Here are a few programs that will help you in preventing unauthorized access to your server:
One is a set of programs that work together, they are BFD and APF\
BFD stands for Brute Force Detector. This software will look at people who are attempting to log into your server, and if they exceed a set number of failed login attempts, the program will then email you all their attempts, and will additionally, add the offending IP address to your iptables firewall, thereby blocking any further "Brute Force" attempts at gaining access to your server.
Gotta run, but will post more on this subject soon...
