Configuring authentication and authorization in RHEL

Red Hat Enterprise Linux 8

Using SSSD, authselect, and sssctl to configure authentication and authorization

Red Hat Customer Content Services

Abstract

This documentation collection provides instructions on how to configure authentication and authorization on a Red Hat Enterprise Linux 8 host.

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Chapter 1. Configuring user authentication using authselect

authselect is a utility that allows you to configure system identity and authentication sources by selecting a specific profile. Profile is a set of files that describes how the resulting Pluggable Authentication Modules (PAM) and Network Security Services (NSS) configuration will look like. You can choose the default profile set or create a custom profile.

1.1. What is authselect used for

You can use the authselect utility to configure user authentication on a Red Hat Enterprise Linux 8 host.

You can configure identity information and authentication sources and providers by selecting one of the ready-made profiles:

  • The default sssd profile enables the System Security Services Daemon (SSSD) for systems that use LDAP authentication.
  • The winbind profile enables the Winbind utility for systems directly integrated with Microsoft Active Directory.
  • The nis profile ensures compatibility with legacy Network Information Service (NIS) systems.
  • The minimal profile serves only local users and groups directly from system files, which allows administrators to remove network authentication services that are no longer needed.

After selecting an authselect profile for a given host, the profile is applied to every user logging into the host.

Red Hat recommends using authselect in semi-centralized identity management environments, for example if your organization utilizes LDAP, Winbind, or NIS databases to authenticate users to use services in your domain.

Warning

Do not use authselect if your host is part of Red Hat Enterprise Linux Identity Management (IdM). Joining your host to an IdM domain with the ipa-client-install command automatically configures SSSD authentication on your host.

Similarly, do not use authselect if your host is part of Active Directory via SSSD. Calling the realm join command to join your host to an Active Directory domain automatically configures SSSD authentication on your host.

1.1.1. Files and directories authselect modifies

The authconfig utility, used in previous Red Hat Enterprise Linux versions, created and modified many different configuration files, making troubleshooting more difficult. Authselect simplifies testing and troubleshooting because it only modifies the following files and directories:

/etc/nsswitch.conf

The GNU C Library and other applications use this Name Service Switch (NSS) configuration file to determine the sources from which to obtain name-service information in a range of categories, and in what order. Each category of information is identified by a database name.

/etc/pam.d/* files

Linux-PAM (Pluggable Authentication Modules) is a system of modules that handle the authentication tasks of applications (services) on the system. The nature of the authentication is dynamically configurable: the system administrator can choose how individual service-providing applications will authenticate users.

The configuration files in the /etc/pam.d/ directory list the PAMs that will perform authentication tasks required by a service, and the appropriate behavior of the PAM-API in the event that individual PAMs fail.

Among other things, these files contain information about:

  • user password lockout conditions
  • the ability to authenticate with a smart card
  • the ability to authenticate with a fingerprint reader

/etc/dconf/db/distro.d/* files

This directory holds configuration profiles for the dconf utility, which you can use to manage settings for the GNOME Desktop Graphical User Interface (GUI).

1.1.2. Data providers in /etc/nsswitch.conf

The default sssd profile establishes SSSD as a source of information by creating sss entries in /etc/nsswitch.conf:

passwd:     sss files
group:      sss files
netgroup:   sss files
automount:  sss files
services:   sss files
...

This means that the system first looks to SSSD if information concerning one of those items is requested:

  • passwd for user information
  • group for user group information
  • netgroup for NIS netgroup information
  • automount for NFS automount information
  • services for information regarding services

Only if the requested information is not found in the sssd cache and on the server providing authentication, or if sssd is not running, the system looks at the local files, that is /etc/*.

For example, if information is requested about a user ID, the user ID is first searched in the sssd cache. If it is not found there, the /etc/passwd file is consulted. Analogically, if a user’s group affiliation is requested, it is first searched in the sssd cache and only if not found there, the /etc/group file is consulted.

In practice, the local files database is not normally consulted. The most important exception is the case of the root user, which is never handled by sssd but by files.

1.2. Choosing an authselect profile

As a system administrator, you can select a profile for the authselect utility for a specific host. The profile will be applied to every user logging into the host.

Prerequisites

  • You need root credentials to run authselect commands

Procedure

  • Select the authselect profile that is appropriate for your authentication provider. For example, for logging into the network of a company that uses LDAP, choose sssd.

    # authselect select sssd
    • (Optional) You can modify the default profile settings by adding the following options to the authselect select sssd or authselect select winbind command, for example:

      • with-faillock
      • with-smartcard
      • with-fingerprint

    To see the full list of available options, see Converting your scripts from authconfig to authselect or the authselect-migration(7) man page.

Note

Make sure that the configuration files that are relevant for your profile are configured properly before finishing the authselect select procedure. For example, if the sssd daemon is not configured correctly and active, running authselect select results in only local users being able to authenticate, using pam_unix.

