CY2 64kv2
  • Gemalto Cyberflex Access 64k V2c
  • Gemalto GemPCKey USB form factor key
  • Giesecke & Devrient (G&D) SmartCafe Expert 6.0 (SCP03)
  • Giesecke & Devrient (G&D) SmartCafe Expert 7.0 (SCP03)
  • Safenet 330J
  • Safenet SC650 (SCP01)
  • Siemens Card CardOS M4.4
  • Yubikey 4

Readers

  • HP Keyboard KUS1206 with built in Smart Card reader
  • Omnikey 3121 reader
  • Omnikey 3121 with PID 0x3022 reader
  • Reiner SCT cyberJack RFID komfort reader
  • SCR331 CCID reader

4.5. One-Time Passwords

One-time password (OTP) is a password that is valid for only one authentication session; it becomes invalid after use. Unlike traditional static passwords that stay the same for a longer period of time, OTPs keep changing. OTPs are used as part of two-factor authentication: the first step requires the user to authenticate with a traditional static password, and the second step prompts for an OTP issued by a recognized authentication token.
Authentication using an OTP combined with a static password is considered safer than authentication using a static password alone. Because an OTP can only be used for successful authentication once, even if a potential intruder intercepts the OTP during login, the intercepted OTP will already be invalid by that point.

One-Time Passwords in Red Hat Enterprise Linux

Red Hat Identity Management supports OTP authentication for IdM users. For more information, see the Linux Domain Identity, Authentication, and Policy Guide.

4.6. Configuring Fingerprints Using authconfig

4.6.1. Using Fingerprint Authentication in the UI

When there is appropriate hardware available, the Enable fingerprint reader support option allows fingerprint scans to be used to authenticate local users in addition to other credentials.
Fingerprint Options

Figure 4.4. Fingerprint Options

4.6.2. Configuring Fingerprint Authentication in the Command Line

There is one option to enable support for fingerprint readers. This option can be used alone or in conjunction with other authconfig settings, like LDAP user stores.
[root@server ~]# authconfig --enablefingerprint --update

Chapter 5. Managing Kickstart and Configuration Files Using authconfig

The --update option updates all of the configuration files with the configuration changes. There are a couple of alternative options with slightly different behavior:
  • --kickstart writes the updated configuration to a kickstart file.
  • --test displays the full configuration with changes, but does not edit any configuration files.
Additionally, authconfig can be used to back up and restore previous configurations. All archives are saved to a unique subdirectory in the /var/lib/authconfig/ directory. For example, the --savebackup option gives the backup directory as 2011-07-01:
[root@server ~]# authconfig --savebackup=2011-07-01
This backs up all of the authentication configuration files beneath the /var/lib/authconfig/backup-2011-07-01 directory.
Any of the saved backups can be used to restore the configuration using the --restorebackup option, giving the name of the manually saved configuration:
[root@server ~]# authconfig --restorebackup=2011-07-01
Additionally, authconfig automatically makes a backup of the configuration before it applies any changes (with the --update option). The configuration can be restored from the most recent automatic backup, without having to specify the exact backup, using the --restorelastbackup option.

Chapter 6. Enabling Custom Home Directories Using authconfig

If LDAP users have home directories that are not in /home and the system is configured to create home directories the first time users log in, then these directories are created with the wrong permissions.
  1. Apply the correct SELinux context and permissions from the /home directory to the home directory that is created on the local system. For example:
    [root@server ~]# semanage fcontext -a -e /home /home/locale
  2. Install the oddjob-mkhomedir package on the system.
    This package provides the pam_oddjob_mkhomedir.so library, which the authconfig command uses to create home directories. The pam_oddjob_mkhomedir.so library, unlike the default pam_mkhomedir.so library, can create SELinux labels.
    The authconfig command automatically uses the pam_oddjob_mkhomedir.so library if it is available. Otherwise, it will default to using pam_mkhomedir.so.
  3. Make sure the oddjobd service is running.
  4. Run the authconfig command and enable home directories. In the command line, this is done through the --enablemkhomedir option.
    [root@server ~]# authconfig --enablemkhomedir --update
    In the UI, there is an option in the Advanced Options tab (Create home directories on the first login) to create a home directory automatically the first time that a user logs in.
    Home Directory Option

    Figure 6.1. Home Directory Option

    This option is beneficial with accounts that are managed centrally, such as with LDAP. However, this option should not be selected if a system like automount is used to manage user home directories.
If home directories were created before the home directory configuration was changed, then correct the permissions and SELinux contexts. For example:
[root@server ~]# semanage fcontext -a -e /home /home/locale
# restorecon -R -v /home/locale

Part II. Identity and Authentication Stores

Chapter 7. Configuring SSSD

7.1. Introduction to SSSD

7.1.1. How SSSD Works

The System Security Services Daemon (SSSD) is a system service to access remote directories and authentication mechanisms. It connects a local system (an SSSD client) to an external back-end system (a provider). This provides the SSSD client with access to identity and authentication remote services using an SSSD provider. For example, these remote services include: an LDAP directory, an Identity Management (IdM) or Active Directory (AD) domain, or a Kerberos realm.
For this purpose, SSSD:
  1. Connects the client to an identity store to retrieve authentication information.
  2. Uses the obtained authentication information to create a local cache of users and credentials on the client.
Users on the local system are then able to authenticate using the user accounts stored in the external back-end system.
SSSD does not create user accounts on the local system. Instead, it uses the identities from the external data store and lets the users access the local system.
How SSSD works

Figure 7.1. How SSSD works

SSSD can also provide caches for several system services, such as Name Service Switch (NSS) or Pluggable Authentication Modules (PAM).

7.1.2. Benefits of Using SSSD

Reduced load on identity and authentication servers
When requesting information, SSSD clients contact SSSD, which checks its cache. SSSD contacts the servers only if the information is not available in the cache.
Offline authentication
SSSD optionally keeps a cache of user identities and credentials retrieved from remote services. In this setup, users can successfully authenticate to resources even if the remote server or the SSSD client are offline.
A single user account: improved consistency of the authentication process
With SSSD, it is not necessary to maintain both a central account and a local user account for offline authentication.
Remote users often have multiple user accounts. For example, to connect to a virtual private network (VPN), remote users have one account for the local system and another account for the VPN system.
Thanks to caching and offline authentication, remote users can connect to network resources simply by authenticating to their local machine. SSSD then maintains their network credentials.

7.2. Using Multiple SSSD Configuration Files on a Per-client Basis

The default configuration file for SSSD is /etc/sssd/sssd.conf. Apart from this file, SSSD can read its configuration from all *.conf files in the /etc/sssd/conf.d/ directory.
For example, this allows you to use the default /etc/sssd/sssd.conf file on all clients and add additional settings in further configuration files to extend the functionality individually on a per-client basis.

How SSSD Processes the Configuration Files

SSSD reads the configuration files in this order:
  1. The primary /etc/sssd/sssd.conf file
  2. Other *.conf files in /etc/sssd/conf.d/, in alphabetical order
If the same parameter appears in multiple configuration files, SSSD uses the last read parameter.

Note

SSSD does not read hidden files (files starting with .) in the conf.d directory.

7.3. Configuring Identity and Authentication Providers for SSSD

7.3.1. Introduction to Identity and Authentication Providers for SSSD

SSSD Domains. Identity and Authentication Providers

Identity and authentication providers are configured as domains in the SSSD configuration file. A single domain can be used as:
  • An identity provider (for user information)
  • An authentication provider (for authentication requests)
  • An access control provider (for authorization requests)
  • A combination of these providers (if all the corresponding operations are performed within a single server)
You can configure multiple domains for SSSD. At least one domain must be configured, otherwise SSSD will not start.
The access_provider option in the /etc/sssd/sssd.conf file sets the access control provider used for the domain. By default, the option is set to permit, which always allows all access. See the sssd.conf(5) man page for details.

Proxy Providers

A proxy provider works as an intermediary relay between SSSD and resources that SSSD would otherwise not be able to use. When using a proxy provider, SSSD connects to the proxy service, and the proxy loads the specified libraries.
Using a proxy provider, you can configure SSSD to use:
  • Alternative authentication methods, such as a fingerprint scanner
  • Legacy systems, such as NIS
  • A local system account defined in /etc/passwd and remote authentication

Available Combinations of Identity and Authentication Providers

Table 7.1. Available Combinations of Identity and Authentication Providers

Identity Provider Authentication Provider
Identity Management [a] Identity Management [a]
Active Directory [a] Active Directory [a]
LDAP LDAP
LDAP Kerberos
proxy proxy
proxy LDAP
proxy Kerberos
[a] An extension of the LDAP provider type.
Note that this guide does not describe all provider types. See the following additional resources for more information:

7.3.2. Configuring an LDAP Domain for SSSD

Prerequisites

  • Install SSSD.
    # yum install sssd

Configure SSSD to Discover the LDAP Domain

  1. Open the /etc/sssd/sssd.conf file.
  2. Create a [domain] section for the LDAP domain:
    [domain/LDAP_domain_name]
  3. Specify if you want to use the LDAP server as an identity provider, an authentication provider, or both.
    1. To use the LDAP server as an identity provider, set the id_provider option to ldap.
    2. To use the LDAP server as an authentication provider, set the auth_provider option to ldap.
    For example, to use the LDAP server as both:
    [domain/LDAP_domain_name]
    id_provider = ldap
    auth_provider = ldap
  4. Specify the LDAP server. Choose one of the following:
    1. To explicitly define the server, specify the server's URI with the ldap_uri option:
      [domain/LDAP_domain_name]
      id_provider = ldap
      auth_provider = ldap
      
      ldap_uri = ldap://ldap.example.com
      The ldap_uri option also accepts the IP address of the server. However, using an IP address instead of the server name might cause TLS/SSL connections to fail. See Configuring an SSSD Provider to Use an IP Address in the Certificate Subject Name in Red Hat Knowledgebase.
    2. To configure SSSD to discover the server dynamically using DNS service discovery, see Section 7.4.3, “Configuring DNS Service Discovery”.
    Optionally, specify backup servers in the ldap_backup_uri option as well.
  5. Specify the LDAP server's search base in the ldap_search_base option:
    [domain/LDAP_domain_name]
    id_provider = ldap
    auth_provider = ldap
    
    ldap_uri = ldap://ldap.example.com
    ldap_search_base = dc=example,dc=com
  6. Specify a way to establish a secure connection to the LDAP server. The recommended method is to use a TLS connection. To do this, enable the ldap_id_use_start_tls option, and use these CA certificate-related options:
    • ldap_tls_reqcert specifies if the client requests a server certificate and what checks are performed on the certificate
    • ldap_tls_cacert specifies the file containing the certificate
    [domain/LDAP_domain_name]
    id_provider = ldap
    auth_provider = ldap
    
    ldap_uri = ldaps://ldap.example.com
    ldap_search_base = dc=example,dc=com
    
    ldap_id_use_start_tls = true
    ldap_tls_reqcert = demand
    ldap_tls_cacert = /etc/pki/tls/certs/ca-bundle.crt

    Note

    SSSD always uses an encrypted channel for authentication, which ensures that passwords are never sent over the network unencrypted. With ldap_id_use_start_tls = true, identity lookups (such as commands based on the id or getent utilities) are also encrypted.
  7. Add the new domain to the domains option in the [sssd] section. The option lists the domains that SSSD queries. For example:
    domains = LDAP_domain_name, domain2

Additional Resources

The above procedure shows the basic options for an LDAP provider. For more details, see:
  • the sssd.conf(5) man page, which describes global options available for all types of domains
  • the sssd-ldap(5) man page, which describes options specific to LDAP

7.3.3. Configuring the Files Provider for SSSD

The files provider mirrors the content of the /etc/passwd and /etc/groups files to make users and groups from these files available through SSSD. This enables you to set the sss database as the first source for users and groups in the /etc/nsswitch.conf file:
passwd:     sss files
group:      sss files
With this setting, and if the files provider is configured in /etc/sssd/sssd.conf, Red Hat Enterprise Linux sends all queries for users and groups first to SSSD. If SSSD is not running or SSSD cannot find the requested entry, the system falls back to look up users and groups in the local files. If you store most users and groups in a central database, such as an LDAP directory, this setting increases speed of users and groups lookups.

Prerequisites

  • Install SSSD.
    # yum install sssd

Configure SSSD to Discover the Files Domain

  1. Add the following section to the /etc/sssd/sssd.conf file:
    [domain/files]
    id_provider = files
  2. Optionally, set the sss database as the first source for user and group lookups in the /etc/sssd/sssd.conf file:
    passwd:     sss files
    group:      sss files
  3. Configure the system in the way that the sssd service starts when the system boots:
    # systemctl enable sssd
  4. Restart the sssd service:
    # systemctl restart sssd

Additional Resources

The above procedure shows the basic options for the files provider. For more details, see:
  • the sssd.conf(5) man page, which describes global options available for all types of domains
  • the sssd-files(5) man page, which describes options specific to the files provider

7.3.4. Configuring a Proxy Provider for SSSD

Prerequisites

  • Install SSSD.
    # yum install sssd

Configure SSSD to Discover the Proxy Domain

  1. Open the /etc/sssd/sssd.conf file.
  2. Create a [domain] section for the proxy provider:
    [domain/proxy_name]
  3. To specify an authentication provider:
    1. Set the auth_provider option to proxy.
    2. Use the proxy_pam_target option to specify a PAM service as the authentication proxy.
    For example:
    [domain/proxy_name]
    auth_provider = proxy
    proxy_pam_target = sssdpamproxy

    Important

    Ensure that the proxy PAM stack does not recursively include pam_sss.so.
  4. To specify an identity provider:
    1. Set the id_provider option to proxy.
    2. Use the proxy_lib_name option to specify an NSS library as the identity proxy.
    For example:
    [domain/proxy_name]
    id_provider = proxy
    proxy_lib_name = nis
  5. Add the new domain to the domains option in the [sssd] section. The option lists the domains that SSSD queries. For example:
    domains = proxy_name, domain2

Additional Resources

The above procedure shows the basic options for a proxy provider. For more details, see the sssd.conf(5) man page, which describes global options available for all types of domains and other proxy-related options.

7.3.5. Configuring a Kerberos Authentication Provider

Prerequisites

  • Install SSSD.
    # yum install sssd

Configure SSSD to Discover the Kerberos Domain

  1. Open the /etc/sssd/sssd.conf file.
  2. Create a [domain] section for the SSSD domain.
    [domain/Kerberos_domain_name]
  3. Specify an identity provider. For example, for details on configuring an LDAP identity provider, see Section 7.3.2, “Configuring an LDAP Domain for SSSD”.
    If the Kerberos principal names are not available in the specified identity provider, SSSD constructs the principals using the format username@REALM.
  4. Specify the Kerberos authentication provider details:
    1. Set the auth_provider option to krb5.
      [domain/Kerberos_domain_name]
      id_provider = ldap
      auth_provider = krb5
    2. Specify the Kerberos server:
      1. To explicitly define the server, use the krb5_server option. The options accepts the host name or IP address of the server:
        [domain/Kerberos_domain_name]
        id_provider = ldap
        auth_provider = krb5
        
        krb5_server = kdc.example.com
      2. To configure SSSD to discover the server dynamically using DNS service discovery, see Section 7.4.3, “Configuring DNS Service Discovery”.
      Optionally, specify backup servers in the krb5_backup_server option as well.
    3. If the Change Password service is not running on the KDC specified in krb5_server or krb5_backup_server, use the krb5_passwd option to specify the server where the service is running.
      [domain/Kerberos_domain_name]
      id_provider = ldap
      auth_provider = krb5
      
      krb5_server = kdc.example.com
      krb5_backup_server = kerberos.example.com
      krb5_passwd = kerberos.admin.example.com
      If krb5_passwd is not used, SSSD uses the KDC specified in krb5_server or krb5_backup_server.
    4. Use the krb5_realm option to specify the name of the Kerberos realm.
      [domain/Kerberos_domain_name]
      id_provider = ldap
      auth_provider = krb5
      
      krb5_server = kerberos.example.com
      krb5_backup_server = kerberos2.example.com
      krb5_passwd = kerberos.admin.example.com
      krb5_realm = EXAMPLE.COM
  5. Add the new domain to the domains option in the [sssd] section. The option lists the domains that SSSD queries. For example:
    domains = Kerberos_domain_name, domain2

Additional Resources

The above procedure shows the basic options for a Kerberos provider. For more details, see:
  • the sssd.conf(5) man page, which describes global options available for all types of domains
  • the sssd-krb5(5) man page, which describes options specific to Kerberos

7.4. Additional Configuration for Identity and Authentication Providers

7.4.1. Adjusting User Name Formats

7.4.1.1. Defining the Regular Expression for Parsing Full User Names

SSSD parses full user name strings into the user name and domain components. By default, SSSD interprets full user names in the format user_name@domain_name based on the following regular expression in Python syntax:
(?P<name>[^@]+)@?(?P<domain>[^@]*$)

Note

For Identity Management and Active Directory providers, the default user name format is user_name@domain_name or NetBIOS_name\user_name.
To adjust how SSSD interprets full user names:
  1. Open the /etc/sssd/sssd.conf file.
  2. Use the re_expression option to define a custom regular expression.
    1. To define the regular expressions globally for all domains, add re_expression to the [sssd] section of sssd.conf.
    2. To define the regular expressions individually for a particular domain, add re_expression to the corresponding domain section of sssd.conf.
For example, to configure a regular expression for the LDAP domain:
[domain/LDAP]
[... file truncated ...]
re_expression = (?P<domain>[^\\]*?)\\?(?P<name>[^\\]+$)
For details, see the descriptions for re_expression in the SPECIAL SECTIONS and DOMAIN SECTIONS parts of the sssd.conf(5) man page.

7.4.1.2. Defining How SSSD Prints Full User Names

If the use_fully_qualified_names option is enabled in the /etc/sssd/sssd.conf file, SSSD prints full user names in the format name@domain based on the following expansion by default:
%1$s@%2$s

Note

If use_fully_qualified_names is not set or is explicitly set to false for trusted domains, only the user name is printed, without the domain component.
To adjust the format in which SSSD prints full user names:
  1. Open the /etc/sssd/sssd.conf file.
  2. Use the full_name_format option to define the expansion for the full user name format:
    1. To define the expansion globally for all domains, add full_name_format to the [sssd] section of sssd.conf.
    2. To define the expansion individually for a particular domain, add full_name_format to the corresponding domain section of sssd.conf.
For details, see the descriptions for full_name_format in the SPECIAL SECTIONS and DOMAIN SECTIONS parts of the sssd.conf(5) man page.
In some name configurations, SSSD could strip the domain component of the name, which can cause authentication errors. Because of this, if you set full_name_format to a non-standard value, a warning will prompt you to change it to a more standard format.

7.4.2. Enabling Offline Authentication

SSSD does not cache user credentials by default. When processing authentication requests, SSSD always contacts the identity provider. If the provider is unavailable, user authentication fails.

Important

SSSD never caches passwords in plain text. It stores only a hash of the password.
To ensure that users can authenticate even when the identity provider is unavailable, enable credential caching:
  1. Open the /etc/sssd/sssd.conf file.
  2. In a domain section, add the cache_credentials = true setting:
    [domain/domain_name]
    cache_credentials = true
  3. Optional, but recommended. Configure a time limit for how long SSSD allows offline authentication if the identity provider is unavailable.
    1. Configure the PAM service to work with SSSD. See Section 7.5.2, “Configuring Services: PAM”.
    2. Use the offline_credentials_expiration option to specify the time limit. For example, to specify that users are able to authenticate offline for 3 days since the last successful login:
      [pam]
      offline_credentials_expiration = 3
For details on offline_credentials_expiration, see the sssd.conf(5) man page.

