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Implement the TCP 3-Way Handshake Using Raw Sockets

hard2/8

Implement a UDP Client and Server Using Only Syscalls Implement TCP Data Transfer with Sequence Numbers and ACKs

Language

Compiler

2.0s Limit 256MB

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Implementation

Implement a simulation of the TCP 3-way handshake.

Requirements:

Simulate client and server state machines
Show SYN, SYN-ACK, and ACK segments with sequence numbers
Track state transitions: CLOSED -> SYN_SENT -> ESTABLISHED (client)
Track state transitions: LISTEN -> SYN_RECEIVED -> ESTABLISHED (server)
Print each segment with flags and sequence numbers
Show how ISN is chosen and how ACK numbers relate to sequence numbers
Handle the handshake as a step-by-step C program
Use raw socket concepts (even if simulated in userspace)

Why This Matters

The Linux kernel's tcp_v4_syn_recv() implements exactly this state machine, and SYN flood attacks work by exploiting the half-open SYN_RECEIVED state you track here — which is also why Linux ships SYN cookies as a defense. Kevin Mitnick's famous 1994 attack against Tsutomu Shimomura relied on predicting the ISN this task has you compute.

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main.c

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