Forwarding  port at the remote host

In case of remote servers have no “white” address and works over NAT (network address translation) we still need to have a secure tunnel. But remote destinations in this case can be not detected. Instead is possible to arrange a secure connection to a well-known central host. And redirect ports at the central host for these clients to connect peer over NAT. The difference is to make port 5000 at remote host available to redirect to local port 9000 at our host. 
 

Forwarding  port at remote host  script body:

#!/usr/bin/env bash
nc -l 9000 &
PID0=$!
ssh alexey@localhost -L 5000:localhost:9000 sleep 4 &
PID1=$!
sleep 1
echo done | nc -N localhost 5000
sleep 1
kill -9 $PID0 $PID1 2> /dev/null

Forwarding  port at remote host script executing console:

All actions at the script are equal to the previous case. But the meaning of redirection is different. We redirect remote host port 5000 to localhost port 9000 and create a secure tunnel to pass connections from remote host port 5000 to localhost port 9000. That makes it available to work in a secure way even with hosts over NAT.

SSH is a secure shell standard client utility for Linux. It is used to establish secure connections to remote (or even local) ssh servers. But some programs are not designed flexible enough to be processed by ssh trivial way: the program can work with local connections only or some related network addresses be hard to code defined.  That is why SSH contains several options to redirect secure traffic to match use cases like that. Let’s go from case to case to see how does it work.