.
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Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. 451 Unavailable For Legal Reasons . . . . . . . . . . . . . . . 3
4. Identifying Blocking Entities . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
This document specifies a Hypertext Transfer Protocol (HTTP) status
code for use when a server operator has a received a legal demand to
deny access to a 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.
Feedback should occur on the ietf-http-wg@w3.org mailing list.
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:
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HTTP/1.1 451 Unavailable For Legal Reasons
Content-Type: text/html
Unavailable For Legal Reasons
Unavailable For Legal Reasons
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.
The use of the 451 status code implies neither the existence nor non-
existence 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 which 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.
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5. Security Considerations
5.1. 451 Unavailable for Legal Reasons
The 451 status code is optional; clients cannot rely upon its use.
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 should be updated with the following
entry:
o Code: 451
o Description: Unavailable for Legal Reasons
o Specification: [ this document ]
7. 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,
.
[RFC4084] Klensin, J., "Terminology for Describing Internet
Connectivity", BCP 104, RFC 4084, DOI 10.17487/RFC4084,
May 2005, .
[RFC4924] Aboba, B., Ed. and E. Davies, "Reflections on Internet
Transparency", RFC 4924, DOI 10.17487/RFC4924, July 2007,
.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/
RFC5988, October 2010,
.
[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,
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.
Appendix A. 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/
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