7.4.3. Configuring DNS Service Discovery

If the identity or authentication server is not explicitly defined in the /etc/sssd/sssd.conf file, SSSD can discover the server dynamically using DNS service discovery [1].
For example, if sssd.conf includes the id_provider = ldap setting, but the ldap_uri option does not specify any host name or IP address, SSSD uses DNS service discovery to discover the server dynamically.

Note

SSSD cannot dynamically discover backup servers, only the primary server.

Configuring SSSD for DNS Service Discovery

  1. Open the /etc/sssd/sssd.conf file.
  2. Set the primary server value to _srv_. For an LDAP provider, the primary server is set using the ldap_uri option:
    [domain/domain_name]
    id_provider = ldap
    ldap_uri = _srv_
  3. Enable service discovery in the password change provider by setting a service type:
    [domain/domain_name]
    id_provider = ldap
    ldap_uri = _srv_
    
    chpass_provider = ldap
    ldap_chpass_dns_service_name = ldap
  4. Optional. By default, the service discovery uses the domain portion of the system host name as the domain name. To use a different DNS domain, specify the domain name in the dns_discovery_domain option.
  5. Optional. By default, the service discovery scans for the LDAP service type. To use a different service type, specify the type in the ldap_dns_service_name option.
  6. Optional. By default, SSSD attempts to look up an IPv4 address. If the attempt fails, SSSD attempts to look up an IPv6 address. To customize this behavior, use the lookup_family_order option. See the sssd.conf(5) man page for details.
  7. For every service with which you want to use service discovery, add a DNS record to the DNS server:
    _service._protocol._domain TTL priority weight port host_name

7.4.4. Defining Access Control Using the simple Access Provider

The simple access provider allows or denies access based on a list of user names or groups. It enables you to restrict access to specific machines.
For example, on company laptops, you can use the simple access provider to restrict access to only a specific user or a specific group. Other users or groups will not be allowed to log in even if they authenticate successfully against the configured authentication provider.

Configuring simple Access Provider Rules

  1. Open the /etc/sssd/sssd.conf file.
  2. Set the access_provider option to simple:
    [domain/domain_name]
    access_provider = simple
  3. Define the access control rules for users. Choose one of the following:
    1. To allow access to users, use the simple_allow_users option.
    2. To deny access to users, use the simple_deny_users option.

      Important

      Allowing access to specific users is considered safer than denying. If you deny access to specific users, you automatically allow access to everyone else.
  4. Define the access control rules for groups. Choose one of the following:
    1. To allow access to groups, use the simple_allow_groups option.
    2. To deny access to groups, use the simple_deny_groups option.

      Important

      Allowing access to specific groups is considered safer than denying. If you deny access to specific groups, you automatically allow access to everyone else.
The following example allows access to user1, user2, and members of group1, while denying access to all other users.
[domain/domain_name]
access_provider = simple
simple_allow_users = user1, user2
simple_allow_groups = group1
For details, see the sssd-simple(5) man page.

7.4.5. Defining Access Control Using the LDAP Access Filter

When the access_provider option is set in /etc/sssd/sssd.conf, SSSD uses the specified access provider to evaluate which users are granted access to the system. If the access provider you are using is an extension of the LDAP provider type, you can also specify an LDAP access control filter that a user must match in order to be allowed access to the system.
For example, when using an Active Directory (AD) server as the access provider, you can restrict access to the Linux system only to specified AD users. All other users that do not match the specified filter will be denied access.

Note

The access filter is applied on the LDAP user entry only. Therefore, using this type of access control on nested groups might not work. To apply access control on nested groups, see Section 7.4.4, “Defining Access Control Using the simple Access Provider”.

Important

When using offline caching, SSSD checks if the user's most recent online login attempt was successful. Users who logged in successfully during the most recent online login will still be able to log in offline, even if they do not match the access filter.

Configuring SSSD to Apply an LDAP Access Filter

  1. Open the /etc/sssd/sssd.conf file.
  2. In the [domain] section, specify the LDAP access control filter.
    • For an LDAP access provider, use the ldap_access_filter option. See the sssd-ldap(5) man page for details.
    • For an AD access provider, use the ad_access_filter option. See the sssd-ad(5) man page for details.
    For example, to allow access only to AD users who belong to the admins user group and have a unixHomeDirectory attribute set:
    [domain/AD_domain_name]
    access provider = ad
    [... file truncated ...]
    ad_access_filter = (&(memberOf=cn=admins,ou=groups,dc=example,dc=com)(unixHomeDirectory=*))
SSSD can also check results by the authorizedService or host attribute in an entry. In fact, all options — LDAP filter, authorizedService, and host — can be evaluated, depending on the user entry and the configuration. The ldap_access_order parameter lists all access control methods to use, in order of how they should be evaluated.
[domain/example.com]
access_provider = ldap
ldap_access_filter = memberOf=cn=allowedusers,ou=Groups,dc=example,dc=com
ldap_access_order = filter, host, authorized_service
The attributes in the user entry to use to evaluate authorized services or allowed hosts can be customized. Additional access control parameters are listed in the sssd-ldap(5) man page.

7.5. Configuring System Services for SSSD

SSSD provides interfaces towards several system services. Most notably:
Name Service Switch (NSS)
Pluggable Authentication Modules (PAM)
OpenSSH
See Configuring SSSD to Provide a Cache for the OpenSSH Services in the Linux Domain Identity, Authentication, and Policy Guide.
autofs
sudo

7.5.1. Configuring Services: NSS

How SSSD Works with NSS

The Name Service Switch (NSS) service maps system identities and services with configuration sources: it provides a central configuration store where services can look up sources for various configuration and name resolution mechanisms.
SSSD can use NSS as a provider for several types of NSS maps. Most notably:
  • User information (the passwd map)
  • Groups (the groups map)
  • Netgroups (the netgroups map)
  • Services (the services map)

Prerequisites

  • Install SSSD.
    # yum install sssd

Configure NSS Services to Use SSSD

  1. Use the authconfig utility to enable SSSD:
    [root@server ~]# authconfig --enablesssd --update
    This updates the /etc/nsswitch.conf file to enable the following NSS maps to use SSSD:
    passwd:     files sss
    shadow:     files sss
    group:      files sss
    
    netgroup:   files sss
  2. Open /etc/nsswitch.conf and add sss to the services map line:
    services: file sss

Configure SSSD to Work with NSS

  1. Open the /etc/sssd/sssd.conf file.
  2. In the [sssd] section, make sure that NSS is listed as one of the services that works with SSSD.
    [sssd]
    [... file truncated ...]
    services = nss, pam
  3. In the [nss] section, configure how SSSD interacts with NSS. For example:
    [nss]
    filter_groups = root
    filter_users = root
    entry_cache_timeout = 300
    entry_cache_nowait_percentage = 75
    For a complete list of available options, see NSS configuration options in the sssd.conf(5) man page.
  4. Restart SSSD.
    # systemctl restart sssd.service

Test That the Integration Works Correctly

Display information about a user with these commands:
  • id user
  • getent passwd user

7.5.2. Configuring Services: PAM

Warning

A mistake in the PAM configuration file can lock users out of the system completely. Always back up the configuration files before performing any changes, and keep a session open so that you can revert any changes.

Configure PAM to Use SSSD

  • Use the authconfig utility to enable SSSD:
    # authconfig --enablesssdauth --update
    This updates the PAM configuration to reference the SSSD modules, usually in the /etc/pam.d/system-auth and /etc/pam.d/password-auth files. For example:
    [... file truncated ...]
    auth		required	pam_env.so
    auth		sufficient	pam_unix.so nullok try_first_pass
    auth		requisite	pam_succeed_if.so uid >= 500 quiet
    auth        	sufficient	pam_sss.so use_first_pass
    auth		required	pam_deny.so
    [... file truncated ...]
For details, see the pam.conf(5) or pam(8) man pages.

Configure SSSD to Work with PAM

  1. Open the /etc/sssd/sssd.conf file.
  2. In the [sssd] section, make sure that NSS is listed as one of the services that works with SSSD.
    [sssd]
    [... file truncated ...]
    services = nss, pam
  3. In the [pam] section, configure how SSSD interacts with PAM. For example:
    [pam]
    offline_credentials_expiration = 2
    offline_failed_login_attempts = 3
    offline_failed_login_delay = 5
    For a complete list of available options, see PAM configuration options in the sssd.conf(5) man page.
  4. Restart SSSD.
    # systemctl restart sssd.service

Test That the Integration Works Correctly

  • Try logging in as a user.
  • Use the sssctl user-checks user_name auth command to check your SSSD configuration. For details, use the sssctl user-checks --help command.

7.5.3. Configuring Services: autofs

How SSSD Works with automount

The automount utility can mount and unmount NFS file systems automatically (on-demand mounting), which saves system resources. For details on automount, see autofs in the Storage Administration Guide.
You can configure automount to point to SSSD. In this setup:
  1. When a user attempts to mount a directory, SSSD contacts LDAP to obtain the required information about the current automount configuration.
  2. SSSD stores the information required by automount in a cache, so that users can mount directories even when the LDAP server is offline.

Configure autofs to Use SSSD

  1. Install the autofs package.
    # yum install autofs
  2. Open the /etc/nsswitch.conf file.
  3. On the automount line, change the location where to look for the automount map information from ldap to sss:
    automount: files sss

Configure SSSD to Work with autofs

  1. Open the /etc/sssd/sssd.conf file.
  2. In the [sssd] section, add autofs to the list of services that SSSD manages.
    [sssd]
    services = nss,pam,autofs
  3. Create a new [autofs] section. You can leave it empty.
    [autofs]
    For a list of available options, see AUTOFS configuration options in the sssd.conf(5) man page.
  4. Make sure an LDAP domain is available in sssd.conf, so that SSSD can read the automount information from LDAP. See Section 7.3.2, “Configuring an LDAP Domain for SSSD”.
    The [domain] section of sssd.conf accepts several autofs-related options. For example:
    [domain/LDAP]
    [... file truncated ...]
    autofs_provider=ldap
    ldap_autofs_search_base=cn=automount,dc=example,dc=com
    ldap_autofs_map_object_class=automountMap
    ldap_autofs_entry_object_class=automount
    ldap_autofs_map_name=automountMapName
    ldap_autofs_entry_key=automountKey
    ldap_autofs_entry_value=automountInformation
    For a complete list of available options, see DOMAIN SECTIONS in the sssd.conf(5) man page.
    If you do not provide additional autofs options, the configuration depends on the identity provider settings.
  5. Restart SSSD.
    # systemctl restart sssd.service

Test the Configuration

  • Use the automount -m command to print the maps from SSSD.

7.5.4. Configuring Services: sudo

How SSSD Works with sudo

The sudo utility gives administrative access to specified users. For more information about sudo, see The sudo Command in the System Administrator's Guide.
You can configure sudo to point to SSSD. In this setup:
  1. When a user attempts a sudo operation, SSSD contacts LDAP to obtain the required information about the current sudo configuration.
  2. SSSD stores the sudo information in a cache, so that users can perform sudo operations even when the LDAP server is offline.
SSSD only caches sudo rules which apply to the local system, depending on the value of the sudoHost attribute. See the sssd-sudo(5) man page for details.

Configure sudo to Use SSSD

  1. Open the /etc/nsswitch.conf file.
  2. Add SSSD to the list on the sudoers line.
    sudoers: files sss

Configure SSSD to Work with sudo

  1. Open the /etc/sssd/sssd.conf file.
  2. In the [sssd] section, add sudo to the list of services that SSSD manages.
    [sssd]
    services = nss,pam,sudo
  3. Create a new [sudo] section. You can leave it empty.
    [sudo]
    For a list of available options, see SUDO configuration options in the sssd.conf(5) man page.
  4. Make sure an LDAP domain is available in sssd.conf, so that SSSD can read the sudo information from LDAP. See Section 7.3.2, “Configuring an LDAP Domain for SSSD”.
    The [domain] section for the LDAP domain must include these sudo-related parameters:
    [domain/LDAP]
    [... file truncated ...]
    sudo_provider = ldap
    ldap_sudo_search_base = ou=sudoers,dc=example,dc=com

    Note

    Setting Identity Management as the ID provider automatically enables the sudo provider. In this situation, it is not necessary to specify sudo_provider = ipa.
    For a complete list of available options, see DOMAIN SECTIONS in the sssd.conf(5) man page.
    For options available for a sudo provider, see the sssd-ldap(5) man page.
  5. Restart SSSD.
    # systemctl restart sssd.service

7.6. SSSD Client-side Views

SSSD enables you to create a client-side view to specify new values for POSIX user or group attributes. The view takes effect only on the local machine where the overrides are configured. You can configure client-side overrides for all id_provider values, except ipa. If you are using the ipa provider, define ID views centrally in IdM. See the corresponding section in the Linux Domain Identity, Authentication, and Policy Guide.
For information about a potential negative impact on the SSSD performance, see the corresponding section in the Linux Domain Identity, Authentication, and Policy Guide.

Note

After creating the first override using the sss_override user-add, sss_override group-add, or sss_override user-import command, restart SSSD for the changes to take effect:
# systemctl restart sssd

7.6.1. Defining a Different Attribute Value for a User Account

As an administrator, you configured an existing host to use accounts from LDAP. However, a user's new ID in LDAP is different from the user's previous ID on the local system. You can configure a client-side view to override the UID instead of changing the permissions on existing files.
To override the UID of the user account with UID 6666:
  1. Optional. Display the current UID of the user account:
    # id user
    uid=1241400014(user_name) gid=1241400014(user_name) Groups=1241400014(user_name)
  2. Override the account's UID with 6666:
    # sss_override user-add user -u 6666
  3. Wait until the in-memory cache has been expired. To expire it manually:
    # sss_cache --users
  4. Verify that the new UID is applied:
    # id user
    uid=6666(user_name) gid=1241400014(user_name) Groups=1241400014(user_name)
  5. Optional. Display the overrides for the user:
    # sss_override user-show user
    user@ldap.example.com::6666:::::
For a list of attributes you can override, list the command-line options by adding --help to the command:
# sss_override user-add --help

7.6.2. Listing All Overrides on a Host

As an administrator, you want to list all user and group overrides on a host to verify that the correct attributes are overridden.
To list all user overrides:
# sss_override user-find
user1@ldap.example.com::8000::::/bin/zsh:
user2@ldap.example.com::8001::::/bin/bash:
...
To list all group overrides:
# sss_override group-find
group1@ldap.example.com::7000
group2@ldap.example.com::7001
...

7.6.3. Removing a Local Override

You previously created an override for the shell of the user account, that is defined in the global LDAP directory. To remove the override for the account, run:
# sss_override user-del user
The changes take effect immediately.
To remove an override for a group, run:
# sss_override group-del group

Note

When you remove overrides for a user or group, all overrides for this object are removed.

7.6.4. Exporting and Importing Local Views

Client-side views are stored in the local SSSD cache. You can export user and group views from the cache to a file to create a backup. For example, when you remove the SSSD cache, you can restore the views later again.
To back up user and group views:
# sss_override user-export /var/lib/sss/backup/sssd_user_overrides.bak
# sss_override group-export /var/lib/sss/backup/sssd_group_overrides.bak
To restore user and group view:
# sss_override user-import /var/lib/sss/backup/sssd_user_overrides.bak
# sss_override group-import /var/lib/sss/backup/sssd_group_overrides.bak

7.7. Downgrading SSSD

When downgrading — either downgrading the version of SSSD or downgrading the operating system itself — then the existing SSSD cache needs to be removed. If the cache is not removed, then SSSD process is dead but a PID file remains. The SSSD logs show that it cannot connect to any of its associated domains because the cache version is unrecognized.
(Wed Nov 28 21:25:50 2012) [sssd] [sysdb_domain_init_internal] (0x0010): Unknown DB version [0.14], expected [0.10] for domain AD!
Users are then no longer recognized and are unable to authenticate to domain services and hosts.
After downgrading the SSSD version:
  1. Delete the existing cache database files.
    [root@server ~]# rm -rf /var/lib/sss/db/*
  2. Restart the SSSD process.
    [root@server ~]# systemctl restart sssd.service

7.8. Using NSCD with SSSD

SSSD is not designed to be used with the NSCD daemon. Even though SSSD does not directly conflict with NSCD, using both services can result in unexpected behavior, especially with how long entries are cached.
The most common evidence of a problem is conflicts with NFS. When using Network Manager to manage network connections, it may take several minutes for the network interface to come up. During this time, various services attempt to start. If these services start before the network is up and the DNS servers are available, these services fail to identify the forward or reverse DNS entries they need. These services will read an incorrect or possibly empty resolv.conf file. This file is typically only read once, and so any changes made to this file are not automatically applied. This can cause NFS locking to fail on the machine where the NSCD service is running, unless that service is manually restarted.
To avoid this problem, enable caching for hosts and services in the /etc/nscd.conf file and rely on the SSSD cache for the passwd, group, services, and netgroup entries.
Change the /etc/nscd.conf file:
enable-cache hosts yes
enable-cache passwd no
enable-cache group no
enable-cache netgroup no
enable-cache services no
With NSCD answering hosts requests, these entries will be cached by NSCD and returned by NSCD during the boot process. All other entries are handled by SSSD.

7.9. Additional Resources

  • A complete list of SSSD-related man pages is available in the SEE ALSO section in the sssd(8) man page.
  • Troubleshooting advice: Section A.1, “Troubleshooting SSSD”.
  • A procedure for configuring SSSD to process password expiration warnings sent by the server and display them to users on the local system: Setting Password Expiry in Red Hat Knowledgebase
  • An SSSD client can automatically create a GID for every user retrieved from an LDAP server, and at the same time ensure that the GID matches the user's UID unless the GID number is already taken. To see how automatic creation of GIDs can be set up on an SSSD client which is directly integrated into Active Directory, see the corresponding section in the Windows Integration Guide.


[1] DNS service discovery enables applications to check the SRV records in a given domain for certain services of a certain type, and then returns any servers that match the required type. DNS service discovery is defined in RFC 2782.

Chapter 8. Using realmd to Connect to an Identity Domain

The realmd system provides a clear and simple way to discover and join identity domains. It does not connect to the domain itself but configures underlying Linux system services, such as SSSD or Winbind, to connect to the domain.
The Windows Integration Guide describes using realmd to connect to a Microsoft Active Directory (AD) domain. The same procedures apply to using realmd to connect to non-AD identity domains. See the corresponding chapter in the Windows Integration Guide.

Chapter 9. LDAP Servers

LDAP (Lightweight Directory Access Protocol) is a set of open protocols used to access centrally stored information over a network. It is based on the X.500 standard for directory sharing, but is less complex and resource-intensive. For this reason, LDAP is sometimes referred to as X.500 Lite.
Like X.500, LDAP organizes information in a hierarchical manner using directories. These directories can store a variety of information such as names, addresses, or phone numbers, and can even be used in a manner similar to the Network Information Service (NIS), enabling anyone to access their account from any machine on the LDAP enabled network.
LDAP is commonly used for centrally managed users and groups, user authentication, or system configuration. It can also serve as a virtual phone directory, allowing users to easily access contact information for other users. Additionally, it can refer a user to other LDAP servers throughout the world, and thus provide an ad-hoc global repository of information. However, it is most frequently used within individual organizations such as universities, government departments, and private companies.

9.1. Red Hat Directory Server

Red Hat Directory Server is an LDAP-compliant server that centralizes user identity and application information. It provides an operating system-independent and network-based registry for storing application settings, user profiles, group data, policies, and access control information.

Note

You require a current Red Hat Directory Server subscription to install and update Directory Server.
For further details about setting up and using Directory Server, see:

9.2. OpenLDAP

This section covers the installation and configuration of OpenLDAP 2.4, an open source implementation of the LDAPv2 and LDAPv3 protocols.

