Cloudflare + Dynamic DNS (ddclient) = Super Awesome

Instructions

# This guide will show you how to configure ddclient to dynamically update your DNS at Cloudflare.
# This cookbook makes use of a number of online guides - all referenced at the end.

# Note all commands require root, so if you aren't already logged in as root, either run:
sudo su -
# to switch to the root user or prefix every command with:
sudo <COMMAND...>

# Cloudflare requires ddclient to be version 3.8.2 or you will get a 'cloudflare protocol didn't exist'.
# Debian 10/Bullseye has [3.9.1](https://packages.debian.org/bullseye/ddclient) so should be good to go.

# Installation - be sure to use from a "proper" interactive terminal (e.g. SSH in)
apt-get update
apt-get install -y ddclient

# During install, you will be asked some questions. Answer them like this:

- Dynamic DNS service provider:
other

- Dynamic DNS update protocol:
cloudflare

- Username:
<YOUR CLOUDFLARE EMAIL ADDRESS>

- Password:
<YOUR CLOUDFLARE API KEY>
(see https://www.cloudflare.com/a/account/<MY_ACCOUNT>)

- Re-enter password:
<YOUR CLOUDFLARE API KEY>

- IP address discovery method:
Web-based IP discovery service

- Hosts to update (comma-separated):
<DOMAIN AND/OR SUB-DOMAIN NAME/S TO UPDATE>

# Good news, you are technically good to go! Test your setup by: 
ddclient -daemon=0 -debug -verbose -noquiet

# Check the log and see if there are any messages. Most likely issues will be with config file (/etc/ddclient.conf),
# so either update manually or re-run through questions like this:

dpkg-reconfigure ddclient

# Retest with the above line until it works without any error

# By default, it will run every 5 minutes, whoich should be fine. If you wish to change that, i can be configured
# in /etc/default/ddclient

# You cna view the status of the ddclient update service with this command:
systemclt status ddclient

# Resources:
#   Original gist:
#     https://gist.github.com/benscabbia/a6b158974219f96ecb1e87b97890c5fe
#   Other resources (from original gist):
#     https://www.cloudflare.com/technical-resources/#ddclient
#     https://jenssegers.com/84/dynamic-dns-for-cloudflare-with-ddclient
#     https://samhobbs.co.uk/2015/01/dynamic-dns-ddclient-raspberry-pi-and-ubuntu