Verification Steps

  1. Verify sss entries for SSSD are present in /etc/nsswitch.conf:

    passwd:     sss files
    group:      sss files
    netgroup:   sss files
    automount:  sss files
    services:   sss files
    ...
  2. Review the contents of the /etc/pam.d/system-auth file for pam_sss.so entries:

    # Generated by authselect on Tue Sep 11 22:59:06 2018
    # Do not modify this file manually.
    
    auth        required        pam_env.so
    auth        required        pam_faildelay.so delay=2000000
    auth        [default=1 ignore=ignore success=ok]    pam_succeed_if.so uid >= 1000 quiet
    auth        [default=1 ignore=ignore success=ok]    pam_localuser.so
    auth        sufficient      pam_unix.so nullok try_first_pass
    auth        requisite       pam_succeed_if.so uid >= 1000 quiet_success
    auth        sufficient      pam_sss.so forward_pass
    auth        required        pam_deny.so
    
    account     required        pam_unix.so
    account     sufficient      pam_localuser.so
    ...

1.3. Modifying a ready-made authselect profile

As a system administrator, you can modify one of the default profiles to suit your needs.

You can modify any of the items in the /etc/authselect/user-nsswitch.conf file with the exception of:

  • passwd
  • group
  • netgroup
  • automount
  • services

Running authselect select profile_name afterwards will result in transferring permissible changes from /etc/authselect/user-nsswitch.conf to the /etc/nsswitch.conf file. Unacceptable changes are overwritten by the default profile configuration.

Important

Do not modify the /etc/nsswitch.conf file directly.

Procedure

  1. Select an authselect profile, for example:

    # authselect select sssd
  2. Edit the /etc/authselect/user-nsswitch.conf file with your desired changes.
  3. Apply the changes from the /etc/authselect/user-nsswitch.conf file:

    # authselect apply-changes

Verification steps

  • Review the /etc/nsswitch.conf file to verify that the changes from /etc/authselect/user-nsswitch.conf have been propagated there.

Additional Resources

1.4. Creating and deploying your own authselect profile

As a system administrator, you can create and deploy a custom profile by making a customized copy of one of the default profiles.

This is particularly useful if Modifying a ready-made authselect profile is not enough for your needs. When you deploy a custom profile, the profile is applied to every user logging into the given host.

Procedure

  1. Create your custom profile by using the authselect create-profile command. For example, to create a custom profile called user-profile based on the ready-made sssd profile but one in which you can configure the items in the /etc/nsswitch.conf file yourself:

    # authselect create-profile user-profile -b sssd --symlink-meta --symlink-pam
    New profile was created at /etc/authselect/custom/user-profile

    Including the --symlink-pam option in the command means that PAM templates will be symbolic links to the origin profile files instead of their copy; including the --symlink-meta option means that meta files, such as README and REQUIREMENTS will be symbolic links to the origin profile files instead of their copy. This ensures that all future updates to the PAM templates and meta files in the original profile will be reflected in your custom profile, too.

    The command creates a copy of the /etc/nsswitch.conf file in the /etc/authselect/custom/user-profile/ directory.

  2. Configure the /etc/authselect/custom/user-profile/nsswitch.conf file.
  3. Select the custom profile by running the authselect select command, and adding custom/name_of_the_profile as a parameter. For example, to select the user-profile profile:

    # authselect select custom/user-profile

    Selecting the user-profile profile for your machine means that if the sssd profile is subsequently updated by Red Hat, you will benefit from all the updates with the exception of updates made to the /etc/nsswitch.conf file.

    Example 1.1. Creating a profile

    The following procedure shows how to create a profile based on the sssd profile which only consults the local static table lookup for hostnames in the /etc/hosts file, not in the dns or myhostname databases.

    1. Edit the /etc/nsswitch.conf file by editing the following line:

      hosts:      files
    2. Create a custom profile based on sssd that excludes changes to /etc/nsswitch.conf:

      # authselect create-profile user-profile -b sssd --symlink-meta --symlink-pam
    3. Select the profile:

      # authselect select custom/user-profile
    4. Optionally, check that selecting the custom profile has

      • created the /etc/pam.d/system-auth file according to the chosen sssd profile
      • left the configuration in the /etc/nsswitch.conf unchanged:

        hosts:      files
        Note

        Running authselect select sssd would, in contrast, result in hosts: files dns myhostname

Additional Resources

1.5. Converting your scripts from authconfig to authselect

If you use ipa-client-install or realm join to join a domain, you can safely remove any authconfig call in your scripts. If this is not possible, replace each authconfig call with its equivalent authselect call. In doing that, select the correct profile and the appropriate options. In addition, edit the necessary configuration files:

  • /etc/krb5.conf
  • /etc/sssd/sssd.conf (for the sssd profile) or /etc/samba/smb.conf (for the winbind profile)

Relation of authconfig options to authselect profiles and Authselect profile option equivalents of authconfig options show the authselect equivalents of authconfig options.

Table 1.1. Relation of authconfig options to authselect profiles

Authconfig optionsAuthselect profile

--enableldap --enableldapauth

sssd

--enablesssd --enablesssdauth

sssd

--enablekrb5

sssd

--enablewinbind --enablewinbindauth

winbind

--enablenis

nis

Table 1.2. Authselect profile option equivalents of authconfig options

Authconfig optionAuthselect profile feature

--enablesmartcard

with-smartcard

--enablefingerprint

with-fingerprint

--enableecryptfs

with-ecryptfs

--enablemkhomedir

with-mkhomedir

--enablefaillock

with-faillock

--enablepamaccess

with-pamaccess

--enablewinbindkrb5

with-krb5

Examples of authselect command equivalents to authconfig commands shows example transformations of Kickstart calls to authconfig into Kickstart calls to authselect.

