User lookup via OpenSSH's AuthorizedPrincipalsCommand

DETAILS: Tier: Free, Premium, Ultimate Offering: Self-managed

The default SSH authentication for GitLab requires users to upload their SSH public keys before they can use the SSH transport.

In centralized (for example, corporate) environments this can be a hassle operationally, particularly if the SSH keys are temporary keys issued to the user, including ones that expire 24 hours after issuing.

In such setups some external automated process is needed to constantly upload the new keys to GitLab.

WARNING: OpenSSH version 6.9+ is required because AuthorizedKeysCommand must be able to accept a fingerprint. Check the version of OpenSSH on your server.

Why use OpenSSH certificates?

By using OpenSSH certificates all the information about what user on GitLab owns the key is encoded in the key itself, and OpenSSH itself guarantees that users can't fake this, since they'd need to have access to the private CA signing key.

When correctly set up, this does away with the requirement of uploading user SSH keys to GitLab entirely.

Setting up SSH certificate lookup via GitLab Shell

How to fully set up SSH certificates is outside the scope of this document. See OpenSSH'sPROTOCOL.certkeys for how it works, for example RedHat's documentation about it.

We assume that you already have SSH certificates set up, and have added the TrustedUserCAKeys of your CA to your sshd_config, for example:

TrustedUserCAKeys /etc/security/mycompany_user_ca.pub

Usually TrustedUserCAKeys would not be scoped under a Match User git in such a setup, since it would also be used for system logins to the GitLab server itself, but your setup may vary. If the CA is only used for GitLab consider putting this in the Match User git section (described below).

The SSH certificates being issued by that CA must have a "key ID" corresponding to that user's username on GitLab, for example (some output omitted for brevity):

$ ssh-add -L | grep cert | ssh-keygen -L -f -

(stdin):1:
        Type: ssh-rsa-cert-v01@openssh.com user certificate
        Public key: RSA-CERT SHA256:[...]
        Signing CA: RSA SHA256:[...]
        Key ID: "aearnfjord"
        Serial: 8289829611021396489
        Valid: from 2018-07-18T09:49:00 to 2018-07-19T09:50:34
        Principals:
                sshUsers
                [...]
        [...]

Technically that's not strictly true, for example, it could be prod-aearnfjord if it's a SSH certificate you'd usually sign in to servers as the prod-aearnfjord user, but then you must specify your own AuthorizedPrincipalsCommand to do that mapping instead of using our provided default.

The important part is that the AuthorizedPrincipalsCommand must be able to map from the "key ID" to a GitLab username in some way, the default command we ship assumes there's a 1=1 mapping between the two, since the whole point of this is to allow us to extract a GitLab username from the key itself, instead of relying on something like the default public key to username mapping.

Then, in your sshd_config set up AuthorizedPrincipalsCommand for the git user. Hopefully you can use the default one shipped with GitLab:

Match User git
    AuthorizedPrincipalsCommandUser root
    AuthorizedPrincipalsCommand /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell-authorized-principals-check %i sshUsers

This command emits output that looks something like:

command="/opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell username-{KEY_ID}",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty {PRINCIPAL}

Where {KEY_ID} is the %i argument passed to the script (for example, aeanfjord), and {PRINCIPAL} is the principal passed to it (for example, sshUsers).

You need to customize the sshUsers part of that. It should be some principal that's guaranteed to be part of the key for all users who can sign in to GitLab, or you must provide a list of principals, one of which is present for the user, for example:

    [...]
    AuthorizedPrincipalsCommand /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell-authorized-principals-check %i sshUsers windowsUsers

Principals and security

You can supply as many principals as you want, these are turned into multiple lines of authorized_keys output, as described in the AuthorizedPrincipalsFile documentation in sshd_config(5).

Usually when using the AuthorizedKeysCommand with OpenSSH the principal is some "group" that's allowed to sign in to that server. However with GitLab it's only used to appease OpenSSH's requirement for it, we effectively only care about the "key ID" being correct. Once that's extracted GitLab enforces its own ACLs for that user (for example, what projects the user can access).

It's therefore fine to be overly generous in what you accept. For example, if the user has no access to GitLab, an error is produced with a message about an invalid user. message about this being an invalid user.

Interaction with the authorized_keys file

If SSH certificates are set up as described above, they can be used with the authorized_keys file so that the authorized_keys file serves as a fallback.

When the AuthorizedPrincipalsCommand is unable to authenticate a user, OpenSSH reverts to checking the ~/.ssh/authorized_keys file or using the AuthorizedKeysCommand. Therefore, you might still need to use Fast lookup of authorized SSH keys in the database with SSH certificates.

For most users, SSH certificates handle authentication by using the AuthorizedPrincipalsCommand, with the ~/.ssh/authorized_keys file primarily serving as a fallback for specific cases such as deploy keys. However, depending on your setup, you might find that using the AuthorizedPrincipalsCommand exclusively for typical users is sufficient. In such cases, the authorized_keys file is only necessary for automated deployment key access or other specific scenarios.

Consider the balance between the number of keys for typical users (especially if they are frequently renewed) and deploy keys to help you determine whether maintaining the authorized_keys fallback is necessary for your environment.

Other security caveats

Users can still bypass SSH certificate authentication by manually uploading an SSH public key to their profile, relying on the ~/.ssh/authorized_keys fallback to authenticate it.

There's an open issue to add a setting that prevents users from uploading SSH keys that are not deploy keys.

You can build a check to enforce this restriction yourself. For example, provide a custom AuthorizedKeysCommand which checks if the discovered key-ID returned from gitlab-shell-authorized-keys-check is a deploy key or not (all non-deploy keys should be refused).

Disabling the global warning about users lacking SSH keys

By default GitLab shows a "You won't be able to pull or push project code via SSH" warning to users who have not uploaded an SSH key to their profile.

This is counterproductive when using SSH certificates, since users aren't expected to upload their own keys.

To disable this warning globally, go to "Application settings -> Account and limit settings" and disable the "Show user add SSH key message" setting.

This setting was added specifically for use with SSH certificates, but can be turned off without using them if you'd like to hide the warning for some other reason.