WebFinger
draft-ietf-appsawg-webfinger-02

Network Working Group                                      Paul E. Jones
Internet Draft                                         Gonzalo Salgueiro
Intended status: Standards Track                           Cisco Systems
Expires: April 22, 2013                                     Joseph Smarr
                                                                  Google
                                                        October 22, 2012

                                 WebFinger
                    draft-ietf-appsawg-webfinger-02.txt

Abstract

   This specification defines the WebFinger protocol.  WebFinger may be
   used to discover information about people on the Internet, such as a
   person's personal profile address, identity service, telephone
   number, or preferred avatar.  WebFinger may also be used to discover
   information about objects on the network, such as the amount of toner
   in a printer or the physical location of a server.

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 http://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."

   This Internet-Draft will expire on April 22, 2013.

Copyright Notice

   Copyright (c) 2012 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

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   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. Terminology....................................................3
   3. Overview.......................................................3
   4. Example Uses of WebFinger......................................4
      4.1. Locating a User's Blog....................................4
      4.2. Simplifying the Login Process.............................7
      4.3. Retrieving Device Information.............................8
   5. WebFinger Protocol.............................................8
      5.1. Performing a WebFinger Query..............................9
      5.2. The Web Host Metadata "resource" Parameter...............10
      5.3. The Web Host Metadata "rel" Parameter....................12
      5.4. WebFinger and URIs.......................................14
   6. The "acct" Link Relation......................................14
      6.1. Purpose for the "acct" Link Relation.....................14
      6.2. Example Message Exchange Using the "acct" Link Relation..15
   7. Cross-Origin Resource Sharing (CORS)..........................16
   8. Controlling Access to Information.............................17
   9. Hosted and Distributed WebFinger Services.....................17
      9.1. Hosting the Entire Domain................................17
      9.2. Distributed WebFinger Services...........................18
   10. Web Host Metadata Interoperability Considerations............20
   11. Security Considerations......................................20
   12. IANA Considerations..........................................21
      12.1. Registration of the "acct" Link Relation Type...........21
   13. Acknowledgments..............................................21
   14. References...................................................21
      14.1. Normative References....................................21
      14.2. Informative References..................................22
   APPENDIX A: XRD Usage (Non-normative)............................24
      A.1. How XRD Documents are Requested via WebFinger............24
      A.2. WebFinger Example using XRDs.............................24
      A.3. Security Considerations Related to XRDs..................25
   Author's Addresses...............................................26

1. Introduction

   There is a utility found on UNIX systems called "finger" [14] that
   allows a person to access information about another person or entity
   that has a UNIX account.  The information queried might be on the
   same computer or a computer anywhere in the world.  What is returned
   via "finger" is simply a plain text file that contains unstructured
   information provided by the queried user, stored in a file named
   .plan in the user's home directory.

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   WebFinger borrows the concept of the legacy finger protocol, but
   introduces a very different approach to sharing information.  Rather
   than return a simple unstructured text file, Webfinger uses
   structured documents that contain link relations.  These link
   relations point to information and might return properties related to
   information a user or entity on the Internet wishes to expose.  For a
   person, the kinds of information that might be exposed include a
   personal profile address, identity service, telephone number, or
   preferred avatar.  WebFinger may also be used to discover information
   about objects on the network, such as the amount of toner in a
   printer or the physical location of a server.

   Information returned via WebFinger might be for direct human
   consumption (e.g., another user's phone number) or it might be used
   by systems to help carry out some operation (e.g., facilitate logging
   into a web site by determining a user's identity service).

2. Terminology

   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 [1].

   WebFinger makes heavy use of "Link Relations".  Briefly, a Link
   Relation is an attribute and value pair used on the Internet wherein
   the attribute identifies the type of link to which the associated
   value refers.  In Hypertext Transfer Protocol (HTTP) [2] and Web
   Linking [4], the attribute is a "rel" and the value is an "href".

