gitlab-org--gitlab-foss/doc/integration/kerberos.md

# Kerberos integration **(STARTER ONLY)**

GitLab can integrate with [Kerberos](https://web.mit.edu/kerberos/) as an authentication mechanism.

## Overview

[Kerberos](https://web.mit.edu/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)](http://web.mit.edu/). 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](http://web.mit.edu/sipb/doc/working/guide/guide/node20.html) 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:

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

### Configure GitLab

**Installations from source**

NOTE: **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](../administration/restart_gitlab.md#installations-from-source) 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](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure) 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/index.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/index.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`.

### Custom allowed realms

[Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/9962) 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 will be ignored and an LDAP identity will not be linked.

**For Omnibus installations**

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

   ```ruby
   gitlab_rails['kerberos_simple_ldap_linking_allowed_realms'] = ['example.com','kerberos.example.com']
   ```

1. Save the file and [reconfigure](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure)
   GitLab for the changes to take effect.

---

**For installations from source**

1. Edit `config/gitlab.yml`:

   ```yaml
   kerberos:
     simple_ldap_linking_allowed_realms: ['example.com','kerberos.example.com']
   ```

1. Save the file and [restart](../administration/restart_gitlab.md#installations-from-source)
   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, 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](../administration/restart_gitlab.md#installations-from-source) 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](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure) 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:
     # Rest of configuration omitted
     # ...
     providers:
       - { name: 'kerberos' }  # <-- remove this line
   ```

1. [Restart GitLab](../administration/restart_gitlab.md#installations-from-source) 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](../administration/restart_gitlab.md#omnibus-gitlab-reconfigure) 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](http://nginx.org/en/docs/http/ngx_http_core_module.html#large_client_header_buffers).

## 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:

```plaintext
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 '<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:

```shell
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

- <https://help.ubuntu.com/community/Kerberos>
- <http://blog.manula.org/2012/04/setting-up-kerberos-server-with-debian.html>
- <https://www.roguelynn.com/words/explain-like-im-5-kerberos/>