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The following socat command-line works as expected when entered at a shell prompt:

# /usr/bin/socat UDP-RECV:4321 STDOUT

It listens on UDP port 4321 and writes everything received to standard output.

The following is an attempt at starting this command as a systemd service with the intention that it writes received data to the systemd journal (the default destination for a service's standard output):

# /etc/systemd/system/socat.service
[Service]
ExecStart=/usr/bin/socat UDP-RECV:4321 STDOUT

However, socat exits immediately when this service is started:

Process: 7425 ExecStart=/usr/bin/socat UDP-RECV:4321 STDOUT (code=exited, status=0/SUCCESS)

Researching the problem (running socat with -d -d -d -d) revals that it's getting an EOF on standard output:

N starting data transfer loop with FDs [5,5] and [1,1]
N socket 2 (fd 1) is at EOF
I close(5)
N exiting with status 0

Is it possible to use socat as a systemd service?

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3
  • Of course you can run socat as a systemd service, there's nothing that stops you from doing that. But I really wonder what are your expectations regarding the stdout stream. You certainly don't expect a system service to be connected with a terminal session, do you? – Pavel Šimerda Jul 26 '15 at 20:51
  • I know I can of course run socat as a systemd service. But something is causing it to exit and that looks like an EOF on stdout. I certainly don't expect a systemd service to be connected with a terminal session. If you know anything about systemd then you'll know that processes' stdouts are by default connected to the journal... just like I said in my question. – starfry Jul 27 '15 at 8:40
  • Relax, no need for confrontation. I see there's now an accepted answer and is indeed related to the standard streams, so nothing unexpected. – Pavel Šimerda Aug 5 '15 at 13:49
8

The problem is due to socat being bi-directional by default. It attempts to read its standard input which is /dev/null, it gets an EOF and exits.

The solution is to use the -u option:

ExecStart=/usr/bin/socat -u UDP-RECV:4321 STDOUT

This tells socat to run unidirectionally from UDP-RECV:4321 to STDOUT.

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