checking whether server is listening or not with netcat

[sanaz]

Hi all,

I'm trying to do some experiments with net cat, i want to send a file via for several times and I want to measure throughput. The problem is that each time Client and server are connected and sending is done, the connection is closed and I server should listen again. Consider following commands:

[root vm1]# nc -l 12345 > /dev/null

and

[root vm2 ~]# dd if=/dev/zero bs=2000 count=500 | nc -v 192.168.122.193 12345

each time I should run the command

[root vm1]# nc -l 12345 > /dev/null

again, I cannot put this command in a loop. I tried it and I got :

nc -l 12345 > /dev/null
nc: getaddrinfo: Servname not supported for ai_socktype
nc -l 12345 > /dev/null
nc: getaddrinfo: Servname not supported for ai_socktype
nc -l 12345 > /dev/null
.........

How can I automate this?

[Rubberman]

The -l port option turns nc into a server, so you should not need to restart it (which is why you are getting errors in the last examples of your post) unless you give it a timeout option and the timeout expires. IE, start nc -l port and then on the other host you can do the dd | nc as often as you want, though bear in mind that nc will handle multiple connections at the same time, so if one is not finished sending, and you start another, your timing results will be skewed.

[sanaz]

That is not my case, server stop after first onnection for e.g second time I got this error in the client:

nc: connect to 192.168.122.193 port 12345 (tcp) failed: Connection refused

[Rubberman]

This (from the nc man page) may help:

     -k      Forces nc to stay listening for another connection after its current connection is completed.  It is an
             error to use this option without the -l option.

[sanaz]

Thanks a lot for your help. It worked.