TCP SYNs

Tags: networks

What Are?

• https://blog.cloudflare.com/syn-packet-handling-in-the-wild/

• Two queues in TCP, one a SYN queue and one Accept Queue

  

• Syn queue responsible for storing inbound syn packets

  • https://elixir.free-electrons.com/linux/v4.14.12/source/include/net/inet_sock.h#L73

Blogs

• https://www.snellman.net/blog/archive/2017-07-20-s3-mystery/
• https://kognitio.com/blog/syn-cookies-ate-my-dog-breaking-tcp-on-linux/

Commits

• https://github.com/freebsd/freebsd/commit/56ba0a68edde7b38323f81f9d56998b10668dd9e
• https://github.com/freebsd/freebsd/commit/fc2be30b217171175bae209edb4f697cc0cb542d

Research

• https://link.springer.com/chapter/10.1007/978-3-642-36516-4_14

Handshake Process

1. SYN - Client sends a server a SYN packet with a random sequence number
2. SYN-ACK - Server responds with a SYN-ACK packet and increments the sequence number, and sets its own sequence number
3. Finally the client sends an ACK and acks the server sequence number

• Both syns and acks are 32 bit ints