Note

Starting with Red Hat Enterprise Linux 7.4, the openldap-server package has been deprecated and will not be included in a future major release of Red Hat Enterprise Linux. For this reason, migrate to Identity Management included in Red Hat Enterprise Linux or to Red Hat Directory Server. For further details about Identity Management, see Linux Domain Identity, Authentication, and Policy Guide. For further details about Directory Server, see Section 9.1, “Red Hat Directory Server”.

9.2.1. Introduction to LDAP

Using a client-server architecture, LDAP provides a reliable means to create a central information directory accessible from the network. When a client attempts to modify information within this directory, the server verifies the user has permission to make the change, and then adds or updates the entry as requested. To ensure the communication is secure, the Transport Layer Security (TLS) cryptographic protocol can be used to prevent an attacker from intercepting the transmission.

Important

The OpenLDAP suite in Red Hat Enterprise Linux 7.5 and later no longer uses Mozilla implementation of Network Security Services (NSS). Instead, it uses the OpenSSL. OpenLDAP continues to work with existing NSS database configuration.

Important

Due to the vulnerability described in Resolution for POODLE SSLv3.0 vulnerability (CVE-2014-3566) for components that do not allow SSLv3 to be disabled via configuration settings, Red Hat recommends that you do not rely on the SSLv3 protocol for security. OpenLDAP is one of the system components that do not provide configuration parameters that allow SSLv3 to be effectively disabled. To mitigate the risk, it is recommended that you use the stunnel command to provide a secure tunnel, and disable stunnel from using SSLv3. For more information on using stunnel, see the Red Hat Enterprise Linux 7 Security Guide.
The LDAP server supports several database systems, which gives administrators the flexibility to choose the best suited solution for the type of information they are planning to serve. Because of a well-defined client Application Programming Interface (API), the number of applications able to communicate with an LDAP server is numerous, and increasing in both quantity and quality.

9.2.1.1. LDAP Terminology

The following is a list of LDAP-specific terms that are used within this chapter:
entry
A single unit within an LDAP directory. Each entry is identified by its unique Distinguished Name (DN).
attribute
Information directly associated with an entry. For example, if an organization is represented as an LDAP entry, attributes associated with this organization might include an address, a fax number, and so on. Similarly, people can be represented as entries with common attributes such as personal telephone number or email address.
An attribute can either have a single value, or an unordered space-separated list of values. While certain attributes are optional, others are required. Required attributes are specified using the objectClass definition, and can be found in schema files located in the /etc/openldap/slapd.d/cn=config/cn=schema/ directory.
The assertion of an attribute and its corresponding value is also referred to as a Relative Distinguished Name (RDN). Unlike distinguished names that are unique globally, a relative distinguished name is only unique per entry.
LDIF
The LDAP Data Interchange Format (LDIF) is a plain text representation of an LDAP entry. It takes the following form:
[id] dn: distinguished_name
attribute_type: attribute_valueattribute_type: attribute_value…
…
The optional id is a number determined by the application that is used to edit the entry. Each entry can contain as many attribute_type and attribute_value pairs as needed, as long as they are all defined in a corresponding schema file. A blank line indicates the end of an entry.

9.2.1.2. OpenLDAP Features

OpenLDAP suite provides a number of important features:
  • LDAPv3 Support — Many of the changes in the protocol since LDAP version 2 are designed to make LDAP more secure. Among other improvements, this includes the support for Simple Authentication and Security Layer (SASL), Transport Layer Security (TLS), and Secure Sockets Layer (SSL) protocols.
  • LDAP Over IPC — The use of inter-process communication (IPC) enhances security by eliminating the need to communicate over a network.
  • IPv6 Support — OpenLDAP is compliant with Internet Protocol version 6 (IPv6), the next generation of the Internet Protocol.
  • LDIFv1 Support — OpenLDAP is fully compliant with LDIF version 1.
  • Updated C API — The current C API improves the way programmers can connect to and use LDAP directory servers.
  • Enhanced Standalone LDAP Server — This includes an updated access control system, thread pooling, better tools, and much more.

9.2.1.3. OpenLDAP Server Setup

The typical steps to set up an LDAP server on Red Hat Enterprise Linux are as follows:
  1. Install the OpenLDAP suite. See Section 9.2.2, “Installing the OpenLDAP Suite” for more information on required packages.
  2. Customize the configuration as described in Section 9.2.3, “Configuring an OpenLDAP Server”.
  3. Start the slapd service as described in Section 9.2.5, “Running an OpenLDAP Server”.
  4. Use the ldapadd utility to add entries to the LDAP directory.
  5. Use the ldapsearch utility to verify that the slapd service is accessing the information correctly.

9.2.2. Installing the OpenLDAP Suite

The suite of OpenLDAP libraries and tools is provided by the following packages:

Table 9.1. List of OpenLDAP packages

Package Description
openldap A package containing the libraries necessary to run the OpenLDAP server and client applications.
openldap-clients A package containing the command line utilities for viewing and modifying directories on an LDAP server.
openldap-servers A package containing both the services and utilities to configure and run an LDAP server. This includes the Standalone LDAP Daemon, slapd.
compat-openldap A package containing the OpenLDAP compatibility libraries.
Additionally, the following packages are commonly used along with the LDAP server:

Table 9.2. List of commonly installed additional LDAP packages

Package Description
nss-pam-ldapd A package containing nslcd, a local LDAP name service that allows a user to perform local LDAP queries.
mod_ldap
A package containing the mod_authnz_ldap and mod_ldap modules. The mod_authnz_ldap module is the LDAP authorization module for the Apache HTTP Server. This module can authenticate users' credentials against an LDAP directory, and can enforce access control based on the user name, full DN, group membership, an arbitrary attribute, or a complete filter string. The mod_ldap module contained in the same package provides a configurable shared memory cache, to avoid repeated directory access across many HTTP requests, and also support for SSL/TLS. Note that this package is provided by the Optional channel. See Adding the Optional and Supplementary Repositories in the System Administrator's Guide for more information on Red Hat additional channels.
To install these packages, use the yum command in the following form:
yum install package
For example, to perform the basic LDAP server installation, type the following at a shell prompt:
~]# yum install openldap openldap-clients openldap-servers
Note that you must have superuser privileges (that is, you must be logged in as root) to run this command. For more information on how to install new packages in Red Hat Enterprise Linux, see Installing Packages in the System Administrator's Guide.

9.2.2.1. Overview of OpenLDAP Server Utilities

To perform administrative tasks, the openldap-servers package installs the following utilities along with the slapd service:

Table 9.3. List of OpenLDAP server utilities

Command Description
slapacl Allows you to check the access to a list of attributes.
slapadd Allows you to add entries from an LDIF file to an LDAP directory.
slapauth Allows you to check a list of IDs for authentication and authorization permissions.
slapcat Allows you to pull entries from an LDAP directory in the default format and save them in an LDIF file.
slapdn Allows you to check a list of Distinguished Names (DNs) based on available schema syntax.
slapindex Allows you to re-index the slapd directory based on the current content. Run this utility whenever you change indexing options in the configuration file.
slappasswd Allows you to create an encrypted user password to be used with the ldapmodify utility, or in the slapd configuration file.
slapschema Allows you to check the compliance of a database with the corresponding schema.
slaptest Allows you to check the LDAP server configuration.
For a detailed description of these utilities and their usage, see the corresponding manual pages as referred to in the section called “Installed Documentation”.

Important

Although only root can run slapadd, the slapd service runs as the ldap user. Because of this, the directory server is unable to modify any files created by slapadd. To correct this issue, after running the slapdadd utility, type the following at a shell prompt:
~]# chown -R ldap:ldap /var/lib/ldap

Warning

To preserve the data integrity, stop the slapd service before using slapadd, slapcat, or slapindex. You can do so by typing the following at a shell prompt:
~]# systemctl stop slapd.service
For more information on how to start, stop, restart, and check the current status of the slapd service, see Section 9.2.5, “Running an OpenLDAP Server”.

9.2.2.2. Overview of OpenLDAP Client Utilities

The openldap-clients package installs the following utilities which can be used to add, modify, and delete entries in an LDAP directory:

Table 9.4. List of OpenLDAP client utilities

Command Description
ldapadd Allows you to add entries to an LDAP directory, either from a file, or from standard input. It is a symbolic link to ldapmodify -a.
ldapcompare Allows you to compare given attribute with an LDAP directory entry.
ldapdelete Allows you to delete entries from an LDAP directory.
ldapexop Allows you to perform extended LDAP operations.
ldapmodify Allows you to modify entries in an LDAP directory, either from a file, or from standard input.
ldapmodrdn Allows you to modify the RDN value of an LDAP directory entry.
ldappasswd Allows you to set or change the password for an LDAP user.
ldapsearch Allows you to search LDAP directory entries.
ldapurl Allows you to compose or decompose LDAP URLs.
ldapwhoami Allows you to perform a whoami operation on an LDAP server.
With the exception of ldapsearch, each of these utilities is more easily used by referencing a file containing the changes to be made rather than typing a command for each entry to be changed within an LDAP directory. The format of such a file is outlined in the man page for each utility.

9.2.2.3. Overview of Common LDAP Client Applications

Although there are various graphical LDAP clients capable of creating and modifying directories on the server, none of them is included in Red Hat Enterprise Linux. Popular applications that can access directories in a read-only mode include Mozilla Thunderbird, Evolution, or Ekiga.

9.2.3. Configuring an OpenLDAP Server

By default, the OpenLDAP configuration is stored in the /etc/openldap/ directory. The following table highlights the most important directories and files within this directory:

Table 9.5. List of OpenLDAP configuration files and directories

Path Description
/etc/openldap/ldap.conf The configuration file for client applications that use the OpenLDAP libraries. This includes ldapadd, ldapsearch, Evolution, and so on.
/etc/openldap/slapd.d/ The directory containing the slapd configuration.
Note that OpenLDAP no longer reads its configuration from the /etc/openldap/slapd.conf file. Instead, it uses a configuration database located in the /etc/openldap/slapd.d/ directory. If you have an existing slapd.conf file from a previous installation, you can convert it to the new format by running the following command:
~]# slaptest -f /etc/openldap/slapd.conf -F /etc/openldap/slapd.d/
The slapd configuration consists of LDIF entries organized in a hierarchical directory structure, and the recommended way to edit these entries is to use the server utilities described in Section 9.2.2.1, “Overview of OpenLDAP Server Utilities”.

Important

An error in an LDIF file can render the slapd service unable to start. Because of this, it is strongly advised that you avoid editing the LDIF files within the /etc/openldap/slapd.d/ directly.

9.2.3.1. Changing the Global Configuration

Global configuration options for the LDAP server are stored in the /etc/openldap/slapd.d/cn=config.ldif file. The following directives are commonly used:
olcAllows
The olcAllows directive allows you to specify which features to enable. It takes the following form:
olcAllows: feature
It accepts a space-separated list of features as described in Table 9.6, “Available olcAllows options”. The default option is bind_v2.

Table 9.6. Available olcAllows options

Option Description
bind_v2 Enables the acceptance of LDAP version 2 bind requests.
bind_anon_cred Enables an anonymous bind when the Distinguished Name (DN) is empty.
bind_anon_dn Enables an anonymous bind when the Distinguished Name (DN) is not empty.
update_anon Enables processing of anonymous update operations.
proxy_authz_anon Enables processing of anonymous proxy authorization control.

Example 9.1. Using the olcAllows directive

olcAllows: bind_v2 update_anon
olcConnMaxPending
The olcConnMaxPending directive allows you to specify the maximum number of pending requests for an anonymous session. It takes the following form:
olcConnMaxPending: number
The default option is 100.

Example 9.2. Using the olcConnMaxPending directive

olcConnMaxPending: 100
olcConnMaxPendingAuth
The olcConnMaxPendingAuth directive allows you to specify the maximum number of pending requests for an authenticated session. It takes the following form:
olcConnMaxPendingAuth: number
The default option is 1000.

Example 9.3. Using the olcConnMaxPendingAuth directive

olcConnMaxPendingAuth: 1000
olcDisallows
The olcDisallows directive allows you to specify which features to disable. It takes the following form:
olcDisallows: feature
It accepts a space-separated list of features as described in Table 9.7, “Available olcDisallows options”. No features are disabled by default.

Table 9.7. Available olcDisallows options

Option Description
bind_anon Disables the acceptance of anonymous bind requests.
bind_simple Disables the simple bind authentication mechanism.
tls_2_anon Disables the enforcing of an anonymous session when the STARTTLS command is received.
tls_authc Disallows the STARTTLS command when authenticated.

Example 9.4. Using the olcDisallows directive

olcDisallows: bind_anon
olcIdleTimeout
The olcIdleTimeout directive allows you to specify how many seconds to wait before closing an idle connection. It takes the following form:
olcIdleTimeout: number
This option is disabled by default (that is, set to 0).

Example 9.5. Using the olcIdleTimeout directive

olcIdleTimeout: 180
olcLogFile
The olcLogFile directive allows you to specify a file in which to write log messages. It takes the following form:
olcLogFile: file_name
The log messages are written to standard error by default.

Example 9.6. Using the olcLogFile directive

olcLogFile: /var/log/slapd.log
olcReferral
The olcReferral option allows you to specify a URL of a server to process the request in case the server is not able to handle it. It takes the following form:
olcReferral: URL
This option is disabled by default.

Example 9.7. Using the olcReferral directive

olcReferral: ldap://root.openldap.org
olcWriteTimeout
The olcWriteTimeout option allows you to specify how many seconds to wait before closing a connection with an outstanding write request. It takes the following form:
olcWriteTimeout
This option is disabled by default (that is, set to 0).

Example 9.8. Using the olcWriteTimeout directive

olcWriteTimeout: 180

9.2.3.2. The Front End Configuration

The OpenLDAP front end configuration is stored in the etc/openldap/slapd.d/cn=config/olcDatabase={-1}frontend.ldif file and defines global database options, such as access control lists (ACL). For details, see the Global Database Options section in the slapd-config(5) man page.

9.2.3.3. The Monitor Back End

The /etc/openldap/slapd.d/cn=config/olcDatabase={1}monitor.ldif file controls the OpenLDAP monitor back end. If enabled, it is automatically generated and dynamically updated by OpenLDAP with information about the running status of the daemon. The suffix is cn=Monitor and cannot be changed. For further details, see the slapd-monitor(5) man page.

9.2.3.4. Database-Specific Configuration

By default, the OpenLDAP server uses the hdb database back end. Besides that it uses a hierarchical database layout which supports subtree renames, it is identical to the bdb back end and uses the same configuration options. The configuration for this database back end is stored in the /etc/openldap/slapd.d/cn=config/olcDatabase={2}hdb.ldif file.
For a list of other back end databases, see the slapd.backends(5) man page. Database-specific settings you find in the man page for the individual back ends. For example:
# man slapd-hdb

Note

The bdb and hdb back ends are deprecated. Consider using the mdb back end for new installations instead.
The following directives are commonly used in a database-specific configuration:
olcReadOnly
The olcReadOnly directive allows you to use the database in a read-only mode. It takes the following form:
olcReadOnly: boolean
It accepts either TRUE (enable the read-only mode), or FALSE (enable modifications of the database). The default option is FALSE.

Example 9.9. Using the olcReadOnly directive

olcReadOnly: TRUE
olcRootDN
The olcRootDN directive allows you to specify the user that is unrestricted by access controls or administrative limit parameters set for operations on the LDAP directory. It takes the following form:
olcRootDN: distinguished_name
It accepts a Distinguished Name (DN). The default option is cn=Manager,dn=my-domain,dc=com.

Example 9.10. Using the olcRootDN directive

olcRootDN: cn=root,dn=example,dn=com
olcRootPW
The olcRootPW directive allows you to set a password for the user that is specified using the olcRootDN directive. It takes the following form:
olcRootPW: password
It accepts either a plain text string, or a hash. To generate a hash, type the following at a shell prompt:
~]$ slappaswd
New password:
Re-enter new password:
{SSHA}WczWsyPEnMchFf1GRTweq2q7XJcvmSxD

Example 9.11. Using the olcRootPW directive

olcRootPW: {SSHA}WczWsyPEnMchFf1GRTweq2q7XJcvmSxD
olcSuffix
The olcSuffix directive allows you to specify the domain for which to provide information. It takes the following form:
olcSuffix: domain_name
It accepts a fully qualified domain name (FQDN). The default option is dc=my-domain,dc=com.

Example 9.12. Using the olcSuffix directive

olcSuffix: dc=example,dc=com

9.2.3.5. Extending Schema

Since OpenLDAP 2.3, the /etc/openldap/slapd.d/ directory also contains LDAP definitions that were previously located in /etc/openldap/schema/. It is possible to extend the schema used by OpenLDAP to support additional attribute types and object classes using the default schema files as a guide. However, this task is beyond the scope of this chapter. For more information on this topic, see http://www.openldap.org/doc/admin/schema.html.

9.2.3.6. Establishing a Secure Connection

The OpenLDAP suite and servers can be secured using the Transport Layer Security (TLS) framework. TLS is a cryptographic protocol designed to provide communication security over the network. OpenLDAP suite in Red Hat Enterprise Linux 7 uses OpenSSL as the TLS implementation.
To establish a secure connection using TLS, obtain the required certificates. Then, a number of options must be configured on both the client and the server. At minimum, a server must be configured with the Certificate Authority (CA) certificates and also its own server certificate and private key. The clients must be configured with the name of the file containing all the trusted CA certificates.
Typically, a server only needs to sign a single CA certificate. A client may want to connect to a variety of secure servers, therefore it is common to specify a list of several trusted CAs in its configuration.
Server Configuration
This section lists global configuration directives for slapd that need to be specified in the /etc/openldap/slapd.d/cn=config.ldif file on an OpenLDAP server in order to establish TLS.
While the old style configuration uses a single file, normally installed as /usr/local/etc/openldap/slapd.conf, the new style uses a slapd back end database to store the configuration. The configuration database normally resides in the /usr/local/etc/openldap/slapd.d/ directory.
The following directives are also valid for establishing SSL. In addition to TLS directives, you need to enable a port dedicated to SSL on the server side – typically it is port 636. To do so, edit the /etc/sysconfig/slapd file and append the ldaps:/// string to the list of URLs specified with the SLAPD_URLS directive.
olcTLSCACertificateFile
The olcTLSCACertificateFile directive specifies the file encoded with privacy-enhanced mail (PEM) schema that contains trusted CA certificates. The directive takes the following form:
olcTLSCACertificateFile: path
Replace path with the path to the CA certificate file.
olcTLSCACertificatePath
The olcTLSCACertificatePath directive specifies the path to a directory containing individual CA certificates in separate files. This directory must be specially managed with the OpenSSL c_rehash utility that generates symbolic links with the hashed names that point to the actual certificate files. In general, it is simpler to use the olcTLSCACertificateFile directive instead.
The directive takes the following form:
olcTLSCACertificatePath: path
Replace path with a path to the directory containing the CA certificate files. The specified directory must be managed with the OpenSSL c_rehash utility.
olcTLSCertificateFile
The olcTLSCertificateFile directive specifies the file that contains the slapd server certificate. The directive takes the following form:
olcTLSCertificateFile: path
Replace path with a path to the server certificate file of the slapd service.
olcTLSCertificateKeyFile
The olcTLSCertificateKeyFile directive specifies the file that contains the private key that matches the certificate stored in the file specified with olcTLSCertificateFile. Note that the current implementation does not support encrypted private keys, and therefore the containing file must be sufficiently protected. The directive takes the following form:
olcTLSCertificateKeyFile: path
Replace path with a path to the private key file.
Client Configuration
Specify the following directives in the /etc/openldap/ldap.conf configuration file on the client system. Most of these directives are parallel to the server configuration options. Directives in/etc/openldap/ldap.conf are configured on a system-wide basis, however, individual users may override them in their ~/.ldaprc files.
The same directives can be used to establish an SSL connection. The ldaps:// string must be used instead of ldap:// in OpenLDAP commands such as ldapsearch. This forces commands to use the default port for SSL, port 636, configured on the server.
TLS_CACERT
The TLS_CACERT directive specifies a file containing certificates for all of the Certificate Authorities the client will recognize. This is equivalent to the olcTLSCACertificateFile directive on a server. TLS_CACERT should always be specified before TLS_CACERTDIR in /etc/openldap/ldap.conf. The directive takes the following form:
TLS_CACERT path
Replace path with a path to the CA certificate file.
TLS_CACERTDIR
The TLS_CACERTDIR directive specifies the path to a directory that contains Certificate Authority certificates in separate files. As with olcTLSCACertificatePath on a server, the specified directory must be managed with the OpenSSL c_rehash utility.
TLS_CACERTDIR directory
Replace directory with a path to the directory containing CA certificate files.
TLS_CERT
The TLS_CERT specifies the file that contains a client certificate. This directive can only be specified in a user's ~/.ldaprc file. The directive takes the following form:
TLS_CERT path
Replace path with a path to the client certificate file.
TLS_KEY
The TLS_KEY specifies the file that contains the private key that matches the certificate stored in the file specified with the TLS_CERT directive. As with olcTLSCertificateFile on a server, encrypted key files are not supported, so the file itself must be carefully protected. This option is only configurable in a user's ~/.ldaprc file.
The TLS_KEY directive takes the following form:
TLS_KEY path
Replace path with a path to the client certificate file.

