packet-sniffer/logger

Snort is an open source network intrusion detection system, capable  of
performing  real-time traffic analysis and packet logging on IP networks.
It  can  perform  protocol analysis and content searching/matching in order 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 uses a flexible rules language to describe traffic that it should collect
or pass, as well as a detection engine  that  utilizes a modular plugin
architecture.  Snort has a real- time alerting capability as well,
incorporating  alerting mechanisms  for  syslog,  user  specified  files, a
UNIX socket, or WinPopup messages to  Windows clients using Samba's smbclient.