Troubleshooting ssh connections problem

Micky found this which was key to debugging our connection problem— one way to watch what is happening with the SSH connection attempt is by opening a port just for this use on the server:

sudo /usr/sbin/sshd -d -p 2222

Then from the client:

ssh -vv -p 2222 example.com

Per http://www.unixlore.net/articles/troubleshooting-ssh-connections.html

In this case, the problem was (as suspected) a holdover from an apparently successful change of the user's username with usermod:

sudo usermod -l newusername -m -d /home/newusername oldusername

Every usual place on Debian Linux — the /home directory of course, the /etc/passwd file — had the rename perfectly. Looking at the terminal output on the server while the test connection to port 2222 (instead of 22) took place, we could spot the error:

debug1: Could not open authorized keys '/var/lib/monkeysphere/authorized_keys/newusername': No such file or directory

Our server setup uses monkeysphere. Yours probably doesn't. But for us, moving the oldusername file to the username wasn't enough, we also needed sudo monkeysphere-authentication update-users.

But the important lesson is how to see what the problem is!