HTTP 451

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In computer networking, HTTP 451 Unavailable For Legal Reasons is a proposed standard error status code of the HTTP protocol to be displayed when the user requests a resource which cannot be served for legal reasons, such as a web page censored by a government. The number 451 is a reference to Ray Bradbury's 1953 dystopian novel Fahrenheit 451, in which books are outlawed.[2] 451 provides more information than HTTP 403, which is often used for the same purpose.[3] This status code is currently a proposed standard in RFC 7725 but is not yet formally a part of HTTP, as of RFC 9110.

A 451 status code returned by the Defence Distributed website to a client in Pennsylvania, 30 July 2018.[1]

Examples of situations where an HTTP 451 error code could be displayed include web pages deemed a danger to national security, or web pages deemed to violate copyright, privacy, blasphemy laws, or any other law or court order.

After introduction of the GDPR in the EEA it became common practice for websites located outside the EEA to serve HTTP 451 errors to EEA visitors instead of trying to comply with this new privacy law. For instance, many regional U.S. news sites no longer serve web browsers from the EU.[4][5][6]

The RFC is specific that a 451 response does not indicate whether the resource exists but requests for it have been blocked, if the resource has been removed for legal reasons and no longer exists, or even if the resource has never existed, but any discussion of its topic has been legally forbidden (see injunction).[7] Some sites have previously returned HTTP 404 (missing) or similar if they are not legally permitted to disclose that the resource has been removed. It is used in the United Kingdom by some Internet service providers utilising the Internet Watch Foundation blacklist, returning a 404 message or another error message instead of showing a message indicating the site is blocked.[8][9]

The status code was formally proposed in 2013 by Tim Bray, following earlier informal proposals by Chris Applegate[10] in 2008 and Terence Eden[11] in 2012. It was approved by the IETF on December 18, 2015.[12] It was published as in the Proposed Standard RFC 7725 in February 2016.

HTTP 451 was mentioned by the BBC's From Our Own Correspondent programme, as an indication of the effects of sanctions on Sudan and the inability to access Airbnb, the App Store, or other Western web services.[13]