"psad makes use of Netfilter log messages to detect,
alert, and (optionally) block port scans and other suspect traffic.
For tcp scans psad analyzes tcp flags to determine the scan type
(syn, fin, xmas, etc.) and corresponding command line options that
could be supplied to nmap to generate such a scan. In addition,
psad makes use of many tcp, udp, and icmp signatures contained
within the Snort intrusion detection system."