ssh-tunnel.service

[Unit]
Description=SSH Reverse Tunnel
ConditionPathExists=|/usr/bin
After=network.target

[Service]
User=pi
# SSH Env Vars -- UPDATE FOR YOUR ENVIRONMENT
Environment=keyFilePath="" #path to private key identity file
Environment=SSHremotePort="22" #remote side SSH service port. Normally 22, change if different
Environment=SSHTunnelPort="2222" #remote side port that the tunnel will be bound to (think high port numbers, e.g. 2222, 5555, etc.)
Environment=SSHlocalPort="22" #port where SSH runs on the originating side (normally 22, change if different)
Environment=identity="username@host" #username and IP/host of remote side 

ExecStart=/usr/bin/ssh -p "$SSHremotePort" -NTC -o ServerAliveInterval=60 -o ExitOnForwardFailure=yes -o StrictHostKeyChecking=no -i "$keyFilePath" -R "$SSHTunnelPort":localhost:"$SSHlocalPort" "$identity"

# Restart every >2 seconds to avoid StartLimitInterval failure
RestartSec=3
Restart=always

[Install]
WantedBy=multi-user.target