forward proxy configuration question

Hi,

I want to achieve something quite simple:
- tell my web client to use a remote Apache server as a proxy
- make the proxy forward domain1.com to d1.com, domain2 to d2.com

I know there are other ways to achieve this kind of masquerading: e.g. 
change the /etc/hosts on the web client or redirect.
But I want maximum transparency on the client side.

I have tried with mod_proxy and mod_rewrite, but no success so far.
If anyone has a simple example to show me how to do that, that would be 
great.

regards,

Arnaud