Cloudflare as DynDNS (DynamicDNS) Provider in 2019 for Ubuntu

ddclient

# Configuration for ddclient scripts 
# generated from debconf on Mon Jun  3 17:01:32 UTC 2019
#
# /etc/default/ddclient

# Set to "true" if ddclient should be run every time DHCP client ('dhclient'
# from package isc-dhcp-client) updates the systems IP address.
run_dhclient="false"

# Set to "true" if ddclient should be run every time a new ppp connection is 
# established. This might be useful, if you are using dial-on-demand.
run_ipup="false"

# Set to "true" if ddclient should run in daemon mode
# If this is changed to true, run_ipup and run_dhclient must be set to false.
run_daemon="true"

# Set the time interval between the updates of the dynamic DNS name in seconds.
# This option only takes effect if the ddclient runs in daemon mode.
daemon_interval="600"

ddclient.conf

# Configuration file for ddclient generated by debconf
#
# /etc/ddclient/ddclient.conf

daemon=1800
protocol=cloudflare
use=web
server=api.cloudflare.com/client/v4
ssl=yes
login=*e-mail address for cloudflare login*
password=*public cloudflare API Key*
zone=*root domain e.g. xyz.com*
*dynDNS name: server_home.xyz.com*

gistfile1.txt

Following steps were needed:

Add your DynDNS Domain as an A-Record to Cloudflare: server_home.xyz.com -> 1.2.3.245 (The IP address does not matter, just use your own.)
apt-get install ddclient libdata-validate-ip-perl
wget the newest ddclient version (Ver. 3.9.0 as of this writing)
unpack
replace the file in /usr/sbin/ddclient with the new binary
use the two config files