p0f

Enumeration

P0f v2 is a versatile passive OS fingerprinting tool. P0f can identify the operating system on machines that connect to your box (SYN mode), machines you connect to (SYN+ACK mode), machine you cannot connect to (RST+ mode), and machines whose communications you can observe. P0f can also do many other tricks, and can detect or measure the following: firewall presence, NAT use (useful for policy enforcement), existence of a load balancer setup, the distance to the remote system and its uptime, other guy's network hookup (DSL, OC3, avian carriers) and his ISP. All this even when the device in question is behind an overzealous packet firewall. P0f does not generate ANY additional network traffic, direct or indirect. No name lookups, no mysterious probes, no ARIN queries, nothing.

http://lcamtuf.coredump.cx/p0f.shtml

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