3. Overview

   WebFinger enables the discovery of information about accounts,
   devices, and other entities that are associated with a host.
   Discover involves two distinct steps that may be optimized as a
   single step, as will be explained later.  The first step is to query
   the host to find out how to discover information about accounts,
   devices, and other entities associated with that host.  The second
   step is to query explicitly for a specific resource (e.g., user
   account) to discover a set of link relations that point to resource-
   specific information about the entity being queried.

   This protocol makes heavy use of well-known URIs as defined in RFC
   5785 [3] and "Link Relations" as defined in RFC 5988 [4].  Further,
   the protocol builds on RFC 6415 [11], which provides the foundation
   for the procedures described in this document.

   Briefly, a link is a typed connection between two web resources that
   are identified by Internationalized Resource Identifiers (IRIs) [13];
   this connection consists of a context IRI, a link relation type, a

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   target IRI, and optionally some target attributes, resulting in
   statements of the form "{context IRI} has a {relation type} resource
   at {target IRI}, which has {target attributes}".  When used in the
   Link HTTP header, the context IRI is the IRI of the requested
   resource, the relation type is the value of the "rel" parameter, the
   target IRI is URI-Reference contained in the Link header, and the
   target attributes are the parameters such as "hreflang", "media",
   "title", "title*", "type", and any other link-extension parameters.

   The framework for WebFinger consists of several building blocks:

   1. To query the host, one requests a web host metadata document
      located at the well-known URI /.well-known/host-meta or /.well-
      known/host-meta.json (referred to as the host-meta resources) at
      the host.
   2. The web server at the host returns a JavaScript Object Notation
      (JSON) [5] Resource Descriptor (JRD) or an Extensible Resource
      Descriptor (XRD) [10] document, including a Link-based Resource
      Descriptor Document (LRDD) link relation.
   3. To discover information about accounts, devices, or other entities
      associated with the host, one requests the actual Link-based
      Resource Descriptor Document associated with a particular URI at
      the host (e.g., an "acct" URI, "http" URI, or "mailto" URI).
   4. The web server at the host returns a JRD or XRD document for the
      requested URI, which includes link relations pointing to resources
      that contain more detailed information about the entity.

   This model is illustrated in the examples in Section 4, then
   described more formally in Section 5.  Steps 2 and 3 above can be
   accomplished simultaneously by utilizing the "resource" parameter
   defined in Section 5.2.

4. Example Uses of WebFinger

   In this section, we describe just a few sample uses for WebFinger and
   show what the protocol looks like.  This is not an exhaustive list of
   possible uses and the entire section should be considered non-
   normative.  The list of potential use cases is virtually unlimited
   since a user can share any kind of machine-consumable information via
   WebFinger.

   All of the following examples utilize JRDs, as that is the only
   mandatory format required to be supported by WebFinger servers.  For
   completeness, an example utilizing XRDs is presented in Appendix A.

4.1. Locating a User's Blog

   Assume you receive an email from Bob and he refers to something he
   posted on his blog, but you do not know where Bob's blog is located.

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   It would be simple to discover the address of Bob's blog if he makes
   that information available via WebFinger.

   Let's assume your email client discovers that blog automatically for
   you.  After receiving the message from Bob (bob@example.com), your
   email client performs the following steps behind the scenes.

   First, your email client tries to get the host metadata information
   for the host example.com.  It does this by issuing the following
   HTTPS query to example.com:

     GET /.well-known/host-meta.json HTTP/1.1
     Host: example.com

   The server replies with a JRD document:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "links" :
       [
         {
           "rel" : "lrdd",
           "type" : "application/json",
           "template" : "https://example.com/lrdd/?f=json&uri={uri}"
         }
       ]
     }

   The client then processes the received JRD in accordance with the Web
   Host Metadata procedures.  The client will see the LRDD link relation
   and issue a query with the user's account URI [6] or other URI that
   serves as an alias for the account.  (The account URI is discussed in
   Section 4.2.)  The query might look like this:

     GET /lrdd/?f=json&uri=acct%3Abob%40example.com HTTP/1.1
     Host: example.com

   The server might then respond with a message like this:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "expires" : "2012-10-12T20:56:11Z",
       "subject" : "acct:bob@example.com",

