Systemd service to run autossh

If you have some devices behind a firewall or NAT (like most home networks) but want to access it from the outside you can use the autossh tool to continually expose their local sshd to a server running on the public internet.

SSH key for port forwarding only

When creating more permanent ssh tunnels it is often a good idea to have an ssh keypaire that can only be used for tunneling and not for obtaining a real shell.