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452 lines
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Internet Engineering Task Force (IETF) M. Nottingham
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Request for Comments: 5785 E. Hammer-Lahav
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Updates: 2616, 2818 April 2010
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Category: Standards Track
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ISSN: 2070-1721
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Defining Well-Known Uniform Resource Identifiers (URIs)
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Abstract
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This memo defines a path prefix for "well-known locations",
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"/.well-known/", in selected Uniform Resource Identifier (URI)
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schemes.
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Status of This Memo
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This is an Internet Standards Track document.
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This document is a product of the Internet Engineering Task Force
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(IETF). It represents the consensus of the IETF community. It has
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received public review and has been approved for publication by the
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Internet Engineering Steering Group (IESG). Further information on
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Internet Standards is available in Section 2 of RFC 5741.
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Information about the current status of this document, any errata,
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and how to provide feedback on it may be obtained at
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http://www.rfc-editor.org/info/rfc5785.
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Copyright Notice
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Copyright (c) 2010 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
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Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
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described in the Simplified BSD License.
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Nottingham & Hammer-Lahav Standards Track [Page 1]
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RFC 5785 Defining Well-Known URIs April 2010
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Table of Contents
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1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
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1.1. Appropriate Use of Well-Known URIs . . . . . . . . . . . . 3
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2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
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3. Well-Known URIs . . . . . . . . . . . . . . . . . . . . . . . . 3
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4. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
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5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
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5.1. The Well-Known URI Registry . . . . . . . . . . . . . . . . 4
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5.1.1. Registration Template . . . . . . . . . . . . . . . . . 5
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6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
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6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
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6.2. Informative References . . . . . . . . . . . . . . . . . . 5
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Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 7
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Appendix B. Frequently Asked Questions . . . . . . . . . . . . . . 7
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1. Introduction
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It is increasingly common for Web-based protocols to require the
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discovery of policy or other information about a host ("site-wide
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metadata") before making a request. For example, the Robots
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Exclusion Protocol <http://www.robotstxt.org/> specifies a way for
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automated processes to obtain permission to access resources;
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likewise, the Platform for Privacy Preferences [W3C.REC-P3P-20020416]
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tells user-agents how to discover privacy policy beforehand.
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While there are several ways to access per-resource metadata (e.g.,
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HTTP headers, WebDAV's PROPFIND [RFC4918]), the perceived overhead
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(either in terms of client-perceived latency and/or deployment
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difficulties) associated with them often precludes their use in these
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scenarios.
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When this happens, it is common to designate a "well-known location"
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for such data, so that it can be easily located. However, this
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approach has the drawback of risking collisions, both with other such
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designated "well-known locations" and with pre-existing resources.
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To address this, this memo defines a path prefix in HTTP(S) URIs for
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these "well-known locations", "/.well-known/". Future specifications
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that need to define a resource for such site-wide metadata can
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register their use to avoid collisions and minimise impingement upon
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sites' URI space.
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Nottingham & Hammer-Lahav Standards Track [Page 2]
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RFC 5785 Defining Well-Known URIs April 2010
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1.1. Appropriate Use of Well-Known URIs
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There are a number of possible ways that applications could use Well-
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known URIs. However, in keeping with the Architecture of the World-
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Wide Web [W3C.REC-webarch-20041215], well-known URIs are not intended
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for general information retrieval or establishment of large URI
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namespaces on the Web. Rather, they are designed to facilitate
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discovery of information on a site when it isn't practical to use
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other mechanisms; for example, when discovering policy that needs to
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be evaluated before a resource is accessed, or when using multiple
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round-trips is judged detrimental to performance.
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As such, the well-known URI space was created with the expectation
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that it will be used to make site-wide policy information and other
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metadata available directly (if sufficiently concise), or provide
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references to other URIs that provide such metadata.
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2. Notational Conventions
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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document are to be interpreted as described in RFC 2119 [RFC2119].
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3. Well-Known URIs
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A well-known URI is a URI [RFC3986] whose path component begins with
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the characters "/.well-known/", and whose scheme is "HTTP", "HTTPS",
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or another scheme that has explicitly been specified to use well-
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known URIs.
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Applications that wish to mint new well-known URIs MUST register
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them, following the procedures in Section 5.1.
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For example, if an application registers the name 'example', the
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corresponding well-known URI on 'http://www.example.com/' would be
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'http://www.example.com/.well-known/example'.
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Registered names MUST conform to the segment-nz production in
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[RFC3986].
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Note that this specification defines neither how to determine the
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authority to use for a particular context, nor the scope of the
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metadata discovered by dereferencing the well-known URI; both should
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be defined by the application itself.
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Typically, a registration will reference a specification that defines
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the format and associated media type to be obtained by dereferencing
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the well-known URI.
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Nottingham & Hammer-Lahav Standards Track [Page 3]
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RFC 5785 Defining Well-Known URIs April 2010
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It MAY also contain additional information, such as the syntax of
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additional path components, query strings and/or fragment identifiers
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to be appended to the well-known URI, or protocol-specific details
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(e.g., HTTP [RFC2616] method handling).
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Note that this specification does not define a format or media-type
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for the resource located at "/.well-known/" and clients should not
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expect a resource to exist at that location.
