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Versions: 00 01 02 03 04 05 06 07 08 09 10 11            Standards Track
Network Working Group                                      M. Nottingham
Internet-Draft                                             April 5, 2018
Obsoletes: 5785 (if approved)
Intended status: Standards Track
Expires: October 7, 2018

        Defining 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
   https://github.com/mnot/I-D/labels/rfc5785bis [1].

   The most recent (often, unpublished) draft is at
   https://mnot.github.io/I-D/rfc5785bis/ [2].

   Recent changes are listed at https://github.com/mnot/I-D/commits/gh-
   pages/rfc5785bis [3].

   See also the draft's current status in the IETF datatracker, at
   https://datatracker.ietf.org/doc/draft-nottingham-rfc5785bis/ [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 https://datatracker.ietf.org/drafts/current/.

   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 October 7, 2018.

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

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
   (http://www.robotstxt.org/ [5]) 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 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

   When this happens, one solution is designating 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).

   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.

   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", "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

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

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   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 "example.com-metadata", not "metadata".

   At a minimum, a registration will reference a specification that
   defines the format and associated media type 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.

   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 only valid when rooted in the top of the path's
   hierarchy; they MUST NOT be used in other parts of the path.  For
   example, "/.well-known/example" is a valid use, but "/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
   "https://www.iana.org/assignments/well-known-uris/".  Registration
   requests can be made by following the instructions located there or
   by sending an email to the "wellknown-uri-review@ietf.org" 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".

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   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.

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

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

   Registrations MUST reference a freely available, stable

   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,
   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 the Web

   In particular, 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 is potentially 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

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   Storage [W3C.REC-webstorage-20160419] and many capabilities.
   Applications defining well-known locations should not assume that
   they have sole access to these mechanisms.

   Applications defining well-known URIs should not assume or require
   that they are the only application using the origin, since this is a
   common deployment pattern; instead, they should use appropriate
   mechanisms to mitigate the risks of co-existing with Web
   applications, such as (but not limited to):

   o  Using Strict Transport Security [RFC6797] to assure that HTTPS is

   o  Using Content-Security-Policy [W3C.WD-CSP3-20160913] to constrain
      the capabilities of content, thereby mitigating Cross-Site
      Scripting attacks (which are possible if client-provided data is
      exposed in any part of a response in the application)

   o  Using X-Frame-Options [RFC7034] to prevent content from being
      included in a HTML frame from another origin, thereby enabling

   o  Using Referrer-Policy [W3C.CR-referrer-policy-20170126] to prevent
      sensitive data in URLs from being leaked in the Referer request

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

4.2.  Scoping Applications

   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; 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
   "https://example.com/.well-known/example" to apply policy to
   "https://department.example.com", "https://www.example.com" or even
   "https://www.example.com:8000" assumes a relationship between hosts
   where there may be none, or there may be conflicting motivations.

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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.

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: * Conformance to the requirements in Section 3 * The
   availability and stability of the specifying document * The security
   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.

   IANA should replace all references to RFC 5988 in that registry have
   been replaced with references to this document.

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,

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

   [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

   [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,

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

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797,
              DOI 10.17487/RFC6797, November 2012,

   [RFC7034]  Ross, D. and T. Gondrom, "HTTP Header Field X-Frame-
              Options", RFC 7034, DOI 10.17487/RFC7034, October 2013,

   [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,

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

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

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

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

6.3.  URIs

   [1] https://github.com/mnot/I-D/labels/rfc5785bis

   [2] https://mnot.github.io/I-D/rfc5785bis/

   [3] https://github.com/mnot/I-D/commits/gh-pages/rfc5785bis

   [4] https://datatracker.ietf.org/doc/draft-nottingham-rfc5785bis/

   [5] http://www.robotstxt.org/

Appendix A.  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 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.

   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.

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   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

Appendix B.  Changes from RFC5785

   o  Allow non-Web well-known locations

   o  Adjust IANA instructions

   o  Update references

   o  Various other clarifications

Author's Address

   Mark Nottingham

   Email: mnot@mnot.net
   URI:   https://www.mnot.net/

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