A most elegant TCP hole punching algorithm

19 hours ago

TCP hole punching connects computers behind NAT routers by knowing each other's WAN IPs, external ports, and connecting simultaneously.
The algorithm uses a deterministic approach to derive metadata from a single parameter, avoiding fixed infrastructure.
A 'bucket' is calculated using Unix timestamp, max clock error, and min run window to synchronize both sides without communication.
Ports are selected using the bucket as a seed for a pseudo-random number generator, ensuring both sides generate the same port list.
Essential socket options for TCP hole punching include SO_REUSEADDR and SO_REUSEPORT to aggressively reuse socket addresses.
Non-blocking sockets with select for polling are recommended for precise timing control during SYN packet exchanges.
The algorithm uses a leader-follower model to choose the winning connection, with the leader sending a single character to confirm.
The entire process is deterministic, requiring only a destination IP and no additional metadata exchange between hosts.
The tool can be tested locally by running commands within a 10-second window to simulate the hole punching process.

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