404 lines
16 KiB
Markdown
404 lines
16 KiB
Markdown
---
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stage: Manage
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group: Access
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info: "To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments"
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type: reference, how-to
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---
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# Kerberos integration **(PREMIUM SELF)**
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GitLab can integrate with [Kerberos](https://web.mit.edu/kerberos/) as an authentication mechanism.
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## Overview
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[Kerberos](https://web.mit.edu/kerberos/) is a secure method for authenticating a request for a service in a
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computer network. Kerberos was developed in the Athena Project at the
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[Massachusetts Institute of Technology (MIT)](https://web.mit.edu/). The name is taken from Greek
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mythology; Kerberos was a three-headed dog who guarded the gates of Hades.
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## Use-cases
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- GitLab can be configured to allow your users to sign with their Kerberos credentials.
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- You can use Kerberos to [prevent](https://web.mit.edu/sipb/doc/working/guide/guide/node20.html) anyone from intercepting or eavesdropping on the transmitted password.
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## Configuration
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For GitLab to offer Kerberos token-based authentication, perform the
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following prerequisites. You still need to configure your system for
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Kerberos usage, such as specifying realms. GitLab makes use of the
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system's Kerberos settings.
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### GitLab keytab
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1. Create a Kerberos Service Principal for the HTTP service on your GitLab server.
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If your GitLab server is `gitlab.example.com` and your Kerberos realm
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`EXAMPLE.COM`, create a Service Principal `HTTP/gitlab.example.com@EXAMPLE.COM`
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in your Kerberos database.
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1. Create a keytab on the GitLab server for the above Service Principal. For example,
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`/etc/http.keytab`.
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The keytab is a sensitive file and must be readable by the GitLab user. Set
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ownership and protect the file appropriately:
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```shell
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sudo chown git /etc/http.keytab
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sudo chmod 0600 /etc/http.keytab
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```
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### Configure GitLab
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#### Installations from source
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NOTE:
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For source installations, make sure the `kerberos` gem group
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[has been installed](../install/installation.md#install-gems).
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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
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authentication. In most cases, you only need to enable Kerberos and specify
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the location of the keytab:
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```yaml
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omniauth:
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enabled: true
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allow_single_sign_on: ['kerberos']
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kerberos:
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# Allow the HTTP Negotiate authentication method for Git clients
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enabled: true
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# Kerberos 5 keytab file. The keytab file must be readable by the GitLab user,
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# and should be different from other keytabs in the system.
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# (default: use default keytab from Krb5 config)
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keytab: /etc/http.keytab
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```
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1. [Restart GitLab](../administration/restart_gitlab.md#installations-from-source) for the changes to take effect.
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#### Omnibus package installations
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1. Edit `/etc/gitlab/gitlab.rb`:
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```ruby
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gitlab_rails['omniauth_allow_single_sign_on'] = ['kerberos']
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gitlab_rails['kerberos_enabled'] = true
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gitlab_rails['kerberos_keytab'] = "/etc/http.keytab"
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```
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1. [Reconfigure GitLab](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to take effect.
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GitLab now offers the `negotiate` authentication method for signing in and
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HTTP Git access, enabling Git clients that support this authentication protocol
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to authenticate with Kerberos tokens.
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#### Enable single sign-on
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See [Initial OmniAuth Configuration](omniauth.md#initial-omniauth-configuration)
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for initial settings to enable single sign-on and add Kerberos servers
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as an identity provider.
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## Create and link Kerberos accounts
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You can either link a Kerberos account to an existing GitLab account, or
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set up GitLab to create a new account when a Kerberos user tries to sign in.
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### Link a Kerberos account to an existing GitLab account
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If you're an administrator, you can link a Kerberos account to an
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existing GitLab account. To do so:
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1. On the top bar, select **Menu >** **{admin}** **Admin**.
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1. On the left sidebar, select **Overview > Users**.
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1. Select a user, then select the **Identities** tab.
