Well-Known URIs: Defining Well-Known Uniform Resource Identifiers (URIs)
The reserved path prefix that lets a site publish one piece of standard information at a fixed address.
Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known Uniform Resource Identifiers (URIs)", RFC 5785, DOI 10.17487/RFC5785, April 2010, <https://www.rfc-editor.org/info/rfc5785>.
@misc{rfc5785,
series = {Request for Comments},
number = 5785,
howpublished = {RFC 5785},
publisher = {RFC Editor},
doi = {10.17487/RFC5785},
url = {https://www.rfc-editor.org/info/rfc5785},
author = {M. Nottingham and E. Hammer-Lahav},
title = {{Defining Well-Known Uniform Resource Identifiers (URIs)}},
pagetotal = 8,
year = 2010,
month = apr,
} RFC text
Contents
Internet Engineering Task Force (IETF) M. Nottingham
Request for Comments: 5785 E. Hammer-Lahav
Updates: 2616, 2818 April 2010
Category: Standards Track
ISSN: 2070-1721
Defining Well-Known Uniform Resource Identifiers (URIs)
Abstract
This memo defines a path prefix for "well-known locations",
"/.well-known/", in selected Uniform Resource Identifier (URI)
schemes.
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
http://www.rfc-editor.org/info/rfc5785.
Copyright Notice
Copyright (c) 2010 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Appropriate Use of Well-Known URIs . . . . . . . . . . . . 3
2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 3
3. Well-Known URIs . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
5.1. The Well-Known URI Registry . . . . . . . . . . . . . . . . 4
5.1.1. Registration Template . . . . . . . . . . . . . . . . . 5
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1. Normative References . . . . . . . . . . . . . . . . . . . 5
6.2. Informative References . . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 7
Appendix B. Frequently Asked Questions . . . . . . . . . . . . . . 7
1. Introduction
It is increasingly common for Web-based protocols to require the
discovery of policy or other information about a host ("site-wide
metadata") before making a request. For example, the Robots
Exclusion Protocol <http://www.robotstxt.org/> specifies a way for
automated processes to obtain permission to access resources;
likewise, the Platform for Privacy Preferences [W3C.REC-P3P-20020416]
tells user-agents how to discover privacy policy beforehand.
While there are several ways to access per-resource metadata (e.g.,
HTTP headers, WebDAV's PROPFIND [RFC4918]), the perceived overhead
(either in terms of client-perceived latency and/or deployment
difficulties) associated with them often precludes their use in these
scenarios.
When this happens, it is common to designate a "well-known location"
for such data, so that it can be easily located. However, this
approach has the drawback of risking collisions, both with other such
designated "well-known locations" and with pre-existing resources.
To address this, this memo defines a path prefix in HTTP(S) URIs for
these "well-known locations", "/.well-known/". Future specifications
that need to define a resource for such site-wide metadata can
register their use to avoid collisions and minimise impingement upon
sites' URI space.
1.1. Appropriate Use of Well-Known URIs
There are a number of possible ways that applications could use Well-
known URIs. However, in keeping with the Architecture of the World-
Wide Web [W3C.REC-webarch-20041215], well-known URIs are not intended
for general information retrieval or establishment of large URI
namespaces on the Web. Rather, they are designed to facilitate
discovery of information on a site when it isn't practical to use
other mechanisms; for example, when discovering policy that needs to
be evaluated before a resource is accessed, or when using multiple
round-trips is judged detrimental to performance.
As such, the well-known URI space was created with the expectation
that it will be used to make site-wide policy information and other
metadata available directly (if sufficiently concise), or provide
references to other URIs that provide such metadata.
2. Notational Conventions
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 RFC 2119 [RFC2119].
3. Well-Known URIs
A well-known URI is a URI [RFC3986] whose path component begins with
the characters "/.well-known/", and whose scheme is "HTTP", "HTTPS",
or another scheme that has explicitly been specified to use well-
known URIs.
