User lookup via OpenSSH's AuthorizedPrincipalsCommand

Available in GitLab Community Edition 11.2.

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

In centralized (e.g. corporate) environments this can be a hassle operationally, particularly if the SSH keys are temporary keys issued to the user, e.g. 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 that version introduced the AuthorizedPrincipalsCommand configuration option. If using CentOS 6, you can follow these instructions to compile an up-to-date version.

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's PROTOCOL.certkeys for how it works, and e.g. 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, e.g.:

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, e.g. (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, e.g. it could be prod-aearnfjord if it's a SSH certificate you'd normally log 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 will emit 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 (e.g. aeanfjord), and {PRINCIPAL} is the principal passed to it (e.g. sshUsers).

You will 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 log in to GitLab, or you must provide a list of principals, one of which is going to be present for the user, e.g.:

    [...]
    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 will be turned into multiple lines of authorized_keys output, as described in the AuthorizedPrincipalsFile documentation in sshd_config(5).

Normally when using the AuthorizedKeysCommand with OpenSSH the principal is some "group" that's allowed to log into 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 will enforce its own ACLs for that user (e.g. what projects the user can access).

So it's OK to e.g. be overly generous in what you accept, since if the user e.g. has no access to GitLab at all it'll just error out with a message about this being an invalid user.

Interaction with the `authorized_keys` file

SSH certificates can be used in conjunction with the authorized_keys file, and if set up as configured above the authorized_keys file will still serve as a fallback.

This is because if the AuthorizedPrincipalsCommand can't authenticate the user, OpenSSH will fall back on ~/.ssh/authorized_keys (or the AuthorizedKeysCommand).

Therefore there may still be a reason to use the "Fast lookup of authorized SSH keys in the database" method in conjunction with this. Since you'll be using SSH certificates for all your normal users, and relying on the ~/.ssh/authorized_keys fallback for deploy keys, if you make use of those.

But you may find that there's no reason to do that, since all your normal users will use the fast AuthorizedPrincipalsCommand path, and only automated deployment key access will fall back on ~/.ssh/authorized_keys, or that you have a lot more keys for normal users (especially if they're renewed) than you have deploy keys.

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 currently no feature to prevent this, but there's an open request for adding it.

Such a restriction can currently be hacked in by e.g. providing 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 will show 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.

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