[Unit]
  Description=SSH tunnel
  After=network-online.target
[Service]
  ExecStart=/usr/bin/ssh -NT \
    -o ServerAliveInterval=60 -o ExitOnForwardFailure=yes \
    -R 4433:127.0.0.1:4430 user@1.2.3.4 -p 22
  RestartSec=15
  Restart=always
[Install]
  WantedBy=network-online.target