How can I direct a url to a subdomain?

I have a rails app that I host for multiple users as useraccount.mysite.com. I want to have the option to allow my users to have their own url (useraccount.com) but still host the app using my shared server. So they should be able to access开发者_如何学Python useraccount.mysite.com or useraccount.com and it should return the same stuff. If they goto useraccount.com, the url should always say useraccount.com and they should never see mysite.com.

Is this possible?