nc (netcat) hangs, waiting for more data, in UDP mode.

Question

I'm trying to send a small string to statsd via nc inside of a read block:

while read line; do
    printf "folder.counter:value|1c" | nc -q 0 -u $host $port
done

Unfortunately, when in UDP mode, nc seems to want to wait indefinitely, even though I've specified -q 0, which the man page says will make the program exit immediately after EOF.

I've tried passing -w 1, but if the data I'm sending comes in at more than one line per second, the data buffers up, and I lose my real time stats (not to mention risking a buffer overflow of some sort).

Is it possible to do what I'm trying to do with netcat, or am I going to need to write something in language which has a statsd library?

This happens in TCP mode now, which is what brought me here. — baitisj, Aug 25 '16 at 22:25

score 9 · Accepted Answer · answered Apr 12 '13 at 20:33

9

I ended up fixing the problem by switching to socat:

while read line; do
    printf "folder.counter:value|1c" | socat -t 0 - UDP:$host:$port
done

answered Apr 12 '13 at 20:33

bshacklett

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As there's been no other input, and this resolved my problem, I'm marking it as the answer. – bshacklett Apr 15 '13 at 13:05
You could have added a 1 second timeout to netcat (adding the arguments -w 1 would have probably worked). – parkamark Feb 28 '17 at 14:00

score 9 · Answer 2 · answered Jun 24 '15 at 10:19

9

You can specify 0 as a timeout value to -w, so it won't wait at all.

answered Jun 24 '15 at 10:19

Dániel

91
1
1

This should be the accepted answer. – Petrus K. Jan 13 '17 at 14:49
6

Zero timeout is unacceptable, the error: `invalid wait-time 0` – AstraSerg Sep 26 '17 at 13:20
@Astraserg no it is not, depending on what netcat/NC you have. Works in BSD nc. – cde Feb 03 '20 at 21:52

score 3 · Answer 3 · edited Oct 16 '19 at 02:17

3

I had same issue; solved it using the -c option:

-c, --close                close connection on EOF from stdin

so something like

while read line; do
    printf "folder.counter:value|1c" | nc -cu $host $port
done

Yeah, does not really make sense to "close" a udp-connection - but that ended up working.

edited Oct 16 '19 at 02:17

womble

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answered Feb 28 '17 at 13:49

Jørn Christensen

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1

Adding `-c` solved my problem with `netcat (The GNU Netcat) 0.7.1` – Noah Sussman Sep 15 '18 at 20:09
Fixed my issue on MacOS. – spoulson Mar 04 '21 at 21:02

score 2 · Answer 4 · answered May 18 '16 at 20:35

2

adding -v option solved my issue. The reason I am not sure.

answered May 18 '16 at 20:35

Kousha

21
1

score 0 · Answer 5 · edited Oct 16 '19 at 02:17

0

For us, it was that we were sending an nc payload from one machine to another via a python script. In the python, when we explicitly encoded the payload in 'UTF-8', it just worked.

edited Oct 16 '19 at 02:17

womble

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answered Oct 16 '19 at 01:50

user1118473

1

nc (netcat) hangs, waiting for more data, in UDP mode.

5 Answers5