Internet Engineering Task Force (IETF) T. Bray Request for Comments: 7725 Textuality Category: Standards Track February 2016 ISSN: 2070-1721
An HTTP Status Code to Report Legal Obstacles
This document specifies a Hypertext Transfer Protocol (HTTP) status code for use when resource access is denied as a consequence of legal demands.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
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This document specifies a Hypertext Transfer Protocol (HTTP) status code for use when a server operator has received a legal demand to deny access to a resource or to a set of resources that includes the requested resource.
This status code can be used to provide transparency in circumstances where issues of law or public policy affect server operations. This transparency may be beneficial both to these operators and to end users.
[RFC4924] discusses the forces working against transparent operation of the Internet; these clearly include legal interventions to restrict access to content. As that document notes, and as Section 4 of [RFC4084] states, such restrictions should be made explicit.
This status code indicates that the server is denying access to the resource as a consequence of a legal demand.
The server in question might not be an origin server. This type of legal demand typically most directly affects the operations of ISPs and search engines.
Responses using this status code SHOULD include an explanation, in the response body, of the details of the legal demand: the party making it, the applicable legislation or regulation, and what classes of person and resource it applies to. For example:
<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>
The use of the 451 status code implies neither the existence nor nonexistence of the resource named in the request. That is to say, it is possible that if the legal demands were removed, a request for the resource still might not succeed.
Note that in many cases clients can still access the denied resource by using technical countermeasures such as a VPN or the Tor network.
A 451 response is cacheable by default, i.e., unless otherwise indicated by the method definition or explicit cache controls; see [RFC7234].
As noted above, when an attempt to access a resource fails with status 451, the entity blocking access might or might not be the origin server. There are a variety of entities in the resource- access path that could choose to deny access -- for example, ISPs, cache providers, and DNS servers.
It is useful, when legal blockages occur, to be able to identify the entities actually implementing the blocking.
When an entity blocks access to a resource and returns status 451, it SHOULD include a "Link" HTTP header field [RFC5988] whose value is a URI reference [RFC3986] identifying itself. When used for this purpose, the "Link" header field MUST have a "rel" parameter whose value is "blocked-by".
The intent is that the header be used to identify the entity actually implementing blockage, not any other entity mandating it. A human- readable response body, as discussed above, is the appropriate location for discussion of administrative and policy issues.
Clients cannot rely upon the use of the 451 status code. It is possible that certain legal authorities might wish to avoid transparency, and not only demand the restriction of access to certain resources, but also avoid disclosing that the demand was made.