Verify which key was used to connect to SSH

Question

I usually connect to a particular server with a specific key, like so:
$ ssh -i key.pem ec2-user@server

Yesterday I realised that the key I have been using isn't actually in ec2-user's authorized_keys file. There is only one key listed, which I do also have on my machine.
I am trying to understand how I've been able to successfully connect using the above command. Is there a way to get the ssh program to explicity describe which key it used to connect?

Ángel · Accepted Answer

When providing -v in the connection command of ssh it will show a couple of lines like debug1: Offering public key: /home/foo/.ssh/id_rsa SHA256:hash debug1: Server accepts key: /home/foo/.ssh/id_rsa SHA256:hash where you can view the key used from the client point of view. At the server it should be logged at /var/log/auth.log like: sshd[1668]: Accepted publickey for from port ssh2: RSA SHA256:` I suspect you will actually have the key appearing on authorized_keys, perhaps in ~/.ssh/id_rsa or similar (they are loeaded by default). If you want to use the key provided, and only that one, even if there are other public key files, or keys on the agent, you should also set IdentitiesOnly=yes, e.g. ssh -oIdentitiesOnly=yes -i key.pem ec2-user@server Although for hosts you frequently connect to, I would recommend setting that up on ~/.ssh/ssh_config: Host server User ec2-user IdentitiesOnly yes IdentityFile key.pem so you can just do ssh server

Verify which key was used to connect to SSH

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