9.2.3.7. Setting Up Replication

Replication is the process of copying updates from one LDAP server (provider) to one or more other servers or clients (consumers). A provider replicates directory updates to consumers, the received updates can be further propagated by the consumer to other servers, so a consumer can also act simultaneously as a provider. Also, a consumer does not have to be an LDAP server, it may be just an LDAP client. In OpenLDAP, you can use several replication modes, most notable are mirror and sync. For more information on OpenLDAP replication modes, see the OpenLDAP Software Administrator's Guide installed with openldap-servers package (see the section called “Installed Documentation”).
To enable a chosen replication mode, use one of the following directives in /etc/openldap/slapd.d/ on both provider and consumers.
olcMirrorMode
The olcMirrorMode directive enables the mirror replication mode. It takes the following form:
olcMirrorMode on
This option needs to be specified both on provider and consumers. Also a serverID must be specified along with syncrepl options. Find a detailed example in the 18.3.4. MirrorMode section of the OpenLDAP Software Administrator's Guide (see the section called “Installed Documentation”).
olcSyncrepl
The olcSyncrepl directive enables the sync replication mode. It takes the following form:
olcSyncrepl on
The sync replication mode requires a specific configuration on both the provider and the consumers. This configuration is thoroughly described in the 18.3.1. Syncrepl section of the OpenLDAP Software Administrator's Guide (see the section called “Installed Documentation”).

9.2.3.8. Loading Modules and Back ends

You can enhance the slapd service with dynamically loaded modules. Support for these modules must be enabled with the --enable-modules option when configuring slapd. Modules are stored in files with the .la extension:
module_name.la
Back ends store or retrieve data in response to LDAP requests. Back ends may be compiled statically into slapd, or when module support is enabled, they may be dynamically loaded. In the latter case, the following naming convention is applied:
back_backend_name.la
To load a module or a back end, use the following directive in /etc/openldap/slapd.d/:
olcModuleLoad
The olcModuleLoad directive specifies a dynamically loadable module to load. It takes the following form:
olcModuleLoad: module
Here, module stands either for a file containing the module, or a back end, that will be loaded.

9.2.4. SELinux Policy for Applications Using LDAP

SELinux is an implementation of a mandatory access control mechanism in the Linux kernel. By default, SELinux prevents applications from accessing an OpenLDAP server. To enable authentication through LDAP, which is required by several applications, the allow_ypbind SELinux Boolean needs to be enabled. Certain applications also demand an enabled authlogin_nsswitch_use_ldap Boolean in this scenario. Execute the following commands to enable the aforementioned Booleans:
~]# setsebool -P allow_ypbind=1
~]# setsebool -P authlogin_nsswitch_use_ldap=1
The -P option makes this setting persistent across system reboots. See the Red Hat Enterprise Linux 7 SELinux User's and Administrator's Guide for more detailed information about SELinux.

9.2.5. Running an OpenLDAP Server

This section describes how to start, stop, restart, and check the current status of the Standalone LDAP Daemon. For more information on how to manage system services in general, see Managing Services with systemd in the System Administrator's Guide.

9.2.5.1. Starting the Service

To start the slapd service in the current session, type the following at a shell prompt as root:
~]# systemctl start slapd.service
To configure the service to start automatically at the boot time, use the following command as root:
~]# systemctl enable slapd.service
ln -s '/usr/lib/systemd/system/slapd.service' '/etc/systemd/system/multi-user.target.wants/slapd.service'

9.2.5.2. Stopping the Service

To stop the running slapd service in the current session, type the following at a shell prompt as root:
~]# systemctl stop slapd.service
To prevent the service from starting automatically at the boot time, type as root:
~]# systemctl disable slapd.service
rm '/etc/systemd/system/multi-user.target.wants/slapd.service'

9.2.5.3. Restarting the Service

To restart the running slapd service, type the following at a shell prompt:
~]# systemctl restart slapd.service
This stops the service and immediately starts it again. Use this command to reload the configuration.

9.2.5.4. Verifying the Service Status

To verify that the slapd service is running, type the following at a shell prompt:
~]$ systemctl is-active slapd.service
active

9.2.6. Configuring a System to Authenticate Using OpenLDAP

In order to configure a system to authenticate using OpenLDAP, make sure that the appropriate packages are installed on both LDAP server and client machines. For information on how to set up the server, follow the instructions in Section 9.2.2, “Installing the OpenLDAP Suite” and Section 9.2.3, “Configuring an OpenLDAP Server”. On a client, type the following at a shell prompt:
~]# yum install openldap openldap-clients nss-pam-ldapd

9.2.6.1. Migrating Old Authentication Information to LDAP Format

The migrationtools package provides a set of shell and Perl scripts to help you migrate authentication information into an LDAP format. To install this package, type the following at a shell prompt:
~]# yum install migrationtools
This will install the scripts to the /usr/share/migrationtools/ directory. Once installed, edit the /usr/share/migrationtools/migrate_common.ph file and change the following lines to reflect the correct domain, for example:
# Default DNS domain
$DEFAULT_MAIL_DOMAIN = "example.com";

# Default base
$DEFAULT_BASE = "dc=example,dc=com";
Alternatively, you can specify the environment variables directly on the command line. For example, to run the migrate_all_online.sh script with the default base set to dc=example,dc=com, type:
~]# export DEFAULT_BASE="dc=example,dc=com" \
/usr/share/migrationtools/migrate_all_online.sh
To decide which script to run in order to migrate the user database, see Table 9.8, “Commonly used LDAP migration scripts”.

Table 9.8. Commonly used LDAP migration scripts

Existing Name Service Is LDAP Running? Script to Use
/etc flat files yes migrate_all_online.sh
/etc flat files no migrate_all_offline.sh
NetInfo yes migrate_all_netinfo_online.sh
NetInfo no migrate_all_netinfo_offline.sh
NIS (YP) yes migrate_all_nis_online.sh
NIS (YP) no migrate_all_nis_offline.sh
For more information on how to use these scripts, see the README and the migration-tools.txt files in the /usr/share/doc/migrationtools-version/ directory.

9.2.7. Additional Resources

The following resources offer additional information on the Lightweight Directory Access Protocol. Before configuring LDAP on your system, it is highly recommended that you review these resources, especially the OpenLDAP Software Administrator's Guide.

Installed Documentation

The following documentation is installed with the openldap-servers package:
  • /usr/share/doc/openldap-servers-version/guide.html — A copy of the OpenLDAP Software Administrator's Guide.
  • /usr/share/doc/openldap-servers-version/README.schema — A README file containing the description of installed schema files.
Additionally, there is also a number of manual pages that are installed with the openldap, openldap-servers, and openldap-clients packages:
Client Applications
  • ldapadd(1) — The manual page for the ldapadd command describes how to add entries to an LDAP directory.
  • ldapdelete(1) — The manual page for the ldapdelete command describes how to delete entries within an LDAP directory.
  • ldapmodify(1) — The manual page for the ldapmodify command describes how to modify entries within an LDAP directory.
  • ldapsearch(1) — The manual page for the ldapsearch command describes how to search for entries within an LDAP directory.
  • ldappasswd(1) — The manual page for the ldappasswd command describes how to set or change the password of an LDAP user.
  • ldapcompare(1) — Describes how to use the ldapcompare tool.
  • ldapwhoami(1) — Describes how to use the ldapwhoami tool.
  • ldapmodrdn(1) — Describes how to modify the RDNs of entries.
Server Applications
  • slapd(8C) — Describes command line options for the LDAP server.
Administrative Applications
  • slapadd(8C) — Describes command line options used to add entries to a slapd database.
  • slapcat(8C) — Describes command line options used to generate an LDIF file from a slapd database.
  • slapindex(8C) — Describes command line options used to regenerate an index based upon the contents of a slapd database.
  • slappasswd(8C) — Describes command line options used to generate user passwords for LDAP directories.
Configuration Files
  • ldap.conf(5) — The manual page for the ldap.conf file describes the format and options available within the configuration file for LDAP clients.
  • slapd-config(5) — Describes the format and options available within the /etc/openldap/slapd.d configuration directory.

Other Resources

Part III. Secure Applications

Chapter 10. Using Pluggable Authentication Modules (PAM)

Pluggable authentication modules (PAMs) are a common framework for authentication and authorization. Most system applications in Red Hat Enterprise Linux depend on underlying PAM configuration for authentication and authorization.

10.1. About PAM

Pluggable Authentication Modules (PAMs) provide a centralized authentication mechanism which system application can use to relay authentication to a centrally configured framework.
PAM is pluggable because there is a PAM module for different types of authentication sources (such as Kerberos, SSSD, NIS, or the local file system). Different authentication sources can be prioritized.
This modular architecture offers administrators a great deal of flexibility in setting authentication policies for the system. PAM is a useful system for developers and administrators for several reasons:
  • PAM provides a common authentication scheme that can be used with a wide variety of applications.
  • PAM provides significant flexibility and control over authentication for system administrators.
  • PAM provides a single, fully-documented library which allows developers to write programs without having to create their own authentication schemes.

10.1.1. Other PAM Resources

PAM has an extensive documentation set with much more detail about both using PAM and writing modules to extend or integrate PAM with other applications. Almost all of the major modules and configuration files with PAM have their own man pages. Additionally, the /usr/share/doc/pam-version#/ directory contains a System Administrators' Guide, a Module Writers' Manual, and the Application Developers' Manual, as well as a copy of the PAM standard, DCE-RFC 86.0.
The libraries for PAM are available at http://www.linux-pam.org. This is the primary distribution website for the Linux-PAM project, containing information on various PAM modules, frequently asked questions, and additional PAM documentation.

10.1.2. Custom PAM Modules

New PAM modules can be created or added at any time for use by PAM-aware applications. PAM-aware programs can immediately use the new module and any methods it defines without being recompiled or otherwise modified. This allows developers and system administrators to use a selection of authentication modules, as well as tests, for different programs without recompiling them.
Documentation on writing modules is included in the /usr/share/doc/pam-devel-version#/ directory.

10.2. About PAM Configuration Files

Each PAM-aware application or service has a file in the /etc/pam.d/ directory. Each file in this directory has the same name as the service to which it controls access. For example, the login program defines its service name as login and installs the /etc/pam.d/login PAM configuration file.

Warning

It is highly recommended to configure PAMs using the authconfig tool instead of manually editing the PAM configuration files.

10.2.1. PAM Configuration File Format

Each PAM configuration file contains a group of directives that define the module (the authentication configuration area) and any controls or arguments with it.
The directives all have a simple syntax that identifies the module purpose (interface) and the configuration settings for the module.
module_interface	control_flag	module_name module_arguments
In a PAM configuration file, the module interface is the first field defined. For example:
auth	required	pam_unix.so
A PAM interface is essentially the type of authentication action which that specific module can perform. Four types of PAM module interface are available, each corresponding to a different aspect of the authentication and authorization process:
  • auth — This module interface authenticates users. For example, it requests and verifies the validity of a password. Modules with this interface can also set credentials, such as group memberships.
  • account — This module interface verifies that access is allowed. For example, it checks if a user account has expired or if a user is allowed to log in at a particular time of day.
  • password — This module interface is used for changing user passwords.
  • session — This module interface configures and manages user sessions. Modules with this interface can also perform additional tasks that are needed to allow access, like mounting a user's home directory and making the user's mailbox available.
An individual module can provide any or all module interfaces. For instance, pam_unix.so provides all four module interfaces.
The module name, such as pam_unix.so, provides PAM with the name of the library containing the specified module interface. The directory name is omitted because the application is linked to the appropriate version of libpam, which can locate the correct version of the module.
All PAM modules generate a success or failure result when called. Control flags tell PAM what to do with the result. Modules can be listed (stacked) in a particular order, and the control flags determine how important the success or failure of a particular module is to the overall goal of authenticating the user to the service.
There are several simple flags[2], which use only a keyword to set the configuration:
  • required — The module result must be successful for authentication to continue. If the test fails at this point, the user is not notified until the results of all module tests that reference that interface are complete.
  • requisite — The module result must be successful for authentication to continue. However, if a test fails at this point, the user is notified immediately with a message reflecting the first failed required or requisite module test.
  • sufficient — The module result is ignored if it fails. However, if the result of a module flagged sufficient is successful and no previous modules flagged required have failed, then no other results are required and the user is authenticated to the service.
  • optional — The module result is ignored. A module flagged as optional only becomes necessary for successful authentication when no other modules reference the interface.
  • include — Unlike the other controls, this does not relate to how the module result is handled. This flag pulls in all lines in the configuration file which match the given parameter and appends them as an argument to the module.
Module interface directives can be stacked, or placed upon one another, so that multiple modules are used together for one purpose.

Note

If a module's control flag uses the sufficient or requisite value, then the order in which the modules are listed is important to the authentication process.
Using stacking, the administrator can require specific conditions to exist before the user is allowed to authenticate. For example, the setup utility normally uses several stacked modules, as seen in its PAM configuration file:
[root@MyServer ~]# cat /etc/pam.d/setup

auth       sufficient	pam_rootok.so
auth       include	system-auth
account    required	pam_permit.so
session	   required	pam_permit.so
  • auth sufficient pam_rootok.so — This line uses the pam_rootok.so module to check whether the current user is root, by verifying that their UID is 0. If this test succeeds, no other modules are consulted and the command is executed. If this test fails, the next module is consulted.
  • auth include system-auth — This line includes the content of the /etc/pam.d/system-auth module and processes this content for authentication.
  • account required pam_permit.so — This line uses the pam_permit.so module to allow the root user or anyone logged in at the console to reboot the system.
  • session required pam_permit.so — This line is related to the session setup. Using pam_permit.so, it ensures that the setup utility does not fail.
PAM uses arguments to pass information to a pluggable module during authentication for some modules.
For example, the pam_pwquality.so module checks how strong a password is and can take several arguments. In the following example, enforce_for_root specifies that even password of the root user must successfully pass the strength check and retry defines that a user will receive three opportunities to enter a strong password.
password	requisite	pam_pwquality.so enforce_for_root retry=3
Invalid arguments are generally ignored and do not otherwise affect the success or failure of the PAM module. Some modules, however, may fail on invalid arguments. Most modules report errors to the journald service. For information on how to use journald and the related journalctl tool, see the System Administrator's Guide.

Note

The journald service was introduced in Red Hat Enterprise Linux 7.1. In previous versions of Red Hat Enterprise Linux, most modules report errors to the /var/log/secure file.

10.2.2. Annotated PAM Configuration Example

Example 10.1, “Simple PAM Configuration” is a sample PAM application configuration file:

Example 10.1. Simple PAM Configuration

#%PAM-1.0
auth		required	pam_securetty.so
auth		required	pam_unix.so nullok
auth		required	pam_nologin.so
account		required	pam_unix.so
password	required	pam_pwquality.so retry=3
password	required	pam_unix.so shadow nullok use_authtok
session		required	pam_unix.so
  • The first line is a comment, indicated by the hash mark (#) at the beginning of the line.
  • Lines two through four stack three modules for login authentication.
    auth required pam_securetty.so — This module ensures that if the user is trying to log in as root, the TTY on which the user is logging in is listed in the /etc/securetty file, if that file exists.
    If the TTY is not listed in the file, any attempt to log in as root fails with a Login incorrect message.
    auth required pam_unix.so nullok — This module prompts the user for a password and then checks the password using the information stored in /etc/passwd and, if it exists, /etc/shadow.
    The argument nullok instructs the pam_unix.so module to allow a blank password.
  • auth required pam_nologin.so — This is the final authentication step. It checks whether the /etc/nologin file exists. If it exists and the user is not root, authentication fails.

    Note

    In this example, all three auth modules are checked, even if the first auth module fails. This prevents the user from knowing at what stage their authentication failed. Such knowledge in the hands of an attacker could allow them to more easily deduce how to crack the system.
  • account required pam_unix.so — This module performs any necessary account verification. For example, if shadow passwords have been enabled, the account interface of the pam_unix.so module checks to see if the account has expired or if the user has not changed the password within the allowed grace period.
  • password required pam_pwquality.so retry=3 — If a password has expired, the password component of the pam_pwquality.so module prompts for a new password. It then tests the newly created password to see whether it can easily be determined by a dictionary-based password cracking program.
    The argument retry=3 specifies that if the test fails the first time, the user has two more chances to create a strong password.
  • password required pam_unix.so shadow nullok use_authtok — This line specifies that if the program changes the user's password, using the password interface of the pam_unix.so module.
    • The argument shadow instructs the module to create shadow passwords when updating a user's password.
    • The argument nullok instructs the module to allow the user to change their password from a blank password, otherwise a null password is treated as an account lock.
    • The final argument on this line, use_authtok, provides a good example of the importance of order when stacking PAM modules. This argument instructs the module not to prompt the user for a new password. Instead, it accepts any password that was recorded by a previous password module. In this way, all new passwords must pass the pam_pwquality.so test for secure passwords before being accepted.
  • session required pam_unix.so — The final line instructs the session interface of the pam_unix.so module to manage the session. This module logs the user name and the service type to /var/log/secure at the beginning and end of each session. This module can be supplemented by stacking it with other session modules for additional functionality.

