SSH login without user name?

Aarkon@feddit.de to Linux@lemmy.ml – 33 points –
docs.gitlab.com

I was reading GitLab's documentation (see link) on how to write to a repository from within the CI pipeline and noticed something: The described Docker executor is able to authenticate e.g. against the Git repository with only a private SSH key, being told absolutely nothing about the user's name it is associated with.
If I'm correct, that would mean that technically, I could authenticate to an SSH server without supplying my name if I use a private key?

I know that when I don't supply a user explicitly like ssh user@server or via .ssh/config, the active environment's user is used automatically, that's not what I'm asking.

The public key contains a user name/email address string, I'm aware, is the same information also encoded into the private key as well? If yes, I don't see the need to hand that info to an SSH call. If no, how does the SSH server know which public key it's supposed to use to challenge my private key ownership? It would have to iterate over all saved keys, which sounds rather inefficient to me and potentially unsafe (timing attacks etc.).

I hope I'm somewhat clear, for some reason I find it really hard to phrase this question.

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Technically, you always use a username, however in case of Gitlab that SSH username is always git. When an SSH client connects to server, it offers an authentication method. Gitlab accepts that method only if it is a publickey and the fingerprint of the offered key maps to the known Gitlab user.

It's a blessing and a curse. I have two gitlab accounts on the same server - private and work. I can't use the same key for both as the key is used to distinguish git users, and git doesn't make it easy to select which key you want to use to pull or clone particular repo.

git config core.sshCommand 'ssh -i '

Hmm, with a similar technique one could even create git command aliases for running git with specific ssh private keys

Better than that, git config supports conditional includes, based on a repo URL or path on disk. So you can have a gitconfig per organization or whatever, which specifies an sshCommand and thus an ssh key.

Just a thought: why don't you just use two different aliases for the Server in your .ssh/config with your two differing ssh keys, that way you can just use two different "hostnames" that have different ssh keys specified

I could but then i can't simply copy the "clone" url from the projects page without editing it.

I think the best solution for me may be to have a separate user account in the OS for work and personal projects. That way i can just log off at the end of the day and use personal account for personal stuff.

You might not even have to log out: Just change the user in the terminal: su - user2