Veja o LEIA-ME do ddclient:
USING DDCLIENT WITH cron
If you have not configured ddclient to use daemon-mode, you'll need to
configure cron to force an update once a month so that the dns entry will
not become stale.
## configure cron to force an update twice a month
cp sample-etc_cron.d_ddclient /etc/cron.d/ddclient
vi /etc/cron.d/ddclient