Why is HTTP_REFERER empty or What causes a blank referrer field?

It’s possible to get an empty HTTP_REFERER. The scenarios under which the referrer field will/may be empty when the end-user

  • entered the site URL in browser address bar itself.
  • visited the site by a browser-maintained bookmark.
  • visited the site as first page in the window/tab.
  • switched from a https URL to a http URL.
  • switched from a https URL to a different https URL.
  • has security software installed (antivirus/firewall/etc) which strips the referrer from all requests.
  • is behind a proxy which strips the referrer from all requests.
  • visited the site programmatically (like, curl) without setting the referrer header (searchbots!).

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