Question

I don’t understand: how are webserver and trackers like Google Analytics able to track referrals?

Is it part of HTTP?

Is it some (un)specified behavior of the browsers?

Apparently every time you click on a link on a web page, the original web page is passed along the request.

What is the exact mechanism behind that? Is it specified by some spec?

I’ve read a few docs and I’ve played with my own Tomcat server and my own Google Analytics account, but I don’t understand how the “magic” happens.

Bonus (totally related) question: if, on my own website (served by Tomcat), I put a link to another site, does the other site see my website as the “referrer” without me doing anything special in Tomcat?

Answer 1

Referer (misspelled in the spec) is an HTTP header. It’s a standard header that all major HTTP clients support (though some proxy servers and firewalls can be configured to strip it or mangle it). When you click on a link, your browser sends an HTTP request that contains the page being requested and the page on which the link was found, among other things.

Since this is a client/request header, the server is irrelevant, and yes, clicking a link on a page hosted on your own server would result in that page’s URL being sent to the other site’s server, though your server may not necessarily be accessible from that other site, depending on your network configuration.