Take this situation: you have a cheap (or even free), low-powered remote server and a considerably better homeserver with more storage and power. For certain services that require more power, you'd obviously want to run them on that homeserver. However, what if you don't want to, or can't, directly open ports onto your home network, or you if you simply want to keep all of your site on one IP? This is where SSH port forwarding comes in handy: using ssh to forward the open port from a service from your local server to the remote one, where it can be exposed to the rest of the internet.

SSH Remote Port Forwarding

SSH remote port forwarding is built right into ssh itself, and is quite simple:

ssh -R 5505:localhost:4404 user@remote.host

When this command is run on the local server, it will create an ssh connection to the remote server, as per usual. Additionally, it will open the port 5505 on the remote server, which will forward all traffic to port 4404 on the local server. This command by itself is already everything you'd need to forward most ports easily to your remote server, of course, remember to open the port on your remote server's firewall, if applicable.

However to ensure that that port is exposed properly on the remote server, you'd want to make sure that it is listening to all external traffic. You can fix this by setting GatewayPorts yes in /etc/ssh/sshd_config on the remote server. (don't forget to restart sshd after editing the config)

Persistent ssh forwarding

The above is all well and good, but you'd need to keep an interactive ssh connection up at all times (not impossible with a tool like screen or tmux), so it isn't the most ideal solution. To get around this, you can create a service to run on the local server to forward requested ports to the remote server.

To begin, I'd recommend creating two users, one on each server. For sake of example, lets all them bridge, though any other names like tunnel or whatever would work too. I'd recommend to avoid giving these users passwords, that way they can only be signed in through key based authentication. Of course you will still be able to log into them as root using su - bridge

Next you should create an ssh keypair on the local server(ssh-keygen) and place the contents of your public key into .ssh/authorized_keys. This will make sure that only the local server will be able to ssh into the remote using that key.

Then, create a script for your ssh port forwarding. I placed mine directly in the home folder of my bridge user of my local server, though it only matters that the bridge user can execute it. In your script you must use the -nT flag on your ssh command. These will allow you to run this script as a service, by preventing a virtual terminal being allocated. (read man ssh for more info)

Here is my example of a script that you could use:

#!/bin/sh

PORTS="8080 25565"
DEST="bridge@remote.host"
SSH_PORT="22"
IDENTITY_FILE="~/.ssh"

/usr/bin/ssh -nNT $(echo $PORTS | awk -v host=$LOCALHOST '{for (i = 1; i <= NF; i++){ printf "-R %d:%s:%d ",$i,host,$i}}') -p $SSH_PORT -i $IDENTITY_FILE $DEST

Next you'd want to run this script as a service. Check your distro's service system how to do this if you have any trouble.

Systemd service

Say that the script you made was /home/bridge/tunnel.sh, you should create a user service with systemd for the bridge user.

To do this create the following file in /home/bridge/.config/systemd/user/tunnel.service:

[Unit]
Description=SSH tunnel

[Service]
ExecStart=/home/bridge/tunnel.sh
RestartSec=5
Restart=always
KillMode=mixed

[Install]
WantedBy=default.target

Then enable and start the service with: systemd --user enable tunnel.service and system --user start tunnel.service. Ensure that it is running with systemd --user status tunnel

Forwarding ports smaller than 1024

As you may know, TCP/IP port numbers below 1024 are special in that normal users are not able to open, and hence forward from on the remote server. To bypass this, you can change your services to run at different ports, though this may not always be possible, for example with ports 80 and 443 for http and https respectively. One solution to this is run a server on the remote that will proxy requests from port 80 to a different port (say port 8080).

This can be achieved using socat.

Say you forwarded traffic from port 8080 on the remote to port 80 on the local, you could then, on the remote server run sudo socat TCP-LISTEN:80,fork TCP:localhost:8080 to listen to traffic on port 80 and forward it to 8080, which will forward back to the local server. Here is an example of this in practice, forwarding port 80 and 443, by forwarding ports 8080 and 8443:

/usr/bin/ssh -nT -R 8443:localhost:443 -R 8080:localhost:80 -i $IDENTITY_FILE -p $SSH_PORT $DEST "(sudo socat TCP-LISTEN:80,fork TCP:localhost:8080) & sudo socat TCP-LISTEN:443,fork TCP:localhost:8443"

However this command assumes that the remote user has access to sudo with NO PASSWORD. Alternatively you could create a similar service (this time as a system service) on the remote server running the socat commands.

originally i was going to make this blog on b.davidovski.xyz using nanoblogger (you might be able to still see the start of that) but nb itself seemed quite dead, and i couldn't really be asked to customise it all myself. So i made my own script to generate this static site: kblg. Right now its probably just the bare minimum needed for this, but I am planning to add more things to it as I go along (including rss, if anyone would be interested?)

anyway thats all for now, cya

~davidovski