Although I don’t consider fail2ban something to rely on for security is just another piece of the puzzle when securing a server. When configuring fail2ban for a debian server I noticed that the defaults rules are missing failures if DIGEST is used for authentication instead of basic.
I added the following to
[client <HOST>] (Digest: )?user .* (authentication failure|not found|password mismatch)
Restart fail2ban and try entering a wrong user/password while checking the fail2ban log. It should catch the try. (Be sure you don’t lock yourself out 🙂 )