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TCP SYN Floods: Why Servers Need Stateless SYN Cookies

dev.to Linux2026-10-28 18:360 views

TCP SYN Floods & SYN Cookies

The classic TCP 3-Way Handshake has an inherent asymmetry that attackers exploit for denial-of-service.

Every incoming SYN allocates a socket structure in the kernel's SYN queue. If ACKs never arrive, the queue fills and all new connections drop.

By setting:

ISN = Hash(s_ip, d_ip, s_port, d_port, counter) + MSS index

the server remains completely stateless until the client sends the final ACK!

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