  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 capability, with alerts being sent
to syslog or a separate "alert" file.   As of version 1.1 it can
also send WinPopup messages via Samba.

Homepage:
https://www.snort.org/
