Bro is a system for detecting Network Intruders in real-time by the guys
that brought you tcpdump, libpcap, and flex.

Bro is a stand-alone system for detecting network intruders in real-time
by passively monitoring a network link over which the intruder's traffic
transits.  Bro is divided into an "event engine" that reduces a
kernel-filtered network traffic stream into a series of higher-level
events, and a "policy script interpreter" that interprets event handlers
written in a specialized language used to express a site's security policy.
Event handlers can update state information, synthesize new events, record
information to disk, and generate real-time notifications via `syslog'.

Bro is documented in the the USENIX 1998 Security Conference proceedings.

-- David
   obrien@cs.ucdavis.edu