#!/bin/sh

# Domain name, username, and password from your Google Domains control panel
MyGDomain=
Username=
Password=
# WAN interface name
WANInterface=

# Record current WAN IP address from the WAN network interface
CurrWANIP=$(ip -o -4 addr show $WANInterface | awk -F '[ /]+' '{print $4}')
# Record IP which your Google domain is currently resolving to
GDNSIP=$(dig $MyGDomain +short)

# Check if current external IP address matches the address your Google domain resolves to
if [ "$CurrWANIP" == "$GDNSIP" ]; then
        # If the IP addresses match, your domain is up to date. Exit.
        logger "Google Domains DDNS: WAN IP ($CurrWANIP) unchanged, no update necessary"
else
        # If the IP addresses do not match, send a POST request over HTTPS to the Google DDNS API to update the record
        ipupdate=$(curl -s https://$Username:$Password@domains.google.com/nic/update?hostname=$MyGDomain&myip=$CurrWANIP)
        # Check if the updated IP returned in the HTTP response matches the current external IP
        if [ "$ipupdate" == "good $CurrWANIP" ]; then
         # If IP addresses match, log and exit
         logger "Google Domains DDNS: DNS updated to $CurrWANIP from $GDNSIP"
        else
         # If IP addresses do not match, log out output to stderr the HTTP response
         logger -s "Google Domains DDNS: Error: $ipupdate"
        fi
fi