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lsof

search : find process for socket, list open files and sockets

I used to use netstat to find the process that owned a socket port :

netstat -tulpn | grep <port>

Result : 

tcp        0      0 0.0.0.0:6666            0.0.0.0:*               LISTEN      3832228/./debug/bac

It seems it yields the same result as root. Just as lsof :

lsof seems shorter and easier : 

lsof -i :6666

COMMAND     PID USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
backend 3832228 kees    3u  IPv4 30243139      0t0  TCP *:6666 (LISTEN)

Also you can print all open files of that process with 

lsof -p 3832228

Note that here sudo may be handy to suppress some warnings about files it cannot 'stat'.

problem with Unable to bind: Address already in use

This test run sheds a light on the problem. Note that we already tried the socket.SO_REUSEADDR setting. But just after a crash when you restart the server it will say address in use.

If we use lsof to find the port in question it seems like the calling side is the one holding the port.

kees@hoek:~/projects/aivrp/broker$ lsof -i :7777
COMMAND     PID USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
worker  3683252 kees    3u  IPv6 63643637      0t0  TCP hoek:37212->hoek:7777 (CLOSE_WAIT)

So it is worker that hangs around, not broker which listens on port 7777. And indeed when we stop the worker debugger session, the port is freed :

kees@hoek:~/projects/aivrp/broker$ lsof -i :7777 # output nothing , so start again
kees@hoek:~/projects/aivrp/broker$ gdb ./debug/broker 
Reading symbols from ./debug/broker...
(gdb) r
Listening 3 -1 -1