yes, for example this website. if i can put some html that will render an image, that image comes from my server that i control. but that image isn't an image we expect instead it's a php script with another content-type (png). in the same time, same place i load the image and other code and point the whole thing to this site. again if i put $_server['remote_addr'] into that script, user ip's will be for grabbing what i want is that none of all unique user ip addresses are intercepted but just a canonical ip address(which is the ip of the server). for each request that is made.
but i want a configuration for that (it doesn't have to be exhaustive). if it were a variable for an apache server.
same thing with email and p2p messages