Table 1.3. Examples of authselect command equivalents to authconfig commands

authconfig commandauthselect equivalent

authconfig --enableldap --enableldapauth --enablefaillock --updateall

authselect select sssd with-faillock

authconfig --enablesssd --enablesssdauth --enablesmartcard --smartcardmodule=sssd --updateall

authselect select sssd with-smartcard

authconfig --enableecryptfs --enablepamaccess --updateall

authselect select sssd with-ecryptfs with-pamaccess

authconfig --enablewinbind --enablewinbindauth --winbindjoin=Administrator --updateall

realm join -U Administrator --client-software=winbind WINBINDDOMAIN

Chapter 2. Understanding SSSD and its benefits

The System Security Services Daemon (SSSD) is a system service to access remote directories and authentication mechanisms. The following chapters outline how SSSD works, what are the benefits of using it, how the configuration files are processed, as well as what identity and authentication providers you can configure.

2.1. How SSSD works

The System Security Services Daemon (SSSD) is a system service that allows you to access remote directories and authentication mechanisms. You can connect a local system, an SSSD client, to an external back-end system, a provider.

For example:

  • An LDAP directory
  • An Identity Management (IdM) domain
  • An Active Directory (AD) domain
  • A Kerberos realm

SSSD works in two stages:

  1. It connects the client to a remote provider to retrieve identity and authentication information.
  2. It 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 remote provider.

SSSD does not create user accounts on the local system. However, SSSD can be configured to create home directories for IdM users. Once created, an IdM user home directory and its contents on the client are not deleted when the user logs out.

Figure 2.1. How SSSD works

A flow chart displaying a local system (an SSSD client) with an "SSSD cache" on the left and a remote system (provider) on the right. An arrow originating from the remote system and pointing inside the SSSD cache of the local system is labeled to explain that SSSD retrieves and stores information about users from the remote system.

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

2.2. Benefits of using SSSD

Using the System Security Services Daemon (SSSD) provides multiple benefits regarding user identity retrieval and user authentication.

Offline authentication
SSSD optionally keeps a cache of user identities and credentials retrieved from remote providers. In this setup, a user - provided they have already authenticated once against the remote provider at the start of the session - can successfully authenticate to resources even if the remote provider or the 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. The conditions are:

  • In a particular session, the user must have logged in at least once: the client must be connected to the remote provider when the user logs in for the first time.
  • Caching must be enabled in SSSD.

    Without SSSD, 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. In this scenario, you must first authenticate on the private network to fetch the user from the remote server and cache the user credentials locally.

    With SSSD, 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.

Reduced load on identity and authentication providers
When requesting information, the clients first check the local SSSD cache. SSSD contacts the remote providers only if the information is not available in the cache.

2.3. 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.

This combination 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.

2.4. Identity and authentication providers for SSSD

You can connect an SSSD client to the external identity and authentication providers, for example an LDAP directory, an Identity Management (IdM), Active Directory (AD) domain, or a Kerberos realm. The SSSD client then get access to identity and authentication remote services using the SSSD provider. You can configure SSSD to use different identity and authentication providers or a combination of them.

Identity and Authentication Providers as SSSD domains

Identity and authentication providers are configured as domains in the SSSD configuration file, /etc/sssd/sssd.conf. The providers are listed in the [domain/name of the domain] or [domain/default] section of the file.

A single domain can be configured as one of the following providers:

  • An identity provider, which supplies user information such as UID and GID.

    • Specify a domain as the identity provider by using the id_provider option in the [domain/name of the domain] section of the /etc/sssd/sssd.conf file.
  • An authentication provider, which handles authentication requests.

    • Specify a domain as the authentication provider by using the auth_provider option in the [domain/name of the domain] section of /etc/sssd/sssd.conf.
  • An access control provider, which handles authorization requests.

    • Specify a domain as the access control provider using the access_provider option in the [domain/name of the domain] section of /etc/sssd/sssd.conf. By default, the option is set to permit, which always allows all access. See the sssd.conf(5) man page for details.
  • A combination of these providers, for example if all the corresponding operations are performed within a single server.

    • In this case, the id_provider, auth_provider, and access_provider options are all listed in the same [domain/name of the domain] or [domain/default] section of /etc/sssd/sssd.conf.
Note

You can configure multiple domains for SSSD. You must configure at least one domain, otherwise SSSD will not start.

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.

You can configure SSSD to use a proxy provider in order to enable:

  • Alternative authentication methods, such as a fingerprint scanner
  • Legacy systems, such as NIS
  • A local system account defined in the /etc/passwd file as an identity provider and a remote authentication provider, for example Kerberos

Available Combinations of Identity and Authentication Providers

You can configure SSSD to use the following combinations of identity and authentication providers.

Table 2.1. Available Combinations of Identity and Authentication Providers

Identity ProviderAuthentication Provider

Identity Management [a]

Identity Management

Active Directory

Active Directory

LDAP

LDAP

LDAP

Kerberos

Proxy

Proxy

Proxy

LDAP

Proxy

Kerberos

[a] An extension of the LDAP provider type.


