Open source utility that sends a series of TCP packets to a host and guesses the host OS from how it responds?

Question

I recall an open source utility that sends a series of oddly formated TCP packets to a host and guesses the host OS from how it responds. But I can't recall the name.

Answer 1

there is nmap, which is an active tool that sends out packets to the host (as such the host will able to "see" this scan)

There is another tool out there, called p0f. THis one takes a packet dump (which could be gathered through sniffing or legitimate interaction with the host (e.g. make a http request)) and guesses the OS based on the dump. It is a passive tool, that can not be detected by the host you try to determine the OS for.

Answer 2

You're probably thinking of 'nmap'.

http://www.insecure.org/nmap/