10.3. PAM and Administrative Credential Caching

A number of graphical administrative tools in Red Hat Enterprise Linux, such as the GNOME's control-center, provide users with elevated privileges for up to five minutes using the pam_timestamp.so module. It is important to understand how this mechanism works, because a user who walks away from a terminal while pam_timestamp.so is in effect leaves the machine open to manipulation by anyone with physical access to the console.
In the PAM timestamp scheme, the graphical administrative application prompts the user for the root password when it is launched. When the user has been authenticated, the pam_timestamp.so module creates a timestamp file. By default, this is created in the /var/run/sudo/ directory. If the timestamp file already exists, graphical administrative programs do not prompt for a password. Instead, the pam_timestamp.so module freshens the timestamp file, reserving an extra five minutes of unchallenged administrative access for the user.
You can verify the actual state of the timestamp file by inspecting the file in the /var/run/sudo/user directory. For the desktop, the relevant file is unknown:root. If it is present and its timestamp is less than five minutes old, the credentials are valid.
The existence of the timestamp file is indicated by an authentication icon, which appears in the notification area of the panel.
The Authentication Icon

Figure 10.1. The Authentication Icon

10.3.1. Removing the Timestamp File

Before abandoning a console where a PAM timestamp is active, it is recommended that the timestamp file be destroyed. To do this from a graphical environment, click the authentication icon on the panel. This causes a dialog box to appear. Click the Forget Authorization button to destroy the active timestamp file.
Dismiss Authentication Dialog

Figure 10.2. Dismiss Authentication Dialog

The PAM timestamp file has some important characteristics:
  • If logged in to the system remotely using ssh, use the /sbin/pam_timestamp_check -k root command to destroy the timestamp file.
  • Run the /sbin/pam_timestamp_check -k root command from the same terminal window where the privileged application was launched.
  • The logged in user who originally invoked the pam_timestamp.so module must be the user who runs the /sbin/pam_timestamp_check -k command. Do not run this command as root.
  • Killing the credentials on the desktop without using the Forget Authorization action on the icon can be done with the /sbin/pam_timestamp_chec command.
    /sbin/pam_timestamp_check -k root </dev/null >/dev/null 2>/dev/null
    Any other method only removes the credentials from the PTY where the command was run.
Refer to the pam_timestamp_check man page for more information about destroying the timestamp file using pam_timestamp_check.

10.3.2. Common pam_timestamp Directives

The pam_timestamp.so module accepts several directives, with two used most commonly:
  • timestamp_timeout — Specifies the period (in seconds) for which the timestamp file is valid. The default value is 300 (five minutes).
  • timestampdir — Specifies the directory in which the timestamp file is stored. The default value is /var/run/sudo/.

10.4. Restricting Domains for PAM services

Important

This feature requires SSSD to be running on the system.
SSSD enables you to restrict which domains can be accessed by PAM services. SSSD evaluates authentication requests from PAM services based on the user the particular PAM service is running as. Whether the PAM service can access an SSSD domain depends on whether the PAM service user is able to access the domain.
An example use case is an environment where external users are allowed to authenticate to an FTP server. The FTP server is running as a separate non-privileged user that should only be able to authenticate to a selected SSSD domain, separate from internal company accounts. With this feature, the administrator can allow the FTP user to only authenticate to selected domains specified in the FTP PAM configuration file.

Note

This functionality is similar to legacy PAM modules, such as pam_ldap, which were able to use a separate configuration file as a parameter for a PAM module.

Options to Restrict Access to Domains

The following options are available to restrict access to selected domains:
pam_trusted_users in /etc/sssd/sssd.conf
This option accepts a list of numerical UIDs or user names representing the PAM services that are to be trusted by SSSD. The default setting is all, which means all service users are trusted and can access any domain.
pam_public_domains in /etc/sssd/sssd.conf
This option accepts a list of public SSSD domains. Public domains are domains accessible even for untrusted PAM service users. The option also accepts the all and none values. The default value is none, which means no domains are public and untrusted service users therefore cannot access any domain.
domains for PAM configuration files
This option specifies a list of domains against which a PAM service can authenticate. If you use domains without specifying any domain, the PAM service will not be able to authenticate against any domain, for example:
auth     required   pam_sss.so domains=
If domains is not used in the PAM configuration file, the PAM service is able to authenticate against all domains, on the condition that the service is running under a trusted user.
The domains option in the /etc/sssd/sssd.conf SSSD configuration file also specifies a list of domains to which SSSD attempts to authenticate. Note that the domains option in a PAM configuration file cannot extend the list of domains in sssd.conf, it can only restrict the sssd.conf list of domains by specifying a shorter list. Therefore, if a domain is specified in the PAM file but not in sssd.conf, the PAM service will not be able to authenticate against the domain.
The default settings pam_trusted_users = all and pam_public_domains = none specify that all PAM service users are trusted and can access any domain. The domains option for PAM configuration files can be used in this situation to restrict the domains that can be accessed.
If you specify a domain using domains in the PAM configuration file while sssd.conf contains pam_public_domains, it might be required to specify the domain in pam_public_domains as well. If pam_public_domains is used but does not include the required domain, the PAM service will not be able to successfully authenticate against the domain if it is running under an untrusted user.

Note

Domain restrictions defined in a PAM configuration file only apply to authentication actions, not to user lookups.
For more information about the pam_trusted_users and pam_public_domains options, see the sssd.conf(5) man page. For more information about the domains option used in PAM configuration files, see the pam_sss(8) man page.

Example 10.2. Restricting Domains for a PAM Service

To restrict the domains against which a PAM service can authenticate:
  1. Make sure SSSD is configured to access the required domain or domains. The domains against which SSSD can authenticate are defined in the domains option in the /etc/sssd/sssd.conf file.
    [sssd]
    domains = domain1, domain2, domain3
    
  2. Specify the domain or domains to which a PAM service will be able to authenticate. To do this, set the domains option in the PAM configuration file. For example:
    auth        sufficient    pam_sss.so forward_pass domains=domain1
    account     [default=bad success=ok user_unknown=ignore] pam_sss.so
    password    sufficient    pam_sss.so use_authtok
    
The PAM service is now only allowed to authenticate against domain1.


[2] There are many complex control flags that can be set. These are set in attribute=value pairs; a complete list of attributes is available in the pam.d manpage.

Chapter 11. Using Kerberos

Maintaining system security and integrity within a network is critical, and it encompasses every user, application, service, and server within the network infrastructure. It requires an understanding of everything that is running on the network and the manner in which these services are used. At the core of maintaining this security is maintaining access to these applications and services and enforcing that access.
Kerberos is an authentication protocol significantly safer than normal password-based authentication. With Kerberos, passwords are never sent over the network, even when services are accessed on other machines.
Kerberos provides a mechanism that allows both users and machines to identify themselves to network and receive defined, limited access to the areas and services that the administrator configured. Kerberos authenticates entities by verifying their identity, and Kerberos also secures this authenticating data so that it cannot be accessed and used or tampered with by an outsider.

11.1. About Kerberos

Kerberos uses symmetric-key cryptography[3] to authenticate users to network services, which means passwords are never actually sent over the network.
Consequently, when users authenticate to network services using Kerberos, unauthorized users attempting to gather passwords by monitoring network traffic are effectively thwarted.

11.1.1. The Basics of How Kerberos Works

Most conventional network services use password-based authentication schemes, where a user supplies a password to access a given network server. However, the transmission of authentication information for many services is unencrypted. For such a scheme to be secure, the network has to be inaccessible to outsiders, and all computers and users on the network must be trusted and trustworthy.
With simple, password-based authentication, a network that is connected to the Internet cannot be assumed to be secure. Any attacker who gains access to the network can use a simple packet analyzer, or packet sniffer, to intercept user names and passwords, compromising user accounts and, therefore, the integrity of the entire security infrastructure.
Kerberos eliminates the transmission of unencrypted passwords across the network and removes the potential threat of an attacker sniffing the network.
Rather than authenticating each user to each network service separately as with simple password authentication, Kerberos uses symmetric encryption and a trusted third party (a key distribution center or KDC) to authenticate users to a suite of network services. The computers managed by that KDC and any secondary KDCs constitute a realm.
When a user authenticates to the KDC, the KDC sends a set of credentials (a ticket) specific to that session back to the user's machine, and any Kerberos-aware services look for the ticket on the user's machine rather than requiring the user to authenticate using a password.
As shown in Figure 11.1, “Kerberos Authentication”, each user is identified to the KDC with a unique identity, called a principal. When a user on a Kerberos-aware network logs into his workstation, his principal is sent to the KDC as part of a request for a ticket-granting ticket (or TGT) from the authentication server. This request can be sent by the login program so that it is transparent to the user or can be sent manually by a user through the kinit program after the user logs in.
The KDC then checks for the principal in its database. If the principal is found, the KDC creates a TGT, encrypts it using the user's key, and sends the TGT to that user.
Kerberos Authentication

Figure 11.1. Kerberos Authentication

The login or kinit program on the client then decrypts the TGT using the user's key, which it computes from the user's password. The user's key is used only on the client machine and is not transmitted over the network. The ticket (or credentials) sent by the KDC are stored in a local store, the credential cache (ccache), which can be checked by Kerberos-aware services. Red Hat Enterprise Linux 7 supports the following types of credential caches:
  • The persistent KEYRING ccache type, the default cache in Red Hat Enterprise Linux 7
  • The System Security Services Daemon (SSSD) Kerberos Credential Manager (KCM), an alternative option since Red Hat Enterprise Linux 7.4
  • FILE
  • DIR
  • MEMORY
With SSSD KCM, the Kerberos caches are not stored in a passive store, but managed by a daemon. In this setup, the Kerberos library, which is typically used by applications such as kinit, is a KCM client and the daemon is referred to as a KCM server.
Having the Kerberos credential caches managed by the SSSD KCM daemon has several advantages:
  • The daemon is stateful and can perform tasks such as Kerberos credential cache renewals or reaping old ccaches. Renewals and tracking are possible not only for tickets that SSSD itself acquired, typically via a login through pam_sss.so, but also for tickets acquired, for example, though kinit.
  • Since the process runs in user space, it is subject to UID namespacing, unlike the Kernel KEYRING.
  • Unlike the Kernel KEYRING-based cache, which is entirely dependent on the UID of the caller and which, in a containerized environment, is shared among all containers, the KCM server’s entry point is a UNIX socket that can be bind-mounted only to selected containers.
After authentication, servers can check an unencrypted list of recognized principals and their keys rather than checking kinit; this is kept in a keytab.
The TGT is set to expire after a certain period of time (usually 10 to 24 hours) and is stored in the client machine's credential cache. An expiration time is set so that a compromised TGT is of use to an attacker for only a short period of time. After the TGT has been issued, the user does not have to enter their password again until the TGT expires or until they log out and log in again.
Whenever the user needs access to a network service, the client software uses the TGT to request a new ticket for that specific service from the ticket-granting server (TGS). The service ticket is then used to authenticate the user to that service transparently.

11.1.2. About Kerberos Principal Names

The principal identifies not only the user or service, but also the realm that the entity belongs to. A principal name has two parts, the identifier and the realm:
identifier@REALM
For a user, the identifier is only the Kerberos user name. For a service, the identifier is a combination of the service name and the host name of the machine it runs on:
service/FQDN@REALM
The service name is a case-sensitive string that is specific to the service type, like host, ldap, http, and DNS. Not all services have obvious principal identifiers; the sshd daemon, for example, uses the host service principal.
The host principal is usually stored in /etc/krb5.keytab.
When Kerberos requests a ticket, it always resolves the domain name aliases (DNS CNAME records) to the corresponding DNS address (A or AAAA records). The host name from the address record is then used when service or host principals are created.
For example:
www.example.com		CNAME		web-01.example.com
web-01.example.com		A		192.0.2.145
A service attempts to connect to the host using its CNAME alias:
$ ssh www.example.com
The Kerberos server requests a ticket for the resolved host name, web-01.example.com@EXAMPLE.COM, so the host principal must be host/web-01.example.com@EXAMPLE.COM.

11.1.3. About the Domain-to-Realm Mapping

When a client attempts to access a service running on a particular server, it knows the name of the service (host) and the name of the server (foo.example.com), but because more than one realm can be deployed on the network, it must guess at the name of the Kerberos realm in which the service resides.
By default, the name of the realm is taken to be the DNS domain name of the server in all capital letters.
foo.example.org → EXAMPLE.ORG
foo.example.com → EXAMPLE.COM
foo.hq.example.com → HQ.EXAMPLE.COM
In some configurations, this will be sufficient, but in others, the realm name which is derived will be the name of a non-existent realm. In these cases, the mapping from the server's DNS domain name to the name of its realm must be specified in the domain_realm section of the client system's /etc/krb5.conf file. For example:
[domain_realm]
.example.com = EXAMPLE.COM
example.com = EXAMPLE.COM
The configuration specifies two mappings. The first mapping specifies that any system in the example.com DNS domain belongs to the EXAMPLE.COM realm. The second specifies that a system with the exact name example.com is also in the realm. The distinction between a domain and a specific host is marked by the presence or lack of an initial period character. The mapping can also be stored directly in DNS using the "_kerberos TXT" records, for example:
$ORIGIN example.com
_kerberos		TXT	"EXAMPLE.COM"

11.1.4. Environmental Requirements

Kerberos relies on being able to resolve machine names. Thus, it requires a working domain name service (DNS). Both DNS entries and hosts on the network must be properly configured, which is covered in the Kerberos documentation in /usr/share/doc/krb5-server-version-number.
Applications that accept Kerberos authentication require time synchronization. You can set up approximate clock synchronization between the machines on the network using a service such as ntpd. For information on the ntpd service, see the documentation in /usr/share/doc/ntp-version-number/html/index.html or the ntpd(8) man page.

Note

Kerberos clients running Red Hat Enterprise Linux 7 support automatic time adjustment with the KDC and have no strict timing requirements. This enables better tolerance to clocking differences when deploying IdM clients with Red Hat Enterprise Linux 7.

11.1.5. Considerations for Deploying Kerberos

Although Kerberos removes a common and severe security threat, it is difficult to implement for a variety of reasons:
  • Kerberos assumes that each user is trusted but is using an untrusted host on an untrusted network. Its primary goal is to prevent unencrypted passwords from being transmitted across that network. However, if anyone other than the proper user has access to the one host that issues tickets used for authentication — the KDC — the entire Kerberos authentication system are at risk.
  • For an application to use Kerberos, its source must be modified to make the appropriate calls into the Kerberos libraries. Applications modified in this way are considered to be Kerberos-aware. For some applications, this can be quite problematic due to the size of the application or its design. For other incompatible applications, changes must be made to the way in which the server and client communicate. Again, this can require extensive programming. Closed source applications that do not have Kerberos support by default are often the most problematic.
  • To secure a network with Kerberos, one must either use Kerberos-aware versions of all client and server applications that transmit passwords unencrypted, or not use that client and server application at all.
  • Migrating user passwords from a standard UNIX password database, such as /etc/passwd or /etc/shadow, to a Kerberos password database can be tedious. There is no automated mechanism to perform this task. Migration methods can vary substantially depending on the particular way Kerberos is deployed. That is why it is recommended that you use the Identity Management feature; it has specialized tools and methods for migration.

Warning

The Kerberos system can be compromised if a user on the network authenticates against a non-Kerberos aware service by transmitting a password in plain text. The use of non-Kerberos aware services (including telnet and FTP) is highly discouraged. Other encrypted protocols, such as SSH or SSL-secured services, are preferred to unencrypted services, but this is still not ideal.

11.1.6. Additional Resources for Kerberos

Kerberos can be a complex service to implement, with a lot of flexibility in how it is deployed. Table 11.1, “External Kerberos Documentation” and Table 11.2, “Important Kerberos Man Pages” list of a few of the most important or most useful sources for more information on using Kerberos.
Any of the manpage files can be opened by running man command_name.

Table 11.2. Important Kerberos Man Pages

Manpage Description
Client Applications
kerberos An introduction to the Kerberos system which describes how credentials work and provides recommendations for obtaining and destroying Kerberos tickets. The bottom of the man page references a number of related man pages.
kinit Describes how to use this command to obtain and cache a ticket-granting ticket.
kdestroy Describes how to use this command to destroy Kerberos credentials.
klist Describes how to use this command to list cached Kerberos credentials.
Administrative Applications
kadmin Describes how to use this command to administer the Kerberos V5 database.
kdb5_util Describes how to use this command to create and perform low-level administrative functions on the Kerberos V5 database.
Server Applications
krb5kdc Describes available command line options for the Kerberos V5 KDC.
kadmind Describes available command line options for the Kerberos V5 administration server.
Configuration Files
krb5.conf Describes the format and options available within the configuration file for the Kerberos V5 library.
kdc.conf Describes the format and options available within the configuration file for the Kerberos V5 AS and KDC.

11.2. Configuring the Kerberos KDC

Install the master KDC first and then install any necessary secondary servers after the master is set up.

Important

Setting up Kerberos KDC manually is not recommended. The recommended way to introduce Kerberos into Red Hat Enterprise Linux environments is to use the Identity Management feature.

11.2.1. Configuring the Master KDC Server

Important

The KDC system should be a dedicated machine. This machine needs to be very secure — if possible, it should not run any services other than the KDC.
  1. Install the required packages for the KDC:
    [root@server ~]# yum install krb5-server krb5-libs krb5-workstation
  2. Edit the /etc/krb5.conf and /var/kerberos/krb5kdc/kdc.conf configuration files to reflect the realm name and domain-to-realm mappings. For example:
    [logging]
     default = FILE:/var/log/krb5libs.log
     kdc = FILE:/var/log/krb5kdc.log
     admin_server = FILE:/var/log/kadmind.log
    
    [libdefaults]
     default_realm = EXAMPLE.COM
     dns_lookup_realm = false
     dns_lookup_kdc = false
     ticket_lifetime = 24h
     renew_lifetime = 7d
     forwardable = true
     allow_weak_crypto = true
    
    [realms]
      EXAMPLE.COM = {
      kdc = kdc.example.com.:88
      admin_server = kdc.example.com
      default_domain = example.com
     }
    
    [domain_realm]
     .example.com = EXAMPLE.COM
     example.com = EXAMPLE.COM
    A simple realm can be constructed by replacing instances of EXAMPLE.COM and example.com with the correct domain name — being certain to keep uppercase and lowercase names in the correct format — and by changing the KDC from kerberos.example.com to the name of the Kerberos server. By convention, all realm names are uppercase and all DNS host names and domain names are lowercase. The man pages of these configuration files have full details about the file formats.
  3. Create the database using the kdb5_util utility.
    [root@server ~]# kdb5_util create -s
    The create command creates the database that stores keys for the Kerberos realm. The -s argument creates a stash file in which the master server key is stored. If no stash file is present from which to read the key, the Kerberos server (krb5kdc) prompts the user for the master server password (which can be used to regenerate the key) every time it starts.
  4. Edit the /var/kerberos/krb5kdc/kadm5.acl file. This file is used by kadmind to determine which principals have administrative access to the Kerberos database and their level of access. For example:
    */admin@EXAMPLE.COM  *
    Most users are represented in the database by a single principal (with a NULL, or empty, instance, such as joe@EXAMPLE.COM). In this configuration, users with a second principal with an instance of admin (for example, joe/admin@EXAMPLE.COM) are able to exert full administrative control over the realm's Kerberos database.
    After kadmind has been started on the server, any user can access its services by running kadmin on any of the clients or servers in the realm. However, only users listed in the kadm5.acl file can modify the database in any way, except for changing their own passwords.

    Note

    The kadmin utility communicates with the kadmind server over the network, and uses Kerberos to handle authentication. Consequently, the first principal must already exist before connecting to the server over the network to administer it. Create the first principal with the kadmin.local command, which is specifically designed to be used on the same host as the KDC and does not use Kerberos for authentication.
  5. Create the first principal using kadmin.local at the KDC terminal:
    [root@server ~]# kadmin.local -q "addprinc username/admin"
  6. Start Kerberos using the following commands:
    [root@server ~]# systemctl start krb5kdc.service
    [root@server ~]# systemctl start kadmin.service
  7. Add principals for the users using the addprinc command within kadmin. kadmin and kadmin.local are command line interfaces to the KDC. As such, many commands — such as addprinc — are available after launching the kadmin program. Refer to the kadmin man page for more information.
  8. Verify that the KDC is issuing tickets. First, run kinit to obtain a ticket and store it in a credential cache file. Next, use klist to view the list of credentials in the cache and use kdestroy to destroy the cache and the credentials it contains.

