I was setting up SSH to be able to contact Gitlab repositories.
But at the very end, when comes the step to test if the connection is
succesful, I followed the Gitlab
documentation
(use the command ssh -T git@gitlab.example.com) and it was
not working.
I had no trouble setting it up for Github a few days before, and the setup is pretty similar. I could not pinpoint what was going on at first.
I figured I must have gone a bit fast following the docs, recreated a new pair of keys, tried again, and still no luck.
I went to the Github documentation to set up SSH, started from scratch again. And still, when it was time to test the connection, it would not be working.
And then, after at least 30min of trying to understand why, the realisation came to me (and maybe you understand as soon as you saw the command): the domain in the doc is not a real one, so I was trying again and again to test my SSH connection again a non-existent domain.
So if I had skipped that step and went straight to cloning a repository, it would have worked!
Lesson for this time: even when following a step-by-step guide for something that should be trivial, pay attention to what the command contains to not waste time debugging afterward.