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       "aliases" :
       [
         "http://www.example.com/~bob/"
       ],
       "links" :
       [
         {
           "rel" : "http://webfinger.net/rel/avatar",
           "href" : "http://www.example.com/~bob/bob.jpg"
         },
         {
           "rel" : "http://webfinger.net/rel/profile-page",
           "href" : "http://www.example.com/~bob/"
         },
         {
           "rel" : "http://packetizer.com/rel/blog",
           "href" : "http://blogs.example.com/bob/"
         },
         {
           "rel" : "vcard",
           "href" : "http://www.example.com/~bob/bob.vcf"
         }
       ]
     }

   The email client might take note of the "blog" link relation in the
   above JRD document that refers to Bob's blog.  This URL would then be
   presented to you so that you could then visit his blog.  The email
   client might also note that Bob has published an avatar link relation
   and use that picture to represent Bob inside the email client.
   Lastly, the client might consider the vcard [16] link relation in
   order to update contact information for Bob.

   Note in the above example that an alias is provided that can also be
   used to return information about the user's account.  Had the "http:"
   URI shown as an alias been used to query for information about Bob,
   the query would have appeared as:

     GET /lrdd/?uri=http%3A%2F%2Fwww.example.com%2F~bob%2F HTTP/1.1
     Host: example.com

   The response would have been substantially the same, with the subject
   and alias information changed as necessary.  Other information, such
   as the expiration time might also change, but the set of link
   relations and properties would be the same with either response.

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4.2. Simplifying the Login Process

   OpenID (http://www.openid.net) is great for allowing users to log
   into a web site, though one criticism is that it is challenging for
   users to remember the URI they are assigned.  WebFinger can help
   address this issue by allowing users to use user@domain-style
   addresses.  Using a user's account URI, a web site can perform a
   query to discover the associated OpenID identifier for a user.

   Let's assume Carol is trying to use OpenID to log into a blog.  The
   blog server might issue the following query to discover the OpenID
   identity provider URL for Carol and to get Carol's avatar.  In this
   example, we utilize the "rel" and "resource" parameters as described
   in sections 5.2 and 5.3:

     GET /.well-known/host-meta.json?\
         rel=avatar%20\
         http%3A%3F%3Fspecs.openid.net%3Fauth%3F2.0%3Fprovider&\
         resource=acct%3Acarol%40example.com HTTP/1.1
     Host: example.com

   The server might return a response like this:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "subject" : "acct:carol@example.com",
       "links" :
       [
         {
           "rel" : "http://webfinger.net/rel/avatar",
           "href" : "http://example.com/~alice/alice.jpg"
         },
         {
           "rel" : "http://specs.openid.net/auth/2.0/provider",
           "href" : "https://openid.example.com/carol"
         }
       ]
     }

   At this point, the blog server knows that Carol's OpenID identifier
   is https://openid.example.com/carol and could then proceed with the
   login process as usual.  Her avatar can also be displayed for the
   benefit of other users on the blog.

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4.3. Retrieving Device Information

   While the examples thus far have been focused on information about
   humans, WebFinger does not limit queries to only those that use the
   account URI scheme.  Any URI scheme that contains host information
   MAY be used with WebFinger.  Let's suppose there are devices on the
   network like printers and you would like to check the current toner
   level for a particular printer identified via the URI like
   device:p1.example.com.  While the "device" URI scheme is not
   presently specified, we use it here only for illustrative purposes.

   Following the procedures similar to those above, a query may be
   issued to get link relations specific to this URI like this:

     GET /.well-known/host-meta.json?resource=\
                       device%3Ap1.example.com HTTP/1.1
     Host: example.com

   The link relations that are returned may be quite different than
   those for user accounts.  Perhaps we may see a response like this:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "subject" : "device:p1.example.com",
       "links" :
       [
         {
           "rel" : "tipsi",
           "href" : "http://192.168.1.5/npap/"
         }
       ]
     }

   While this example is entirely fictitious, you can imagine that
   perhaps the Transport Independent, Printer/System Interface [18] may
   be enhanced with a web interface that allows a device that
   understands the TIP/SI web interface specification to query the
   printer for toner levels.

