How to use nc to listen on a TCP port indefinitely?

Question

I want to read the data arriving on a specific TCP port indefinitely and dump the data into a file. I have tried many different variations of nc but it is not working out for me. For the most part I am not getting any error on the console. Below is what I have tried:

nc -lv 12345 >> ../myfile
nc -v 12345 >> ../myfile
nc -v -p 12345 >> ../myfile
nc -v localhost -p 12345 >> ../myfile

Appreciate your help..

Answer 1

I want to read the data arriving on a specific TCP port indefinitely

This doesn't really make sense... if you were after all data arriving on a UDP port, then fine - TCP however is a connection-based protocol. Once a client connects, and you accept(), you end up with two sockets - one listening, and one connected to the client... nc will close the original listening socket, and deal one-on-one with the client.

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If you'd like to be able to listen for multiple clients connecting, then try using socat:

socat TCP-LISTEN:12345,fork FD:1 >> ../myfile

This will setup a listening socket and fork a new process on connection - keeping the listening socket listening. All data received will be written to stdout (file descriptor 1), which is redirected to ../myfile for appending.

NOTE: data will be received from any number of clients, in no guaranteed order with no framing... i.e: it'll be a mess of jumbled data if you have more than one client at a time.

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If you want socat to handle the file for you, then you can use one of CREATE or OPEN (see the man page):

socat TCP-LISTEN:12345,fork OPEN:../myfile,append

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If you are content with a "one client at a time" approach, then put nc in a loop:

while :; do nc -l 12345; done >> ../myfile

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Test this with nc:

nc localhost 12345

Answer 2

the screen on the left

nc -l 1234 starts the server.

the screen on the right nc 127.0.0.1 1234 connects.

it looks like you got your commands wrong.

Worth knowing also whether you're using the gnu one or the bsd one, mine is the bsd one. man nc shows "NC(1) BSD General Commands Manual". To start the server on the gnu one it might need -l with -p. To connect the client to the server on whatever port, will never use -p.

You can use -v as well

Your immediate problem was that you weren't getting it running at all.

Your question asks about it being indefinite, and i'd note though that if you do ctrl-c at the client side then besides breaking the connection, it stops the server. You can put that server listening in a loop. That works. So then the server continues listening and is still there if you try to connect again after a ctrl-c from the client.