This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Did you check the output of Request.getRemoteHost() & getRemoteAddr() methods? If these methods doesn't return the server1 IP address then you can query for HTTP header fields "Referer" & "Via". Even if these fields doesn't work then you have to add a field to the request from server1 before forwarding to the server2.