ddclient

ddclient is a small full-featured client requiring only Perl. Supported
features include daemon operation, manual and automatic updates, static
and dynamic updates, optimized updates for multiple addresses, MX, wild
cards, abuse avoidance, retry for failed updates, and status updates to
syslog and through e-mail. ddclient can obtain the IP address from any 
interface, through a Web-based IP detection service, and for multiple  
routers using custom FW definitions. It also provides full support for 
DynDNS.org's NIC2 protocol. Support is also included for other dynamic 
DNS services. Comes with sample scripts for use with DHCP, PPP, and    
cron.