WordPress Multisite and Domain Mapping

I am using a multisite WordPress installation. The install includes a blog that's a subdirectory of my main web site. I am using the Domain Mapping plugin to map an existing domain hosted on linode to the mysite.com/subdir blog. Both the primary domain (mysite.com) and the subdir blog are hosted at linode. The desired result I want is for subdir.com to resolve to the mysite.com/subdir blog. How can I use DNS to point subdir.com to mysite.com/subdir? I've tried adding an A record and a CNAME record to my subdir.com master account. However, CNAME gives an error about a duplicate domain and an A record makes no difference.