Fail2Ban

Fail2ban is a program that parses logs and and block servers that try to abuse your system. While it doesn't replace a firewall, it's a good complement as it prevents people from trying thousands of password on your server.

Installation
Debian Stretch (currently in testing) contain a much nicer version of fail2ban than Jessie (current stable). Configuration has been simplified a lot between the two releases and installing the version from stretch will save you migration pain later.

Defaults
Create file

Other rules
Rules specific to one program are documented on the program page. You can see the list on the fail2ban category page.