Snort is a libpcap-based packet sniffer/logger which can be used as a
lightweight network intrusion detection system. It features rules based logging
and can perform protocol analysis, content searching/matching and can be used
to detect a variety of attacks and probes, such as buffer overflows, stealth
port scans, CGI attacks, SMB probes, OS fingerprinting attempts, and much more.
Snort has a real-time alerting capabilty, with alerts being sent to syslog, a
separate "alert" file, or as a WinPopup message via Samba's smbclient

Snort compiled with flexresp+mysql+postgresql+inline+prelude support.