referrers

Referrers, in web terminology, refer to the sources of traffic to a web resource. A referrer is the URL of the page that contained a hyperlink to the current resource. The HTTP header that carries this information is historically named Referer, a misspelling from the original specification, but it is widely used in practice by servers and analytics tools.

How it works: When a user clicks a link, the browser may include the Referer header in the resulting request, providing the destination with the address of the page that initiated the navigation. This enables traffic attribution, allowing site owners to see which pages or domains brought visitors. Privacy-conscious users and sites can reduce or suppress referrer data through browser settings or security policies.

Referrer policy and privacy: The Referrer-Policy header lets developers control how much information is sent with requests. Policy values range from no-referrer, which sends no referrer, to origin, which only reveals the origin of the referring page, to full or origin-when-cross-origin. These controls help mitigate leakage of sensitive information from URLs, such as query parameters, and support privacy requirements.

Applications and limitations: Referrer data is widely used for web analytics, attribution, and marketing measurement, helping organizations understand traffic sources and user journeys. However, referrers are not guaranteed to be present: users may configure browsers to omit them, traffic may pass through intermediaries that strip them, and some requests (such as those from non-browser clients) may not include a header at all. Referrers can also be manipulated, so they should be treated as one data point among others.

See also: HTTP header, Referer, Referrer-Policy, web analytics.