Zeek (formerly known as Bro) is an open-source, Unix-based Network
Intrusion Detection System (NIDS) that passively monitors network
traffic and looks for suspicious activity. Zeek detects intrusions
by first parsing network traffic to extract its application-level
semantics and then executing event-oriented analyzers that compare
the activity with patterns deemed troublesome. Its analysis includes
detection of specific attacks (including those defined by signatures,
but also those defined in terms of events) and unusual activities
(e.g., certain hosts connecting to certain services, or patterns
of failed connection attempts).

Zeek is documented in the USENIX 1998 Security Conference proceedings
(as Bro).