Mirroring Website in Real Time

Hi, I had my domain A.com pointed to the main server. Now, I would like to have an IP domain or domain B.com as a mirror of A.com in real time without having the full data on my new server, only to direct the traffic from new server to main server. Is that possible? I heard about the script for reverse proxy. But not sure how to apply those though...
Any suggestion will be appreciated..