Reverse ssh tunnel systemd service

reverse-ssh-tunnel.service

[Unit]
Description=Reverse SSH tunnel to scratch.
After=network-online.target
Before=multi-user.target
DefaultDependencies=no

[Service]
Restart=always
RestartSec=10
ExecStart=/usr/bin/ssh \
    -i /etc/tunnel/id_rsa \
    -o StrictHostKeyChecking=no \
    -o UserKnownHostsFile=/dev/null \
    -o ServerAliveInterval=20 \
    -o ServerAliveCountMax=1 \
    -o ExitOnForwardFailure=yes \
    -N \
    -T \
    -R 2222:localhost:22 \
    [email protected]

[Install]
WantedBy=multi-user.target

Runs as root, though you could set the user to something unprivileged and put an ssh key in an appropriate spot. -N means "don't run any commands", -T means "disable tty allocation".