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This is question is somehow related to https://superuser.com/questions/270698/how-is-it-called-when-listeningconnecting-two-sockets-and-exchanging-data-betw

In the test setup I have three terminal windows open I run in

Term1: "nc -l 55545"
Term2: "nc -l 55546"
Term3: "socat tcp:localhost:55545 tcp:localhost:55546"

Input to Term1 now appears in Term2 and input to Term2 appears in Term1.

This is the desired behavior. How can I achieve this behavior by only using nc?

When I run in

Term3: "nc localhost 55545 | nc localhost 55546"

then Input to Term1 appears in Term2 but input to Term2 appears in Term3. How can I make the pipe bidirectional? If possible without temporary files.

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1 Answer 1

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14

This comes straight from the Wikipedia page on netcat. In Term3, you would run:

mkfifo backpipe
nc localhost 55545 0<backpipe | nc localhost 55546 1>backpipe

This does pretty much exactly what you want. It uses a FIFO to get output from the left-hand side back into the right-hand side. It's not, strictly speaking, a temporary file -- a FIFO is a named pipe between two processes.

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  • or do I have that wrong? seems like you need to reference the variable with $ sign and you can leave off the 0 and 1 file descriptors?
    – Alexander Mills
    Apr 21, 2018 at 6:58
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    You can of course always use implicit file descriptors. I was just being explicit when I wrote this 7 years ago. I'm not sure what variable you're referring to; there are no variables in my answer.
    – larsks
    Apr 21, 2018 at 18:48
  • nice that works, I follow
    – Alexander Mills
    Apr 21, 2018 at 20:23

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