p0f is a tool written by superhuman Michal Zalewski. The main purpose of p0f is to passively analyze and categorize arbitrary network traffic. You can feed p0f any packet and in return it will derive knowledge about the operating system that sent the packet.
One of the features that caught our attention was the concise yet explanatory signature format used to describe TCP SYN packets.
The p0f SYN signature is a simple string consisting of colon separated values. This string cleanly describes a SYN packet in a human-readable way. The format is pretty smart, skipping the varying TCP fields and keeping focus only on the essence of the SYN packet, extracting the interesting bits from it.
We are using this on daily basis to categorize the packets that we, at CloudFlare, see when we are a target of a SYN flood. To defeat SYN attacks we want to discriminate the packets that are part of an attack from legitimate traffic. One of the ways we do this uses p0f.
We want to rate limit attack packets, and in effect prioritize processing of other, hopefully legitimate, ones. The p0f SYN signatures give us a language to describe and distinguish different types of SYN packets.