How to blogger/wordpress/tumlr allow users to mask url
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I am creating a saas application. Users have their own url like user.mysaasapp.com

To allow user to have own url, i used mod rewrite. Something like http://mysaasapp.com/?user=user to user.mysaasapp.com (This is working perfectly)

Now, i want to give user the flexibility to mask their url. This mean that user can direct their subdomain (saas.theirdomain.com) to user.mysaasapp.com. How do i go about doing so. 

I read many articles(cname) on this but still have not have a definite solution. I am wondering how big companies like wordpress/google does that.

e.g http://www.tumblr.com/docs/en/custom_domains

e.g http://developer.uservoice.com/docs/site/domain-aliasing/

I realize that companies would take this process.
1) Get user to add cname
2) Get user to enter their domain in the saas app (maybe under custom domain)

What is needed to my done to my side. Thanks!