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# User lookup via OpenSSH's AuthorizedPrincipalsCommand
> [Available in](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19911) 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](fast_ssh_key_lookup.html#compiling-a-custom-version-of-openssh-for-centos-6)
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 setup SSH certificates is outside the scope of this
document. See [OpenSSH's
PROTOCOL.certkeys](https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?annotate=HEAD)
for how it works, and e.g. [RedHat's documentation about
it](https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/deployment_guide/sec-using_openssh_certificate_authentication).
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 setup 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"](fast_ssh_key_lookup.html) 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](https://gitlab.com/gitlab-org/gitlab-ce/issues/49218).
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).
|
