The Linux IP Personality patch fools OS detection by changing some characteristics of the network traffic. Among the things that can be changed are the TCP Initial Sequence Number (ISN), TCP initial window size, TCP options (their types, but also their order in the packet), answers to some pathological TCP packets, and answers to some UDP packets.
cbfbf6d82b30b4ebbcf446bf81bdeae0049b1de667b7eb94c362ebb6b5163ebe