SSH reverse tunnel as systemd service

ssh_reverse_tunnel.sh

#!/usr/bin/env bash

: '
SOURCE node creates a tunnel to DESTINATION
This allows connections from DESTINATION to SOURCE with: ssh $USERNAME@localhost -p 19999
'

USERNAME=tunnel
PRIVKEY=/etc/tunnel/id_rsa
DESTINATION=ssh.example.com

cat << EOF | sudo tee /etc/systemd/system/sshtunnel.service
[Unit]
Description=SSH Tunnel
After=network.target

[Service]
Restart=always
RestartSec=20
# User=sshtunnel
ExecStart=/usr/bin/ssh -N -p 22 -o PubkeyAuthentication=yes -o ServerAliveInterval=60 -i $PRIVKEY -R 19999:localhost:22 $USERNAME@$DESTINATION

[Install]
WantedBy=multi-user.target
EOF

sudo chmod a+x /etc/systemd/system/sshtunnel.service
sudo chmod a+r /etc/systemd/system/sshtunnel.service
# sudo useradd sshtunnel

sudo systemctl enable sshtunnel.service
sudo systemctl start sshtunnel.service