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Absolute OpenBSD, 2nd Edition by Michael W. Lucas

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Sanitizing Traffic

All sorts of weird traffic arrives at Internet hosts. Some of that traffic is broken garbage. Other parts tell you that someone else is running broken garbage.

PF tries to sanitize and normalize traffic before otherwise processing it. The normalizations include discarding illegal packets, packet reassembly, and packet modification.

Illegal Packets

Some of the random stuff that arrives at a host is garbage. If a packet is shorter than the IP header, it can’t be a real IP packet, and if a TCP packet is too short to include a full TCP header, it can’t be a real packet.

If the packet length doesn’t match the length given in the header, it’s somehow corrupt. PF has no way to figure out where these packets came from, or if they’re ...

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