How to use multiple SSH keys in an `authorized_keys` file for a user on my server?

Question

I have 3 public ssh keys that correspond to 3 users that work on my server. I need to use those 3 public keys for the git user on my server, so that they can access git repositories on the server.

I ran the following in an attempt to make the git user accessible via ssh:

 cat /home/user_one/.ssh/authorized_keys >> /var/git/.ssh/authorized_keys
 cat /home/user_two/.ssh/authorized_keys >> /var/git/.ssh/authorized_keys
 cat /home/user_three/.ssh/authorized_keys >> /var/git/.ssh/authorized_keys

However, we get a git@domain.com: Permission denied (publickey) error when trying to ssh in as the git user, or trying to push to a git repository.

Notes: /var/git is the home directory of the git user. The server is on Debian 11.

We can connect to each of the users individually, why can't we connect to the git user?

Answer 1

The sshd logfile on the git machine will likely tell you what it's finding wrong with the permissions on the authorized_keys file. I have a 3-point checklist I follow after I've appended to a user's authorized keys file like you describe:

1. Make sure the git user's home dir is not writable by the world or group. This usually means 755, or 751, or 711 permissions on the home dir.
2. Make sure the git user's .ssh dir is very locked down. Usually 700 permissions.
3. Make sure the git user's authorized_keys file is not readable or writable by anyone but the user. Usually 600 permissions and owned by the user and the user's primary group.