Applications that wish to mint new well-known URIs MUST register
them, following the procedures in Section 5.1.
For example, if an application registers the name 'example', the
corresponding well-known URI on 'http://www.example.com/' would be
'http://www.example.com/.well-known/example'.
Registered names MUST conform to the segment-nz production in
[RFC3986].
Note that this specification defines neither how to determine the
authority to use for a particular context, nor the scope of the
metadata discovered by dereferencing the well-known URI; both should
be defined by the application itself.
Typically, a registration will reference a specification that defines
the format and associated media type to be obtained by dereferencing
the well-known URI.
It MAY also contain additional information, such as the syntax of
additional path components, query strings and/or fragment identifiers
to be appended to the well-known URI, or protocol-specific details
(e.g., HTTP [RFC2616] method handling).
Note that this specification does not define a format or media-type
for the resource located at "/.well-known/" and clients should not
expect a resource to exist at that location.
4. Security Considerations
This memo does not specify the scope of applicability of metadata or
policy obtained from a well-known URI, and does not specify how to
discover a well-known URI for a particular application. Individual
applications using this mechanism must define both aspects.
Applications minting new well-known URIs, as well as administrators
deploying them, will need to consider several security-related
issues, including (but not limited to) exposure of sensitive data,
denial-of-service attacks (in addition to normal load issues), server
and client authentication, vulnerability to DNS rebinding attacks,
and attacks where limited access to a server grants the ability to
affect how well-known URIs are served.
5. IANA Considerations
5.1. The Well-Known URI Registry
This document establishes the well-known URI registry.
Well-known URIs are registered on the advice of one or more
Designated Experts (appointed by the IESG or their delegate), with a
Specification Required (using terminology from [RFC5226]). However,
to allow for the allocation of values prior to publication, the
Designated Expert(s) may approve registration once they are satisfied
that such a specification will be published.
Registration requests should be sent to the
wellknown-uri-review@ietf.org mailing list for review and comment,
with an appropriate subject (e.g., "Request for well-known URI:
example").
Before a period of 14 days has passed, the Designated Expert(s) will
either approve or deny the registration request, communicating this
decision both to the review list and to IANA. Denials should include
an explanation and, if applicable, suggestions as to how to make the
request successful. Registration requests that are undetermined for
a period longer than 21 days can be brought to the IESG's attention
(using the iesg@iesg.org mailing list) for resolution.
5.1.1. Registration Template
URI suffix: The name requested for the well-known URI, relative to
"/.well-known/"; e.g., "example".
Change controller: For Standards-Track RFCs, state "IETF". For
others, give the name of the responsible party. Other details
(e.g., postal address, e-mail address, home page URI) may also be
included.
Specification document(s): Reference to the document that specifies
the field, preferably including a URI that can be used to retrieve
a copy of the document. An indication of the relevant sections
may also be included, but is not required.
Related information: Optionally, citations to additional documents
containing further relevant information.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
6.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter,
L., Leach, P., and T. Berners-Lee, "Hypertext Transfer
Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC4918] Dusseault, L., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
[W3C.REC-P3P-20020416]
Marchiori, M., "The Platform for Privacy Preferences 1.0
(P3P1.0) Specification", World Wide Web Consortium
Recommendation REC-P3P-20020416, April 2002,
<http://www.w3.org/TR/2002/ REC-P3P-20020416>.
[W3C.REC-webarch-20041215]
Jacobs, I. and N. Walsh, "Architecture of the World Wide
Web, Volume One", World Wide Web Consortium
Recommendation REC- webarch-20041215, December 2004,
<http:// www.w3.org/TR/2004/REC-webarch-20041215>.
