bash script to update dynamic dns ip address on google domains from a Zyxel C3000Z modem

update_dynamic_dns

#!/usr/bin/env bash

#
# Update Google Domains Dynamic DNS from Zyxel C3000Z modem information.
# Tested on a Centurylink issued modem, YMMV
#
# See https://support.google.com/domains/answer/6147083?hl=en for 
# Google Domains help and credentials information
#

MODEM_BASE_URL="${MODEM_BASE_URL:-http://192.168.0.1}"

LOGINURL="$MODEM_BASE_URL/login.cgi"
STATUSURL="$MODEM_BASE_URL/modemstatus_connectionstatus.html"

# Create tempfile for cookies, and clean up on exit
export COOKIEFILE="$(mktemp)"
trap "rm -f ${COOKIEFILE}" EXIT

USER="${USERNAME:-admin}"
PASS="${PASSWORD:-fakepassword}"

# Login
curl --cookie-jar "$COOKIEFILE" -d "admin_username=$USER" -d "admin_password=$PASS" -XPOST "$LOGINURL" -o /dev/null

# Status
STATUSLINE="$(
  curl --cookie "$COOKIEFILE" "$STATUSURL" | \
    sed -e '/var allStatus/s/^var allStatus = "\(.*\)".*$/\1/p;d' | \
    sed -e 's/||/\t/g'
)"

# Parse
IFS='\t' read -ra VARS <<< "$STATUSLINE"

IPv4="${VARS[9]}"
IPv6="${VARS[10]}"

echo "IPv4: $IPv4"
echo "IPv6: $IPv6"

USER="${GD_USERNAME:-fakeusrname}"
PASS="${GD_PASSWORD:-fakepssword}"
DYNHOST="${DYNHOST:-dyn.example.com}"
IPADDR="${IPv4}"

# Check 
EXISTING="$(dig +short $DYNHOST)"
[ "$EXISTING" == "$IPv4" ] && echo "up-to-date" && exit


# Optionally update
curl -u "$USER:$PASS" "https://domains.google.com/nic/update?hostname=${DYNHOST}&myip=${IPADDR}"