5. WebFinger Protocol

   WebFinger does not actually introduce a new protocol, per se.
   Rather, it builds upon the existing Web Host Metadata specification
   and leverages the Cross-Origin Resource Sharing (CORS) [9]
   specification.

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   While WebFinger strives to maintain backward-compatibility with RFC
   6415, this specification introduces a fundamental change in
   requirements.  Specifically, support for server-side production of
   JSON Resource Descriptor (JRD) documents is mandatory and support for
   server-side production Extensible Resource Descriptor (XRD) documents
   is optional.  Please refer to Section 10 for interoperability
   considerations.

5.1. Performing a WebFinger Query

   The first step a client performs in executing a WebFinger query is to
   query for the host metadata using HTTPS or HTTP.  The procedures are
   defined in the Web Host Metadata specification.  It is strongly
   RECOMMENDED that WebFinger servers return content using secure
   (HTTPS) connections.  Clients MUST first attempt queries using HTTPS
   before attempting a query using HTTP.

   WebFinger clients MUST locate the LRDD link relation and perform a
   query for that link relation, if present.  All other link templates
   found must be processed to form a complete resource descriptor.  The
   processing rules in Section 4.2 of RFC 6415 MUST be followed.

   WebFinger servers MAY accept requests for both JRD and XRD documents,
   but MUST support requests for JRD documents.  For interoperability
   with RFC 6415 implementations, the default representation returned by
   a server via the resource at /.well-known/host-meta MUST be an XRD
   document if XRD is supported by the server and a JRD document is not
   explicitly requested by the client.  The default format returned via
   the resource /.well-known/host-meta.json MUST be a JRD document.

   As per RFC 6415, a JRD document MUST be returned by the WebFinger
   server if the client explicitly requests it by querying /.well-
   known/host-meta.json or by querying /.well-known/host-meta and
   including an "Accept" header in the HTTP request with a type of
   "application/json" [5].  Additionally, the server MUST return a JRD
   document if it does not support production of XRD documents (or any
   other format requested by the client).  Servers MUST indicate the
   type of document returned using the "Content-Type" header in the HTTP
   response.

   To avoid the possibility of receiving the wrong document format,
   WebFinger clients SHOULD submit queries to the server via the /.well-
   known/host-meta.json resource.

   If the client requests a JRD document when querying for host
   metadata, the WebFinger server MUST assume that the client will want
   a JRD document when querying the LRDD resource.  Thus when the
   WebFinger server returns a JRD document containing host metadata that
   contains an LRDD link relation, it MUST include a URI for the LRDD

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   resource(s) that will return a JRD document.  Likewise, if a client
   requests an XRD document when querying the host metadata resource,
   the server MUST, unless unable due to external factors, return LRDD
   link relations that would return XRD documents.

   It is important to note that unless the "resource" parameter is used
   as per section 5.2, it is the responsibility of the client to process
   each of the LRDD link relations as per Section 4.2 of RFC 6415 if a
   server returns multiple LRDD link relations.  Multiple LRDD link
   relations in a server response do not represent alternative URIs for
   the same LRDD document.

   If the client queries the LRDD resource and provides a URI for which
   the server has no information, the server MUST return a 404 status
   code.  Likewise, any query to a URI in the resource descriptor that
   is unknown to the server MUST result in the server returning a 404
   status code.

   WebFinger servers MAY include cache validators in a response to
   enable conditional requests by clients and/or expiration times as per
   RFC 2616 section 13.

5.2. The Web Host Metadata "resource" Parameter

   In addition to the traditional processing logic for processing host
   metadata information, WebFinger defines the "resource" parameter for
   querying for host metadata and returning all of the link relations
   from LRDD and other resource-specific link templates in a single
   response.  This parameter essentially pushes the work to the server
   to form a complete resource descriptor for the specified resource.

