[RFC Home] [TEXT|PDF|HTML] [Tracker] [IPR] [Errata] [Info page]

Errata Exist
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

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.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 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 2016

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  451 Unavailable For Legal Reasons . . . . . . . . . . . . . .   2
   4.  Identifying Blocking Entities . . . . . . . . . . . . . . . .   3
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  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

   [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.

2.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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

    <head><title>Unavailable For Legal Reasons</title></head>
     <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>

   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

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 2016

5.  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

6.  IANA Considerations

   The HTTP Status Codes Registry has been updated with the following

   o  Code: 451

   o  Description: Unavailable For Legal Reasons

   o  Specification: RFC 7725

   The Link Relation Type Registry has been updated with the following

   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 7725

7.  References

7.1.  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,

   [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,

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,

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,

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,


   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

   Email: tbray@textuality.com
   URI:   http://www.tbray.org/ongoing/

Bray                         Standards Track                    [Page 5]