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Internet Engineering Task Force (IETF)                           T. BrayRequest for Comments: 7725                                    TextualityCategory: Standards Track                                  February 2016ISSN: 2070-1721An HTTP Status Code to Report Legal ObstaclesAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7725.Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Bray                         Standards Track                    [Page 1]

RFC 7725                     HTTP-status-451               February 2016Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .23.  451 Unavailable For Legal Reasons . . . . . . . . . . . . . .24.  Identifying Blocking Entities . . . . . . . . . . . . . . . .35.  Security Considerations . . . . . . . . . . . . . . . . . . .46.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .47.  References  . . . . . . . . . . . . . . . . . . . . . . . . .4   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .5   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .51.  Introduction   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 asSection 4   of [RFC4084] states, such restrictions should be made explicit.2.  Requirements   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].3.  451 Unavailable For Legal Reasons   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:Bray                         Standards Track                    [Page 2]

RFC 7725                     HTTP-status-451               February 2016   HTTP/1.1 451 Unavailable For Legal Reasons   Link: <https://spqr.example.org/legislatione>; 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>   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].4.  Identifying Blocking Entities   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.Bray                         Standards Track                    [Page 3]

RFC 7725                     HTTP-status-451               February 20165.  Security Considerations   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.6.  IANA Considerations   The HTTP Status Codes Registry has been updated with the following   entry:   o  Code: 451   o  Description: Unavailable For Legal Reasons   o  Specification:RFC 7725   The Link Relation Type Registry has been updated with the following   entry:   o  Relation Name: blocked-by   o  Description: Identifies the entity that blocks access to a      resource following receipt of a legal demand.   o  Reference:RFC 77257.  References7.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform              Resource Identifier (URI): Generic Syntax", STD 66,RFC 3986, DOI 10.17487/RFC3986, January 2005,              <http://www.rfc-editor.org/info/rfc3986>.   [RFC5988]  Nottingham, M., "Web Linking",RFC 5988,              DOI 10.17487/RFC5988, October 2010,              <http://www.rfc-editor.org/info/rfc5988>.Bray                         Standards Track                    [Page 4]

RFC 7725                     HTTP-status-451               February 2016   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",RFC 7234, DOI 10.17487/RFC7234, June 2014,              <http://www.rfc-editor.org/info/rfc7234>.7.2.  Informative References   [RFC4084]  Klensin, J., "Terminology for Describing Internet              Connectivity",BCP 104,RFC 4084, DOI 10.17487/RFC4084,              May 2005, <http://www.rfc-editor.org/info/rfc4084>.   [RFC4924]  Aboba, B., Ed. and E. Davies, "Reflections on Internet              Transparency",RFC 4924, DOI 10.17487/RFC4924, July 2007,              <http://www.rfc-editor.org/info/rfc4924>.Acknowledgements   Thanks to Terence Eden, who observed that the existing status code   403 was not really suitable for this situation, and suggested the   creation of a new status code.   Thanks also to Ray Bradbury.Author's Address   Tim Bray   Textuality   Email: tbray@textuality.com   URI:http://www.tbray.org/ongoing/Bray                         Standards Track                    [Page 5]