Kerberos integration

Note: Available in GitLab Starter.

GitLab can integrate with Kerberos as an authentication mechanism.

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 will make use of the system's Kerberos settings.

GitLab keytab

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.
Create a keytab on the GitLab server for the above Service Principal, e.g. /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.

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

Restart GitLab for the changes to take effect.

Omnibus package installations

Edit /etc/gitlab/gitlab.rb:

gitlab_rails['omniauth_enabled'] = true
gitlab_rails['omniauth_allow_single_sign_on'] = ['kerberos']

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

Reconfigure GitLab for the changes to take effect.

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

Creating and linking Kerberos accounts

The Administrative user can navigate to Admin > Users > Example User > Identities and attach a Kerberos account. Existing GitLab users can go to Profile > Account and attach a Kerberos account. If you want to allow users without a GitLab account to login, you should enable the option allow_single_sign_on as described in the Configure GitLab section. Then, the first time a user signs in with Kerberos credentials, GitLab will create a new GitLab user associated with the email, which is built from the Kerberos username and realm. User accounts will be created automatically when authentication was successful.

Linking Kerberos and LDAP accounts together

If your users log in with Kerberos, but you also have LDAP integration enabled, then your users will be automatically linked to their LDAP accounts on first login. 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 GitLab's 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 will only work 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.

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, i.e., without having to send their password with each operation.

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 (e.g. 8443) while the standard port will keep offering only basic authentication.

For source installations with HTTPS

Edit the NGINX configuration file for GitLab (e.g., /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;

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 (e.g. 8443) and uncomment the following lines
  # to dedicate this port to Kerberos authentication. (default: false)
  use_dedicated_port: true
  port: 8443
  https: true

Restart GitLab and NGINX for the changes to take effect.

For Omnibus package installations

Edit /etc/gitlab/gitlab.rb:

gitlab_rails['kerberos_use_dedicated_port'] = true
gitlab_rails['kerberos_port'] = 8443
gitlab_rails['kerberos_https'] = true

Reconfigure GitLab for the changes to take effect.

After this change, all Git remote URLs will 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 will 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

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

omniauth:
  # ...
  providers:
    - { name: 'kerberos' } # <-- remove this line

Restart GitLab for the changes to take effect.

For Omnibus installations

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
]
```
Reconfigure GitLab for the changes to take effect.

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 will fail 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 will fall 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 will work in this case, while Linux clients will fail. They use reverse DNS while detecting the Kerberos realm. If they get the wrong realm, then ordinary Kerberos mechanisms will fail, so the client will fall 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 acces 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