install ddclient package.

Edit /etc/ddclient.conf

An at the end add this:

##
## DNS-O-Matic account-configuration
##
use=web, web=myip.dnsomatic.com
server=updates.dnsomatic.com, \
protocol=dyndns2, \
login=username, \
password=password    \
all.dnsomatic.com

Of course login and password must be changed to suite your own ones.