title: How to give cname forward support to saas software
body: <p>I have a webapp where users can create their account and use the service.  Now I want to give them a custom domain facility where
  app.customer1web.com   points_to      myservice.com  with userid  customer1
  once he sets up the custom domain, for the world it looks like my service is running on his machine.
  Many services like blogger, wp.com, tumblr give
  this feature.</p>
  
  <p>how do i do that?
  I am using java to write my web app.
   How do i map domain name to userid when request comes in? </p>