Failed? You mean you got the proxy's address? Well, that's to be expected since the proxy IS the client of the server. There's no way of knowing more than that from HTTP. You might be able to do more using JavaScript since that will run in the browser, not on the server.
There's no way of knowing more than that from HTTP
As a matter of fact there is at least one: it's X-Forwarded-For header which although is not a standard header, is very popular, and there are quite a lot of proxies giving away the client's IP this way.