autossh and systemd to start a reverse ssh tunnel

I have a machine behind my firewall. When I turn this machine on, I want it to make a reverse ssh connection to another machine, which is available from outside the firewall. I use autossh to make and maintain the connection, and systemd to start the task on boot. The systemd unit file looks like:


cat /etc/systemd/system/autossh.service
[Unit]
Description=Keeps a tunnel to 'MYEXTERNALDOMAIN.com' open
After=network-online.target

[Service]
Environment=AUTOSSH_GATETIME=0
ExecStart=/usr/bin/autossh -M 0 -N -q -o "ServerAliveInterval 60" -o "ServerAliveCountMax 3" -i /root/.ssh/id_rsa -R 22222:localhost:22 -p 23 USER@MYEXTERNALDOMAIN.com

[Install]
WantedBy=multi-user.target

And then do:

systemctl start autossh
systemctl enable autossh