How to do “netcat” relays properly so that I have more control?

Question

How to use the netcat to have relays? Like following I tried but not working:

cd /tmp
mknod backpipe p
nc -l -p 7007 0<backpipe | nc 192.168.1.101 9001 | tee backpipe

1. I want to receive data on 7007 must be listening mode
   $ another application is putting packets here

2. what ever comes to 7007 I need to move to 9001 (which is also a listening port)

   $ telnet localhost 9001
   

   will show 7007 packets?

Any idea?

Optional: will it be also possible that I can put some extra header packets? to simulate http headers as image content type?

Answer 1

If you do:

nc -l -p 7007 | nc -l -p 9001

Then anything that comes in to port 7007 will be piped to the second netcat and be relayed to your telnet session on port 9001.

Injecting headers requires knowing the underlying protocol, at least to figure out "message" boundaries, so it's not trivial. If you know how to do it, you can inject your code to do so between the two pipes:

nc -l -p 7007 | ./my_filter | nc -l -p 9001

./my_filter will get the input on stdin, and anything it writes to stdout will show up on port 9001.