Kerberos integration

GitLab can integrate with Kerberos as an authentication mechanism.

caution
GitLab CI/CD does not work with a Kerberos-enabled GitLab instance due to an unresolved bug in Git CLI that fails to use job token authentication from the GitLab Runners.

Overview

Kerberos is a secure method for authenticating a request for a service in a computer network. Kerberos was developed in the Athena Project at the Massachusetts Institute of Technology (MIT). The name is taken from Greek mythology; Kerberos was a three-headed dog who guarded the gates of Hades.

Use-cases

  • GitLab can be configured to allow your users to sign with their Kerberos credentials.
  • You can use Kerberos to prevent anyone from intercepting or eavesdropping on the transmitted password.

Configuration

For GitLab to offer Kerberos token-based authentication, perform the following prerequisites. You still need to configure your system for Kerberos usage, such as specifying realms. GitLab makes use of the system’s Kerberos settings.

GitLab keytab

  1. Create a Kerberos Service Principal for the HTTP service on your GitLab server. If your GitLab server is gitlab.example.com and your Kerberos realm EXAMPLE.COM, create a Service Principal HTTP/gitlab.example.com@EXAMPLE.COM in your Kerberos database.
  2. Create a keytab on the GitLab server for the above Service Principal. For example, /etc/http.keytab.

The keytab is a sensitive file and must be readable by the GitLab user. Set ownership and protect the file appropriately:

sudo chown git /etc/http.keytab
sudo chmod 0600 /etc/http.keytab

Configure GitLab

Installations from source

note
For source installations, make sure the kerberos gem group has been installed.
  1. Edit the kerberos section of gitlab.yml to enable Kerberos ticket-based authentication. In most cases, you only need to enable Kerberos and specify the location of the keytab:

    omniauth:
      enabled: true
      allow_single_sign_on: ['kerberos']
    
    kerberos:
      # Allow the HTTP Negotiate authentication method for Git clients
      enabled: true
    
      # Kerberos 5 keytab file. The keytab file must be readable by the GitLab user,
      # and should be different from other keytabs in the system.
      # (default: use default keytab from Krb5 config)
      keytab: /etc/http.keytab
    
  2. Restart GitLab for the changes to take effect.

Omnibus package installations

  1. Edit /etc/gitlab/gitlab.rb:

    gitlab_rails['omniauth_allow_single_sign_on'] = ['kerberos']
    
    gitlab_rails['kerberos_enabled'] = true
    gitlab_rails['kerberos_keytab'] = "/etc/http.keytab"
    

    To avoid GitLab creating users automatically on their first sign in through Kerberos, don’t set kerberos for gitlab_rails['omniauth_allow_single_sign_on'].

  2. Reconfigure GitLab for the changes to take effect.

GitLab now offers the negotiate authentication method for signing in and HTTP Git access, enabling Git clients that support this authentication protocol to authenticate with Kerberos tokens.

Enable single sign-on

See Initial OmniAuth Configuration for initial settings to enable single sign-on and add Kerberos servers as an identity provider.

You can either link a Kerberos account to an existing GitLab account, or set up GitLab to create a new account when a Kerberos user tries to sign in.

If you’re an administrator, you can link a Kerberos account to an existing GitLab account. To do so:

  1. On the top bar, select Menu > Admin.
  2. On the left sidebar, select Overview > Users.
  3. Select a user, then select the Identities tab.
  4. Select ‘Kerberos SPNEGO’ in the ‘Provider’ dropdown box.
  5. Make sure the Identifier corresponds to the Kerberos username.
  6. Select Save changes.

If you’re not an administrator:

  1. In the top-right corner, select your avatar.
  2. Select Edit profile.
  3. On the left sidebar, select Account.
  4. In the Social sign-in section, select Connect Kerberos SPNEGO. If you don’t see a Social sign-in Kerberos option, follow the requirements in Enable single sign-on.

In either case, you should now be able to sign in to your GitLab account with your Kerberos credentials.

Create accounts on first sign-in

The first time users sign in to GitLab with their Kerberos accounts, GitLab creates a matching account. Before you continue, review the Initial OmniAuth Configuration options in Omnibus and GitLab source. You must also include kerberos.

