- Why use OpenSSH certificates?
- Setting up SSH certificate lookup via GitLab Shell
- Principals and security
- Interaction with the
authorized_keys
file - Other security caveats
- Disabling the global warning about users lacking SSH keys
User lookup via OpenSSH’s AuthorizedPrincipalsCommand
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.
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
currently no feature to prevent this,
but there’s an open request for adding it.
Such a restriction can currently be hacked in by, for example, 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 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.