[1] To list and verify the status of the domains using the sssctl utility, your host should be enrolled in Identity Management (IdM) that is in a trust agreement with an Active Directory (AD) forest.

Chapter 3. Configuring SSSD to use LDAP and require TLS authentication

The System Security Services Daemon (SSSD) is a daemon that manages identity data retrieval and authentication on a RHEL 8 host. A system administrator can configure the host to use a standalone LDAP server as the user account database. The administrator can also specify the requirement that the connection with the LDAP server must be encrypted with a TLS certificate.

3.1. An OpenLDAP client using SSSD to retrieve data from LDAP in an encrypted way

The authentication method of the LDAP objects can be either a Kerberos password or an LDAP password. Note that the questions of authentication and authorization of the LDAP objects are not addressed in this chapter.

Important

Configuring SSSD with LDAP is a complex procedure requiring a high level of expertise in SSSD and LDAP. Consider using an integrated and automated solution such as Active Directory or Red Hat Identity Management (IdM) instead. For details about IdM, see Planning Identity Management.

3.2. Configuring SSSD to use LDAP and require TLS authentication

Complete this procedure to configure your Red Hat Enterprise Linux (RHEL) system as an OpenLDAP client.

Use the following client configuration:

  • The RHEL system authenticates users stored in an OpenLDAP user account database.
  • The RHEL system uses the System Security Services Daemon (SSSD) service to retrieve user data.
  • The RHEL system communicates with the OpenLDAP server over a TLS-encrypted connection.
Note

You can alternatively use this procedure to configure your RHEL system as a client of a Red Hat Directory Server.

Prerequisites

  • The OpenLDAP server is installed and configured with user information.
  • You have root permissions on the host you are configuring as the LDAP client.
  • On the host you are configuring as the LDAP client, the /etc/sssd/sssd.conf file has been created and configured to specify ldap as the autofs_provider and the id_provider.
  • You have a PEM-formatted copy of the root CA signing certificate chain from the Certificate Authority that issued the OpenLDAP server certificate, stored in a local file named core-dirsrv.ca.pem.

Procedure

  1. Install the requisite packages:

    # dnf -y install openldap-clients sssd sssd-ldap oddjob-mkhomedir
  2. Switch the authentication provider to sssd:

    # authselect select sssd with-mkhomedir
  3. Copy the core-dirsrv.ca.pem file containing the root CA signing certificate chain from the Certificate Authority that issued the OpenLDAP server’s SSL/TLS certificate into the /etc/openldap/certs folder.

    # cp core-dirsrv.ca.pem /etc/openldap/certs
  4. Add the URL and suffix of your LDAP server to the /etc/openldap/ldap.conf file:

    URI ldap://ldap-server.example.com/
    BASE dc=example,dc=com
  5. In the /etc/openldap/ldap.conf file, add a line pointing the TLS_CACERT parameter to /etc/openldap/certs/core-dirsrv.ca.pem:

    # When no CA certificates are specified the Shared System Certificates
    # are in use. In order to have these available along with the ones specified
    # by TLS_CACERTDIR one has to include them explicitly:
    TLS_CACERT /etc/openldap/certs/core-dirsrv.ca.pem
  6. In the /etc/sssd/sssd.conf file, add your environment values to the ldap_uri and ldap_search_base parameters:

    [domain/default]
    id_provider = ldap
    autofs_provider = ldap
    auth_provider = ldap
    chpass_provider = ldap
    ldap_uri = ldap://ldap-server.example.com/
    ldap_search_base = dc=example,dc=com
    ldap_id_use_start_tls = True
    cache_credentials = True
    ldap_tls_cacertdir = /etc/openldap/certs
    ldap_tls_reqcert = allow
    
    [sssd]
    services = nss, pam, autofs
    domains = default
    
    [nss]
    homedir_substring = /home
    …
  7. In /etc/sssd/sssd.conf, specify the TLS authentication requirement by modifying the ldap_tls_cacert and ldap_tls_reqcert values in the [domain] section:

    …
    cache_credentials = True
    ldap_tls_cacert = /etc/openldap/certs/core-dirsrv.ca.pem
    ldap_tls_reqcert = hard
  8. Change the permissions on the /etc/sssd/sssd.conf file:

    # chmod 600 /etc/sssd/sssd.conf
  9. Restart and enable the SSSD service and the oddjobd daemon:

    # systemctl restart sssd oddjobd
    # systemctl enable sssd oddjobd
  10. (Optional) If your LDAP server uses the deprecated TLS 1.0 or TLS 1.1 protocols, switch the system-wide cryptographic policy on the client system to the LEGACY level to allow RHEL 8 to communicate using these protocols:

    # update-crypto-policies --set LEGACY

    For more details, see the Deprecated Functionality section in the RHEL 8.0 Release Notes.

Verification steps

  • Verify you can retrieve user data from your LDAP server by using the id command and specifying an LDAP user:

    # id ldap_user
    uid=17388(ldap_user) gid=45367(sysadmins) groups=45367(sysadmins),25395(engineers),10(wheel),1202200000(admins)

The system administrator can now query users from LDAP using the id command. The command returns a correct user ID and group membership.

