The Do Not Track (DNT) header is the proposed HTTP header field DNT that requests that a web application disable either its tracking or cross-site user tracking (the ambiguity remains unresolved) of an individual user. The Do Not Track header was originally proposed in 2009 by researchers Christopher Soghoian, Sid Stamm, and Dan Kaminsky. Efforts to standardize Do Not Track by the W3C have so far been unsuccessful.
In December 2010, Microsoft announced support for the DNT mechanism in its Internet Explorer 9 web browser. Mozilla's Firefox became the first browser to implement the feature, while Internet Explorer, Apple's Safari, Opera and Google Chrome all later added support.
The header field name is DNT and it currently accepts three values: 1 in case the user does not want to be tracked (opt out), 0 in case the user consents to being tracked (opt in), or null (no header sent) if the user has not expressed a preference. The default behavior required by the standard is not to send the header unless the user enables the setting via their browser or their choice is implied by use of that specific browser.