HTTP referer

In HTTP, "Referer" (a misspelling of "Referrer") is an optional HTTP header field that identifies the address of the web page (i.e., the URI or IRI) from which the resource has been requested. By checking the referrer, the server providing the new web page can see where the request originated.

In the most common situation, this means that when a user clicks a hyperlink in a web browser, causing the browser to send a request to the server holding the destination web page, the request may include the Referer field, which indicates the last page the user was on (the one where they clicked the link).

Web sites and web servers log the content of the received Referer field to identify the web page from which the user followed a link, for promotional or statistical purposes.^{[citation needed]} This entails a loss of privacy for the user and may introduce a security risk. To mitigate security risks, browsers have been steadily reducing the amount of information sent in Referer. As of March 2021, by default Chrome, Chromium-based Edge, Firefox, Safari default to sending only the origin in cross-origin requests, stripping out everything but the domain name.

The misspelling of referrer was introduced in the original proposal by computer scientist Phillip Hallam-Baker to incorporate the "Referer" header field into the HTTP specification. The misspelling was set in stone by the time (May 1996) of its incorporation into the Request for Comments standards document RFC 1945 (which 'reflects common usage of the protocol referred to as "HTTP/1.0"' at that time); document co-author Roy Fielding remarked in March 1995 that "neither one (referer or referrer) is understood 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 Referrer-Policy HTTP header or the Document Object Model.

When visiting a web page, the referrer or referring page is the URL of the previous web page from which a link was followed.

More generally, a referrer is the URL of a previous item which led to this request. For example, the referrer for an image 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 reduces these problems but can still in some cases divulge the user's last-visited web page.

Many blogs publish 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.