tedster - 11:58 pm on Feb 19, 2010 (gmt 0)
This discussion from two years ago [webmasterworld.com] is still definitive, as far as I know. It is part of our Hot Topics area [webmasterworld.com], which is always pinned to the top of this forum's index page.
The key to understanding a rankings hijack by a proxy server is that googlebot (legitimate googlebot) is crawling your content using the proxy server's domain. A key defense strategy is offered by incredibill in an earlier thread:
How you stop this problem is to use REVERSE and FORWARD DNS to validate that Googlebot is coming from an actual Google IP, which returns a domain name that contains ".googlebot.com". If the reverse DNS of the IP that claims to be Googlebot doesn't contain ".googlebot.com", you simply bounce them.