Figure out why `nc` allows two instances to listen to the same `address:port`

[yaobinwen]

Description

On my machine:

• nc -v -l 127.0.0.1 8888 can succeed.
• On the same machine, running nc -v -l 127.0.0.1 8888 again can still succeed.

But I remember TCP doesn't allow listening to the same address/port at the same time. What's going on?

TODO

• [x] https://stackoverflow.com/questions/1694144/can-two-applications-listen-to-the-same-port
• [x] https://stackoverflow.com/questions/37570647/how-netcat-can-listen-to-the-same-port-on-the-same-host-from-two-different-termi
• [x] How to resolve the undefined reference res_init?

[yaobinwen]

Update (2023-07-10)

So this answer tells why:

So the socket is setup with the options SO_REUSEADDR and SO_REUSEPORT, which allow multiple processes to bind to the same port and same listening address. ... The goal of this option is to allow an easy form of load balancing: incoming connections to the port will be redirected to one of the processes (apparently at random).

This answer also recommends the detailed answer here. But this detailed answer provides so much information that I need to study further in another session.