Networking – netcat udp address already in use

So there are multiple things called netcat; ubuntu even has /etc/alternatives symbolic-link-hackery for it.

I think part of your problem is that UDP doesn't do sessions; I've copied in part of the file /usr/share/doc/netcat-traditional/README.gz below which does a pretty good job of explaining.

UDP connections are opened instead of TCP when -u is specified. These aren't really "connections" per se since UDP is a connectionless protocol, although netcat does internally use the "connected UDP socket" mechanism that most kernels support. Although netcat claims that an outgoing UDP connection is "open" immediately, no data is sent until something is read from standard input. Only thereafter is it possible to determine whether there really is a UDP server on the other end, and often you just can't tell. Most UDP protocols use timeouts and retries to do their thing and in many cases won't bother answering at all, so you should specify a timeout and hope for the best. You will get more out of UDP connections if standard input is fed from a source of data that looks like various kinds of server requests.

OK so maybe that's not all that great of an explanation but it's what I could find.

If you haven't yet, you might want to experiment with any netcat options you can find that would have to do with waiting... have you experimented with:

• using -l as well as -u to ensure you're in "listening" mode

• -vv to see exactly what's happening

• -q -1 ...which should "wait forever" even after receiving EOF (hopefully, listening again?)

Ok, I think at least I got something with socat - namely, the option fork needs to be appended to the server line:

$ socat - udp4-listen:5000,reuseaddr,fork

... and then, in another terminal, we can call echo piping into socat client line multiple times on command line, as it will exit immediately (well, after half a second :)):

$ echo "hello" | socat - udp-sendto:127.0.0.1:5000
$ echo "hello" | socat - udp-sendto:127.0.0.1:5000
$ echo "hello" | socat - udp-sendto:127.0.0.1:5000

... and going back to the first terminal, we can see that the server has successfully shown all three hellos:

$ socat - udp4-listen:5000,reuseaddr,fork
hello
hello
hello
^C

Note that even with a fork-ed socat server, the line echo "hello" | nc -u 127.0.0.1 5000 will still 'lock' as if waiting for user input; however, now after Ctrl-C and re-running the command, i.e.

$ echo "hello" | nc -u 127.0.0.1 5000
^C
$ echo "hello" | nc -u 127.0.0.1 5000
^C
$ echo "hello" | nc -u 127.0.0.1 5000
^C

... the fork-ed socat server will show three hellos without the need to be restarted..

Seemingly, this openBSD netcat doesn't have a fork option - but I'm not sure if it has one that is corresponding to it..

Anyways, hope this helps someone,
Cheers!