alt Hacker NewsA most elegant TCP hole punching algorithm
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- you know each others IP's (or have a way to signal it)
- can't decide on a port in the same message
- don't suffer from NAT port randomization
I'm not saying it will never happen, but the Venn diagram of this being the minimum complexity solution just doesn't seem very large?
This is a great algorithm!
In this era where AI is eating away at how deterministic computers are, I really appreciate reading about an elegant solution to a real problem using deterministic logic.
> Many home routers try to preserve the source port in external mappings. This is a property called “equal delta mapping” – it won’t work on all routers but for our algorithm we’re sacrificing coverage for simplicity.
It is precisely this point that has flummoxed me when connecting my p2p wireguard config[1] with a friend that uses a pfsense router, no matter what we tried, pfsense always chooses a random source port.
But in the simple case this blog outlines, if both ends use the same source port, this method punches through 2 firewalls effortlessly:
If you're asking "where is the listener", you don't need one: https://datatracker.ietf.org/doc/html/rfc9293#simul_connect
Needing to punch holes in NAT is one of the most idiotic own-goals in the entire field of networking.
NAT is effectively your router doing DHCP with a 17-bit suffix (16-bit port + 1 bit for UDP vs TCP) to each of your applications and then not telling you the address it gave you or how long it is good for (which is what a regular DHCP lease does). This is in addition to it, most likely, already doing regular DHCP and allocating you a IP address that it does tell you about, but which is basically worthless since routing to just that prefix without the hidden suffix goes into a black hole.
If you could just ask your router for a lease on a chunk of IP+NAT addresses that you could allocate to your applications and rotate them as they expire, you would not need this horrifying mess.
The router would just need to maintain the last-leg routing table (what a concept, a router doing routing with routing tables) just like it already does DHCP.
The applications would have short-term stable addresses that they could just tell their peers and just directly tell the router/firewall to block anybody except the desired peer short-term address.
Does TCP hole punching actually work with common CPEs and CG-NATs?
I don’t think I’ve ever seen it done successfully and have often wondered if it’s for a lack of use cases or due to its bad success rate and complexity compared to UDP hole punching.
That said, I really wish there was a standardized way to do it. Some sort of explicit (or at least implicit but unambiguous) indicator to all firewalls that a connection from a given host/port pair is desired for the next few seconds. Basically a lightweight, in-band port mapping protocol.
It could have well been an official recommendation to facilitate TCP hole punching, but I guess it’s too late now, as firewall behaviors have had decades to evolve into different directions.