Caching Only Server - Without Proxying

[Manish Kathuria <manish@xxxxxxxxxxxx>]

Hi,

I am setting up a internet gateway for a small ISP who wants a 
transparent caching server configured in such a manner that the outgoing 
requests from his customers retain their IP addresses (instead of 
getting changed to the proxy server's IP address) while the web content 
is cached to service future requests. I have gone through the available 
documentation but haven't been able to figure out how it can be done or 
if it is possible. Is there any other open source caching server which 
can meet this requirement ?

Thanks,

Manish Kathuria