Appendix A. Acknowledgements
We would like to acknowledge the contributions of everyone who
provided feedback and use cases for this document; in particular,
Phil Archer, Dirk Balfanz, Adam Barth, Tim Bray, Brian Eaton, Brad
Fitzpatrick, Joe Gregorio, Paul Hoffman, Barry Leiba, Ashok Malhotra,
Breno de Medeiros, John Panzer, and Drummond Reed. However, they are
not responsible for errors and omissions.
Appendix B. Frequently Asked Questions
1. Aren't well-known locations bad for the Web?
They are, but for various reasons -- both technical and social --
they are commonly used and their use is increasing. This memo
defines a "sandbox" for them, to reduce the risks of collision and
to minimise the impact upon pre-existing URIs on sites.
2. Why /.well-known?
It's short, descriptive, and according to search indices, not
widely used.
3. What impact does this have on existing mechanisms, such as P3P and
robots.txt?
None, until they choose to use this mechanism.
4. Why aren't per-directory well-known locations defined?
Allowing every URI path segment to have a well-known location
(e.g., "/images/.well-known/") would increase the risks of
colliding with a pre-existing URI on a site, and generally these
solutions are found not to scale well, because they're too
"chatty".
Authors' Addresses
Mark Nottingham
EMail: mnot@mnot.net
URI: http://www.mnot.net/
Eran Hammer-Lahav
EMail: eran@hueniverse.com
URI: http://hueniverse.com/
ISO Plain Language
On this page
§1Why a Reserved Prefix
It is increasingly common for web-based protocols to require the discovery of policy or other information about a host, that is site-wide metadata, before making a request. The Robots Exclusion Protocol, for example, specifies a way for automated processes to obtain permission to access resources, and the Platform for Privacy Preferences tells user agents how to discover a privacy policy beforehand.
There are several ways to reach per-resource metadata, such as HTTP headers or WebDAV's PROPFIND method, but the perceived overhead of those, in terms of client-perceived latency or deployment difficulty, often rules them out in these situations. When that happens, it is common to designate a well-known location for the data so that it can be easily found. The drawback is that this risks collisions, both with other well-known locations and with pre-existing resources on the site.
The RFC addresses that by defining a path prefix for HTTP and HTTPS URIs, /.well-known/. Future specifications that need a resource for site-wide metadata can register their use, which avoids collisions and minimises the impingement on a site's own URI space.
The RFC's appendix answers the objections it anticipates. Well-known locations are, it admits, bad for the web in various reasons, both technical and social, but they are commonly used and their use is increasing, and this memo defines a sandbox for them that reduces the risk of collision. The prefix was chosen because it is short, descriptive and, according to search indices, not widely used. The impact on existing mechanisms such as P3P and robots.txt is nil until they choose to use the mechanism. Per-directory well-known locations were not defined because allowing every path segment to have one would raise the risk of colliding with an existing URI and would not scale well, being too chatty.
Without a shared convention, each protocol picks its own path, and a site can end up with a page that collides with the convention. Reserving one prefix and keeping a registry of the names inside it means a collision has to be somebody's mistake rather than everyone's.
§1.1Appropriate Use of Well-Known URIs
There are a number of ways applications could use well-known URIs. However, in keeping with the Architecture of the World Wide Web, well-known URIs are not intended for general information retrieval or for the establishment of large URI namespaces on the web. They are designed to facilitate the discovery of information on a site when it is not practical to use other mechanisms: for example, when discovering policy that has to be evaluated before a resource is accessed, or when using multiple round trips is judged detrimental to performance.
The well-known URI space was created with the expectation that it would be used to make site-wide policy information and other metadata available directly, if sufficiently concise, or to provide references to other URIs that provide such metadata.
This is for one small file per site, fetched once before something else happens, not for a filing system or a place to put everything a site publishes.
- For site-wide metadataTo make policy or other information about a site discoverable before a request is made, when other mechanisms would be too slow or too hard to deploy.
- Not for general retrievalWell-known URIs are not intended for general information retrieval or for establishing large URI namespaces on the web.