Chapter 4. SSSD Client-side View

SSSD provides the sss_override utility, which allows you to create a local view that displays values for POSIX user or group attributes that are specific to your local machine. You can configure overrides for all id_provider values, except ipa.

If you are using the ipa provider, define ID views centrally in IPA. For more information, see ID Views section.

For information about a potential negative impact on the SSSD performance, see Potential Negative Impact on SSSD Performance section.

4.1. Overriding the LDAP username attribute

As an administrator, you can configure an existing host to use accounts from LDAP. However, the values for a user (name, UID, GID, home directory, shell) in LDAP are different from the values on the local system. You can override the LDAP username attribute by defining a secondary username with the following procedure.

Prerequisites

  • root access
  • Installed sssd-tools

Procedure

  1. Display the current information for the user:

    # id username

    Replace username with the name of the user.

  2. Add the secondary username:

    # sss_override user-add username -n secondary-username

    Replace username with the name of the user and replace secondary-username with the new username.

  3. After creating the first override using the sss_override user-add command, restart SSSD for the changes to take effect:

    # systemctl restart sssd

Verification steps

  • Verify that the new username is added:

    # id secondary-username
  • Optional. Display the overrides for the user:

    # sss_override user-show user-name
    user@ldap.example.com:secondary-username::::::

    Example 4.1. Defining a secondary username

    To add a secondary username sarah for the user sjones:

    1. Display the current information for the user sjones:

      # id sjones
      uid=1001(sjones) gid=6003 groups=6003,10(wheel)
    2. Add the secondary username:

      # sss_override user-add sjones -n sarah
    3. Verify that the new username has been added and overrides for the user display correctly:

      # id sarah
      uid=1001(sjones) gid=6003(sjones) groups=6003(sjones),10(wheel)
      
      # sss_override user-show sjones
      user@ldap.example.com:sarah::::::

Additional resources

  • sss_override man page

4.2. Overriding the LDAP UID attribute

As an administrator, you can configure an existing host to use accounts from LDAP. However, the values for a user (name, UID, GID, home directory, shell) in LDAP are different from the values on the local system. You can override the LDAP UID attribute by defining a different UID with the following procedure.

Prerequisites

  • root access
  • Installed sssd-tools

Procedure

  1. Display the current UID of the user:

    # id -u user-name

    Replace user-name with the name of the user.

  2. Override the UID of the user’s account:

    # sss_override user-add user-name -u new-UID

    Replace user-name with the name of the user and replace new-UID with the new UID number.

  3. Expire the in-memory cache:

    # sss_cache --users
  4. After creating the first override using the sss_override user-add command, restart SSSD for the changes to take effect:

    # systemctl restart sssd

Verification steps

  • Verify that the new UID has been applied:

    # id -u user-name
  • Optional. Display the overrides for the user:

    # sss_override user-show user-name
    user@ldap.example.com::new-UID:::::

    Example 4.2. Overriding the UID of the user

    To override the UID of the user sarah with UID 6666:

    1. Display the current UID of the user sarah:

      # id -u sarah
      1001
    2. Override the UID of the user sarah's account with UID 6666:

      # sss_override user-add sarah -u 6666
    3. Manually expire the in-memory cache:

      # sss_cache --users
    4. Restart SSSD for the changes to take effect:

      # systemctl restart sssd
    5. Verify that the new UID is applied and overrides for the user display correctly:

      # id sarah
      6666
      
      # sss_override user-show sarah
      user@ldap.example.com::6666:::::

Additional resources

  • sss_override man page

4.3. Overriding the LDAP GID attribute

As an administrator, you can configure an existing host to use accounts from LDAP. However, the values for a user (name, UID, GID, home directory, shell) in LDAP are different from the values on the local system. You can override the LDAP GID attribute by defining a different GID with the following procedure.

Prerequisites

  • root access
  • Installed sssd-tools

Procedure

  1. Display the current GID of the user:

    # id -g user-name

    Replace user-name with the name of the user.

  2. Override the GID of the user’s account:

    # sss_override user-add user-name -u new-GID

    Replace user-name with the name of the user and replace new-GID with the new GID number.

  3. Expire the in-memory cache:

    # sss_cache --users
  4. After creating the first override using the sss_override user-add command, restart SSSD for the changes to take effect:

    # systemctl restart sssd

Verification steps

  • Verify that the new GID is applied:

    # id -g user-name
  • Optional. Display the overrides for the user:

    # sss_override user-show user-name
    user@ldap.example.com:::6666::::

    Example 4.3. Overriding the GID of the user

    To override the GID of the user sarah with GID 6666:

    1. Display the current GID of the user sarah:

      # id -g sarah
      6003
    2. Override the GID of the user sarah's account with GID 6666:

      # sss_override user-add sarah -g 6666
    3. Manually expire the in-memory cache:

      # sss_cache --users
    4. If this is your first override, restart SSSD for the changes to take effect:

      # systemctl restart sssd
    5. Verify that the new GID is applied and overrides for the user display correctly:

      # id -g sarah
      6666
      
      # sss_override user-show sarah
      user@ldap.example.com::6666:::::

Additional resources

  • sss_override man page

4.4. Overriding the LDAP home directory attribute

As an administrator, you can configure an existing host to use accounts from LDAP. However, the values for a user (name, UID, GID, home directory, shell) in LDAP are different from the values on the local system. You can override the LDAP home directory attribute by defining a different home directory with the following procedure.