   WebFinger servers compliant with this specification MUST support for
   the "resource" parameter.  Note that an RFC 6415-compliant server
   might not implement the "resource" parameter, though the server would
   respond to queries from the client as described in RFC 6415.  Thus,
   WebFinger clients MUST check the server response to ensure that the
   "resource" parameter is supported as explained below.

   To utilize the host-meta "resource" parameter, a WebFinger client
   issues a request to /.well-known/host-meta.json (RECOMMENDED) or
   /.well-known/host-meta as usual, but then appends a "resource"
   parameter as shown in this example:

     GET /.well-known/host-meta.json?resource=\
                            acct%3Abob%40example.com HTTP/1.1
     Host: example.com

   When processing this request, the WebFinger server MUST

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       *  Return a 404 status code if the URI provided in the resource
          parameter is unknown to the server; and

       *  Set the "Subject" returned in the response to the value of the
          "resource" parameter if the URI provided in the resource
          parameter is known to the server; and

       *  Collect and expand all resource-specific link relations,
          including those returned by querying for any LRDD link
          relations, discard any host-wide link relations, and return a
          complete resource descriptor following the processing rules in
          Section 4.2 of RFC 6415; and

   It is not the responsibility of the WebFinger server to verify, for
   example, that a URI pointing to a person's avatar is a valid URI.  If
   the WebFinger server needs to query an LRDD resource to collect
   additional resource-specific information, any errors (e.g., 500 or
   404) MUST be ignored by the server.  When a request for an LRDD
   fails, the server MUST NOT attempt to augment missing resource
   information or return a "template" type link relation to a client
   that utilizes the "resource" parameter.  Note that a WebFinger server
   might be implemented such that all LRDD resource-specific information
   can be resolved without issuing HTTP requests.  How a WebFinger
   server collects and expands such resource-specific link relations is
   an implementation matter.

   The WebFinger client MUST verify support for the "resource" parameter
   by checking the value of the Subject returned in the response.  If
   the Subject matches the value of the "resource" parameter, then the
   "resource" parameter is supported by the server.  The Subject would
   be absent if the "resource" parameter is not supported.

   For illustrative purposes, the following is an example usage of the
   "resource" parameter that aligns with the example in Section 1.1.1 of
   RFC 6415.  The WebFinger client would issue this request:

     GET /.well-known/host-meta.json?resource=\
                            http%3A%2F%2Fexample.com%2Fxy HTTP/1.1
     Host: example.com

   Note: The "\" character shown above and used throughout this document
   indicates that the line breaks at this point and continues on the
   next line.  The content of the next line should be concatenated to
   the previous line without any whitespace characters, replacing the
   "\" character.  This is shown only to avoid line wrapping in this
   document.

   The WebFinger server would reply with this response:

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     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "subject" : "http://example.com/xy",
       "properties" :
       {
         "http://spec.example.net/color" : "red"
       },
       "links" :
       [
         {
           "rel" : "hub",
           "href" : "http://example.com/hub"
         },
         {
           "rel" : "hub",
           "href" : "http://example.com/another/hub"
         },
         {
           "rel" : "author",
           "href" : "http://example.com/john"
         },
         {
           "rel" : "author",
           "href" : "http://example.com/author?\
                              q=http%3A%2F%2Fexample.com%2Fxy"
         }
       ]
     }

5.3. The Web Host Metadata "rel" Parameter

   WebFinger also defines the "rel" parameter for use when querying for
   host metadata or resource-specific information.  It is used to return
   a subset of the information that would otherwise be returned without
   the "rel" parameter.  When the "rel" parameter is used, only the link
   relations that match the space-separated list of link relations
   provided via "rel" are included in the list of links returned in the
   resource descriptor.  All other information normally present in a
   resource descriptor is present in the resource descriptor, even when
   "rel" is employed.

   The purpose of the "rel" parameter is to return a subset of
   resource's link relations.  It is not intended to reduce the work
   required of a server to produce a response.  That said, use of the
   parameter might reduce processing requirements on either the client
   or server, and it might also reduce the bandwidth required to convey

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   the partial resource descriptor, especially if there are numerous
   link relation values to convey for a given resource.

