What URL does Recovery HD connect to in order to download OS X?
Have a look at: OS X: Ports and hosts used by OS X Recovery 


  In order to reinstall OS X using the OS X Recovery or Lion Internet
  Recovery, you must be able to connect to the Internet. If your
  computer is behind an external proxy server or firewall, your network
  administrator will need to ensure that you can resolve DNS for the
  following hosts and contact them on ports 80 (HTTP) and 443 (HTTPS):
  
  osrecovery.apple.com
  oscdn.apple.com
  
  Note: Each hostname may resolve to a different IP address each time it
  is used.