Prerequisites

  • root access
  • Installed sssd-tools

Procedure

  1. Display the current home directory of the user:

    # getent passwd user-name
    user-name:x:XXXX:XXXX::/home/home-directory:/bin/bash

    Replace user-name with the name of the user.

  2. Override the home directory of the user:

    # sss_override user-add user-name -h new-home-directory

    Replace user-name with the name of the user and replace new-home-directory with the new home directory.

  3. Restart SSSD for the changes to take effect:

    # systemctl restart sssd

Verification steps

  • Verify that the new home directory is defined:

    # getent passwd user-name
    user-name:x:XXXX:XXXX::/home/new-home-directory:/bin/bash
  • Optional. Display the overrides for the user:

    # sss_override user-show user-name
    user@ldap.example.com:::::::new-home-directory::

    Example 4.4. Overriding the home directory of the user

    To override the home directory of the user sarah with admin:

    1. Display the current home directory of the user sarah:

      # getent passwd sarah
      sarah:x:1001:6003::sarah:/bin/bash
    2. Override the home directory of the user sarah with new home directory admin:

      # sss_override user-add sarah -h admin
    3. Restart SSSD for the changes to take effect:

      # systemctl restart sssd
    4. Verify that the new home directory is defined and overrides for the user display correctly:

      # getent passwd sarah
      sarah:x:1001:6003::admin:/bin/bash
      
      # sss_override user-show user-name
      user@ldap.example.com:::::::admin::

Additional resources

  • sss_override man page

4.5. Overriding the LDAP shell attribute

As an administrator, you can configure an existing host to use accounts from LDAP. However, the values for a user (name, UID, GID, home directory, shell) in LDAP are different from the values on the local system. You can override the LDAP shell attribute by defining a different shell with the following procedure.

Prerequisites

  • root access
  • Installed sssd-tools

Procedure

  1. Display the current shell of the user:

    # getent passwd user-name
    user-name:x:XXXX:XXXX::/home/home-directory:/bin/bash

    Replace user-name with the name of the user.

  2. Override the shell of the user:

    # sss_override user-add user-name -s new-shell

    Replace user-name with the name of the user and replace new-shell with the new shell.

  3. Restart SSSD for the changes to take effect:

    # systemctl restart sssd

Verification steps

  • Verify that the new shell is defined:

    # getent passwd user-name
    user-name:x:XXXX:XXXX::/home/home-directory:new-shell
  • Optional. Display the overrides for the user:

    # sss_override user-show user-name
    user@ldap.example.com::::::new-shell:

    Example 4.5. Overriding the shell of the user

    To change the shell of the user sarah from /bin/bash to sbin/nologin:

    1. Display the current shell of the user sarah:

      # getent passwd sarah
      sarah:x:1001:6003::sarah:/bin/bash
    2. Override the shell of the user sarah with new /sbin/nologin shell:

      # sss_override user-add sarah -s /sbin/nologin
    3. Restart SSSD for the changes to take effect:

      # systemctl restart sssd
    4. Verify that the new shell is defined and overrides for the user display correctly:

      # getent passwd sarah
      sarah:x:1001:6003::sarah:/sbin/nologin
      
      # sss_override user-show user-name
      user@ldap.example.com::::::/sbin/nologin:

Additional resources

  • sss_override man page

4.6. Listing overrides on a host

As an administrator, you can list all user and group overrides on a host to verify that the correct attributes have been overridden.

Prerequisites

  • root access
  • Installed sssd-tools

Procedure

  • List all user overrides:

    # sss_override user-find
    user1@ldap.example.com::8000::::/bin/zsh:
    user2@ldap.example.com::8001::::/bin/bash:
    ...
  • List all group overrides:

    # sss_override group-find
    group1@ldap.example.com::7000
    group2@ldap.example.com::7001
    ...

4.7. Removing a local override

If you want to remove local override that is defined in the global LDAP directory, use the following procedure.

Prerequisites

  • root access
  • Installed sssd-tools

Procedure

  • To remove the override for a user account, use:

    # sss_override user-del user-name

    Replace user-name with the name of the user. The changes take effect immediately.

  • To remove an override for a group, use:

    # sss_override group-del group-name
  • After removing the first override using the sss_override user-del or sss_override group-del command, restart SSSD for the changes to take effect:

    # systemctl restart sssd

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

4.8. Exporting and importing local view

Your local overrides are stored in the local SSSD cache. You can export user and group overrides from this cache to a file to create a backup. This ensures that even if the cache is cleared, you can restore the configurations later.

Prerequisites

  • root access
  • Installed sssd-tools

Procedure

  • To back up user and group view, use:

    # 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, use:

    # sss_override user-import /var/lib/sss/backup/sssd_user_overrides.bak
    # sss_override group-import /var/lib/sss/backup/sssd_group_overrides.bak

Chapter 5. Configuring a RHEL host to use AD as an authentication provider

As a system administrator, you can use Active Directory (AD) as the authentication provider for a Red Hat Enterprise Linux (RHEL) host without joining the host to AD.

This can be done if, for example:

  • You do not want to grant AD administrators the control over enabling and disabling the host.
  • The host, which can be a corporate PC, is only meant to be used by one user in your company.
Important

Implement this procedure only in the rare cases where this approach is preferred.

