synsanity is a netfilter (iptables) target for high performance lockless SYN cookies for SYN flood mitigation, as used in production at GitHub.
前人的作法 (SYNPROXY) 以 module 形式運作，需要過濾每一個封包，而這在 GitHub 這種規模上會導致效能不足並且 kernel panic：
This is quite an intrusive way of solving the problem since it touches every packet during the entire connection, but it does successfully mitigate SYN floods. Unfortunately we found that in practise under our load and with the amount of malformed packets we receive, it quickly broke down and caused a kernel panic.
GitHub 所開發的 synsanity 則是透過 netfilter (iptables) 的 target，只處理 initial packets，在撰寫的時候考慮多 CPU 的 lock 問題：