Merlin + DuckDNS with Double NAT

ddns-start

#!/bin/sh

# register a subdomain at https://www.duckdns.org/ to get your token
# put 'hostname|token' in the 'Host Name' field under DDNS (custom)
# e.g. myhost|abcdefgh-1234-5678-9876-f71b0ed7a7fe

DDNS_HOSTNAME_FIELD=$(nvram get ddns_hostname_x)
SUBDOMAIN=$(echo "$DDNS_HOSTNAME_FIELD" | awk -F'|' '{print $1}')
TOKEN=$(echo "$DDNS_HOSTNAME_FIELD" | awk -F'|' '{print $2}')
IP="$(curl -fs4 https://myip.dnsomatic.com/)"

if [ -z "${IP}" ]
then
  logger "The IP is empty! Not calling the update line..."
else
  REQUEST_URL="https://www.duckdns.org/update?domains=${SUBDOMAIN}&token=${TOKEN}&ip=${IP}"
  logger "DDNS Request URL: ${REQUEST_URL}"

  RESULT=$(curl --silent "${REQUEST_URL}" 2>&1)
  logger "DDNS Request result: $RESULT"

  if [[ "$RESULT" == "OK" ]];
    then                     
          /sbin/ddns_custom_updated 1
        else                         
          /sbin/ddns_custom_updated 0
  fi                                 
fi