Netcat stops listening for UDP traffic

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?)

You can use socat for that. It has very nice option fork:

fork After establishing a connection, handles its channel in a child process and keeps the parent process attempting to produce more connections, either by listening or by connecting in a loop (example).

Client (yes this you run from the client):

$ ssh -L 7753:localhost:7753 YourServer.com "/usr/bin/socat tcp4-listen:7753,reuseaddr,fork UDP:8.8.8.8:53"

Client:

$ sudo socat udp4-listen:53,reuseaddr,fork tcp:localhost:7753

$ dig @127.0.0.1 google.com