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Referer is a common misspelling of the word "referrer", so common in fact that it made it into the official specification of HTTP - the communication protocol of the world wide web. When visiting a webpage, the referer (sic) or referring page is the URL of the previous webpage from which a link was followed. More generally, it is the URL of a previous item which led to this request - the referer for an image, for example, is generally the HTML page on which it is to be displayed. The referer is part of the HTTP request sent by the browser program to the web server.
Some web sites log referers, for advertising or other purposes: the referring Web site may receive a small payment for people who follow an ad ("clickthroughs"). Some browsers and security software can be configured so that referer information is never sent out, to protect the privacy of the user. This can in turn cause problems: some servers block parts of their site to browser requests that don't contain the right referer information, for example in order to ensure that users always enter through the official portal and not by following a direct link.
Reference
- W3C: RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1 (ftp://ftp.isi.edu/in-notes/rfc2616.txt)
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