3

I need to use nc in a weird way, where I want the server to first send a file, and then receive a file, and this repeats. I would like to have every instance of the server server close after some seconds. How can I do this in a script? I already have a client that can interact with the server.

4 Answers 4

7

Some netcat implementations have a timeout option. For the one on my system, it is -w. Thus

$ nc -l -p 2000 -w 5

will listen on port 2000 and exit in 5 seconds unless a connection is attempted on that port.

2
  • most nc's suck these days, though, because they are not kept. ncat - by the nmap people - is the exception, i think. and, of course, there is socat if you have the determination to learn a new programming language.
    – mikeserv
    Jan 5, 2016 at 15:06
  • 1
    This is incorrect. According to man nc(1): "The -w flag has no effect on the -l option, i.e. nc will listen forever for a connection, with or without the -w flag" Mar 17, 2019 at 11:38
4

Using bash timeout:

$ timeout 3s nc -l -p 2000
5
  • That will kill the nc after 3s even if it had accepted a connection.
    – user313992
    Dec 25, 2019 at 7:47
  • ... which may actually be what the OP was asking, but it's not what the other answers do.
    – user313992
    Dec 25, 2019 at 7:58
  • actually I didn't try to accept connection... it was perfect for my use case (I need the port to accept connection just to make remote script happy).
    – chenchuk
    Dec 25, 2019 at 19:04
  • You should clearly explain what it's doing. The question is unclear and people come here with different ideas -- in this case the other two answers do something completely different than yours: they will wait up to N seconds for a connection, and if a connection happens within the N seconds, they will accept it and will continue servicing it even after the N seconds have passed.
    – user313992
    Dec 25, 2019 at 22:39
  • in my use case, I got remote ansible script which check 9092 port (kafka) and fails because kafka fails. since I don't care about this, I need a mechanism to make the port open. using nc in a loop did the job for me. ansinle tries to reach the port for 5 min, I made it to make ansible happy and continue execution. (after this I fix Kafka which is not relevant here)
    – chenchuk
    Dec 26, 2019 at 8:21
3

Because of the documentation :The -w flag has no effect on the -l option, i.e. nc will listen forever for a connection, with or without the -w flag

I tried nc ncat socat, no one can set timeout for server mode.

As far as I know, only busybox nc can follow -w option in server mode.

So you have to download busybox which compiled with CONFIG_NC_SERVER=y option, or compile it by your self.

So that you can

$busybox nc -w 10 -l -p 9999

But my system's busybox didn't compile with CONFIG_NC_SERVER=y option, and I don't want compile it. So I use this solution:

portnum=9999
(sleep 10 ;echo "T" | nc -w 1 127.0.0.1 $portnum) | nc -N -l -p $portnum 

After 10 seconds, send "T" to localhost:9999

The full code is

#!/bin/sh
portnum=9999
testmsg="Hello_World"
if [ "$( (sleep 10 ;echo "T" | nc -w 1 127.0.0.1 $portnum) | nc -N -l -p $portnum )" = $testmsg ]; then
  echo "Test pass"
else
  echo "Test not pass"
fi

If server received Hello_World in 10 seconds, it will print Test pass. Otherwise print Test not pass

You can try open another console, and type

echo "Hello_World" | nc -w 1 127.0.0.1 9999

to test it.

1

As stated by peterph the timeout (-w) while listening (-l) for connections works (only) with traditional nc implementations.

On Ubuntu such an implementation can be installed with apt install netcat-traditional and called with nc.traditional.

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