2

I'm looking for a way to set a specific SSH key while logging to a remote host with a particular username. Is this possible within the SSH config file?

For example:

Use key "id_rsa-test" for username "testuser":

ssh testuser@host1
ssh testuser@host2
ssh testuser@host3

Use key "id_rsa" for all other users.

ssh root@host1
ssh admin@host2
ssh user@1.2.3.4

Is there a way to configure this?

2

Thank you @xhienne (comment link) for your suggestion.

Adding the following to ssh_config appears to have solved the problem:

Match User testuser
  IdentityFile ~/.ssh/id_rsa-test

This block should be placed above Host * to send id_rsa-test ahead of id_rsa for testuser.

  • You're welcome. You may even accept your own answer. – xhienne Jul 27 '17 at 1:42
1

Assuming you're using OpenSSH you can configure your ~/.ssh/config file for each, then you won't have to specify a user either, you can create your own name:

host host1
    User testuser
    IdentityFile ~/.ssh/id_rsa-test

then when you do ssh host1 it will do that automatically, or, since you will use host1 as other users too you can do

host test-host1
    User testuser
    IdentityFile ~/.ssh/id_rsa-test
    Hostname host1

then do ssh test-host1

and you can do the same for each host you want.

Any host that isn't matched by an entry in your ssh_config(5) file will use the default behavior--or use the defaults that are specified in ssh_config at the global setting, i.e., not inside a host block

ssh_config doesn't support matching on usernames, but you could write a shell script to detect that for you. Here's a fragile one in bash that assumes your username/host is always the first argument and will break if it is not:

ssh() {
    if [[ "$1" =~ ^testuser@ ]]; then
        command ssh -i ~/.ssh/id_rsa-test "$@"
    else
        command ssh "$@"
    fi
}
  • Is it possible to use host * and / or create a rule that can match the username? Assuming there are lots of hosts, making it inefficient to create configs for each. – user2248259 Jul 27 '17 at 0:50
  • @user2248259 I don't know of a way to use the username as part of the pattern to match in ssh_config. I suppose you could always try writing a shell function to check the username and add appropriate arguments as desired if no one else has a config-based answer and you don't want to have different host entries for each – Eric Renouf Jul 27 '17 at 0:57
  • @user2248259 I provided a fragile example shell function that could try to help with this situation – Eric Renouf Jul 27 '17 at 1:03
  • 1
    @Eric, you mean ssh -i, right? Not ssh -I. Moreover, in ssh_config, the Match directive allows to check against the user part. – xhienne Jul 27 '17 at 1:26
  • @xhienne yes, I did mean that, and I didn't know that about the match directive! – Eric Renouf Jul 27 '17 at 1:27

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