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Well-Known Uniform Resource Identifiers (URIs)

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8615.
Author Mark Nottingham
Last updated 2019-02-21 (Latest revision 2018-10-03)
RFC stream Internet Engineering Task Force (IETF)
Stream WG state (None)
Document shepherd Martin Thomson
Shepherd write-up Show Last changed 2018-09-05
IESG IESG state Became RFC 8615 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Needs a YES. Needs 9 more YES or NO OBJECTION positions to pass.
Responsible AD Alexey Melnikov
Send notices to Martin Thomson <>
IANA IANA review state IANA OK - Actions Needed
Network Working Group                                      M. Nottingham
Internet-Draft                                           October 4, 2018
Obsoletes: 5785, 8307 (if approved)
Updates: 7230, 6455 (if approved)
Intended status: Standards Track
Expires: April 7, 2019

             Well-Known Uniform Resource Identifiers (URIs)


   This memo defines a path prefix for "well-known locations", "/.well-
   known/", in selected Uniform Resource Identifier (URI) schemes.

Note to Readers

   _RFC EDITOR: please remove this section before publication_

   This draft is a proposed revision of RFC5875.

   The issues list for this draft can be found at [1].

   The most recent (often, unpublished) draft is at [2].

   Recent changes are listed at
   pages/rfc5785bis [3].

   See also the draft's current status in the IETF datatracker, at [4].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on April 7, 2019.

Copyright Notice

   Copyright (c) 2018 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
   ( 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
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Well-Known URIs . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Registering Well-Known URIs . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
     4.1.  Interaction with Web Browsing . . . . . . . . . . . . . .   6
     4.2.  Scoping Applications  . . . . . . . . . . . . . . . . . .   7
     4.3.  Hidden Capabilities . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  The Well-Known URI Registry . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
     6.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Frequently Asked Questions . . . . . . . . . . . . .  10
   Appendix B.  Changes from RFC5785 . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Some applications on the Web require the discovery of information
   about an origin [RFC6454] (sometimes called "site-wide metadata")
   before making a request.  For example, the Robots Exclusion Protocol
   ( [5]) specifies a way for automated
   processes to obtain permission to access resources; likewise, the
   Platform for Privacy Preferences [P3P] tells user-agents how to
   discover privacy policy before interacting with an origin server.

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   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

   At the same time, it has become more popular to use HTTP as a
   substrate for non-Web protocols.  Sometimes, such protocols need a
   way to locate one or more resources on a given host.

   When this happens, one solution is to designate a "well-known
   location" for data or services related to the origin overall, 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 resources that the origin has created (or wishes
   to create).  Furthermore, defining well-known locations usurp's the
   origin's control over its own URI space [RFC7320].

   To address these uses, 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 metadata can
   register their use to avoid collisions and minimise impingement upon
   origins' URI space.

   Well-known URIs can also be used with other URI schemes, but only
   when those schemes' definitions explicitly allow it.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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" [RFC7230],
   "https" [RFC7230], "ws" [RFC6455], "wss" [RFC6455], 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 '' would be

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   Registered names MUST conform to the segment-nz production in
   [RFC3986].  This means they cannot contain the "/" character.

   Registered names for a specific application SHOULD be correspondingly
   precise; "squatting" on generic terms is not encouraged.  For
   example, if the Example application wants a well-known location for
   metadata, an appropriate registered name might be "example-metadata"
   or even "", not "metadata".

   At a minimum, a registration will reference a specification that
   defines the format and associated media type(s) to be obtained by
   dereferencing the well-known URI, along with the URI scheme(s) that
   the well-known URI can be used with.  If no URI schemes are
   explicitly specified, "http" and "https" are assumed.

   Typically, applications will use the default port for the given
   scheme; if an alternative port is used, it MUST be explicitly
   specified by the application in question.

   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 [RFC7231] method handling).

   Note that this specification defines neither how to determine the
   hostname to use to find the well-known URI for a particular
   application, nor the scope of the metadata discovered by
   dereferencing the well-known URI; both should be defined by the
   application itself.

   Also, 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.

   Well-known URIs are rooted in the top of the path's hierarchy; they
   are not well-known by definition in other parts of the path.  For
   example, "/.well-known/example" is a well-known URI, whereas
   "/foo/.well-known/example" is not.

   See also Section 4 for Security Considerations regarding well-known

3.1.  Registering Well-Known URIs

   The "Well-Known URIs" registry is located at
   "".  Registration
   requests can be made by following the instructions located there or

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   by sending an email to the "" mailing

   Registration requests consist of at least the following information:

   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

   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.

   Status:  One of "permanent" or "provisional".  See guidance below.

   Related information:  Optionally, citations to additional documents
      containing further relevant information.

   General requirements for registered relation types are described in
   Section 3.

   Standards-defined values have a status of "permanent".  Other values
   can also be registered as permanent, if the Experts find that they
   are in use, in consultation with the community.  Other values should
   be registered as "provisional".

   Provisional entries can be removed by the Experts if - in
   consultation with the community - the Experts find that they are not
   in use.  The Experts can change a provisional entry's status to
   permanent at any time.

   Note that well-known URIs can be registered by third parties
   (including the expert(s)), if the expert(s) determines that an
   unregistered well-known URI is widely deployed and not likely to be
   registered in a timely manner otherwise.  Such registrations still
   are subject to the requirements defined, including the need to
   reference a specification.

4.  Security Considerations

   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,

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   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.

4.1.  Interaction with Web Browsing

   Applications using well-known URIs for "http" or "https" URLs need to
   be aware that well-known resources will be accessible to Web
   browsers, and therefore are able to be manipulated by content
   obtained from other parts of that origin.  If an attacker is able to
   inject content (e.g., through a Cross-Site Scripting vulnerability),
   they will be able to make potentially arbitrary requests to the well-
   known resource.

