Yeah,a don't use masking. Masking is bad.
Stick this javascript somewhere on your page (either in Advanced Layout Options, or in the "HTML Body" field of a menu category)
<script type="text/javascript">
if(!location.hostname.match(/www\.preferreddomain\.com/i))
{
location.href = "http://www.preferreddomain.com" + location.pathname + location.search + location.hash;
}
</script>
Note: the
\. in the first line are not typos.
Make sure you replace "preferreddomain" with your domainname, and make sure you spell it right when you save, or your site will become completely unusable (unless I fix it behind the scenes).
This script will redirect all requests to your site that aren't "www.domainname.com" to www.domainname.com. This includes if someone types "followers.dkpsystem.com" it'll still redirect to "www.domainname.com"