    Note

    By default, kinit attempts to authenticate using the same system login user name (not the Kerberos server). If that user name does not correspond to a principal in the Kerberos database, kinit issues an error message. If that happens, supply kinit with the name of the correct principal as an argument on the command line:
    kinit principal

11.2.2. Setting up Secondary KDCs

When there are multiple KDCs for a given realm, one KDC (the master KDC) keeps a writable copy of the realm database and runs kadmind. The master KDC is also the realm's admin server. Additional secondary KDCs keep read-only copies of the database and run kpropd.
The master and slave propagation procedure entails the master KDC dumping its database to a temporary dump file and then transmitting that file to each of its slaves, which then overwrite their previously received read-only copies of the database with the contents of the dump file.
To set up a secondary KDC:
  1. Install the required packages for the KDC:
    [root@slavekdc ~]# yum install krb5-server krb5-libs krb5-workstation
  2. Copy the master KDC's krb5.conf and kdc.conf files to the secondary KDC.
  3. Start kadmin.local from a root shell on the master KDC.
    1. Use the kadmin.local add_principal command to create a new entry for the master KDC's host service.
      [root@slavekdc ~]# kadmin.local -r EXAMPLE.COM
       Authenticating as principal root/admin@EXAMPLE.COM with password.
      kadmin: add_principal -randkey host/masterkdc.example.com
      Principal "host/masterkdc.example.com@EXAMPLE.COM" created.
      kadmin: ktadd host/masterkdc.example.com
      Entry for principal host/masterkdc.example.com with kvno 3, encryption type Triple DES cbc mode with HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab.
      Entry for principal host/masterkdc.example.com with kvno 3, encryption type ArcFour with HMAC/md5 added to keytab WRFILE:/etc/krb5.keytab.
      Entry for principal host/masterkdc.example.com with kvno 3, encryption type DES with HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab.
      Entry for principal host/masterkdc.example.com with kvno 3, encryption type DES cbc mode with RSA-MD5 added to keytab WRFILE:/etc/krb5.keytab.
      kadmin: quit
    2. Use the kadmin.local ktadd command to set a random key for the service and store the random key in the master's default keytab file.

      Note

      This key is used by the kprop command to authenticate to the secondary servers. You will only need to do this once, regardless of how many secondary KDC servers you install.
  4. Start kadmin from a root shell on the secondary KDC.
    1. Use the kadmin.local add_principal command to create a new entry for the secondary KDC's host service.
      [root@slavekdc ~]# kadmin -p jsmith/admin@EXAMPLE.COM -r EXAMPLE.COM
      Authenticating as principal jsmith/admin@EXAMPLE.COM with password.
      Password for jsmith/admin@EXAMPLE.COM:
      kadmin: add_principal -randkey host/slavekdc.example.com
      Principal "host/slavekdc.example.com@EXAMPLE.COM" created.
      kadmin: ktadd host/slavekdc.example.com@EXAMPLE.COM
      Entry for principal host/slavekdc.example.com with kvno 3, encryption type Triple DES cbc mode with HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab.
      Entry for principal host/slavekdc.example.com with kvno 3, encryption type ArcFour with HMAC/md5 added to keytab WRFILE:/etc/krb5.keytab.
      Entry for principal host/slavekdc.example.com with kvno 3, encryption type DES with HMAC/sha1 added to keytab WRFILE:/etc/krb5.keytab.
      Entry for principal host/slavekdc.example.com with kvno 3, encryption type DES cbc mode with RSA-MD5 added to keytab WRFILE:/etc/krb5.keytab.
      kadmin: quit
    2. Use the kadmin.local ktadd command to set a random key for the service and store the random key in the secondary KDC server's default keytab file. This key is used by the kpropd service when authenticating clients.
  5. With its service key, the secondary KDC could authenticate any client which would connect to it. Obviously, not all potential clients should be allowed to provide the kprop service with a new realm database. To restrict access, the kprop service on the secondary KDC will only accept updates from clients whose principal names are listed in /var/kerberos/krb5kdc/kpropd.acl.
    Add the master KDC's host service's name to that file.
    [root@slavekdc ~]# echo host/masterkdc.example.com@EXAMPLE.COM > /var/kerberos/krb5kdc/kpropd.acl
  6. Once the secondary KDC has obtained a copy of the database, it will also need the master key which was used to encrypt it. If the KDC database's master key is stored in a stash file on the master KDC (typically named /var/kerberos/krb5kdc/.k5.REALM), either copy it to the secondary KDC using any available secure method, or create a dummy database and identical stash file on the secondary KDC by running kdb5_util create -s and supplying the same password. The dummy database will be overwritten by the first successful database propagation.
  7. Ensure that the secondary KDC's firewall allows the master KDC to contact it using TCP on port 754 (krb5_prop), and start the kprop service.
  8. Verify that the kadmin service is disabled.
  9. Perform a manual database propagation test by dumping the realm database on the master KDC to the default data file which the kprop command will read (/var/kerberos/krb5kdc/slave_datatrans).
    [root@masterkdc ~]# kdb5_util dump /var/kerberos/krb5kdc/slave_datatrans
  10. Use the kprop command to transmit its contents to the secondary KDC.
    [root@slavekdc ~]# kprop slavekdc.example.com
  11. Using kinit, verify that the client system is able to correctly obtain the initial credentials from the KDC.
    The /etc/krb5.conf for the client should list only the secondary KDC in its list of KDCs.
    [realms]
      EXAMPLE.COM = {
      kdc = slavekdc.example.com.:88
      admin_server = kdc.example.com
      default_domain = example.com
     }
  12. Create a script which dumps the realm database and runs the kprop command to transmit the database to each secondary KDC in turn, and configure the cron service to run the script periodically.

11.2.3. Kerberos Key Distribution Center Proxy

In some deployments, only the HTTPS port (443 using TCP) is accessible and not the default Kerberos ports. Clients can obtain Kerberos credentials using the IdM HTTPS service as a proxy. This reverse proxy enables accessing Kerberos-authenticated services through HTTPS.
Kerberos Key Distribution Center Proxy (KKDCP) provides this functionality in IdM.

Configuring KKDCP in Your Deployment

On an IdM server, KKDCP is enabled by default.
On an IdM client, you must enable KKDCP:
  1. Reconfigure the /etc/krb5.conf file as described in Section 11.3, “Configuring a Kerberos Client”.
  2. Restart the SSSD service:
    # systemctl restart sssd.service

Verifying That KKDCP Is Enabled on an IdM Server

The KKDCP is automatically enabled each time the Apache web server starts, if the attribute and value pair ipaConfigString=kdcProxyEnabled exists in the directory. In this situation, the symbolic link /etc/httpd/conf.d/ipa-kdc-proxy.conf is created.
To verify if the KKDCP feature is enabled, check that the symbolic link exists:
$ ls -l /etc/httpd/conf.d/ipa-kdc-proxy.conf
lrwxrwxrwx. 1 root root 36 Aug 15 09:37 /etc/httpd/conf.d/ipa-kdc-proxy.conf -> /etc/ipa/kdcproxy/ipa-kdc-proxy.conf

Disabling KKDCP on an IdM Server

  1. Remove the ipaConfigString=kdcProxyEnabled attribute and value pair from the directory:
    # ipa-ldap-updater /usr/share/ipa/kdcproxy-disable.uldif
  2. Restart the httpd service on the IdM server:
    # systemctl restart httpd.service

Additional Resources

11.3. Configuring a Kerberos Client

All that is required to set up a Kerberos 5 client is to install the client packages and provide each client with a valid krb5.conf configuration file. While ssh and slogin are the preferred methods of remotely logging in to client systems, Kerberos-aware versions of rsh and rlogin are still available, with additional configuration changes.
  1. Install the krb5-libs and krb5-workstation packages on all of the client machines.
    [root@server ~]# yum install krb5-workstation krb5-libs
  2. Supply a valid /etc/krb5.conf file for each client. Usually this can be the same krb5.conf file used by the Kerberos Distribution Center (KDC). For example:
    [logging]
     default = FILE:/var/log/krb5libs.log
     kdc = FILE:/var/log/krb5kdc.log
     admin_server = FILE:/var/log/kadmind.log
    
    [libdefaults]
     default_realm = EXAMPLE.COM
     dns_lookup_realm = false
     dns_lookup_kdc = false
     ticket_lifetime = 24h
     renew_lifetime = 7d
     forwardable = true
     allow_weak_crypto = true
    
    [realms]
      EXAMPLE.COM = {
      kdc = kdc.example.com.:88
      admin_server = kdc.example.com
      default_domain = example.com
     }
    
    [domain_realm]
     .example.com = EXAMPLE.COM
     example.com = EXAMPLE.COM
    In some environments, the KDC is only accessible using an HTTPS Kerberos Key Distribution Center Proxy (KKDCP). In this case, make the following changes:
    1. Assign the URL of the KKDCP instead of the host name to the kdc and admin_server options in the [realms] section:
      [realms]
      EXAMPLE.COM = {
        kdc = https://kdc.example.com/KdcProxy
        admin_server = https://kdc.example.com/KdcProxy
        kpasswd_server = https://kdc.example.com/KdcProxy
        default_domain = example.com
      }
      For redundancy, the parameters kdc, admin_server, and kpasswd_server can be added multiple times using different KKDCP servers.
    2. On IdM clients, restart the sssd service to make the changes take effect:
      [root@server ~]# systemctl restart sssd
  3. To use Kerberos-aware rsh and rlogin services, install the rsh package.
  4. Before a workstation can use Kerberos to authenticate users who connect using ssh, rsh, or rlogin, it must have its own host principal in the Kerberos database. The sshd, kshd, and klogind server programs all need access to the keys for the host service's principal.
    1. Using kadmin, add a host principal for the workstation on the KDC. The instance in this case is the host name of the workstation. Use the -randkey option for the kadmin's addprinc command to create the principal and assign it a random key:
      addprinc -randkey host/server.example.com
    2. The keys can be extracted for the workstation by running kadmin on the workstation itself and using the ktadd command.
      ktadd -k /etc/krb5.keytab host/server.example.com
  5. To use other Kerberos-aware network services, install the krb5-server package and start the services. The Kerberos-aware services are listed in Table 11.3, “Common Kerberos-aware Services”.

Table 11.3. Common Kerberos-aware Services

Service Name Usage Information
ssh OpenSSH uses GSS-API to authenticate users to servers if the client's and server's configuration both have GSSAPIAuthentication enabled. If the client also has GSSAPIDelegateCredentials enabled, the user's credentials are made available on the remote system. OpenSSH also contains the sftp tool, which provides an FTP-like interface to SFTP servers and can use GSS-API.
IMAP
The cyrus-imap package uses Kerberos 5 if it also has the cyrus-sasl-gssapi package installed. The cyrus-sasl-gssapi package contains the Cyrus SASL plugins which support GSS-API authentication. Cyrus IMAP functions properly with Kerberos as long as the cyrus user is able to find the proper key in /etc/krb5.keytab, and the root for the principal is set to imap (created with kadmin).
An alternative to cyrus-imap can be found in the dovecot package, which is also included in Red Hat Enterprise Linux. This package contains an IMAP server but does not, to date, support GSS-API and Kerberos.

11.4. Setting up a Kerberos Client for Smart Cards

Smart cards can be used with Kerberos, but it requires additional configuration to recognize the X.509 (SSL) user certificates on the smart cards:
  1. Install the required PKI/OpenSSL package, along with the other client packages:
    [root@server ~]# yum install krb5-pkinit
    [root@server ~]# yum install krb5-workstation krb5-libs
  2. Edit the /etc/krb5.conf configuration file to add a parameter for the public key infrastructure (PKI) to the [realms] section of the configuration. The pkinit_anchors parameter sets the location of the CA certificate bundle file.
    [realms]
      EXAMPLE.COM = {
        kdc = kdc.example.com.:88
        admin_server = kdc.example.com
        default_domain = example.com
        ...
        pkinit_anchors = FILE:/usr/local/example.com.crt
     }
  3. Add the PKI module information to the PAM configuration for both smart card authentication (/etc/pam.d/smartcard-auth) and system authentication (/etc/pam.d/system-auth). The line to be added to both files is as follows:
    auth    optional    pam_krb5.so use_first_pass no_subsequent_prompt preauth_options=X509_user_identity=PKCS11:/usr/lib64/pkcs11/opensc-pkcs11.so
    If the OpenSC module does not work as expected, use the module from the coolkey package: /usr/lib64/pkcs11/libcoolkeypk11.so. In this case, consider contacting Red Hat Technical Support or filing a Bugzilla report about the problem.

11.5. Setting up Cross-Realm Kerberos Trusts

The Kerberos V5 realm is a set of Kerberos principals defined in the Kerberos database on all connected masters and slaves. You must configure cross-realm Kerberos trust if you want principals from different realms to communicate with each other.
A lot of Linux environments, as well as mixed environments, will already have a Kerberos realm deployed for single sign-on, application authentication, and user management. That makes Kerberos a potentially common integration path for different domains and mixed system (such as Windows and Linux) environments, particularly if the Linux environment is not using a more structured domain configuration like Identity Management.

11.5.1. A Trust Relationship

A trust means that the users within one realm are trusted to access the resources in another domain as if they belonged to that realm. This is done by creating a shared key for a single principal that is held in common by both domains.
Basic Trust

Figure 11.2. Basic Trust

In Figure 11.2, “Basic Trust”, the shared principal would belong to Domain B (krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM). When that principal is also added to Domain A, then the clients in Domain A can access the resources in Domain B. The configured principal exists in both realms. That shared principal has three characteristics:
  • It exists in both realms.
  • When a key is created, the same password is used in both realms.
  • The key has the same key version number (kvno).
A cross-realm trust is unidirectional by default. This trust is not automatically reciprocated so that the B.EXAMPLE.COM realm are trusted to authenticate to services in the A.EXAMPLE.COM realm. To establish trust in the other direction, both realms would need to share keys for the krbtgt/A.EXAMPLE.COM@B.EXAMPLE.COM service — an entry with a reverse mapping from the previous example.
A realm can have multiple trusts, both realms that it trusts and realms it is trusted by. With Kerberos trusts, the trust can flow in a chain. If Realm A trusts Realm B and Realm B trusts Realm C, Realm A implicitly trusts Realm C, as well. The trust flows along realms; this is a transitive trust.
Transitive Trust

Figure 11.3. Transitive Trust

The direction of a transitive trust is the trust flow. The trust flow has to be defined, first by recognizing to what realm a service belongs and then by identifying what realms a client must contact to access that service.
A Kerberos principal name is structured in the format service/hostname@REALM. The service is generally a protocol, such as LDAP, IMAP, HTTP, or host. The hostname is the fully-qualified domain name of the host system, and the REALM is the Kerberos realm to which it belongs. Kerberos clients typically use the host name or DNS domain name for Kerberos realm mapping. This mapping can be explicit or implicit. Explicit mapping uses the [domain_realm] section of the /etc/krb5.conf file. With implicit mapping, the domain name is converted to upper case; the converted name is then assumed to be the Kerberos realm to search.
When traversing a trust, Kerberos assumes that each realm is structured like a hierarchical DNS domain, with a root domain and subdomains. This means that the trust flows up to a shared root. Each step, or hop, has a shared key. In Figure 11.4, “Trusts in the Same Domain”, SALES.EXAMPLE.COM shares a key with EXAMPLE.COM, and EXAMPLE.COM shares a key with EVERYWHERE.EXAMPLE.COM.
Trusts in the Same Domain

Figure 11.4. Trusts in the Same Domain

The client treats the realm name as a DNS name, and it determines its trust path by stripping off elements of its own realm name until it reaches the root name. It then begins prepending names until it reaches the service's realm.
Child/Parent Trusts in the Same Domain

Figure 11.5. Child/Parent Trusts in the Same Domain

This is a nature of trusts being transitive. SITE.SALES.EXAMPLE.COM only has a single shared key, with SALES.EXAMPLE.COM. But because of a series of small trusts, there is a large trust flow that allows trust to go from SITE.SALES.EXAMPLE.COM to EVERYWHERE.EXAMPLE.COM.
That trust flow can even go between completely different domains by creating a shared key at the domain level, where the sites share no common suffix.
Trusts in Different Domains

Figure 11.6. Trusts in Different Domains

The [capaths] section

It is also possible to reduce the number of hops and represent very complex trust flows by explicitly defining the flow. The [capaths] section of the /etc/krb5.conf file defines the trust flow between different realms.
The format of the [capaths] section is relatively straightforward: there is a main entry for each realm where a client has a principal, and then inside each realm section is a list of intermediate realms from which the client must obtain credentials.
For example, [capaths] can be used to specify the following process for obtaining credentials:
  1. With credentials from Realm A, the client obtains a krbtgt/A@A ticket from the KDC of Realm A. Using this ticket, the client then asks for the krbtgt/B@A ticket.
    The krbtgt/B@A ticket issued by the KDC of Realm A is a cross-realm ticket granting ticket. It allows the client to ask the KDC of Realm B for a ticket to a service principal of Realm B.
  2. With the krbtgt/B@A ticket, the client asks for the krbtgt/C@B cross-realm ticket.
  3. With the krbtgt/C@B ticket issued by the KDC of Realm B, the client asks for the krbtgt/D@C cross-realm ticket.
  4. With the krbtgt/D@C ticket issued by the KDC of Realm C, the client asks the KDC of Realm D for a ticket to a service principal in Realm D.
After this, the credentials cache contains tickets for krbtgt/A@A, krbtgt/B@A, krbtgt/C@B, krbtgt/D@C, and service/hostname@D. To obtain the service/hostname@D ticket, it was required to obtain the three intermediate cross-realm tickets.
For more information on the [capaths] section, including examples of the [capaths] configuration, see the krb5.conf(5) man page.

11.5.2. Setting up a Realm Trust

In this example, the Kerberos realms are A.EXAMPLE.COM and B.EXAMPLE.COM.
Create the entry for the shared principal for the B realm in the A realm, using kadmin.
[root@server ~]# kadmin -r A.EXAMPLE.COM
kadmin: add_principal krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM
Enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Re-enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM" created.
quit
That means that the A realm will trust the B principal.

Important

It is recommended that you choose very strong passwords for cross-realm principals. Unlike many other passwords, for which the user can be prompted as often as several times a day, the system will not request the password for cross-realm principal frequently from you, which is why it does not need to be easy to memorize.
To create a bidirectional trust, then create principals going the reverse way. Create a principal for the A realm in the B realm, using kadmin.
[root@server ~]# kadmin -r B.EXAMPLE.COM
kadmin: add_principal krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM
Enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Re-enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM" created.
quit
Use the get_principal command to verify that both entries have matching key version numbers (kvno values) and encryption types.

Important

A common, but incorrect, situation is for administrators to try to use the add_principal command's -randkey option to assign a random key instead of a password, dump the new entry from the database of the first realm, and import it into the second. This will not work unless the master keys for the realm databases are identical, as the keys contained in a database dump are themselves encrypted using the master key.


[3] A system where both the client and the server share a common key that is used to encrypt and decrypt network communication.

Chapter 12. Working with certmonger

Part of managing machine authentication is managing machine certificates. The certmonger service manages certificate life cycle for applications and, if properly configured, can work together with a certificate authority (CA) to renew certificates.
The certmonger daemon and its command-line clients simplify the process of generating public/private key pairs, creating certificate requests, and submitting requests to the CA for signing. The certmonger daemon monitors certificates for expiration and can renew certificates that are about to expire. The certificates that certmonger monitors are tracked in files stored in a configurable directory. The default location is /var/lib/certmonger/requests.

Note

The certmonger daemon cannot revoke certificates. A certificate can only be revoked by a relevant Certificate Authority, which needs to invalidate the certificate and update its Certificate Revocation List.

