HTTP Referer not always being passed

Question

I have an application which records users visits. None of these visits are directly accessed, 100% of these visits are referred from another site.

I am passing

Answer 1

When linking from one document to another in Internet Explorer 4.0 and later, the Referer header will not be sent when the link is from an HTTPS page to a non-HTTPS page. The Referer header also will not be sent when the link is from a non-HTTP(S) protocol, such as file://, to another page. for more info go to this link