Just define "*" as a subdomain A record on your Namecheap "all host records" page.App:: DDNS:: Namecheap is on the CPAN if you're interested.
If your Linux distribution does not have a recent version of ddclient, you can download the tar file by following this link.
On my Mac, I would also like to run ddclient regularly.
This will keep my DNS entry updated in the event that my external IP address changes.
I've used the dns o matic program and also tomato firmware to update a few dns accounts with my current ip, it's worked before, but ain't working now.
It is only updating one single account instead of all of the accounts...