Tcpdump allows you to dump the traffic on a network.

Nessus is a remote security scanner for Linux, BSD, Solaris, and other Unices. Open Source (GPL) development on this project has ceased for version 3 but continues for version 2. A fork (OpenVAS) has also been created .

Snort is a lightweight network intrusion detection system, capable of performing real-time traffic analysis and packet logging on IP networks. Features Content searching and matching Protocol analysis Common attack and probe detection Frontends Snort is simply a network monitor, it doesn’t provide a sophisticated GUI through which to see and respond to security events. Several 3rd party open source projects fulfill this purpose however: SnortSnarf sguil BASE

Nmap is a utility for network exploration or security auditing.

Wireshark is a network protocol analyzer, or “packet sniffer”, that captures and shows contents of network frames. It runs on Unix/Linux, and Windows. Wireshark uses the libcap project to capture packets (WinPcap on Windows). Wireshark was previously named Ethereal, this name was scrapped by the developer of the project due to trademark issues. Wireshark groks network protocols, so it can interpret and give meaning to the raw network traffic that is being passed over the wire.