The "referer" header is the one which contains the URL of the resource that submitted the request. At least it's supposed to be. It's possible for a requester to put anything it likes in there, but typically browsers won't lie about it. Perhaps that's what those other resources meant when they said "not possible". Although you didn't say anything about them except that they existed, so who knows? Looking for 100% certainty in the world of the Internet isn't very practical.