With that information at hand:

  1. Include 'kerberos' with the allow_single_sign_on setting.
  2. For now, accept the default block_auto_created_users option, true.
  3. When a user tries to sign in with Kerberos credentials, GitLab creates a new account.
    1. If block_auto_created_users is true, the Kerberos user may see a message like:

      Your account has been blocked. Please contact your GitLab
      administrator if you think this is an error.
      
      1. As an administrator, you can confirm the new, blocked account:
        1. On the top bar, select Menu > Admin.
        2. On the left sidebar, select Overview > Users and review the Blocked tab.
      2. You can enable the user.
    2. If block_auto_created_users is false, the Kerberos user is authenticated and is signed in to GitLab.

caution
We recommend that you retain the default for block_auto_created_users. Kerberos users who create accounts on GitLab without administrator knowledge can be a security risk.

If your users sign in with Kerberos, but you also have LDAP integration enabled, your users are linked to their LDAP accounts on their first sign-in. For this to work, some prerequisites must be met:

The Kerberos username must match the LDAP user’s UID. You can choose which LDAP attribute is used as the UID in the GitLab LDAP configuration but for Active Directory, this should be sAMAccountName.

The Kerberos realm must match the domain part of the LDAP user’s Distinguished Name. For instance, if the Kerberos realm is AD.EXAMPLE.COM, then the LDAP user’s Distinguished Name should end in dc=ad,dc=example,dc=com.

Taken together, these rules mean that linking only works if your users’ Kerberos usernames are of the form foo@AD.EXAMPLE.COM and their LDAP Distinguished Names look like sAMAccountName=foo,dc=ad,dc=example,dc=com.

Custom allowed realms

Introduced in GitLab 13.5.

You can configure custom allowed realms when the user’s Kerberos realm doesn’t match the domain from the user’s LDAP DN. The configuration value must specify all domains that users may be expected to have. Any other domains are ignored and an LDAP identity is not linked.

For Omnibus installations

  1. Edit /etc/gitlab/gitlab.rb:

    gitlab_rails['kerberos_simple_ldap_linking_allowed_realms'] = ['example.com','kerberos.example.com']
    
  2. Save the file and reconfigure GitLab for the changes to take effect.


For installations from source

  1. Edit config/gitlab.yml:

    kerberos:
      simple_ldap_linking_allowed_realms: ['example.com','kerberos.example.com']
    
  2. Save the file and restart GitLab for the changes to take effect.

HTTP Git access

A linked Kerberos account enables you to git pull and git push using your Kerberos account, as well as your standard GitLab credentials.

GitLab users with a linked Kerberos account can also git pull and git push using Kerberos tokens. That is, without having to send their password with each operation.

caution
There is a known issue with libcurl older than version 7.64.1 wherein it doesn’t reuse connections when negotiating. This leads to authorization issues when push is larger than http.postBuffer configuration. Ensure that Git is using at least libcurl 7.64.1 to avoid this. To know the libcurl version installed, run curl-config --version.

HTTP Git access with Kerberos token (passwordless authentication)

Support for Git before 2.4

Until Git version 2.4, the git command uses only the negotiate authentication method if the HTTP server offers it, even if this method fails (such as when the client does not have a Kerberos token). It is thus not possible to fall back to username/password (also known as basic) authentication if Kerberos authentication fails.

For GitLab users to be able to use either basic or negotiate authentication with older Git versions, it is possible to offer Kerberos ticket-based authentication on a different port (for example, 8443) while the standard port offers only basic authentication.

For source installations with HTTPS

  1. Edit the NGINX configuration file for GitLab (for example, /etc/nginx/sites-available/gitlab-ssl) and configure NGINX to listen to port 8443 in addition to the standard HTTPS port:

    server {
      listen 0.0.0.0:443 ssl;
      listen [::]:443 ipv6only=on ssl default_server;
      listen 0.0.0.0:8443 ssl;
      listen [::]:8443 ipv6only=on ssl;
    
  2. Update the kerberos section of gitlab.yml:

    kerberos:
      # Dedicated port: Git before 2.4 does not fall back to Basic authentication if Negotiate fails.
      # To support both Basic and Negotiate methods with older versions of Git, configure
      # nginx to proxy GitLab on an extra port (for example: 8443) and uncomment the following lines
      # to dedicate this port to Kerberos authentication. (default: false)
      use_dedicated_port: true
      port: 8443
      https: true
    
  3. Restart GitLab and NGINX for the changes to take effect.

For Omnibus package installations

  1. Edit /etc/gitlab/gitlab.rb:

    gitlab_rails['kerberos_use_dedicated_port'] = true
    gitlab_rails['kerberos_port'] = 8443
    gitlab_rails['kerberos_https'] = true
    
  2. Reconfigure GitLab for the changes to take effect.

After this change, Git remote URLs have to be updated to https://gitlab.example.com:8443/mygroup/myproject.git in order to use Kerberos ticket-based authentication.

