In what cases will HTTP_REFERER be empty

Question

I know it\'s possible to get an empty HTTP_REFERER. Under what circumstances does this happen? If I get an empty one, does it always mean that the user changed it? Is gettin

Answer 1

BalusC's list is solid. One additional way this field frequently appears empty is when the user is behind a proxy server. This is similar to being behind a firewall but is slightly different so I wanted to mention it for the sake of completeness.