Regularly scanning the ports on your own network prevents intruders from sneaking in, but if you have dozens or hundreds of servers, you’ll need professional help: Dr. Portscan to the rescue.

The powerful Metasploit framework helps you see your network as an intruder would see it. You might discover it is all too easy to get past your own defenses.

Tcpdump is a general-purpose packet sniffer and incident response tool that should be in your tool shed.