How to block the real IP rather than an entire proxy?

["J and T" <j_and_t@xxxxxxxxxxx>]

My question is about blocking IPs from my network. It's a piece of cake to 
block static IPs. But there are times when someone sitting behind a proxy 
whos IP doesn't change, but the proxy in which he is accessing us does. 
There's no way I can figure out how to keep these people out. For example; 
Apache only shows the IP of the visitor and not the real IP of the visitor. 
Here's an example of what I mean using environment variables:

$ENV{'REMOTE_ADDR'};

One might believe this is the IP of the visitor. But...

$ENV{'HTTP_X_FORWARDED_FOR'}

if the above is present it is most likely the true IP of the visitor and the 
"REMOTE_ADDR" is just the proxy in which they are accessing the Net with.

$ENV{'HTTP_CLIENT_IP'}

Which you may need to deal with as well.

So how can I block a visitor who is jumping around through proxies? I don't 
want to block out the proxy as this could ultimately block out all AOL users 
for example.

Thanks!
John

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