Fail2ban

Fail2ban when ssh in telnet port

This are no failures in sense of authentication (because login does not take place).

But if you will that yet, just copy /etc/filter.d/sshd.conf into /etc/filter.d/sshd.local and add following to the failregex:

^%(__prefix_line)sBad protocol version identification '.*' from <HOST> port \d+\s*$
^%(__prefix_line)sDid not receive identification string from <HOST>\s*$

https://github.com/fail2ban/fail2ban/issues/1284