AKA, Poor-mans VPN Method
Use Case
Let's say you have secret service running on port 4444 of you server code.red.com that, I don't
know, is a code scanner with a Web Interface. When you run that scanner, it fires up it's web GUI report
and listens on a local port 4444
Now, you could open up port 4444 to to the world and just access it via code.red.com:4444. Or
you could remote in via VNC for instance, but that would require a graphical desktop and a VNC service
which can be frought with security issues.
Instead, you can create an SSH tunnel as long as you have SSH credentials and access that will let you access that service on you local machine, via a forwarded, or tunneled port.
Creating the Tunnel
The following command will create an SSH tunnel from your machine, port 8080 to your server's local, not publically accessible
port 4444.
ssh -L 8080:localhost:4444 root@code.red.com
If you have ssh access, this will log you in via SSH as usuall, BUT, it will also start listening on a local port 8080.
Explanation
The -L instructs ssh to create a local port forward. Note, this is local to the remote machine, so that
can be a bit confusing at first.
8080 is the port on your local machine that will listen for connections and foward them to port localhost:4444 on your server.
root@code.red.com is the user account and domain of your server. You could easily use an IP address as well.
Tunneling without a shell
If you just want to forward the port, and dont need shell access, you can use the following command:
ssh -N -L 8080:localhost:4444 root@code.red.com
The -N just means, "no login" and ssh will continue to hold that connection until you Ctrl-C out of it.
A nice perk of this methods is that your connection to whatever service is encrpted with the same security as an SSH connection. This allows you to securely access and potentially insecure service from the inside. There are also other kinds of SSH tunneling and port forwarding that I encourage you to look into, but those are beyond the scope of this little writeup.