field whose value is a URI reference identifying itself. The "Link" header field must then have a "rel" parameter whose value is "blocked-by". This is intended to identify the entity implementing the blocking (an ISP, DNS provider, caching system, etc.), not the legal authority mandating the block. At an IETF hackathon, participants used a web crawler to discover that several implementations misunderstood this header and gave the legal authority instead. Additional uses The meaning of "a resource which cannot be served for legal reasons" has been interpreted to extend beyond government censorship: When content cannot be shown in the user's country, due to contractual or licensing restrictions with the content owner, for example, a TV program may not be available to users in some countries. When a publisher refuses to serve content to a user, because the user's country adds regulatory requirements that the publisher refuses to comply with, e.g. websites based outside of the EU may refuse to serve users in the EU because they do not want to comply with the GDPR. Example HTTP/1.1 451 Unavailable For Legal Reasons Link: <https://search.example.net/legal>; rel="blocked-by" Content-Type: text/html <html> <head><title>Unavailable For Legal Reasons</title></head> <body> <h1>Unavailable For Legal Reasons</h1> <p>This request may not be serviced in the Roman Province of Judea due to the Lex Julia Majestatis, which disallows access to resources hosted on servers deemed to be operated by the People's Front of Judea.</p> </body> </html> See also Evil bit Gag order Superinjunction Technological fix References External links RFC 7725 – An HTTP Status Code to Report Legal Obstacles HTTP_451 legal Internet censorship