HTTP referer (originally a misspelling of referrer) is an HTTP header field that identifies the address of the webpage (i.e. the URI or IRI) that linked to the resource being requested. By checking the referer, the new webpage can see where the request originated.
In the most common situation this means that when a user clicks a hyperlink in a web browser, the browser sends a request to the server holding the destination webpage. The request includes the referer field, which indicates the last page the user was on (the one where they clicked the link).
Referer is sometimes used to combat cross-site request forgery, but such security mechanisms are limited by the ease of disabling or forging a referer. A dereferer is a means to strip the details of the referring website from a link request so that the target website cannot identify the page which was clicked on to originate a request.
Origin of the term referer 
The misspelling referer originated in the original proposal by computer scientist Phillip Hallam-Baker to incorporate the field into the HTTP specification. The misspelling was set in stone by the time of its incorporation into the Request for Comments standards document RFC 1945; document co-author Roy Fielding has remarked that neither "referrer" nor the misspelling "referer" were recognized by the standard Unix spell checker of the period. "Referer" has since become a widely used spelling in the industry when discussing HTTP referers; usage of the misspelling is not universal, though, as the correct spelling of "referrer" is used in some web specifications such as the Document Object Model.
When visiting a webpage, the referrer or referring page is the URL of the previous webpage from which a link was followed.
More generally, a referer 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 field is an optional part of the HTTP request sent by the web browser to the web server.
Many websites log referers as part of their attempt to track their users. Most web log analysis software can process this information. Because referer information can violate privacy, some web browsers allow the user to disable the sending of referer information. Some proxy and firewall software will also filter out referer information, to avoid leaking the location of non-public websites. This can, in turn, cause problems: some web servers block parts of their website to web browsers that do not send the right referer information, in an attempt to prevent deep linking or unauthorised use of images (bandwidth theft). Some proxy software has the ability to give the top-level address of the target website as the referer, which usually prevents these problems while still not divulging the user's last-visited website.
Recently many blogs have started publishing referer information in order to link back to people who are linking to them, and hence broaden the conversation. This has led, in turn, to the rise of referer spam: the sending of fake referer information in order to popularize the spammer's website.
Many pornographic paysites use referer information to secure their websites. Only web browsers arriving from a small set of approved (login) pages are given access; this facilitates the sharing of materials among a group of cooperating paysites. Referer spoofing is often used to gain free access to these paysites.
Referer hiding 
Most web servers maintain logs of all traffic, and record the HTTP referer sent by the web browser for each request. This raises a number of privacy concerns, and as a result, a number of systems to prevent web servers being sent the real referring URL have been developed. These systems work either by blanking the referer field or by replacing it with inaccurate data. Generally, Internet-security suites blank the referer data, while web-based servers replace it with a false URL, usually their own. This, of course, raises the problem of referer spam. The technical details of both methods are fairly consistent – software applications act as a proxy server and manipulate the HTTP request, while web-based methods load websites within frames, causing the web browser to send a referer URL of their website address. Some web browsers give their users the option to turn off referer fields in the request header.
Most web browsers do not send the referer field when they are instructed to redirect using the "Refresh" field. This does not include some versions of Opera and many mobile web browsers. However, this method of redirection is discouraged by the World Wide Web Consortium (W3C).
- Kyrnin, Jennifer (2012-04-10). "Referrer - What is a Referrer - How do HTTP Referrers Work?". About.com. Retrieved 2013-03-20.
- Wharton, David (2007-10-29). "XSRF: Checking HTTP Referer Header Is Not Enough". Dell SecureWorks. Retrieved 2013-03-20.
- Hallam-Baker, Phillip (2000-09-21). "Re: Is Al Gore The Father of the Internet?". alt.folklore.computers. Retrieved 2013-03-20.
- Fielding, Roy (1995-03-09). "Re: Referer: (sic)". ietf-http-wg-old. Retrieved 2013-03-20.
- "Hypertext Transfer Protocol -- HTTP/1.1 (RFC 2616 § 14.36)". IETF. June 1999. Retrieved 2013-03-20. "The Referer[sic] request-header field allows the client to specify […] the address (URI) of the resource from which the Request-URI was obtained […]"
- "Network.http.sendRefererHeader". MozillaZine. 2007-06-10. Retrieved 2013-03-20.
- "HTML DOM Document referrer Property". w3schools.com. Retrieved 2013-03-20.
- Gundersen, Bret (2011-10-19). "The Impact of Google Encrypted Search". Adobe Digital Marketing Blog. Retrieved 2013-03-20.
- "HTML Techniques for Web Content Accessibility Guidelines 1.0: The META element". W3C. 2000-11-06. Retrieved 2013-03-20.
- "Hypertext Transfer Protocol -- HTTP/1.1: Encoding Sensitive Information in URI's (RFC 2616 § 15.1.3)". IETF. June 1999. Retrieved 2013-03-20. "Clients SHOULD NOT include a Referer[sic] header field in a (non-secure) HTTP request if the referring page was transferred with a secure protocol"
- "4.12 Links — HTML Living Standard: 220.127.116.11 Link type "noreferrer"". WHATWG. 2013-03-20. Retrieved 2013-03-20.
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