Contributed by
scourgen hung
in #8708.

When a 404 occurs in a web application, knowing the root cause helps fixing the problem: is it someone trying to hack your website by trying different well-known URLs? Is it a dead-link on your website? or on another one? If the 404 comes from your website, fixing it should be relatively easy.

The only source of information about the cause is the HTTP Referer header. And even if it cannot be trusted, it can help nonetheless. That's why as of Symfony 2.4, the HTTP Referer is added in the NotFoundHttpException exception message thrown by the router listener.