Check tunnel from inside Transmission openvpn container

synology-check-transmission-tunnel-kill-if-dead.sh

# based on tunnelChecker from
# https://github.com/mrjackyliang/tunnelChecker
# modified to work on Alpine and doesn't require a script in the container
CONTAINER=transmission

# alpine container will always have ash (even if bash installed)
docker exec -i $CONTAINER sh <<'EOF'
  PORT_FORWARD=12345
  IP_CHECK=http://ipinfo.io/ip

  # address of the tunnel
  VPN_ADDR=$(ip -4 addr show tun0 | grep -Eo '(\d{1,3}\.){3}\d{1,3}' | head -n1)
  # alternate: ifconfig tun0 | grep "inet " | cut -d: -f2 | awk '{print $1}'
  echo "Internal tun0 IP is ${VPN_ADDR}"

  externalip() { curl -sS --interface $1 $IP_CHECK; }

  TUN_IP=$(externalip tun0) # ip visible through the tunnel
  ETH_IP=$(externalip eth0) # ip visible through eth0 (regular ISP IP)
  
  echo "External tun0 (VPN) IP is ${TUN_IP}"
  echo "External eth0 IP is ${ETH_IP}"

  nc -z $VPN_ADDR $PORT_FORWARD
  if [ $? -eq 0 ]; then
    VPN_PORT="open";
  else
    VPN_PORT="closed";
  fi;

  echo "Network Port is ${VPN_PORT}"
  if [ "$TUN_IP" = "$ETH_IP" ] || [ "$VPN_PORT" = "closed" ]; then
    echo "Killing OpenVPN"
    pkill -SIGINT openvpn
  else
    echo "VPN is already working!"
  fi

  exit 0
EOF

exit 0

This can be run as a scheduled task on a Synology to basically kick a transmission container that has lost its VPN port