# Kerberos integration **(STARTER ONLY)** GitLab can integrate with [Kerberos][kerb] as an authentication mechanism. ## Overview [Kerberos][kerb] 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)][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][why-kerb] 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 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. 1. 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](../install/installation.md#install-gems). 1. Edit the `kerberos` section of [`gitlab.yml`](https://gitlab.com/gitlab-org/gitlab/blob/master/config/gitlab.yml.example) to enable Kerberos ticket-based authentication. In most cases, you only need to enable Kerberos and specify the location of the keytab: ```yaml 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 ``` 1. [Restart GitLab] for the changes to take effect. **Omnibus package installations** 1. Edit `/etc/gitlab/gitlab.rb`: ```ruby gitlab_rails['omniauth_allow_single_sign_on'] = ['kerberos'] gitlab_rails['kerberos_enabled'] = true gitlab_rails['kerberos_keytab'] = "/etc/http.keytab" ``` 1. [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](#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](../administration/auth/ldap.md) 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](../administration/auth/ldap.md#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** 1. 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: ```conf 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; ``` 1. Update the `kerberos` section of [`gitlab.yml`](https://gitlab.com/gitlab-org/gitlab/blob/master/config/gitlab.yml.example): ```yaml 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 ``` 1. [Restart GitLab] and NGINX for the changes to take effect. **For Omnibus package installations** 1. Edit `/etc/gitlab/gitlab.rb`: ```ruby gitlab_rails['kerberos_use_dedicated_port'] = true gitlab_rails['kerberos_port'] = 8443 gitlab_rails['kerberos_https'] = true ``` 1. [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](#configuration). 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`](https://gitlab.com/gitlab-org/gitlab/blob/master/config/gitlab.yml.example) and remove the `- { name: 'kerberos' }` line under OmniAuth providers: ```yaml omniauth: # ... providers: - { name: 'kerberos' } # <-- remove this line ``` 1. [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']`: ```ruby gitlab_rails['omniauth_providers'] = [ { "name" => "kerberos" } # <-- remove this entry ] ``` 1. [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][nginx]. ## 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`](https://k5wiki.kerberos.org/wiki/Projects/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 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 ```shell remote: HTTP Basic: Access denied fatal: Authentication failed for '' ``` 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](https://github.com/git/git/blob/master/Documentation/RelNotes/2.11.0.txt#L482-L486) ## Helpful links - - - [restart gitlab]: ../administration/restart_gitlab.md#installations-from-source [reconfigure gitlab]: ../administration/restart_gitlab.md#omnibus-gitlab-reconfigure [nginx]: http://nginx.org/en/docs/http/ngx_http_core_module.html#large_client_header_buffers [kerb]: https://web.mit.edu/kerberos/ [mit]: http://web.mit.edu/ [why-kerb]: http://web.mit.edu/sipb/doc/working/guide/guide/node20.html [ee]: https://about.gitlab.com/pricing/