TCP Client Self-Connect (2013) (sgros.blogspot.com)

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?

Intuitively it doesn't sound like it would work. The program doesn't call listen() or accept() on the socket.

I have separately documented the same thing happening on our work knowledgebase. TCP is always such fun.

Hmm it is not working for MacOS :( I understand that linux and macos network stacks differ. However when I ran the test it and observed the network via wireshark, it seems that my machine does not send a request containing the same source port as the destination port. It just skips it, but anyway MacOS seems to have the same increasing sequential port behavior