Port Scan Attack Detector

 PSAD is a collection of four lightweight system daemons (in Perl and
 C) designed to work with iptables to detect port scans. It features:
  * a set of highly configurable danger thresholds (with sensible
    defaults provided);
  * verbose alert messages that include the source, destination,
    scanned port range, beginning and end times, TCP flags, and
    corresponding Nmap options;
  * reverse DNS information;
  * alerts via email;
  * automatic blocking of offending IP addresses via dynamic firewall
    configuration.
 .
 When combined with fwsnort and the iptables string match extension,
 PSAD is capable of detecting many attacks described in the Snort rule
 set that involve application layer data.