nopaste.txt

Normally, this header is included in most HTTP requests (and
preserved across HTTP-level redirects), except in the following
scenarios:
⋅ After organically entering a new URL into the address bar or
  opening a bookmarked page.
⋅ When the navigation originates from a pseudo-URL document, such
  as data: or javascript:.
⋅ When the request is a result of redirection controlled by the
   Refresh header (but not a Location-based one).
⋅ Whenever the referring site is encrypted but the requested page
  isn’t.  According to RFC 2616 section 15.1.2, this is done for
  privacy reasons, but it does not make a lot of sense. The
  Referer string is still disclosed to third parties when one
  navigates from one encrypted domain to an unrelated encrypted
  one, and rest assured, the use of encryption is not synonymous
  with trustworthiness.
⋅ If the user decides to block or spoof the header by tweaking
  browser settings or installing a privacy-oriented plug-in.