sudo useradd -g nogroup -s /bin/false -m tunnel
sudo -u tunnel mkdir -p ~tunnel/.ssh # and copy your private key here
sudo -u tunnel nano ~tunnel/.ssh/config # add host and key configs here and a myhostsshconf (or other name)
sudo -u tunnel ssh myhostsshconf # just make sure to add your host to `known_hosts`
Create a new systemd service with the same name than an entry in the .ssh/config (myhostsshconf in this example)