sshtun.service

[Unit]
Description=SSH Tunnel
After=network-online.target
StartLimitIntervalSec=3610
StartLimitBurst=20

[Service]
ExecStart=/usr/bin/screen -Dm -S sshtun /usr/bin/ssh -o ServerAliveInterval=60 -R22222:localhost:22 myuser@myserver "ssh localhost -gL2222:localhost:22222"

ExecStop=/usr/bin/screen -S sshtun -X stuff "exit\015"
ExecStop=/usr/bin/screen -wipe

WorkingDirectory=/home/myuser
Restart=always
RestartSec=180s
User=myuser
Group=myuser

[Install]
WantedBy=multi-user.target