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),
– machines whose communications you can observe.
Basically it can fingerprint anything, just by listening, it doesn’t make ANY active connections to the target machine.