[mistwang]

Are all subdomains handled by the same application? If yes, you need to do the following

setup the rails vhost to serve example.com and all its subdomains, http://mysubdomain.example.com/... will go to this vhost.

then add a rewrite rule by extract the subdomain from HTTP_HOST, then rewrite the URL from /controller/action/id to /mysubdomain/controller/action/id