- A sandbox for collisionsReserving one prefix and registering the names inside it keeps two specifications from choosing the same path and keeps them from landing on an existing page.
- Nothing at the prefix itselfNo format or media type is defined for a resource at /.well-known/, and clients should not expect one to exist there.
§3What Counts as a Well-Known URI
A well-known URI is a URI whose path component begins with the characters /.well-known/, and whose scheme is HTTP, HTTPS, or another scheme that has explicitly been specified to use well-known URIs. Applications that wish to mint new well-known URIs MUST register them.
The example the RFC gives: if an application registers the name example, the corresponding well-known URI on http://www.example.com/ is http://www.example.com/.well-known/example. Registered names MUST conform to the segment-nz production of RFC 3986.
The RFC is careful about what it does not settle. It defines neither how to determine the authority to use for a particular context, nor the scope of the metadata discovered by dereferencing the well-known URI; both should be defined by the application itself.
Typically a registration will reference a specification that defines the format and the associated media type to be obtained by dereferencing the well-known URI. It may also contain additional information, such as the syntax of additional path components, query strings or fragment identifiers to be appended to the well-known URI, or protocol-specific details such as how HTTP methods are handled.
One thing the RFC says plainly: it does not define a format or media type for the resource located at /.well-known/ itself, and clients should not expect a resource to exist at that location.
A well-known URI is a path that starts with the reserved prefix, on a web address or on a scheme that has been declared to use the same convention. The name after the prefix has to be registered, and the registration normally points at the document that says what the file contains.
http://www.example.com/.well-known/example§5The Registry and How to Register
The RFC establishes the well-known URI registry. Well-known URIs are registered on the advice of one or more Designated Experts appointed by the IESG or its delegate, with a Specification Required. To allow values to be allocated before publication, the Designated Experts may approve a registration once they are satisfied that such a specification will be published.
Registration requests should be sent to the wellknown-uri-review@ietf.org mailing list for review and comment, with an appropriate subject. Before a period of 14 days has passed, the Designated Experts will either approve or deny the request, communicating the decision both to the review list and to IANA. Denials should include an explanation and, where it applies, suggestions on how to make the request successful. Requests that stay undetermined for longer than 21 days can be brought to the IESG's attention for resolution.
The registration template has four fields. The URI suffix is the name requested for the well-known URI, relative to /.well-known/, for example example. The change controller is IETF for Standards Track RFCs and otherwise the name of the responsible party, with other details such as a postal address, e-mail address or home page URI optionally included. The specification document field references the document that specifies the field, preferably including a URI that can be used to retrieve a copy, with an indication of the relevant sections optional. The related information field optionally carries citations to additional documents with further relevant information.
Registering a name needs a specification to point at, a named person or group in charge of it, and a note of any other documents that help. A reviewer checks the request and approves or refuses it within a fortnight.
| Field | What it holds |
|---|---|
| URI suffix | The name requested for the well-known URI, relative to /.well-known/, for example example |
| Change controller | IETF for Standards Track RFCs, otherwise the name of the responsible party |
| Specification document(s) | Reference to the document that specifies the field, preferably with a URI that can retrieve it |
| Related information | Optional citations to additional documents containing further relevant information |
§4Security Considerations
The RFC does not specify the scope of applicability of metadata or policy obtained from a well-known URI, and does not specify how to discover a well-known URI for a particular application. Individual applications using the mechanism must define both aspects.
Applications minting new well-known URIs, and administrators deploying them, need to consider several security-related issues, including, but not limited to, exposure of sensitive data, denial-of-service attacks in addition to normal load issues, server and client authentication, vulnerability to DNS rebinding attacks, and attacks in which limited access to a server grants the ability to affect how well-known URIs are served.
Publishing a file at a predictable address has its own risks: the file might reveal something, or be a target for flooding, and the scheme as a whole says nothing about who is allowed to read it. Each application that uses it has to decide those things for itself.