Systemd ddclient.service

ddclient.service

#/usr/lib/systemd/system/ddclient.service
[Unit]
Description=ddclient Service
After=network.target

[Service]
Type=forking
PIDFile=/var/run/ddclient.pid
ExecStart=/sbin/ddclient -pid /var/run/ddclient.pid -file /etc/ddclient/ddclient.conf -daemon 300
ExecStop=/usr/bin/pkill -SIGKILL -P /var/run/ddclient.pid

I have modified it to work without forking and pidfile and under separate user. This is for debian based systems, as the paths are different.
/etc/systemd/system/ddclient.service:

[Unit]
Description=ddclient Service
After=network.target

[Service]
User=ddclient
Group=ddclient
ExecStart=/usr/sbin/ddclient -file /etc/ddclient.conf -foreground -daemon 300

[Install]
WantedBy=multi-user.target

install ddclient from apt, create service and create user (the package does have it's own sysV init script/service, this is only necessary if you want to run ddclient under non-root/restricted user)

apt install ddclient

systemctl disable ddclient
rm /etc/init.d/ddclient
# install the service file
systemctl daemon-reload
systemctl enable ddclient

useradd -s /usr/sbin/nologin -r -M ddclient
chown ddclient /etc/ddclient.conf /var/cache/ddclient -R
chmod g+rw /var/cache/ddclient/ddclient.cache