Netcat/socat behavior with piping and UDP?
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 hello
s:
$ 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 hello
s 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!
Your netcat is only reading the output from echo's stdout when you use pipe, it's not "connected" to the keyboard anymore. To get the response you're expecting, you can add your three "hello"'s to a file an run
cat [myfile] | nc -u 127.0.0.1 5000
If you do an strace of the listening nc, it will show that netcat is waiting for the connection, and once it gets it, will connect to that host and port, ignoring all others. You need to add '-k' to keep going and '-w 0' to timeout each connection after 0 seconds. Socat is a better choice, I think.