Persistent SSH Tunnel

ssh-tunnel.monit

check host ssh_tunnel with address 127.0.0.1
  IF FAILED host 127.0.0.1 port 19999 PROTO SSH
    with timeout 20 seconds for 2 cycles
  then alert

ssh_tunnel.service

[Unit]
Description=SSH Reverse tunnel

[Service]
Type=simple
Restart=always
RestartSec=60
ExecStart=/usr/bin/bash -c /path/to/ssh_tunnel.sh
User=USER
Group=systemd-journal

[Install]
WantedBy=network.target

ssh_tunnel.sh

#!/bin/bash

# The SSH endpoint to connect to
ADDR=username@example.com
# A SSH Key for the endpoint
IF=/path/to/key/id_rsa

while true; do
# ssh with Keepalive and -R reverse port forwarding
ssh -i $IF -o "ServerAliveInterval=60" -o "ExitOnForwardFailure=yes" -R 19999:localhost:22 $ADDR -N
sleep 1
done