After a longer than desired struggle with getting sshd to accept my public key, I think a blog post is in order to remind myself not to repeat the same mistakes. Here’s how you should go about debugging your SSH public key authentication woes:

• Getting more debug info when connecting with your ssh client: Add a ‘-v’ option to your ssh command (e.g. ssh chuyeow@remotehost -v -v -v). Add more ‘-v’ for more detailed debug (you can do up to ‘-v -v -v’ I think).
• Debugging on the remote host by running sshd in debug mode: Run ‘/usr/sbin/sshd -d -p 2222′ on the remote host and connect to it. ‘2222′ here is the port number of the sshd process you started on the remote host.
• tail the authentication log: Run ‘tail -f /var/log/auth.log’ on the remotehost. You can watch the log as you try to connect via SSH with your key.
• Make sure your ssh key agent is running: Do a ‘ps aux|grep ssh-agent’. Make sure your key agent is running. If you’re not using ssh-agent (I like keychain from Gentoo, or SSHKeyChain for Mac OS X), do whatever you have to do to ensure that your keychain is running.
• Make sure your private key is added to the ssh key agent: Do a ’ssh-add -l’ to check that ssh-agent has your key. Likewise, if you are using something else, check your keychain application has your private key.
• Check the permissions on your home directory, .ssh directory, and the authorized_keys file: If your ssh server is running with ‘StrictModes on’, it will refuse to use your public keys in the ~/.ssh/authorized_keys file. Your home directory should be writable only by you, ~/.ssh should be 700, and authorized_keys should be 600.

Tailing the authentication log was the clincher for me this time – my problem was the group permissions on the home folder were incorrectly set (the error message I got from auth.log was: ‘Authentication refused: bad ownership or modes for directory /home/chuyeow’). Just had to fix it so it was no longer group-writable. Of course, this can also be fixed by turning setting ‘StrictModes off’ in your sshd config (/etc/ssh/sshd_config), but it’s not really recommended. Plus, you may not always have the rights to edit that file anyway.