In computer networking, HTTP 451 Unavailable For Legal Reasons is an 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. 451 could be described as a more explanatory variant of 403 Forbidden. This status code is standardized in RFC 7725.
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.
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 superinjunction). Some sites have previously returned HTTP 404 (Not Found) or similar if they are not legally permitted to disclose that the resource has been removed. Such a tactic 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.
The status code was formally proposed in 2013 by Tim Bray, following earlier informal proposals by Chris Applegate in 2008 and Terence Eden in 2012. It was approved by the IESG on December 18, 2015. It was published as RFC 7725 in February 2016.
- Flood, Alison (June 22, 2012). "Call for Ray Bradbury to be honoured with internet error message". The Guardian. Retrieved June 22, 2012.
- Ducklin, Paul (August 19, 2013). "HTTP error code 451: "Unavailable For Legal Reasons"". Naked Security. Sophos.
- RFC 7225, §3
- "If the request is for the blocked content then the proxy server will return a 404 error page to the customer" https://wiki.openrightsgroup.org/wiki/Cleanfeed
- "TCP Reset is sent back to the customer instead of content." https://www.theguardian.com/technology/blog/2008/dec/08/internet-censorship-wikipedia-diagram
- "There is no HTTP code for censorship". December 9, 2008. Retrieved December 23, 2015.
- Byrne, Michael (21 December 2015). "The HTTP 451 Error Code for Censorship Is Now an Internet Standard". Vice. Retrieved 21 December 2015.
- "Why 451?". December 18, 2015. Retrieved December 20, 2015.
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