The 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 referrer, 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 referrer field, which indicates the last page the user was on (the one where they clicked the link).
The misspelling of referrer 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 referrers; usage of the misspelling is not universal, though, as the correct spelling "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 referrer is the URL of a previous item which led to this request. The referrer for an image, for example, is generally the HTML page on which it is to be displayed. The referrer field is an optional part of the HTTP request sent by the web browser to the web server.
Many websites log referrers as part of their attempt to track their users. Most web log analysis software can process this information. Because referrer information can violate privacy, some web browsers allow the user to disable the sending of referrer information. Some proxy and firewall software will also filter out referrer 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 referrer 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 referrer, which usually prevents these problems while still not divulging the user's last-visited website.
Recently many blogs have started publishing referrer 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 referrer spam: the sending of fake referrer information in order to popularize the spammer's website.
Many pornographic paysites use referrer 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. Referrer spoofing is often used to gain free access to these paysites.
Most web servers maintain logs of all traffic, and record the HTTP referrer 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 referrer field or by replacing it with inaccurate data. Generally, Internet-security suites blank the referrer data, while web-based servers replace it with a false URL, usually their own. This raises the problem of referrer 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 referrer URL of their website address. Some web browsers give their users the option to turn off referrer fields in the request header.
Most web browsers do not send the referrer 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).
Another referrer hiding method is to convert the original link URL to a Data URI scheme-based URL containing small HTML page with a meta refresh to the original URL. When the user is redirected from the data: page, the original referrer is hidden. The first public implementation of this method is the Darefer app for ownCloud.
Content Security Policy standard version 1.1 introduced a new referrer directive that allows more control over the browser's behavior in regards to the referrer header. Specifically it allows the webmaster to instruct the browser not to block referrer at all, reveal it only when moving with the same origin etc.
The "referrer" [sic] header field allows the user agent to specify a URI reference for the resource from which the target URI was obtained [...]
A user agent MUST NOT send a referrer header field in an unsecured HTTP request if the referring page was received with a secure protocol
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