12.1. certmonger and Certificate Authorities

By default, certmonger can automatically obtain three kinds of certificates that differ in what authority source the certificate employs:
  • Self-signed certificate
    Generating a self-signed certificate does not involve any CA, because each certificate is signed using the certificate's own key. The software that is verifying a self-signed certificate needs to be instructed to trust that certificates directly in order to verify it.
    To obtain a self-signed certificate, run the selfsign-getcert command.
  • Certificate from the Dogtag Certificate System CA as part of Red Hat Enterprise Linux IdM
    To obtain a certificate using an IdM server, run the ipa-getcert command
  • Certificate signed by a local CA present on the system
    The software that is verifying a certificate signed by a local signer needs to be instructed to trust certificates from this local signer in order to verify them.
    To obtain a locally-signed certificate, run the local-getcert command.
Other CAs can also use certmonger to manage certificates, but support must be added to certmonger by creating special CA helpers. For more information on how to create CA helpers, see the certmonger project documentation at https://pagure.io/certmonger/blob/master/f/doc/submit.txt.

12.2. Requesting a Self-signed Certificate with certmonger

To request a certificate with certmonger, use the getcert request utility.
Certificates and keys are stored locally in plain text files with the .pem extension or in an NSS database, identified by the certificate nickname. When requesting a certificate, then, the request should identify the location where the certificate will be stored and the nickname of the certificate. For example:
[root@server ~]# selfsign-getcert request -d /etc/pki/nssdb -n Server-Cert
The /etc/pki/nssdb file is the global NSS database, and Server-Cert is the nickname of this certificate. The certificate nickname must be unique within this database.
The options you can provide with the command to generate a certificate vary depending on what kind of certificate you are requesting and the required configuration for the final certificate, as well as other settings:
  • -r automatically renews the certificate when its expiration date is close if the key pair already exists. This option is used by default.
  • -f stores the certificate in the given file.
  • -k either stores the key in the given file or, if the key file already exists, uses the key in the file.
  • -K gives the Kerberos principal name of the service that will be using the certificate; -K is required when requesting a certificate from an IdM server and optional when requesting a self-signed or locally-signed certificate
  • -N gives the subject name.
  • -D requests a DNS domain name to be included in the certificate as a subjectAltName value.
  • -U sets the extended key usage flag.
  • -A requests an IP address to be included in the certificate as a subjectAltName value.
  • -I sets a name for the task. certmonger uses this name to refer to the combination of storage locations and request options, and it is also displayed in the output of the getcert list command. If you do not specify this option, certmonger assigns an automatically-generated name for the task.
A real CA, such as the one in IdM, can ignore anything that you specify in the signing request using the -K, -N, -D, -U, and -A options according to the CA's own policies. For example, IdM requires that -K and -N agree with the local host name. Certificates generated using the selfsign-getcert and local-getcert commands, on the other hand, agree with the options that you specify because these commands do not enforce any policy.

Example 12.1. Using certmonger for a Service

[root@server ~]# selfsign-getcert request -f /etc/httpd/conf/ssl.crt/server.crt -k /etc/httpd/conf/ssl.key/server.key -N CN=`hostname --fqdn` -D `hostname` -U id-kp-serverAuth

12.3. Requesting a CA-signed Certificate Through SCEP

The Simple Certificate Enrollment Protocol (SCEP) automates and simplifies the process of certificate management with the CA. It lets a client request and retrieve a certificate over HTTP directly from the CA's SCEP service. This process is secured by a one-time PIN that is usually valid only for a limited time.
The following example adds a SCEP CA configuration to certmonger, requests a new certificate, and adds it to the local NSS database.
  1. Add the CA configuration to certmonger:
    [root@server ~]# getcert add-scep-ca -c CA_Name -u SCEP_URL
    • -c: Mandatory nickname for the CA configuration. The same value can later be passed to other getcert commands.
    • -u: URL to the server's SCEP interface.
    • Mandatory parameter when using an HTTPS URL:
      -R CA_Filename: Location of the PEM-formatted copy of the SCEP server's CA certificate, used for the HTTPS encryption.
  2. Verify that the CA configuration has been successfully added:
    [root@server ~]# getcert list-cas -c CA_Name
    CA 'CA_Name':
           is-default: no
           ca-type: EXTERNAL
           helper-location: /usr/libexec/certmonger/scep-submit -u http://SCEP_server_enrollment_interface_URL
           SCEP CA certificate thumbprint (MD5): A67C2D4B 771AC186 FCCA654A 5E55AAF7
           SCEP CA certificate thumbprint (SHA1): FBFF096C 6455E8E9 BD55F4A5 5787C43F 1F512279
    The CA configuration was successfully added, when the CA certificate thumbprints were retrieved over SCEP and shown in the command's output. When accessing the server over unencrypted HTTP, manually compare the thumbprints with the ones displayed at the SCEP server to prevent a Man-in-the-middle attack.
  3. Request a certificate from the CA:
    [root@server ~]# getcert request -I Task_Name -c CA_Name -d /etc/pki/nssdb -n Certificate_Name -N cn="Subject Name" -L one-time_PIN
    • -I: Name of the task. The same value can later be passed to the getcert list command.
    • -c: CA configuration to submit the request to.
    • -d: Directory with the NSS database to store the certificate and key.
    • -n: Nickname of the certificate, used in the NSS database.
    • -N: Subject name in the CSR.
    • -L: Time-limited one-time PIN issued by the CA.
  4. Right after submitting the request, you can verify that a certificate was issued and correctly stored in the local database:
    [root@server ~]# getcert list -I TaskName
    	Request ID 'Task_Name':
            status: MONITORING
            stuck: no
            key pair storage: type=NSSDB,location='/etc/pki/nssdb',nickname='TestCert',token='NSS Certificate DB'
            certificate: type=NSSDB,location='/etc/pki/nssdb',nickname='TestCert',token='NSS Certificate DB'
            signing request thumbprint (MD5): 503A8EDD DE2BE17E 5BAA3A57 D68C9C1B
            signing request thumbprint (SHA1): B411ECE4 D45B883A 75A6F14D 7E3037F1 D53625F4
            CA: AD-Name
            issuer: CN=windows-CA,DC=ad,DC=example,DC=com
            subject: CN=Test Certificate
            expires: 2018-05-06 10:28:06 UTC
            key usage: digitalSignature,keyEncipherment
            eku: iso.org.dod.internet.security.mechanisms.8.2.2
            certificate template/profile: IPSECIntermediateOffline
            pre-save command:
            post-save command:
            track: yes
    	auto-renew: yes
    The status MONITORING signifies a successful retrieval of the issued certificate. The getcert-list(1) man page lists other possible states and their meanings.

12.4. Storing Certificates in NSS Databases

By default, certmonger uses .pem files to store the key and the certificate. To store the key and the certificate in an NSS database, specify the -d and -n with the command you use for requesting the certificate.
  • -d sets the security database location
  • -n gives the certificate nickname which is used for the certificate in the NSS database

Note

The -d and -n options are used instead of the -f and -k options that give the .pem file.
For example:
[root@server ~]# selfsign-getcert request -d /export/alias -n ServerCert ...
Requesting a certificate using ipa-getcert and local-getcert allows you to specify another two options:
  • -F gives the file where the certificate of the CA is to be stored.
  • -a gives the location of the NSS database where the certificate of the CA is to be stored.

Note

If you request a certificate using selfsign-getcert, there is no need to specify the -F and -a options because generating a self-signed certificate does not involve any CA.
Supplying the -F option, the -a option, or both with local-getcert allows you to obtain a copy of the CA certificate that is required in order to verify a certificate issued by the local signer. For example:
[root@server ~]# local-getcert request -F /etc/httpd/conf/ssl.crt/ca.crt -n ServerCert -f /etc/httpd/conf/ssl.crt/server.crt -k /etc/httpd/conf/ssl.key/server.key

12.5. Tracking Certificates with certmonger

certmonger can monitor expiration date of a certificate and automatically renew the certificate at the end of its validity period. To track a certificate in this way, run the getcert start-tracking command.

Note

It is not required that you run getcert start-tracking after running getcert request, because the getcert request command by default automatically tracks and renews the requested certificate. The getcert start-tracking command is intended for situations when you have already obtained the key and certificate through some other process, and therefore you have to manually instruct certmonger to start the tracking.
The getcert start-tracking command takes several options:
  • -r automatically renews the certificate when its expiration date is close if the key pair already exists. This option is used by default.
  • -I sets a name for the tracking request. certmonger uses this name to refer to the combination of storage locations and request options, and it is also displayed in the output of the getcert list command. If you do not specify this option, certmonger assigns an automatically generated a name for the task.
[root@server ~]# getcert start-tracking -I cert1-tracker -d /export/alias -n ServerCert
To cancel tracking for a certificate, run the stop-tracking command.

Chapter 13. Configuring Applications for Single Sign-On

Some common applications, such as browsers and email clients, can be configured to use Kerberos tickets, SSL certifications, or tokens as a means of authenticating users.
The precise procedures to configure any application depend on that application itself. The examples in this chapter (Mozilla Thunderbird and Mozilla Firefox) are intended to give you an idea of how to configure a user application to use Kerberos or other credentials.

13.1. Configuring Firefox to Use Kerberos for Single Sign-On

Firefox can use Kerberos for single sign-on (SSO) to intranet sites and other protected websites. For Firefox to use Kerberos, it first has to be configured to send Kerberos credentials to the appropriate KDC.
Even after Firefox is configured to pass Kerberos credentials, it still requires a valid Kerberos ticket to use. To generate a Kerberos ticket, use the kinit command and supply the user password for the user on the KDC.
[jsmith@host ~] $ kinit
Password for jsmith@EXAMPLE.COM:
To configure Firefox to use Kerberos for SSO:
  1. In the address bar of Firefox, type about:config to display the list of current configuration options.
  2. In the Filter field, type negotiate to restrict the list of options.
  3. Double-click the network.negotiate-auth.trusted-uris entry.
  4. Enter the name of the domain against which to authenticate, including the preceding period (.). If you want to add multiple domains, enter them in a comma-separated list.
    Manual Firefox Configuration

    Figure 13.1. Manual Firefox Configuration

Important

It is not recommended to configure delegation using the network.negotiate-auth.delegation-uris entry in the Firefox configuration options because this enables every Kerberos-aware server to act as the user.

Note

For information about configuring Firefox to use Kerberos in Identity Management, see the corresponding section in the Linux Domain Identity, Authentication, and Policy Guide.

13.2. Certificate Management in Firefox

To manage certificates in Firefox, open the Certificate Manager.
  1. In Mozilla Firefox, open the Firefox menu and click Preferences.
    Firefox Preferences

    Figure 13.2. Firefox Preferences

  2. Open the Advanced section and choose the Certificates tab.
    Certificates Tab in Firefox

    Figure 13.3. Certificates Tab in Firefox

  3. Click View Certificates to open the Certificate Manager.
To import a CA certificate:
  1. Download and save the CA certificate to your computer.
  2. In the Certificate Manager, choose the Authorities tab and click Import.
    Importing the CA Certificate in Firefox

    Figure 13.4. Importing the CA Certificate in Firefox

  3. Select the downloaded CA certificate.
To set the certificate trust relationships:
  1. In the Certificate Manager, under the Authorities tab, select the appropriate certificate and click Edit Trust.
  2. Edit the certificate trust settings.
    Editing the Certificate Trust Settings in Firefox

    Figure 13.5. Editing the Certificate Trust Settings in Firefox

To use a personal certificate for authentication:
  1. In the Certificate Manager, under the Your Certificates tab, click Import.
    Importing a Personal Certificate for Authentication in Firefox

    Figure 13.6. Importing a Personal Certificate for Authentication in Firefox

  2. Select the required certificate from your computer.

13.3. Certificate Management in Email Clients

The following example shows how to manage certificates in the Mozilla Thunderbird email client. It represents a procedure to set up certificates in email clients in general.
  1. In Mozilla Thunderbird, open the Thunderbird main menu and select PreferencesAccount Settings.
  2. Select the Security item, and click View Certificates to open the Certificate Manager.
    Account Settings in Thunderbird

    Figure 13.7. Account Settings in Thunderbird

To import a CA certificate:
  1. Download and save the CA certificate to your computer.
  2. In the Certificate Manager, choose the Authorities tab and click Import.
    Importing the CA Certificate in Thunderbird

    Figure 13.8. Importing the CA Certificate in Thunderbird

  3. Select the downloaded CA certificate.
To set the certificate trust relationships:
  1. In the Certificate Manager, under the Authorities tab, select the appropriate certificate and click Edit Trust.
  2. Edit the certificate trust settings.
    Editing the Certificate Trust Settings in Thunderbird

    Figure 13.9. Editing the Certificate Trust Settings in Thunderbird

To use a personal certificate for authentication:
  1. In the Certificate Manager, under the Your Certificates tab, click Import.
    Importing a Personal Certificate for Authentication in Thunderbird

    Figure 13.10. Importing a Personal Certificate for Authentication in Thunderbird

  2. Select the required certificate from your computer.
  3. Close the Certificate Manager and return to the Security item in Account Settings.
  4. Under the Digital Signing section of the form, click Select to choose your personal certificate to use for signing messages.
  5. Under Encryption, click Select to choose your personal certificate to encrypt and decrypt messages.

Appendix A. Troubleshooting

A.1. Troubleshooting SSSD

A.1.1. Setting Debug Logs for SSSD Domains

Each domain sets its own debug log level. Increasing the log level can provide more information about problems with SSSD or with the domain configuration.
To change the log level, set the debug_level parameter for each section in the sssd.conf file for which to produce extra logs. For example:
[domain/LDAP]
cache_credentials = true
debug_level = 9

Table A.1. Debug Log Levels

Level Description
0 Fatal failures. Anything that would prevent SSSD from starting up or causes it to cease running.
1 Critical failures. An error that does not kill the SSSD, but one that indicates that at least one major feature is not going to work properly.
2 Serious failures. An error announcing that a particular request or operation has failed.
3 Minor failures. These are the errors that would percolate down to cause the operation failure of 2.
4 Configuration settings.
5 Function data.
6 Trace messages for operation functions.
7 Trace messages for internal control functions.
8 Contents of function-internal variables that may be interesting.
9 Extremely low-level tracing information.
To change the debug level while SSSD is running, use the sss_debuglevel utility, which is part of the sssd-tools package. For more information about how it works, see the sss_debuglevel man page.

A.1.2. Checking SSSD Log Files

SSSD uses a number of log files to report information about its operation, located in the /var/log/sssd/ directory. SSSD produces a log file for each domain, as well as an sssd_pam.log and an sssd_nss.log file.
Additionally, the /var/log/secure file logs authentication failures and the reason for the failure.

A.1.3. Problems with SSSD Configuration

Q: SSSD fails to start
Q: I do not see any groups with id or group members with getent group.
Q: Authentication fails against LDAP.
Q: Connecting to LDAP servers on non-standard ports fail.
Q: NSS fails to return user information
Q: NSS returns incorrect user information
Q: Setting the password for the local SSSD user prompts twice for the password
Q: An Active Directory identity provider is properly configured in my sssd.conf file, but SSSD fails to connect to it, with GSS-API errors.
Q: I configured SSSD for central authentication, but now several of my applications (such as Firefox or Adobe) will not start.
Q: SSSD is showing an automount location that I removed.
Q:
SSSD fails to start
A:
SSSD requires that the configuration file be properly set up, with all the required entries, before the daemon will start.
  • SSSD requires at least one properly configured domain before the service will start. Without a domain, attempting to start SSSD returns an error that no domains are configured:
    # sssd -d4 -i
    
    [sssd] [ldb] (3): server_sort:Unable to register control with rootdse!
    [sssd] [confdb_get_domains] (0): No domains configured, fatal error!
    [sssd] [get_monitor_config] (0): No domains configured.
    
    Edit the /etc/sssd/sssd.conf file and create at least one domain.
  • SSSD also requires at least one available service provider before it will start. If the problem is with the service provider configuration, the error message indicates that there are no services configured:
    [sssd] [get_monitor_config] (0): No services configured!
    
    Edit the /etc/sssd/sssd.conf file and configure at least one service provider.

    Important

    SSSD requires that service providers be configured as a comma-separated list in a single services entry in the /etc/sssd/sssd.conf file. If services are listed in multiple entries, only the last entry is recognized by SSSD.
  • SSSD also requires the ownership and permissions of the /etc/sssd/sssd.conf to be set correctly. If the ownership or permissions are set incorrectly, attempt to start SSSD returns these error messages:
    [sssd] [confdb_ldif_from_ini_file] (0x0020): Permission check on config file failed.
    [sssd] [confdb_init_db] (0x0020): Cannot convert INI to LDIF [1]: [Operation not permitted]
    [sssd] [confdb_setup] (0x0010): ConfDB initialization has failed [1]: Operation not permitted
    [sssd] [load_configuration] (0x0010): Unable to setup ConfDB [1]: Operation not permitted
    [sssd] [main] (0x0020): Cannot read config file /etc/sssd/sssd.conf. Please check that the file is accessible only by the owner and owned by root.root.
    
