Sshguard is a small program that monitors services running on your
machine from the log files. When it detects that someone is doing
something bad to a service, sshguard blocks the IP address of the bad
guy with a firewall rule.

Sshguard was started in 2006 to mitigate the growing brute force
attacks to SSH servers. Because of the generality of its infrastructure,
however, it was soon extended to monitor and protect more and more services.