   HTTP and HTTPS also use origins as a security boundary for many other
   mechanisms, including (but not limited to) Cookies [RFC6265], Web
   Storage [WEBSTORAGE] and many capabilities.

   Applications defining well-known locations should not assume that
   they have sole access to these mechanisms, or that they are the only
   application using the origin.  Depending on the nature of the
   application, mitigations can include:

   o  Encrypting sensitive information

   o  Allowing flexibility in the use of identifiers (e.g., Cookie
      names) to avoid collisions with other applications

   o  Using the 'HttpOnly' flag on Cookies to assure that cookies are
      not exposed to browser scripting languages [RFC6265]

   o  Using the 'Path' parameter on Cookies to assure that they are not
      available to other parts of the origin [RFC6265]

   o  Using X-Content-Type-Options: nosniff [FETCH] to assure that
      content under attacker control can't be coaxed into a form that is
      interpreted as active content by a Web browser

   Other good practices include:

   o  Using an application-specific media type in the Content-Type
      header, and requiring clients to fail if it is not used

   o  Using Content-Security-Policy [CSP] to constrain the capabilities
      of active content (such as HTML [HTML5]), thereby mitigating
      Cross-Site Scripting attacks

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   o  Using Referrer-Policy [REFERRER-POLICY] to prevent sensitive data
      in URLs from being leaked in the Referer request header

   o  Avoiding use of compression on any sensitive information (e.g.,
      authentication tokens, passwords), as the scripting environment
      offered by Web browsers allows an attacker to repeatedly probe the
      compression space; if the attacker has access to the path of the
      communication, they can use this capability to recover that

4.2.  Scoping Applications

   This memo does not specify the scope of applicability for the
   information 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; if this is not specified, security issues can arise from
   implementation deviations and confusion about boundaries between

   Applying metadata discovered in a well-known URI to resources other
   than those co-located on the same origin risks administrative as well
   as security issues.  For example, allowing
   "" to apply policy to
   "", "" or even
   "" assumes a relationship between hosts
   where there might be none, giving control to a potential attacker.

   Likewise, specifying that a well-known URI on a particular hostname
   is to be used to bootstrap a protocol can cause a large number of
   undesired requests.  For example, if a well-known HTTPS URI is used
   to find policy about a separate service such as e-mail, it can result
   in a flood of requests to Web servers, even if they don't implement
   the well-known URI.  Such undesired requests can resemble a denial-
   of-services attack.

4.3.  Hidden Capabilities

   Applications using well-known locations should consider that some
   server administrators might be unaware of its existence (especially
   on operating systems that hide directories whose names begin with
   ".").  This means that if an attacker has write access to the .well-
   known directory, they would be able to control its contents, possibly
   without the administrator realising it.

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5.  IANA Considerations

5.1.  The Well-Known URI Registry

   This specification updates the registration procedures for the "Well-
   Known URI" registry, first defined in [RFC5785]; see Section 3.1.

   Well-known URIs are registered on the advice of one or more experts
   (appointed by the IESG or their delegate), with a Specification
   Required (using terminology from [RFC8126]).

   The Experts' primary considerations in evaluating registration
   requests are:

   o  Conformance to the requirements in Section 3

   o  The availability and stability of the specifying document

   o  The considerations outlined in Section 4

   IANA will direct any incoming requests regarding the registry to this
   document and, if defined, the processes established by the expert(s);
   typically, this will mean referring them to the registry Web page.

   Upon publication, IANA should:

   o  Replace all references to RFC 5988 in that registry have been
      replaced with references to this document.

   o  Update the status of all existing registrations to "permanent".

6.  References

6.1.  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,

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011,

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   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

6.2.  Informative References

   [CSP]      West, M., "Content Security Policy Level 3", World Wide
              Web Consortium WD WD-CSP3-20160913, September 2016,

   [FETCH]    WHATWG, "Fetch - Living Standard", n.d.,

   [HTML5]    WHATWG, "HTML - Living Standard", n.d.,

   [P3P]      Marchiori, M., "The Platform for Privacy Preferences 1.0
              (P3P1.0) Specification", World Wide Web Consortium
              Recommendation REC-P3P-20020416, April 2002,

              Eisinger, J. and E. Stark, "Referrer Policy", World Wide
              Web Consortium CR CR-referrer-policy-20170126, January

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918,
              DOI 10.17487/RFC4918, June 2007,

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,

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   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,

   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 7320, DOI 10.17487/RFC7320, July 2014,

              Hickson, I., "Web Storage (Second Edition)", World Wide
              Web Consortium Recommendation REC-webstorage-20160419,
              April 2016,

6.3.  URIs






Appendix A.  Frequently Asked Questions

   Aren't well-known locations bad for the Web?  They are, but for
      various reasons - both technical and social - they are sometimes
      necessary.  This memo defines a "sandbox" for them, to reduce the
      risks of collision and to minimise the impact upon pre-existing
      URIs on sites.

   Why /.well-known?  It's short, descriptive, and according to search
      indices, not widely used.

   What impact does this have on existing mechanisms, such as P3P and
      None, until they choose to use this mechanism.

   Why aren't per-directory well-known locations defined?  Allowing
      every URI path segment to have a well-known location (e.g.,

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      "/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".

Appendix B.  Changes from RFC5785

   o  Allow non-Web well-known locations

   o  Adjust IANA instructions

   o  Update references

   o  Various other clarifications

   o  Add "ws" and "wss" schemes

Author's Address

   Mark Nottingham


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