    Set the correct ownership and permissions of the /etc/sssd/sssd.conf file:
    # chmod 600 /etc/sssd/sssd.conf
    # chown root:root /etc/sssd/sssd.conf
    
Q:
I do not see any groups with id or group members with getent group.
A:
This may be due to an incorrect ldap_schema setting in the [domain/DOMAINNAME] section of sssd.conf.
SSSD supports RFC 2307 and RFC 2307bis schema types. By default, SSSD uses the more common RFC 2307 schema.
The difference between RFC 2307 and RFC 2307bis is the way which group membership is stored in the LDAP server. In an RFC 2307 server, group members are stored as the multi-valued memberuid attribute, which contains the name of the users that are members. In an RFC2307bis server, group members are stored as the multi-valued member or uniqueMember attribute which contains the DN of the user or group that is a member of this group. RFC2307bis allows nested groups to be maintained as well.
If group lookups are not returning any information:
  1. Set ldap_schema to rfc2307bis.
  2. Delete /var/lib/sss/db/cache_DOMAINNAME.ldb.
  3. Restarting SSSD.
If that does not work, add this line to sssd.conf:
ldap_group_name = uniqueMember
Then delete the cache and restart SSSD again.
Q:
Authentication fails against LDAP.
A:
To perform authentication, SSSD requires that the communication channel be encrypted. This means that if sssd.conf is configured to connect over a standard protocol (ldap://), it attempts to encrypt the communication channel with Start TLS. If sssd.conf is configured to connect over a secure protocol (ldaps://), then SSSD uses SSL.
This means that the LDAP server must be configured to run in SSL or TLS. TLS must be enabled for the standard LDAP port (389) or SSL enabled on the secure LDAPS port (636). With either SSL or TLS, the LDAP server must also be configured with a valid certificate trust.
An invalid certificate trust is one of the most common issues with authenticating against LDAP. If the client does not have proper trust of the LDAP server certificate, it is unable to validate the connection, and SSSD refuses to send the password. The LDAP protocol requires that the password be sent in plain text to the LDAP server. Sending the password in plain text over an unencrypted connection is a security problem.
If the certificate is not trusted, a syslog message is written, indicating that TLS encryption could not be started. The certificate configuration can be tested by checking if the LDAP server is accessible apart from SSSD. For example, this tests an anonymous bind over a TLS connection to test.example.com:
$ ldapsearch -x -ZZ -h test.example.com -b dc=example,dc=com
If the certificate trust is not properly configured, the test fails with this error:
ldap_start_tls: Connect error (-11) additional info: TLS error -8179:Unknown code ___f 13
To trust the certificate:
  1. Obtain a copy of the public CA certificate for the certificate authority used to sign the LDAP server certificate and save it to the local system.
  2. Add a line to the sssd.conf file that points to the CA certificate on the filesystem.
    ldap_tls_cacert = /path/to/cacert
  3. If the LDAP server uses a self-signed certificate, remove the ldap_tls_reqcert line from the sssd.conf file.
    This parameter directs SSSD to trust any certificate issued by the CA certificate, which is a security risk with a self-signed CA certificate.
Q:
Connecting to LDAP servers on non-standard ports fail.
A:
When running SELinux in enforcing mode, the client's SELinux policy has to be modified to connect to the LDAP server over the non-standard port. For example:
# semanage port -a -t ldap_port_t -p tcp 1389
Q:
NSS fails to return user information
A:
This usually means that SSSD cannot connect to the NSS service.
  • Ensure that the NSS service is running:
    # service sssd status
    Redirecting to /bin/systemctl status sssd.service
      sssd.service - System Security Services Daemon
       Loaded: loaded (/usr/lib/systemd/system/sssd.service; enabled)
       Active: active (running) since Wed 2015-01-14 10:17:26 CET; 1min 30s ago
       Process: 683 ExecStart=/usr/sbin/sssd -D -f (code=exited, status=0/SUCCESS)
     Main PID: 745 (sssd)
       CGroup: /system.slice/sssd.service
                ├─745 /usr/sbin/sssd -D -f
    	    ├─746 /usr/libexec/sssd/sssd_be --domain default --debug-to-files...
    	    ├─804 /usr/libexec/sssd/sssd_nss --debug-to-files
    	    └─805 /usr/libexec/sssd/sssd_pam --debug-to-files
    NSS service is running when SSSD is in the Active: active (running) state and when the output includes sssd_nss.
  • If NSS is running, make sure that the provider is properly configured in the [nss] section of the /etc/sssd/sssd.conf file. Especially check the filter_users and filter_groups attributes.
  • Make sure that NSS is included in the list of services that SSSD uses.
  • Check the configuration in the /etc/nsswitch.conf file. For more information, see the section called “Configure NSS Services to Use SSSD”.
Q:
NSS returns incorrect user information
A:
If searches are returning the incorrect user information, check that there are not conflicting user names in separate domains. When there are multiple domains, set the use_fully_qualified_domains attribute to true in the /etc/sssd/sssd.conf file. This differentiates between different users in different domains with the same name.
Q:
Setting the password for the local SSSD user prompts twice for the password
A:
When attempting to change a local SSSD user's password, it may prompt for the password twice:
[root@clientF11 tmp]# passwd user1000
Changing password for user user1000.
New password:
Retype new password:
New Password:
Reenter new Password:
passwd: all authentication tokens updated successfully.
This is the result of an incorrect PAM configuration. Ensure that the use_authtok option is correctly configured in your /etc/pam.d/system-auth file. For examples of the correct configuration, see Section 7.5.2, “Configuring Services: PAM”.
Q:
An Active Directory identity provider is properly configured in my sssd.conf file, but SSSD fails to connect to it, with GSS-API errors.
A:
SSSD can only connect with an Active Directory provider using its host name. If the host name is not given, the SSSD client cannot resolve the IP address to the host, and authentication fails.
For example, with this configuration:
[domain/ADEXAMPLE]
debug_level = 0xFFF0
id_provider = ad
ad_server = 172.16.0.1
ad_domain = example.com
krb5_canonicalize = False
The SSSD client returns this GSS-API failure, and the authentication request fails:
(Fri Jul 27 18:27:44 2012) [sssd[be[ADTEST]]] [sasl_bind_send] (0x0020): ldap_sasl_bind failed (-2)[Local error]
(Fri Jul 27 18:27:44 2012) [sssd[be[ADTEST]]] [sasl_bind_send] (0x0080): Extended failure message: [SASL(-1): generic failure: GSSAPI Error: Unspecified GSS failure.  Minor code may provide more information (Cannot determine realm for numeric host address)]
To avoid this error, set the ad_server to the name of the Active Directory host, or use the _srv_ keyword to use the DNS service discovery, as described in Section 7.4.3, “Configuring DNS Service Discovery”.
Q:
I configured SSSD for central authentication, but now several of my applications (such as Firefox or Adobe) will not start.
A:
Even on 64-bit systems, 32-bit applications require a 32-bit version of SSSD client libraries to use to access the password and identity cache. If a 32-bit version of SSSD is not available, but the system is configured to use the SSSD cache, then 32-bit applications can fail to start.
For example, Firefox can fail with permission denied errors:
Failed to contact configuration server. See http://www.gnome.org/projects/gconf/
for information. (Details -  1: IOR file '/tmp/gconfd-somebody/lock/ior'
not opened successfully, no gconfd located: Permission denied 2: IOR
file '/tmp/gconfd-somebody/lock/ior' not opened successfully, no gconfd
located: Permission denied)
For Adobe Reader, the error shows that the current system user is not recognized:
[jsmith@server ~]$ acroread
(acroread:12739): GLib-WARNING **: getpwuid_r(): failed due to unknown
user id (366)
Other applications may show similar user or permissions errors.
Q:
SSSD is showing an automount location that I removed.
A:
The SSSD cache for the automount location persists even if the location is subsequently changed or removed. To update the autofs information in SSSD:
  1. Restart SSSD:
    # systemctl restart sssd

A.1.4. A User Cannot Log In After UID or GID Changed

After a user or group ID changed, SSSD prevents a user from logging in.

What this means:

SSSD recognizes users and groups based on user IDs (UID) and group IDs (GID). When the UID or GID of an existing user or group changes, SSSD does not recognize the user or group.

To fix the problem:

Clear the SSSD cache using the sss_cache utility:
  1. Make sure the sssd-tools is installed.
  2. To clear the SSSD cache for a specific user and leave the rest of the cache records intact:
    # sss_cache --user user_name
    To clear the cache for an entire domain:
    # sss_cache --domain domain_name
The utility invalidates records in the SSSD cache for a user, group, or domain. After this, SSSD retrieves the records from the identity provider to refresh the cache.
For details on sss_cache, see the sss_cache(8) man page.

A.1.5. SSSD Control and Status Utility

SSSD provides the sssctl utility to obtain status information, manage data files during troubleshooting, and other SSSD-related tasks.
  1. To use sssctl, install the sssd-tools package:
    [root@server ~]# yum install sssd-tools
  2. Some options of sssctl use the SSSD InfoPipe responder. To enable it, add ifp to the services option of your /etc/sssd/sssd.conf:
    [sssd]
    services = nss, sudo, pam, ssh, ifp
  3. Restart SSSD:
    [root@server ~]# systemctl restart sssd.service

A.1.5.1. SSSD Configuration Validation

The sssctl config-check command performs a static analysis of the SSSD configuration files. This enables you to validate the /etc/sssd/sssd.conf file and /etc/sssd/conf.d/* files to receive a report before restarting SSSD.
The command performs the following checks on SSSD configuration files:
Permissions
The owner and group owner must be set to root:root and the permissions to 600.
File names
File names in /etc/sssd/conf.d/ must use the suffix .conf and not start with a period (hidden files).
Typographical errors
Typographical errors are checked in section and option names. Note that values are not checked.
Options
Options must be placed in the correct sections.
To verify the configuration, run:
# sssctl config-check
Issues identified by validators: 3
[rule/allowed_sections]: Section [paM] is not allowed. Check for typos.
[rule/allowed_domain_options]: Attribute 'offline_timeoutX' is not allowed in section 'domain/idm.example.com'. Check for typos.
[rule/allowed_sudo_options]: Attribute 'homedir_substring' is not allowed in section 'sudo'. Check for typos.

Messages generated during configuration merging: 2
File /etc/sssd/conf.d/wrong-file-permissions.conf did not pass access check. Skipping.
File configuration.conf.disabled did not match provided patterns. Skipping.

Used configuration snippet files: 1
/etc/sssd/conf.d/sample.conf

A.1.5.2. Domain Information

The domain status displays a list of domains SSSD accesses, and enables you to retrieve their status.
  1. List all domains available within SSSD, including trusted domains:
    [root@server ~]# sssctl domain-list
    idm.example.com
    ad.example.com
    
  2. Retrieve the status of the domain idm.example.com:
    [root@server ~]# sssctl domain-status idm.example.com
    Online status: Online

A.1.5.3. Cached Entries Information

The sssctl command enables you to retrieve information about a cached entry, to investigate and debug cache-related authentication problems.
To query cache information for the user account admin, run:
[root@server ~]# sssctl user-show admin
Name: admin
Cache entry creation date: 07/10/16 16:09:18
Cache entry last update time: 07/14/16 10:13:32
Cache entry expiration time: 07/14/16 11:43:32
Initgroups expiration time: Expired
Cached in InfoPipe: No
To query the cache information for a group or netgroup, use:
[root@server ~]# sssctl group-show groupname
[root@server ~]# sssctl netgroup-show netgroupname

A.1.5.4. Truncating the Log Files

When you debug a problem, you can use sssctl logs-remove to truncate all SSSD log files in the /var/log/sssd/ directory to capture only newly created entries.
[root@server ~]# sssctl logs-remove
Truncating log files...

A.1.5.5. Removing the SSSD Cache

To remove the SSSD cache database files, the sssctl command provides the remove-cache option. Before the databases are removed, the command creates automatically a backup.
Use the following command to back up all local data and remove the SSSD cache:
[root@server ~]# sssctl cache-remove
SSSD must not be running. Stop SSSD now? (yes/no) [yes]
Creating backup of local data...
Removing cache files...
SSSD needs to be running. Start SSSD now? (yes/no) [yes]

Note

The backup stores only local data, such as local overrides, in the /var/lib/sss/backup/ directory.
To automatically import the backed-up content, run sssctl restore-local-data.

A.1.5.6. Obtaining Information about an LDAP Group Takes Long

Operations that involve looking up information about an LDAP group take very long, especially in case of large groups or nested groups.

What this means:

By default, LDAP group information lookups return all members for the group. For operations that involve large groups or nested groups, returning all members makes the process longer.

To fix the problem:

The membership lists returned in group lookups are not used when evaluating whether a user belongs to a group. To improve performance, especially for identity lookups, disable the group membership lookup:
  1. Open the /etc/sssd/sssd.conf file.
  2. In the [domain] section, set the ignore_group_members option to true.
    [domain/domain_name]
    [... file truncated ...]
    ignore_group_members = true

A.2. Troubleshooting sudo with SSSD and sudo Debugging Logs

A.2.1. SSSD and sudo Debug Logging

The debug logging feature enables you to log additional information about SSSD and sudo.

The sudo Debug Log File

To enable sudo debugging:
  1. Add the following lines to /etc/sudo.conf:
    Debug sudo /var/log/sudo_debug.log all@debug
    Debug sudoers.so /var/log/sudo_debug.log all@debug
  2. Run the sudo command as the user you want to debug.
The /var/log/sudo_debug.log file is created automatically and provides detailed information to answer questions like:
  • What information is available about the user and the environment when running the sudo command?
    sudo[22259] settings: debug_flags=all@debug
    sudo[22259] settings: run_shell=true
    sudo[22259] settings: progname=sudo
    sudo[22259] settings: network_addrs=192.0.2.1/255.255.255.0 fe80::250:56ff:feb9:7d6/ffff:ffff:ffff:ffff::
    sudo[22259] user_info: user=user_name
    sudo[22259] user_info: pid=22259
    sudo[22259] user_info: ppid=22172
    sudo[22259] user_info: pgid=22259
    sudo[22259] user_info: tcpgid=22259
    sudo[22259] user_info: sid=22172
    sudo[22259] user_info: uid=10000
    sudo[22259] user_info: euid=0
    sudo[22259] user_info: gid=554801393
    sudo[22259] user_info: egid=554801393
    sudo[22259] user_info: groups=498,6004,6005,7001,106501,554800513,554801107,554801108,554801393,554801503,554802131,554802244,554807670
    sudo[22259] user_info: cwd=/
    sudo[22259] user_info: tty=/dev/pts/1
    sudo[22259] user_info: host=client
    sudo[22259] user_info: lines=31
    sudo[22259] user_info: cols=237
  • What data sources are used to fetch sudo rules?
    sudo[22259] <- sudo_parseln @ ./fileops.c:178 := sudoers: files sss
  • SSSD plug-in starts with this line:
    sudo[22259] <- sudo_sss_open @ ./sssd.c:305 := 0
  • How many rules did SSSD return?
    sudo[22259] Received 3 rule(s)
  • Does a rule match or not?
    sudo[22259] sssd/ldap sudoHost 'ALL' ... MATCH!
    sudo[22259] <- user_in_group @ ./pwutil.c:1010 := false

The SSSD Debug Log Files

To enable SSSD debugging:
  1. Add the debug_level option to the [sudo] and [domain/domain_name] sections of your /etc/sssd/sssd.conf file:
    [domain/domain_name]
    debug_level = 0x3ff0
    ...
    [sudo]
    debug_level = 0x3ff0
  2. Restart SSSD:
    # systemctl restart sssd
  3. Run the sudo command to write the debug information to the log files.
The following log files are created:
The domain log file: /var/log/sssd/sssd_domain_name.log
This log file helps you to answer questions like:
  • How many rules did SSSD return?
    [sdap_sudo_refresh_load_done] (0x0400): Received 4-rules rules
  • What sudo rules did SSSD download from the server?
    [sssd[be[LDAP.PB]]] [sysdb_save_sudorule] (0x0400): Adding sudo rule demo-name
  • Are the matching rules stored in the cache?
    [sdap_sudo_refresh_load_done] (0x0400): Sudoers is successfully stored in cache
  • What filter was used to download the rules from the server?
    [sdap_get_generic_ext_step] (0x0400): calling ldap_search_ext with [(&(objectClass=sudoRole)(|(!(sudoHost=*))(sudoHost=ALL)(sudoHost=client.example.com)(sudoHost=client)(sudoHost=192.0.2.1)(sudoHost=192.0.2.0/24)(sudoHost=2620:52:0:224e:21a:4aff:fe23:1394)(sudoHost=2620:52:0:224e::/64)(sudoHost=fe80::21a:4aff:fe23:1394)(sudoHost=fe80::/64)(sudoHost=+*)(|(sudoHost=*\\*)(sudoHost=*?*)(sudoHost=*\2A*)(sudoHost=*[*]*))))][dc=example,dc=com]
    Use this filter to look up the rules in the IdM database:
    # ldapsearch -x -D "cn=Directory Manager" -W -H ldap://server.example.com -b dc=example,dc=com '(&(objectClass=sudoRole)...)'
The sudo responder log file: /var/log/sssd/sssd_sudo.log
This log file helps you to answer questions like:
  • How many rules did SSSD return?
    [sssd[sudo]] [sudosrv_get_sudorules_from_cache] (0x0400): Returning 4-rules rules for [user@idm.example.com]
  • What filter was applied for searching the cache of SSSD?
    [sudosrv_get_sudorules_query_cache] (0x0200): Searching sysdb with [(&(objectClass=sudoRule)(|(sudoUser=ALL)(sudoUser=user)(sudoUser=#10001)(sudoUser=%group-1)(sudoUser=%user)(sudoUser=+*)))]
  • How do I look up the rules returned from the SSSD cache? Use the following filter to look up the rules:
    # ldbsearch -H /var/lib/sss/db/cache_domain_name.ldb -b cn=sysdb '(&(objectClass=sudoRule)...)'

    Note

    The ldbsearch utility is included in the ldb-tools package.

A.3. Troubleshooting Firefox Kerberos Configuration

If Kerberos authentication is not working, turn on verbose logging for the authentication process.
  1. Close all instances of Firefox.
  2. In a command prompt, export values for the NSPR_LOG_* variables:
    export NSPR_LOG_MODULES=negotiateauth:5
    export NSPR_LOG_FILE=/tmp/moz.log
  3. Restart Firefox from that shell, and visit the website where Kerberos authentication is failing.
  4. Check the /tmp/moz.log file for error messages with nsNegotiateAuth in the message.
There are several common errors that occur with Kerberos authentication.
No credentials found
-1208550944[90039d0]: entering nsNegotiateAuth::GetNextToken()
-1208550944[90039d0]: gss_init_sec_context() failed: Miscellaneous failure
No credentials cache found
This means that no Kerberos tickets are available (meaning that they expired or were not generated). To fix this, run kinit to generate the Kerberos ticket, and then open the website again.
Server not found in Kerberos database
-1208994096[8d683d8]: entering nsAuthGSSAPI::GetNextToken()
-1208994096[8d683d8]: gss_init_sec_context() failed: Miscellaneous failure
Server not found in Kerberos database
This means that the browser is unable to contact the KDC. This is usually a Kerberos configuration problem. The correct entries must be in the [domain_realm] section of the /etc/krb5.conf file to identify the domain. For example:
.example.com = EXAMPLE.COM
example.com = EXAMPLE.COM
No errors are present in the log
An HTTP proxy server could be stripping off the HTTP headers required for Kerberos authentication. Try to connect to the site using HTTPS, which allows the request to pass through unmodified.

Appendix B. Revision History

Note that revision numbers relate to the edition of this manual, not to version numbers of Red Hat Enterprise Linux.

Revision History
Revision 7.0-29Tue Apr 08 2019Marc Muehlfeld
Added Configuring the Files Provider for SSSD and Displaying User Data. Minor fixes and updates.
Revision 7.0-28Fri Oct 26 2018Filip Hanzelka
Preparing document for 7.6 GA publication.
Revision 7.0-27Mon Jun 25 2018Filip Hanzelka
Updated Working with certmonger.
Revision 7.0-26Tue Apr 10 2018Filip Hanzelka
Preparing document for 7.5 GA publication.
Revision 7.0-25Mon Mar 19 2018Lucie Maňásková
Minor updates.
Revision 7.0-24Mon Feb 12 2018Aneta Šteflová Petrová
Minor fixes and updates.
Revision 7.0-23Mon Jan 29 2018Aneta Šteflová Petrová
Minor fixes.
Revision 7.0-22Mon Dec 4 2017Aneta Šteflová Petrová
Updated Smart cards.
Revision 7.0-21Mon Nov 20 2017Aneta Šteflová Petrová
Minor fixes.
Revision 7.0-20Mon Nov 6 2017Aneta Šteflová Petrová
Minor fixes.
Revision 7.0-19Mon Aug 14 2017Aneta Šteflová Petrová
Updated sections that referred to the coolkey package.
Revision 7.0-18Tue Jul 18 2017Aneta Šteflová Petrová
Document version for 7.4 GA publication.
Revision 7.0-17Mon Mar 27 2017Aneta Šteflová Petrová
Fixed broken links.
Revision 7.0-16Mon Feb 27 2017Aneta Šteflová Petrová
Updated Kerberos KDC proxy. Other minor updates.
Revision 7.0-15Wed Dec 7 2016Aneta Šteflová Petrová
Added SSSD client-side views. Other minor updates.
Revision 7.0-14Tue Oct 18 2016Aneta Šteflová Petrová
Version for 7.3 GA publication.
Revision 7.0-13Wed Jul 27 2016Marc Muehlfeld
Added Kerberos over HTTP (kdcproxy), requesting a certificate through SCEP, other minor updates.
Revision 7.0-11Thu Mar 03 2016Aneta Petrová
Added restricting domains for PAM services.
Revision 7.0-10Tue Feb 09 2016Aneta Petrová
Split authconfig chapter into smaller chapters, other minor updates.
Revision 7.0-9Thu Nov 12 2015Aneta Petrová
Version for 7.2 GA release.
Revision 7.0-8Fri Mar 13 2015Tomáš Čapek
Async update with last-minute edits for 7.1.
Revision 7.0-6Wed Feb 25 2015Tomáš Čapek
Version for 7.1 GA release.
Revision 7.0-4Fri Dec 05 2014Tomáš Čapek
Rebuild to update the sort order on the splash page.
Revision 7.0-1July 16, 2014Ella Deon Ballard
Initial draft for RHEL 7.0.

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