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4. Security Considerations
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This memo does not specify the scope of applicability of metadata or
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policy obtained from a well-known URI, and does not specify how to
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discover a well-known URI for a particular application. Individual
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applications using this mechanism must define both aspects.
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Applications minting new well-known URIs, as well as administrators
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deploying them, will need to consider several security-related
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issues, including (but not limited to) exposure of sensitive data,
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denial-of-service attacks (in addition to normal load issues), server
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and client authentication, vulnerability to DNS rebinding attacks,
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and attacks where limited access to a server grants the ability to
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affect how well-known URIs are served.
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5. IANA Considerations
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5.1. The Well-Known URI Registry
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This document establishes the well-known URI registry.
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Well-known URIs are registered on the advice of one or more
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Designated Experts (appointed by the IESG or their delegate), with a
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Specification Required (using terminology from [RFC5226]). However,
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to allow for the allocation of values prior to publication, the
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Designated Expert(s) may approve registration once they are satisfied
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that such a specification will be published.
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Registration requests should be sent to the
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wellknown-uri-review@ietf.org mailing list for review and comment,
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with an appropriate subject (e.g., "Request for well-known URI:
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example").
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Before a period of 14 days has passed, the Designated Expert(s) will
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either approve or deny the registration request, communicating this
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decision both to the review list and to IANA. Denials should include
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an explanation and, if applicable, suggestions as to how to make the
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Nottingham & Hammer-Lahav Standards Track [Page 4]
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RFC 5785 Defining Well-Known URIs April 2010
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request successful. Registration requests that are undetermined for
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a period longer than 21 days can be brought to the IESG's attention
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(using the iesg@iesg.org mailing list) for resolution.
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5.1.1. Registration Template
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URI suffix: The name requested for the well-known URI, relative to
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"/.well-known/"; e.g., "example".
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Change controller: For Standards-Track RFCs, state "IETF". For
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others, give the name of the responsible party. Other details
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(e.g., postal address, e-mail address, home page URI) may also be
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included.
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Specification document(s): Reference to the document that specifies
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the field, preferably including a URI that can be used to retrieve
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a copy of the document. An indication of the relevant sections
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may also be included, but is not required.
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Related information: Optionally, citations to additional documents
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containing further relevant information.
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6. References
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6.1. Normative References
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
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Resource Identifier (URI): Generic Syntax", STD 66,
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RFC 3986, January 2005.
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[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
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IANA Considerations Section in RFCs", BCP 26, RFC 5226,
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May 2008.
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6.2. Informative References
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[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
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L., Leach, P., and T. Berners-Lee, "Hypertext Transfer
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Protocol -- HTTP/1.1", RFC 2616, June 1999.
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[RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed
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Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
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Nottingham & Hammer-Lahav Standards Track [Page 5]
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RFC 5785 Defining Well-Known URIs April 2010
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[W3C.REC-P3P-20020416]
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Marchiori, M., "The Platform for Privacy Preferences 1.0
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(P3P1.0) Specification", World Wide Web Consortium
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Recommendation REC-P3P-20020416, April 2002,
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<http://www.w3.org/TR/2002/ REC-P3P-20020416>.
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[W3C.REC-webarch-20041215]
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Jacobs, I. and N. Walsh, "Architecture of the World Wide
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Web, Volume One", World Wide Web Consortium
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Recommendation REC- webarch-20041215, December 2004,
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<http:// www.w3.org/TR/2004/REC-webarch-20041215>.
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Nottingham & Hammer-Lahav Standards Track [Page 6]
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RFC 5785 Defining Well-Known URIs April 2010
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Appendix A. Acknowledgements
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We would like to acknowledge the contributions of everyone who
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provided feedback and use cases for this document; in particular,
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Phil Archer, Dirk Balfanz, Adam Barth, Tim Bray, Brian Eaton, Brad
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Fitzpatrick, Joe Gregorio, Paul Hoffman, Barry Leiba, Ashok Malhotra,
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Breno de Medeiros, John Panzer, and Drummond Reed. However, they are
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not responsible for errors and omissions.
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Appendix B. Frequently Asked Questions
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1. Aren't well-known locations bad for the Web?
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They are, but for various reasons -- both technical and social --
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they are commonly used and their use is increasing. This memo
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defines a "sandbox" for them, to reduce the risks of collision and
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to minimise the impact upon pre-existing URIs on sites.
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2. Why /.well-known?
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It's short, descriptive, and according to search indices, not
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widely used.
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3. What impact does this have on existing mechanisms, such as P3P and
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robots.txt?
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None, until they choose to use this mechanism.
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4. Why aren't per-directory well-known locations defined?
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Allowing every URI path segment to have a well-known location
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(e.g., "/images/.well-known/") would increase the risks of
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colliding with a pre-existing URI on a site, and generally these
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solutions are found not to scale well, because they're too
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"chatty".
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Nottingham & Hammer-Lahav Standards Track [Page 7]
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RFC 5785 Defining Well-Known URIs April 2010
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Authors' Addresses
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Mark Nottingham
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EMail: mnot@mnot.net
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URI: http://www.mnot.net/
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Eran Hammer-Lahav
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EMail: eran@hueniverse.com
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URI: http://hueniverse.com/
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Nottingham & Hammer-Lahav Standards Track [Page 8]
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