Dynamic DNS POST request that matches the API provided by Google Domains

ddns.sh

#!/bin/sh
#
# Construct a Dynamic DNS POST request that matches Google Domains' API.
# The final URL format should looks like:
# https://username:password@domains.google.com/nic/update?hostname=subdomain.yourdomain.com&myip=1.2.3.4
# See https://support.google.com/domains/answer/6147083
#
# Author: Ray Chen <oldsharp@gmail.com>
# License: Public Domain

# According to the document, we must set a user agent in our request.
# Below is an example for a true user agent(Chrome v45.0.2454.85 on
# GNU/Linux).
UAGENT='Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/45.0.2454.85 Safari/537.36'

# We _must_ fetch our "true" public IP first in case of we may need to
# send a POST request via proxy.
PUBLIC_IP=$(dig +short myip.opendns.com @resolver1.opendns.com)

# Edit these three lines below to fit your demand.
UNAME='ddns_username_provided_by_google_domains'
PASSWD='ddns_password_provided_by_google_domains'
HOST='ddns.example.TLD'

proxychains4 curl --data "hostname=${HOST}&myip=${PUBLIC_IP}" \
    --user-agent "${UAGENT}" \
    https://${UNAME}:${PASSWD}@domains.google.com/nic/update