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1. Select 'Kerberos SPNEGO' in the 'Provider' dropdown box.
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1. Make sure the **Identifier** corresponds to the Kerberos username.
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1. Select **Save changes**.
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If you're not an administrator:
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1. In the top-right corner, select your avatar.
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1. Select **Edit profile**.
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1. In the left sidebar, select **Account**.
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1. In the **Social sign-in** section, select **Connect Kerberos SPNEGO**.
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If you don't see a **Social sign-in** Kerberos option, follow the
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requirements in [Enable single sign-on](#enable-single-sign-on).
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In either case, you should now be able to sign in to your GitLab account
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with your Kerberos credentials.
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### Create accounts on first sign-in
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The first time users sign in to GitLab with their Kerberos accounts,
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GitLab creates a matching account.
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Before you continue, review the [Initial OmniAuth Configuration](omniauth.md#initial-omniauth-configuration) options in Omnibus and GitLab source. You must also include `kerberos`.
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With that information at hand:
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1. Include `'kerberos'` with the `allow_single_sign_on` setting.
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1. For now, accept the default `block_auto_created_users` option, true.
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1. When a user tries to sign in with Kerberos credentials, GitLab
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creates a new account.
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1. If `block_auto_created_users` is true, the Kerberos user may see
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a message like:
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```shell
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Your account has been blocked. Please contact your GitLab
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administrator if you think this is an error.
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```
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1. As an administrator, you can confirm the new, blocked account:
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1. On the top bar, select **Menu >** **{admin}** **Admin**.
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1. On the left sidebar, select **Overview > Users** and review the **Blocked** tab.
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1. You can enable the user.
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1. If `block_auto_created_users` is false, the Kerberos user is
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authenticated and is signed in to GitLab.
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WARNING:
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We recommend that you retain the default for `block_auto_created_users`.
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Kerberos users who create accounts on GitLab without administrator
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knowledge can be a security risk.
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## Link Kerberos and LDAP accounts together
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If your users sign in with Kerberos, but you also have [LDAP integration](../administration/auth/ldap/index.md)
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enabled, your users are linked to their LDAP accounts on their first sign-in.
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For this to work, some prerequisites must be met:
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The Kerberos username must match the LDAP user's UID. You can choose which LDAP
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attribute is used as the UID in the GitLab [LDAP configuration](../administration/auth/ldap/index.md#configuration)
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but for Active Directory, this should be `sAMAccountName`.
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The Kerberos realm must match the domain part of the LDAP user's Distinguished
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Name. For instance, if the Kerberos realm is `AD.EXAMPLE.COM`, then the LDAP
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user's Distinguished Name should end in `dc=ad,dc=example,dc=com`.
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Taken together, these rules mean that linking only works if your users'
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Kerberos usernames are of the form `foo@AD.EXAMPLE.COM` and their
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LDAP Distinguished Names look like `sAMAccountName=foo,dc=ad,dc=example,dc=com`.
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### Custom allowed realms
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[Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/9962) in GitLab 13.5.
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You can configure custom allowed realms when the user's Kerberos realm doesn't
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match the domain from the user's LDAP DN. The configuration value must specify
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all domains that users may be expected to have. Any other domains are
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ignored and an LDAP identity is not linked.
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**For Omnibus installations**
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1. Edit `/etc/gitlab/gitlab.rb`:
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```ruby
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gitlab_rails['kerberos_simple_ldap_linking_allowed_realms'] = ['example.com','kerberos.example.com']
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```
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1. Save the file and [reconfigure](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure)
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GitLab for the changes to take effect.
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---
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**For installations from source**
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1. Edit `config/gitlab.yml`:
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```yaml
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kerberos:
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simple_ldap_linking_allowed_realms: ['example.com','kerberos.example.com']
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```
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1. Save the file and [restart](../administration/restart_gitlab.md#installations-from-source)
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GitLab for the changes to take effect.
