p0f performs passive OS detection by watching SYN packets with tcpdump. Additionally, it is able to determine distance to remote host, and can be used to determine the structure of a foreign or local network. When running on the gateway of a network it is able to gather huge amounts of data and provide useful statistics. On a user-end computer it could be used to track which operating systems are making each connection. p0f supports full tcpdump-style filtering expressions, and has an easily modified fingerprinting database. Tested on Linux, FreeBSD, OpenBSD, NetBSD, SunOS, and Solaris. This particular version is modified by Nerijus Krukauskas to store the data in a database.
