Can confirm that it still works in 2025, and also via netcat.
I wonder if it's also theoretically possible for a pair of clients to connect to each other like this?
Can confirm that it still works in 2025, and also via netcat.
I wonder if it's also theoretically possible for a pair of clients to connect to each other like this?
The tailscale client does this trick to force its way through NAT-to-NAT connections.
"How NAT traversal works" - https://tailscale.com/blog/how-nat-traversal-works > "The benefits of birthdays"
The tailscale article is about UDP, but here we are talking about TCP.
> I wonder if it's also theoretically possible for a pair of clients to connect to each other like this?
RFC793 actually specifically calls this out as one of the supported ways of two endpoints establishing a TCP connection[0]. The fact that it works on the same host via src port collision is just a happy accident.
[0] - https://www.rfc-editor.org/rfc/rfc793.html#section-3.4
Yeah a pair of clients can via “tcp simultaneous open” but you’ll need to arrange for each client to know about the source port of the other. So bypass the usual ephemeral port choice made by the OS. And it’s still highly sensitive to timing.