OpenID Connect OmniAuth provider

GitLab can use OpenID Connect as an OmniAuth provider.

To enable the OpenID Connect OmniAuth provider, you must register your application with an OpenID Connect provider. The OpenID Connect will provide you with a client details and secret for you to use.

On your GitLab server, open the configuration file.

For Omnibus GitLab:
```
sudo editor /etc/gitlab/gitlab.rb
```
For installations from source:
```
cd /home/git/gitlab
sudo -u git -H editor config/gitlab.yml
```
See Initial OmniAuth Configuration for initial settings.

Add the provider configuration.

For Omnibus GitLab:

gitlab_rails['omniauth_providers'] = [
  { 'name' => 'openid_connect',
    'label' => '<your_oidc_label>',
    'args' => {
      'name' => 'openid_connect',
      'scope' => ['openid','profile'],
      'response_type' => 'code',
      'issuer' => '<your_oidc_url>',
      'discovery' => true,
      'client_auth_method' => 'query',
      'uid_field' => '<uid_field>',
      'client_options' => {
        'identifier' => '<your_oidc_client_id>',
        'secret' => '<your_oidc_client_secret>',
        'redirect_uri' => '<your_gitlab_url>/users/auth/openid_connect/callback'
      }
    }
  }
]

For installation from source:

  - { name: 'openid_connect',
      label: '<your_oidc_label>',
      args: {
        name: 'openid_connect',
        scope: ['openid','profile'],
        response_type: 'code',
        issuer: '<your_oidc_url>',
        discovery: true,
        client_auth_method: 'query',
        uid_field: '<uid_field>',
        client_options: {
          identifier: '<your_oidc_client_id>',
          secret: '<your_oidc_client_secret>',
          redirect_uri: '<your_gitlab_url>/users/auth/openid_connect/callback'
        }
      }
    }

NOTE: Note: For more information on each configuration option refer to the OmniAuth OpenID Connect usage documentation and the OpenID Connect Core 1.0 specification.

For the configuration above, change the values for the provider to match your OpenID Connect client setup. Use the following as a guide:
- <your_oidc_label> is the label that will be displayed on the login page.
- <your_oidc_url> (optional) is the URL that points to the OpenID Connect provider. For example, https://example.com/auth/realms/your-realm. If this value is not provided, the URL is constructed from the client_options in the following format: <client_options.scheme>://<client_options.host>:<client_options.port>.
- If discovery is set to true, the OpenID Connect provider will try to auto discover the client options using <your_oidc_url>/.well-known/openid-configuration. Defaults to false.
- client_auth_method (optional) specifies the method used for authenticating the client with the OpenID Connect provider.
  - Supported values are:
    - basic - HTTP Basic Authentication
    - jwt_bearer - JWT based authentication (private key and client secret signing)
    - mtls - Mutual TLS or X.509 certificate validation
    - Any other value will POST the client id and secret in the request body
  - If not specified, defaults to basic.
- <uid_field> (optional) is the field name from the user_info details that will be used as uid value. For example, preferred_username. If this value is not provided or the field with the configured value is missing from the user_info details, the uid will use the sub field.
- client_options are the OpenID Connect client-specific options. Specifically:
  - identifier is the client identifier as configured in the OpenID Connect service provider.
  - secret is the client secret as configured in the OpenID Connect service provider.
  - redirect_uri is the GitLab URL to redirect the user to after successful login. For example, http://example.com/users/auth/openid_connect/callback.
  - end_session_endpoint (optional) is the URL to the endpoint that end the session (logout). Can be provided if auto-discovery disabled or unsuccessful.
  - The following client_options are optional unless auto-discovery is disabled or unsuccessful:
    - authorization_endpoint is the URL to the endpoint that authorizes the end user.
    - token_endpoint is the URL to the endpoint that provides Access Token.
    - userinfo_endpoint is the URL to the endpoint that provides the user information.
    - jwks_uri is the URL to the endpoint where the Token signer publishes its keys.
Save the configuration file.
Reconfigure or restart GitLab for the changes to take effect if you installed GitLab via Omnibus or from source respectively.

On the sign in page, there should now be an OpenID Connect icon below the regular sign in form. Click the icon to begin the authentication process. The OpenID Connect provider will ask the user to sign in and authorize the GitLab application (if confirmation required by the client). If everything goes well, the user will be redirected to GitLab and will be signed in.

Example configurations

The following configurations illustrate how to set up OpenID with different providers with Omnibus GitLab.

Google

See the Google documentation for more details:

 gitlab_rails['omniauth_providers'] = [
 {
   'name' => 'openid_connect',
   'label' => 'Google OpenID',
   'args' => {
     'name' => 'openid_connect',
     'scope' => ['openid', 'profile', 'email'],
     'response_type' => 'code',
     'issuer' => 'https://accounts.google.com',
     'client_auth_method' => 'query',
     'discovery' => true,
     'uid_field' => 'preferred_username',
     'client_options' => {
       'identifier' => '<YOUR PROJECT CLIENT ID>',
       'secret' => '<YOUR PROJECT CLIENT SECRET>',
       'redirect_uri' => 'https://example.com/users/auth/openid_connect/callback',
     }
   }
 }

Troubleshooting

If you're having trouble, here are some tips:

Ensure discovery is set to true. Setting it to false requires specifying all the URLs and keys required to make OpenID work.
Check your system clock to ensure the time is synchronized properly.
As mentioned in the documentation, make sure issuer corresponds to the base URL of the Discovery URL. For example, https://accounts.google.com is used for the URL https://accounts.google.com/.well-known/openid-configuration.
The OpenID Connect client uses HTTP Basic Authentication to send the OAuth2 access token if client_auth_method is not defined or if set to basic. If you are seeing 401 errors upon retrieving the userinfo endpoint, you may want to check your OpenID Web server configuration. For example, for oauth2-server-php, you may need to add a configuration parameter to Apache.