Some times I need to relay TCP connections through some machines, and this can be achieved by nc command with a fifo file :

#!/usr/bin/sh
if [ -z $1 ] || [ -z $2 ] || [ -z $3 ]; then
    echo "Usage: $0 port host port"
    exit
fi

backpipe="backpipe-$1-$2-$3"

if [ -p $backpipe ]; then
    echo "$backpipe exists."
else
    mkfifo $backpipe
fi

while [ 1 ]; do
    echo "listening on port $1 and redirect to $2:$3"
    nc -l $1 0<$backpipe | nc $2 $3 1>$backpipe
    echo "one connection ends, start another."
done

• Transparently relay TCP ports

  $ nano-tcp-relay {host} {port-1} [{port-2} ... {port-n}]
  

  • Works as following

    Internet ---> localhost:{port-1} ---> {host}:{port-1}
    Internet ---> localhost:{port-2} ---> {host}:{port-2}
    ...
    Internet ---> localhost:{port-n} ---> {host}:{port-n}
    

• Port forwarding

  $ nano-tcp-relay {host} {src-port-1}-{dst-port-1} [{src-port-2}-{dst-port-2} ...]
  

  • Works as following

    Internet --> localhost:{src-port-1} ---> {host}:{dst-port-1}
    Internet --> localhost:{src-port-2} ---> {host}:{dst-port-2}
    ...
    Internet --> localhost:{src-port-n} ---> {host}:{dst-port-n}