The best-known HTTP server response code is probably 404 error pointing to a non-existing page.
Tim Bray, a Google employee, presented the RFC, which proposes to introduce a new HTTP Error status code number 451 – showing the message ” Unavailable For Legal Reasons ” – aimed at shining a light on web censorship. It would be used to label the content to which access has been restricted by law which prevent it servicing the request, for example, as a result of judgment or decision of an organization such as RIAA and MPAA, as they already acquire such rights. Since such restrictions typically apply to all operators in a legal jurisdiction, the server in question may or may not be an origin server. The restrictions typically most directly affect the operations of ISPs and search engines.
Error code number is not accidental. The selection of the 451 code is inspired by the famous novel “Fahrenheit 451” by the recently deceased Ray Bradbury.
As it stands, most web-blocking tools return a 403 error (which means access is forbidden) when denying access to censored pages. For instance, UK ISPs, which are now required to block The Pirate Bay, typically return a 403 error code when doing so.
Tim Bray notes in the proposal that many governments might not want such censorship transparency and would likely take steps to disclosure that the restriction exists. As such the 451 status code would be optional and clients (like web browser) are instructed not to rely upon its use.