The IP Personality project is a patch to Linux 2.4 kernels that adds netfilter features: it enables the emulation of other OSes at network level, thus fooling remote OS detection tools such as nmap that rely on network fingerprinting. The characteristics that can be changed are TCP Initial Sequence Number (ISN), TCP initial window size, TCP options (their types, values and order in the packet), IP ID numbers, answers to some pathological TCP packets, and answers to some UDP packets.
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