The Internet Engineering Steering Group (IESG), which helps to review and update standards on the internet, has approved a new status code for web page errors – one that will indicate where the page has been repressed for legal or political reasons.

Status codes are used when transmitting or requesting data over the internet, with the 404 error – ‘page not found’ – most commonly seen by the casual web viewer. The newly-instituted 451 error code will be used in cases where access to a web page is denied as a result of legal demands, such as censorship by a national government or a take-down notice presented to Google.

Mark Nottingham, chair of the IETF HTTP Working Group, explained in an online post that a draft was initially brought by Tim Bray, one of the co-authors of the original XML specification, as it was considered important to spotlight online censorship. The initiative was originally reported in August 2013 with the 451 error message informing users when governments are blocking websites.

The IESG has since approved the publication of An HTTP Status Code to Report Legal Obstacles by Tim Bray. Mr Nottingham explains that while there will be a Request For Comments (RFC), the 451 code can effectively be used from now.

[The] 403 status code says “Forbidden”, but it doesn’t say “I can’t show you that for legal reasons.” Hence, 451 (which is also a great tip of the hat to Ray Bradbury).

Community members had expressed to Nottingham that they wanted the opportunity to find instances of censorship in an automated fashion. Amongst those calling for the examples were Lumen (formerly Chilling Effects) and Article19, believing that use of the 451 status code could help develop new indexes of censorship on the internet.

Nottingham says that the 451 code can be used by network-based intermediaries (such as a firewall) and on the origin web server, and added that 451 was likely to be used more in the latter case for sites such as Google, Twitter and Facebook, often forced to censor content against their will in certain jurisdictions.