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Yesterday I asked a question about catting a file over a UDP socket in bash.

The solution we came up was netcat -c -w 1 -v -u -s 127.0.0.1 239.255.0.1 30001 < test.txt. This worked in the sense that it sent the packets, but there's a problem.

The source file isn't strictly a text file. It's actually a binary file -- the content is mostly text, with embedded non-printable characters and no \n lineendings. Instead, the control character ^C (0x03) is used as a line delimiter.

When netcat was sending packets, it would send as much as possible in a single UDP frame. But I want it to send one UDP frame per ^C-delimited mesasge in the source file.

For example, if my file consists of:

foo^Cbar^Cbaz^C

using netcat would result in one UDP frame being sent. What I want is to send 3 messages:

  1. foo^C
  2. bar^C
  3. baz^C

Is there a way to accomplish this?

I have tried a number of possible solutions, but nothing's worked.

For one I've tried sedding the source file to replace the ^C with ^C\n, but that had no effect:

sed 's/^C/^C\n\0/g' test.txt | netcat -n -vv -c -w 1 -v -u -s 127.0.0.1 239.255.0.2 30002

I also tried catting the files to /dev/udp/ instead of using netcat, but the results were similar.

cat test.txt > /dev/udp/239.255.0.2/30002

Finally I tried using awk to print one line at a time and redirecting that to /dev/udp, but the results were really the same.

It appears that both netcat and cat > /dev/udp both buffer the input until it has a full frame, then sending the frame. That's not what I want.

Can I flush the udp buffer, or some other way send one UDP message per ^C-delimited message in the source file?

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1 Answer 1

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gawk -v 'RS=\03' -v cmd=' socat -u - udp-datagram:239.255.0.1:30001,bind=127.0.0.1' ' {print $0 RT| cmd; close(cmd)}' < file

should work as long as there's not more than 8k in between two ^Cs.

That runs one socat command per record (records being ^C delimited via the record separator variable), with the record plus the record terminator fed to socat via a pipe.

socat reads 8192 bytes at a time, so it's as large a packet it sends can get.

gawk writes to the pipe as a full record per write(2).

Using gawk instead of awk here to make sure it handles NUL bytes properly.

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  • Brilliant, this seems to work exactly as I want. Now that I've read about close in gawk's manpage, I think I understand how it's working as well. Commented Jan 31, 2014 at 16:16

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