Internet Engineering Task Force (IETF) T. Bray
Request for Comments: 7725 Textuality
Category: Standards Track February 2016
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.
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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
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
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.
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:
HTTP/1.1 451 Unavailable For Legal Reasons
Link: <https://spqr.example.org/legislatione>; rel="blocked-by"
<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.
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
7.1. 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,
[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.