What actually reuseaddr option does in socat?

Question

I'm reading the doc about socat here and here.

The example socat -u TCP4-LISTEN:3334,reuseaddr,fork OPEN:/tmp/test.log,creat,append works well with and without option reuseaddr.

What does the reuseaddr do? Why above example works well with and without the reuseaddr? In which cases the reuseaddr is really needed?

Answer 1

reuseaddr allows to bind to address which is in TIME_WAIT state, e.g. after server is killed while client is connected.

Without reuseaddr, server side:

$ socat -u TCP-LISTEN:3334,fork -

Client:

$ socat -u TCP-CONNECT:localhost:3334 -

Back to server:

^C
$ netstat -nta | grep 3334
tcp        0      0 127.0.0.1:3334          127.0.0.1:37578         TIME_WAIT  
$
$ socat -u TCP-LISTEN:3334,fork -
2023/01/14 16:44:19 socat[84239] E bind(5, {AF=2 0.0.0.0:3334}, 16): Address already in use

With reuseaddr (before run wait until TIME_WAIT state passed away, or use different port number):

$ socat -u TCP-LISTEN:3334,fork,reuseaddr -

Client:

$ socat -u TCP-CONNECT:localhost:3334 -

Back to server:

^C
$ netstat -nta | grep 3334
tcp        0      0 127.0.0.1:3334          127.0.0.1:35168         TIME_WAIT  
$ socat -u TCP-LISTEN:3334,fork,reuseaddr -

^^^ No error.

Note reuseaddr should be applied to both initial and subsequent sockets.