Consider fully joining the system to AD or Red Hat Identity Management (IdM) instead. Joining the RHEL host to a domain makes the setup easier to manage. If you are concerned about client access licences related to joining clients into AD directly, consider leveraging an IdM server that is in a trust agreement with AD. For more information on an IdM-AD trust, see Planning a cross-forest trust between IdM and AD and Installing a trust between IdM and AD.

This procedure enables the user named AD_user to log in to the rhel8_host system using the password set in the Active Directory (AD) user database in the example.com domain. In this example, the EXAMPLE.COM Kerberos realm corresponds to the example.com domain.

Prerequisites

  • You have root access to rhel8_host.
  • The AD_user user account exists in the example.com domain.
  • The Kerberos realm is EXAMPLE.COM.
  • rhel8_host has not been joined to AD using the realm join command.

Procedure

  1. Create the AD_user user account locally without assigning a password to it:

    # useradd AD_user
  2. Open the /etc/nsswitch.conf file for editing, and make sure that it contains the following lines:

    passwd:     sss files systemd
    group:      sss files systemd
    shadow:     files sss
  3. Open the /etc/krb5.conf file for editing, and make sure that it contains the following sections and items:

    # To opt out of the system crypto-policies configuration of krb5, remove the
    # symlink at /etc/krb5.conf.d/crypto-policies which will not be recreated.
    includedir /etc/krb5.conf.d/
    
    [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
        ticket_lifetime = 24h
        renew_lifetime = 7d
        forwardable = true
        rdns = false
        pkinit_anchors = /etc/pki/tls/certs/ca-bundle.crt
        spake_preauth_groups = edwards25519
        default_realm = EXAMPLE.COM
        default_ccache_name = KEYRING:persistent:%{uid}
    
    [realms]
     EXAMPLE.COM = {
         kdc = ad.example.com
         admin_server = ad.example.com
     }
    
    [domain_realm]
     .example.com = EXAMPLE.COM
     example.com = EXAMPLE.COM
  4. Create the /etc/sssd/sssd.conf file and insert the following sections and lines into it:

    [sssd]
        services = nss, pam
        domains = EXAMPLE.COM
    
    [domain/EXAMPLE.COM]
        id_provider = files
        auth_provider = krb5
        krb5_realm = EXAMPLE.COM
        krb5_server = ad.example.com
  5. Change the permissions on the /etc/sssd/sssd.conf file:

    # chmod 600 /etc/sssd/sssd.conf
  6. Start the Security System Services Daemon (SSSD):

    # systemctl start sssd
  7. Enable SSSD:

    # systemctl enable sssd
  8. Open the /etc/pam.d/system-auth file, and modify it so that it contains the following sections and lines:

    # Generated by authselect on Wed May  8 08:55:04 2019
    # Do not modify this file manually.
    
    auth        required                                     pam_env.so
    auth        required                                     pam_faildelay.so delay=2000000
    auth        [default=1 ignore=ignore success=ok]         pam_succeed_if.so uid >= 1000 quiet
    auth        [default=1 ignore=ignore success=ok]         pam_localuser.so
    auth        sufficient                                   pam_unix.so nullok try_first_pass
    auth        requisite                                    pam_succeed_if.so uid >= 1000 quiet_success
    auth        sufficient                                   pam_sss.so forward_pass
    auth        required                                     pam_deny.so
    
    account     required                                     pam_unix.so
    account     sufficient                                   pam_localuser.so
    account     sufficient                                   pam_succeed_if.so uid < 1000 quiet
    account     [default=bad success=ok user_unknown=ignore] pam_sss.so
    account     required                                     pam_permit.so
    
    password    requisite                                    pam_pwquality.so try_first_pass local_users_only
    password    sufficient                                   pam_unix.so sha512 shadow nullok try_first_pass use_authtok
    password    sufficient                                   pam_sss.so use_authtok
    password    required                                     pam_deny.so
    
    session     optional                                     pam_keyinit.so revoke
    session     required                                     pam_limits.so
    -session    optional                                     pam_systemd.so
    session     [success=1 default=ignore]                   pam_succeed_if.so service in crond quiet use_uid
    session     required                                     pam_unix.so
    session     optional                                     pam_sss.so
  9. Copy the contents of the /etc/pam.d/system-auth file into the /etc/pam.d/password-auth file. Enter yes to confirm the overwriting of the current contents of the file:

    # cp /etc/pam.d/system-auth /etc/pam.d/password-auth
    cp: overwrite '/etc/pam.d/password-auth'? yes

Verification steps

  1. Request a Kerberos ticket-granting ticket (TGT) for AD_user. Enter the password of AD_user as requested:

    # kinit AD_user
    Password for AD_user@EXAMPLE.COM:
  2. Display the obtained TGT:

    # klist
    Ticket cache: KEYRING:persistent:0:0
    Default principal: AD_user@EXAMPLE.COM
    
    Valid starting     Expires            Service principal
    11/02/20 04:16:38  11/02/20 14:16:38  krbtgt/EXAMPLE.COM@EXAMPLE.COM
    	renew until 18/02/20 04:16:34

AD_user has successfully logged in to rhel8_host using the credentials from the EXAMPLE.COM Kerberos domain.