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## HTTP Git access
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A linked Kerberos account enables you to `git pull` and `git push` using your
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Kerberos account, as well as your standard GitLab credentials.
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GitLab users with a linked Kerberos account can also `git pull` and `git push`
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using Kerberos tokens. That is, without having to send their password with each
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operation.
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WARNING:
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There is a [known issue](https://github.com/curl/curl/issues/1261) with `libcurl`
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older than version 7.64.1 wherein it doesn't reuse connections when negotiating.
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This leads to authorization issues when push is larger than `http.postBuffer`
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configuration. Ensure that Git is using at least `libcurl` 7.64.1 to avoid this. To
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know the `libcurl` version installed, run `curl-config --version`.
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### HTTP Git access with Kerberos token (passwordless authentication)
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#### Support for Git before 2.4
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Until Git version 2.4, the `git` command uses only the `negotiate` authentication
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method if the HTTP server offers it, even if this method fails (such as when
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the client does not have a Kerberos token). It is thus not possible to fall back
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to username/password (also known as `basic`) authentication if Kerberos
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authentication fails.
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For GitLab users to be able to use either `basic` or `negotiate` authentication
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with older Git versions, it is possible to offer Kerberos ticket-based
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authentication on a different port (for example, `8443`) while the standard port
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offers only `basic` authentication.
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**For source installations with HTTPS**
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1. Edit the NGINX configuration file for GitLab
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(for example, `/etc/nginx/sites-available/gitlab-ssl`) and configure NGINX to
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listen to port `8443` in addition to the standard HTTPS port:
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```conf
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server {
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listen 0.0.0.0:443 ssl;
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listen [::]:443 ipv6only=on ssl default_server;
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listen 0.0.0.0:8443 ssl;
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listen [::]:8443 ipv6only=on ssl;
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```
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1. Update the `kerberos` section of [`gitlab.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/gitlab.yml.example):
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```yaml
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kerberos:
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# Dedicated port: Git before 2.4 does not fall back to Basic authentication if Negotiate fails.
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# To support both Basic and Negotiate methods with older versions of Git, configure
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# nginx to proxy GitLab on an extra port (for example: 8443) and uncomment the following lines
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# to dedicate this port to Kerberos authentication. (default: false)
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use_dedicated_port: true
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port: 8443
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https: true
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```
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1. [Restart GitLab](../administration/restart_gitlab.md#installations-from-source) and NGINX for the changes to take effect.
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**For Omnibus package installations**
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1. Edit `/etc/gitlab/gitlab.rb`:
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```ruby
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gitlab_rails['kerberos_use_dedicated_port'] = true
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gitlab_rails['kerberos_port'] = 8443
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gitlab_rails['kerberos_https'] = true
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```
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1. [Reconfigure GitLab](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to take effect.
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After this change, Git remote URLs have to be updated to
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`https://gitlab.example.com:8443/mygroup/myproject.git` in order to use
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Kerberos ticket-based authentication.
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## Upgrading from password-based to ticket-based Kerberos sign-ins
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Prior to GitLab 8.10 Enterprise Edition, users had to submit their
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Kerberos username and password to GitLab when signing in. We plan to
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remove support for password-based Kerberos sign-ins in a future
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release, so we recommend that you upgrade to ticket-based sign-ins.
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Depending on your existing GitLab configuration, the 'Sign in with:
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Kerberos SPNEGO' button may already be visible on your GitLab sign-in
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page. If not, then add the settings [described above](#configuration).
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Once you have verified that the 'Kerberos SPNEGO' button works
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without entering any passwords, you can proceed to disable
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password-based Kerberos sign-ins. To do this you need only need to
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remove the OmniAuth provider named `kerberos` from your `gitlab.yml` /
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`gitlab.rb` file.
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**For installations from source**
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1. Edit [`gitlab.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/gitlab.yml.example) and remove the `- { name: 'kerberos' }` line under OmniAuth
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providers:
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```yaml
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omniauth:
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# Rest of configuration omitted
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# ...