   Support for the "rel" parameter is OPTIONAL, but support is
   RECOMMENDED for the host-meta resources and LRDD resources.

   For illustrative purposes, the following is an example usage of the
   "rel" parameter that aligns with the example in Section 1.1.1 of RFC
   6415.  The WebFinger client would issue this request to receive links
   that are of the type "hub" and "copyright":

     GET /.well-known/host-meta.json?resource=\
           http%3A%2F%2Fexample.com%2Fxy&rel=hub%20copyright HTTP/1.1
     Host: example.com

   The WebFinger server would reply with this response:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "subject" : "http://example.com/xy",
       "properties" :
       {
         "http://spec.example.net/color" : "red"
       },
       "links" :
       [
         {
           "rel" : "hub",
           "href" : "http://example.com/hub"
         },
         {
           "rel" : "hub",
           "href" : "http://example.com/another/hub"
         }
       ]
     }

   Note that in this example, the "author" links are removed, though all
   other content is present.  Since there were no "copyright" links,
   none are returned.

   In the event that a client requests links for link relations that are
   not defined for the specified resource, a resource descriptor MUST be
   returned, void of any links.  When a JRD is returned, the "links"
   array MAY be either absent or empty.  The server MUST NOT return a
   404 status code when a particular link relation specified via "rel"

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   is not defined for the resource, as a 404 status code is reserved for
   indicating that the resource itself (e.g., either /.well-known/host-
   meta.json or the resource indicated via the "resource" parameter)
   does not exist.

5.4. WebFinger and URIs

   Requests for both LRDD documents and host metadata can include a
   parameter specifying the URI of an account, device, or other entity
   (for LRDD this is the "uri" parameter as defined by the operative JRD
   or XRD template and for host metadata this is the "resource"
   parameter).  WebFinger itself is agnostic regarding the scheme of
   such a URI: it could be an "acct" URI [7], an "http" or "https" URI,
   a "mailto" URI, or some other scheme.

   For resources associated with a user account at a host, use of the
   "acct" URI scheme is RECOMMENDED, since it explicitly identifies an
   account accessible via WebFinger.  Further, the "acct" URI scheme is
   not associated with other protocols as, by way of example, the
   "mailto" URI scheme is associated with email.  Since not every host
   offers email service, using the "mailto" URI scheme [8] is not ideal
   for identifying user accounts on all hosts.  That said, use of the
   "mailto" URI scheme would be ideal for use with WebFinger to discover
   mail server configuration information for a user, for example.

   A host MAY utilize one or more URIs that serve as aliases for the
   user's account, such as URIs that use the "http" URI scheme [2].  A
   WebFinger server MUST return substantially the same response to both
   an "acct" URI and any alias URI for the account, including the same
   set of link relations and properties.  In addition, the server SHOULD
   include the entire list aliases for the user's account in the JRD or
   XRD returned when querying the LRDD resource or when utilizing the
   "resource" parameter.

6. The "acct" Link Relation

6.1. Purpose for the "acct" Link Relation

   Users of some services might have an "acct" URI that looks
   significantly different from his or her email address, perhaps using
   an entirely different domain name.  It is also possible for a user to
   have multiple accounts that a user wants to have cross-referenced
   from another account.  To address both of these needs, this
   specification defines the "acct" link relation.

   The "acct" link relation allows a resource descriptor to reference
   one or more other user account URIs.  The "acct" link relation is
   intended to allow a client to incorporate additional link relations
   by reference so that it might utilize a more complete set of link

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   relations for a user.  For example, a user acct:bob@example.com might
   wish to allow a client to discover additional information about him
   by including an "acct" link relation with the URI
   acct:bob@example.net.

   Note that the "acct" link relation does not replace the use of
   standard HTTP 3xx response codes to indicate the new temporary or
   permanent location of a user account.  If a user account is moved to
   a different location, then a 3xx response code SHOULD be used.  Also,
   the "acct" link relation does not replace Link-based Resource
   Descriptor Documents (LRDDs).  A WebFinger server might return
   multiple LRDD link relations for a user, each of which perhaps
   containing link relations that are to be merged to form a complete
   resource descriptor.  The "acct" link relation is different in that
   it would refer to an entirely different, separate resource
   descriptor.  Further, only a client would act consider the "acct"
   link relations as it performs queries, not the WebFinger server.