Upgrading from password-based to ticket-based Kerberos sign-ins

Prior to GitLab 8.10 Enterprise Edition, users had to submit their Kerberos username and password to GitLab when signing in. We plan to remove support for password-based Kerberos sign-ins in a future release, so we recommend that you upgrade to ticket-based sign-ins.

Depending on your existing GitLab configuration, the ‘Sign in with: Kerberos SPNEGO’ button may already be visible on your GitLab sign-in page. If not, then add the settings described above.

Once you have verified that the ‘Kerberos SPNEGO’ button works without entering any passwords, you can proceed to disable password-based Kerberos sign-ins. To do this you need only need to remove the OmniAuth provider named kerberos from your gitlab.yml / gitlab.rb file.

For installations from source

  1. Edit gitlab.yml and remove the - { name: 'kerberos' } line under OmniAuth providers:

    omniauth:
      # Rest of configuration omitted
      # ...
      providers:
        - { name: 'kerberos' }  # <-- remove this line
    
  2. Restart GitLab for the changes to take effect.

For Omnibus installations

  1. Edit /etc/gitlab/gitlab.rb and remove the { "name" => "kerberos" } line under gitlab_rails['omniauth_providers']:

    gitlab_rails['omniauth_providers'] = [
      { "name" => "kerberos" } # <-- remove this entry
    ]
    
  2. Reconfigure GitLab for the changes to take effect.

note
Removing the kerberos OmniAuth provider can also resolve a rare Krb5Auth::Krb5::Exception (No credentials cache found) error (500 error in GitLab) when trying to clone via HTTPS.

Support for Active Directory Kerberos environments

When using Kerberos ticket-based authentication in an Active Directory domain, it may be necessary to increase the maximum header size allowed by NGINX, as extensions to the Kerberos protocol may result in HTTP authentication headers larger than the default size of 8kB. Configure large_client_header_buffers to a larger value in the NGINX configuration.

Troubleshooting

Unsupported GSSAPI mechanism

With Kerberos SPNEGO authentication, the browser is expected to send a list of mechanisms it supports to GitLab. If it doesn’t support any of the mechanisms GitLab supports, authentication fails with a message like this in the log:

OmniauthKerberosSpnegoController: failed to process Negotiate/Kerberos authentication: gss_accept_sec_context did not return GSS_S_COMPLETE: An unsupported mechanism was requested Unknown error

This is usually seen when the browser is unable to contact the Kerberos server directly. It falls back to an unsupported mechanism known as IAKERB, which tries to use the GitLab server as an intermediary to the Kerberos server.

If you’re experiencing this error, ensure there is connectivity between the client machine and the Kerberos server - this is a prerequisite! Traffic may be blocked by a firewall, or the DNS records may be incorrect.

Another failure mode occurs when the forward and reverse DNS records for the GitLab server do not match. Often, Windows clients work in this case while Linux clients fail. They use reverse DNS while detecting the Kerberos realm. If they get the wrong realm then ordinary Kerberos mechanisms fail, so the client falls back to attempting to negotiate IAKERB, leading to the above error message.

To fix this, ensure that the forward and reverse DNS for your GitLab server match. So for instance, if you access GitLab as gitlab.example.com, resolving to IP address 1.2.3.4, then 4.3.2.1.in-addr.arpa must be a PTR record for gitlab.example.com.

Finally, it’s possible that the browser or client machine lack Kerberos support completely. Ensure that the Kerberos libraries are installed and that you can authenticate to other Kerberos services.

HTTP Basic: Access denied when cloning

remote: HTTP Basic: Access denied
fatal: Authentication failed for '<KRB5 path>'

If you are using Git v2.11 or newer and see the above error when cloning, you can set the http.emptyAuth Git option to true to fix this:

git config --global http.emptyAuth true

See also: Git v2.11 release notes