Here is another link for setting up ddclient DynDNSupdate.
Below is the setup for plug computers but can be used with any other platform if you leave out the part which deals with making the root partition writeable.

 mount -o remount,rw / 
 ipkg install ddclient

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)
 ipkg install openssl (which should also install wget-ssl)

edit /opt/etc/ddclient/ddclient.conf

 # 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.
Edit /etc/init.d/rcS to incude /opt/sbin/ddclient

 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.