Chapter 6. Reporting on user access on hosts using SSSD

The Security System Services Daemon (SSSD) tracks which users can or cannot access clients. This chapter describes creating access control reports and displaying user data using the sssctl tool.

Prerequisites

  • SSSD packages are installed in your network environment

6.1. The sssctl command

sssctl is a command-line tool that provides a unified way to obtain information about the Security System Services Daemon (SSSD) status.

You can use the sssctl utility to gather information about:

  • domain state
  • client user authentication
  • user access on clients of a particular domain
  • information about cached content

With the sssctl tool, you can:

  • manage the SSSD cache
  • manage logs
  • check configuration files
Note

The sssctl tool replaces sss_cache and sss_debuglevel tools.

Additional resources

  • sssctl --help

6.2. Generating access control reports using sssctl

You can list the access control rules applied to the machine on which you are running the report because SSSD controls which users can log in to the client.

Note

The access report is not accurate because the tool does not track users locked out by the Key Distribution Center (KDC).

Prerequisites

  • You must be logged in with administrator privileges
  • The sssctl is available on RHEL 7 and RHEL 8 systems

Procedure

  • To generate a report for the idm.example.com domain, enter:

    [root@client1 ~]# sssctl access-report idm.example.com
    1 rule cached
    
    Rule name: example.user
    	Member users: example.user
    	Member services: sshd

6.3. Displaying user authorization details using sssctl

The sssctl user-checks command helps debug problems in applications that use the System Security Services Daemon (SSSD) for user lookup, authentication, and authorization.

The sssctl user-checks [USER_NAME] command displays user data available through Name Service Switch (NSS) and the InfoPipe responder for the D-Bus interface. The displayed data shows whether the user is authorized to log in using the system-auth Pluggable Authentication Module (PAM) service.

The command has two options:

  • -a for a PAM action
  • -s for a PAM service

If you do not define -a and -s options, the sssctl tool uses default options: -a acct -s system-auth.

Prerequisites

  • You must be logged in with administrator privileges
  • The sssctl tool is available on RHEL 7 and RHEL 8 systems

Procedure

  • To display user data for a particular user, enter:

    [root@client1 ~]# sssctl user-checks -a acct -s sshd example.user
    user: example.user
    action: acct
    service: sshd
    ....

Additional resources

  • sssctl user-checks --help

Chapter 7. Querying domain information using SSSD

Security System Services Daemon (SSSD) can list domains in Identity Management (IdM) as well as the domains in Active Directory that is connected to IdM by a cross-forest trust.

7.1. Listing domains using sssctl

You can use the sssctl domain-list command to debug problems with the domain topology.

Note

The status might not be available immediately. If the domain is not visible, repeat the command.

Prerequisites

  • You must be logged in with administrator privileges
  • The sssctl is available on RHEL 7 and RHEL 8 systems

Procedure

  1. To display help for the sssctl command, enter:

    [root@client1 ~]# sssctl --help
    ....
  2. To display a list of available domains, enter:
[root@client1 ~]# sssctl domain-list
implicit_files
idm.example.com
ad.example.com
sub1.ad.example.com

The list includes domains in the cross-forest trust between Active Directory and Identity Management.

7.2. Verifying the domain status using sssctl

You can use the sssctl domain-status command to debug problems with the domain topology.

Note

The status might not be available immediately. If the domain is not visible, repeat the command.

Prerequisites

  • You must be logged in with administrator privileges
  • The sssctl is available on RHEL 7 and RHEL 8 systems

Procedure

  1. To display help for the sssctl command, enter:

    [root@client1 ~]# sssctl --help
  2. To display user data for a particular domain, enter:

    [root@client1 ~]# sssctl domain-status idm.example.com
    Online status: Online
    
    Active servers:
    IPA: server.idm.example.com
    
    Discovered IPA servers:
    - server.idm.example.com

The domain idm.example.com is online and visible from the client where you applied the command.

If the domain is not available, the result is:

[root@client1 ~]# sssctl domain-status ad.example.com
Unable to get online status

Chapter 8. Eliminating typographical errors in local SSSD configuration

You can test if the /etc/sssd/sssd.conf file on your host contains any typographical errors using the sssctl config-check command.

Prerequisites

  • You are logged in as root.
  • The sssd-tools package is installed.

Procedure

  1. Enter the sssctl config-check command:

    # sssctl config-check
    
    Issues identified by validators: 1
    [rule/allowed_domain_options]: Attribute 'ldap_search' is not allowed in section 'domain/example1'. Check for typos.
    
    Messages generated during configuration merging: 0
    
    Used configuration snippet files: 0
  2. Open the /etc/sssd/sssd.conf file and correct the typo. If you, for example, received the error message in the previous step, replace ldap_search with ldap_search_base:

    [...]
    [domain/example1]
    ldap_search_base = dc=example,dc=com
    [...]
  3. Save the file.
  4. Restart SSSD:

    # systemctl restart sssd

Verification steps

  • Enter the sssctl config-check command:

    # sssctl config-check
    
    Issues identified by validators: 0
    
    Messages generated during configuration merging: 0
    
    Used configuration snippet files: 0

The /etc/sssd/sssd.conf file now has no typographical errors.

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