   Since an account may make a reference to one or more different
   accounts, WebFinger clients that support automatic processing of the
   "acct" link relations MUST take steps to avoid loops wherein two
   account URIs, directly or indirectly, refer the client to each other.

   There are no limits on the number of "acct" link relations that might
   be returned in a WebFinger query.

   An "acct" link relation used within the context of a WebFinger query
   for a user's account MUST NOT return "acct" link relations for
   another user.

   Client-side consideration of the "acct" link relation is OPTIONAL and
   WebFinger server MUST NOT assume a client will perform additional
   processing in response to receiving an "acct" link relation.

6.2. Example Message Exchange Using the "acct" Link Relation

   Consider the following non-normative example.

   Suppose Alice receives an email from bob@example.net.  While Bob's
   email identifier might be in the example.net domain, he holds a user
   account in the example.com domain and another account in the
   example.org domain.  His email provider may provide WebFinger
   services, but is unable to serve information from other domains.

   Suppose Alice's client issues the following request:

     GET /.well-known/host-meta.json?resource=\
                            acct%3Abob%40example.net HTTP/1.1
     Host: example.net

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   The response that Alice's client receives back might be:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "subject" : "acct:bob@example.net",
       "links" :
       [
         {
           "rel" : "acct",
           "href" : "acct:bob@example.com"
         },
         {
           "rel" : "acct",
           "href" : "acct:bob@example.org"
         },
         {
           "rel" : "acct",
           "href" : "mailto:bob@example.net"
         }
       ]
     }

   While these link relations provide Alice with very little
   information, Alice's WebFinger client could then perform subsequent
   queries against the URIs acct:bob@example.com, acct:bob@example.org,
   and mailto:bob@example.net in order to get the information Alice is
   seeking.

7. Cross-Origin Resource Sharing (CORS)

   WebFinger is most useful when it is accessible without restrictions
   on the Internet, and that includes web browsers.  Therefore,
   WebFinger servers MUST support Cross-Origin Resource Sharing (CORS)
   [9] when serving content intended for public consumption.
   Specifically, all queries to /.well-known/host-meta.json, /.well-
   known/host-meta, and to any LRDD URIs MUST include the following HTTP
   header in the response:

      Access-Control-Allow-Origin: *

   Enterprise WebFinger servers that wish to restrict access to
   information from external entities SHOULD use a more restrictive
   Access-Control-Allow-Origin header and MAY exclude the header
   entirely.

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8. Controlling Access to Information

   As with all web resources, access to the Host Metadata resource and
   the LRDD resource MAY require authentication.  Further, failure to
   provide required credentials MAY result in the server forbidding
   access or providing a different response than had the client
   authenticated with the server.

   Likewise, a server MAY provide different responses to different
   clients based on other factors, such as whether the client is inside
   or outside a corporate network.  As a concrete example, a query
   performed on the internal corporate network might return link
   relations to employee pictures whereas link relations for employee
   pictures might not be provided to external entities.

   Further, link relations provided in a WebFinger server response MAY
   point to web resources that impose access restrictions.  For example,
   it is possible that the aforementioned corporate server may provide
   both internal and external entities with URIs to employee pictures,
   but further authentication MAY be required in order for the WebFinger
   client to access those picture resources if the request comes from
   outside the corporate network.

   The decisions made with respect to what set of link relations a
   WebFinger server provides to one client versus another and what
   resources require further authentication, as well as the specific
   authentication mechanisms employed, are outside the scope of this
   document.

9. Hosted and Distributed WebFinger Services

9.1. Hosting the Entire Domain

   As with most services provided on the Internet, it is possible for a
   domain owner to utilize "hosted" WebFinger services.  By way of
   example, a domain owner might control most aspects of their domain,
   but use a third-party hosting service email. In the case of email,
   mail servers for a domain are identified by MX records.  An MX record
   points to the mail server to which mail for the domain should be
   delivered.  It does not matter to the sending mail server whether
   those MX records point to a server in the destination domain or a
   different domain.

