Here is another link for setting up ddclient DynDNSupdate.
mount -o remount,rw /
The next two installs are for secure logon, probably a good idea, if not ssl=yes will crash ddclient. If you want to be unsecure, ssl=no for ddlient.conf.
ipkg install perl-io-socket-ssl (which should also install perl-crypt-ssleay)
# Basic configuration file for ddclient daemon=600 cache=/tmp/ddclient.cache pid=/var/run/ddclient.pid use=web, web=checkip.dyndns.com/, web-skip='IP Address' login=<your username> password=<your password> protocol=dyndns2 server=members.dyndns.org wildcard=YES ssl=yes <your hostname you signed up for>
With the above setup, ddclient will run in daemon mode.
mount -o ro,remount /
To secure root or reboot with sync, sync, reboot. Rebooting is a good test for making sure that the rcS modification works. Do ps or ps -ef (if you installed the utils) to see if ddclient is running.