This PHP script will allow you to use your rackspace cloud account as a replacement for the now discontinued free DynDNS domain service.

Setup:

1. Log into mycloud.rackspace.com and add a sub-domain record to your chosen domain which points to your current IP (or any ip, it does not matter at this point)
2. Modify dnsupdate.php with your chosen domain, subdomain and rackspace username, rackspace apikey
3. chmod write permissions to lastip.txt and execute permission to dnsupdate.php
4. To have the script run every 5 minutes just add the following to your crontab:

*/5 * * * * php /home/user/dyndns/dnsupdate.php

The script checks your server's current ip with icanhazip.com and stores your ip in a txt file which it compares on every run. If the ip has changed it will send a DNS modification request to rackspace's DNS API.