"Have you ever wondered how many people are scanning your server
looking for weaknesses? One way to find out is to install the Port
Scan Attack Detector (psad), is a collection of three lightweight
system daemons that alert you to suspicious network activity by
analyzing iptables log files.
"With psad you can:
"Detect port scans
"View a report of all attacks, along with system resources
consumed by PSAD..."