HTTP 451 - Wikipedia, the free encyclopedia
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 an illegal resource, such as a web page censored by a government, with the number 451 being a reference to the 1953 dystopian novel Fahrenheit 451, where books are outlawed.[1] 451 could be described as a more descriptive variant of 403 Forbidden.[2]
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.
However, some sites may be forced to produce HTTP 404 (File 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 (see also: Gag order, superinjunction) by some ISPs utilising the Internet Watch Foundation blacklist, returning a 404 message or another error message instead of showing a message indicating the site is blocked.[3][4]
The status code was formally proposed in 2013 by Tim Bray, based on an informal proposal in a blog post by Terence Eden .[5] It was approved by the IESG on December 18, 2015.[6]