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providers:
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- { name: 'kerberos' } # <-- remove this line
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```
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1. [Restart GitLab](../administration/restart_gitlab.md#installations-from-source) for the changes to take effect.
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**For Omnibus installations**
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1. Edit `/etc/gitlab/gitlab.rb` and remove the `{ "name" => "kerberos" }` line
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under `gitlab_rails['omniauth_providers']`:
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```ruby
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gitlab_rails['omniauth_providers'] = [
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{ "name" => "kerberos" } # <-- remove this entry
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]
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```
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1. [Reconfigure GitLab](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to take effect.
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NOTE:
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Removing the `kerberos` OmniAuth provider can also resolve a rare
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`Krb5Auth::Krb5::Exception (No credentials cache found)` error (`500` error in GitLab)
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when trying to clone via HTTPS.
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## Support for Active Directory Kerberos environments
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When using Kerberos ticket-based authentication in an Active Directory domain,
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it may be necessary to increase the maximum header size allowed by NGINX,
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as extensions to the Kerberos protocol may result in HTTP authentication headers
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larger than the default size of 8kB. Configure `large_client_header_buffers`
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to a larger value in [the NGINX configuration](http://nginx.org/en/docs/http/ngx_http_core_module.html#large_client_header_buffers).
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## Troubleshooting
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### Unsupported GSSAPI mechanism
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With Kerberos SPNEGO authentication, the browser is expected to send a list of
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mechanisms it supports to GitLab. If it doesn't support any of the mechanisms
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GitLab supports, authentication fails with a message like this in the log:
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```plaintext
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OmniauthKerberosSpnegoController: failed to process Negotiate/Kerberos authentication: gss_accept_sec_context did not return GSS_S_COMPLETE: An unsupported mechanism was requested Unknown error
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```
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This is usually seen when the browser is unable to contact the Kerberos server
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directly. It falls back to an unsupported mechanism known as
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[`IAKERB`](https://k5wiki.kerberos.org/wiki/Projects/IAKERB), which tries to use
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the GitLab server as an intermediary to the Kerberos server.
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If you're experiencing this error, ensure there is connectivity between the
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client machine and the Kerberos server - this is a prerequisite! Traffic may be
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blocked by a firewall, or the DNS records may be incorrect.
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Another failure mode occurs when the forward and reverse DNS records for the
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GitLab server do not match. Often, Windows clients work in this case while
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Linux clients fail. They use reverse DNS while detecting the Kerberos
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realm. If they get the wrong realm then ordinary Kerberos mechanisms fail,
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so the client falls back to attempting to negotiate `IAKERB`, leading to the
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above error message.
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To fix this, ensure that the forward and reverse DNS for your GitLab server
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match. So for instance, if you access GitLab as `gitlab.example.com`, resolving
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to IP address `1.2.3.4`, then `4.3.2.1.in-addr.arpa` must be a PTR record for
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`gitlab.example.com`.
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Finally, it's possible that the browser or client machine lack Kerberos support
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completely. Ensure that the Kerberos libraries are installed and that you can
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authenticate to other Kerberos services.
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### HTTP Basic: Access denied when cloning
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```shell
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remote: HTTP Basic: Access denied
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fatal: Authentication failed for '<KRB5 path>'
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```
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If you are using Git v2.11 or newer and see the above error when cloning, you can
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set the `http.emptyAuth` Git option to `true` to fix this:
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```shell
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git config --global http.emptyAuth true
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```
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See also: [Git v2.11 release notes](https://github.com/git/git/blob/master/Documentation/RelNotes/2.11.0.txt#L482-L486)
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## Helpful links
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- <https://help.ubuntu.com/community/Kerberos>
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- <http://blog.manula.org/2012/04/setting-up-kerberos-server-with-debian.html>
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- <https://www.roguelynn.com/words/explain-like-im-5-kerberos/>
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