Fixing fail2ban for sshd on Debian 12

It would seem that due to some sort of packaging or maintainer config dispute the default configuration for fail2ban's SSHd authentication monitoring (and thus blocking) does not work on Debian 12, and it's derivatives such as Ubuntu.

The solution is to configure fail2ban to read (and eventually ban) SSHd authentication failures from "systemd" rather than a log file as previously expected.

This can be done with the following one-liner (it write the quoted text into the fail2ban jail config file):

echo -e "[sshd]\nbackend=systemd\nenabled=true" | tee /etc/fail2ban/jail.local

Next you'll need to restart the fail2ban service, and inspecting the output for any issues:

systemctl restart fail2ban && systemctl status fail2ban