p0f - 3.09b-3 main

p0f performs passive OS detection based on SYN packets. Unlike nmap
and queso, p0f does recognition without sending any data.
Additionally, it is able to determine the distance to the 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 as powerful IDS add-on. p0f
supports full tcpdump-style filtering expressions, and has an
extensible and detailed fingerprinting database.