title: When browser sets the "referrer" in HTTP Request header?
body: <p>I have a login page and users from different domains are redirected to this page for the purpose of SSO. I use the referrer HTTP Header field to know where the user has come from, thus on successful login, I return him/her back to his/her original domain. 
  But sometimes my referrer is not set, while other times it's set. Why?</p>