   Likewise, a domain owner might utilize the services of a third party
   to provide WebFinger services on behalf of its users.  Just as a
   domain owner was required to insert MX records into DNS to allow for
   hosted email serves, the domain owner is required to redirect HTTP(S)
   queries to its domain to allow for hosted WebFinger services.

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   When a query is issued to /.well-known/host-meta.json or /.well-
   known/host-meta, the target domain's web server MUST return a 301,
   302, or 307 response status code that includes a Location header
   pointing to the location of the hosted WebFinger service URL.  The
   WebFinger service URL does not need to point to /.well-known/* on the
   hosting service provider server.  In fact, it should not, as that
   location would be reserved for queries relating to the service
   provider's domain.  WebFinger clients MUST follow all 301, 302, or
   307 redirection requests.

   As an example, let's assume that example.com's WebFinger services are
   hosted by example.net.  Suppose a client issues a query for
   acct:alice@example.com like this:

     GET /.well-known/host-meta.json?
                   resource=acct%3Aalice%40example.com HTTP/1.1
     Host: example.com

   The server might respond with this:

     HTTP/1.1 301 Moved Permanently
     Location: http://wf.example.net/example.org/host-meta.json

   The client should follow the request, re-issuing the request to the
   URL provided in the Location header.

   Note that both of the /.well-known/host-meta.json and /.well-
   known/host-meta resources need to be considered when redirecting
   request to third party service providers.  Those URLs requests SHOULD
   NOT be redirected to the same location and without any
   differentiation, since the default format returned by host-meta.json
   is a JRD and the default format returned by host-meta MAY be XRD.
   Each resource is distinct and should be redirected separately and to
   different service locations or differentiated with a URI parameter.
   Since the "Referer" HTTP header field is not mandatory, service
   providers cannot rely on that header to determine the URL of the
   original request.

9.2. Distributed WebFinger Services

   A domain owner may wish to manage only a part of its WebFinger
   services and WebFinger service providers or the domain owner may wish
   to distribute WebFinger services across a number of WebFinger service
   locations.  The key to enabling this type of distribution is
   placement of resource-specific information in more than one LRDD
   document, each document existing at different locations.

   Assume that the company operating example.com manages its own
   WebFinger services, but also wants to utilize the services of

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   example.org to serve link relations related to some aspects of its
   business.  Suppose a client issued this request:

     GET /.well-known/host-meta.json HTTP/1.1
     Host: example.com

   The server might reply with this JRD document:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "links" :
       [
         {
           "rel" : "lrdd",
           "type" : "application/json",
           "template" : "https://example.com/lrdd/?f=json&uri={uri}"
         },
         {
           "rel" : "lrdd",
           "type" : "application/json",
           "template" : "https://wf.example.org/lrdd/?f=json&uri={uri}"
         }
       ]
     }

   This would indicate to the client that some of the resource-specific
   information is found at example.com and some is found at example.org,
   following those specific URLs.  Observing the rules in Section 4.2 of
   RFC 6415, the client would issue queries to both URLs and construct a
   complete resource descriptor.

   As discussed in Section 5.2, a client may issue a query like this to
   the example.com domain:

     GET /.well-known/host-meta.json?resource=\
                            acct%3Aalice%40example.com HTTP/1.1
     Host: example.com

   In that case, it would be the responsibility of the WebFinger server
   at example.com to query the LRDD URL at example.org and then compose
   a complete descriptor document.  The client that uses the resource
   parameter remains entirely oblivious to the fact that link relation
   information is distributed across multiple servers or domains.

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10. Web Host Metadata Interoperability Considerations

   As noted in Section 3, RFC 6415 required all servers to support the
   production of Extensible Resource Documents (XRDs) and optionally
   support the production of JSON Resource Documents (JRDs). This
   specification reverses that requirement: WebFinger-compliant servers
   MUST support JRD and MAY support XRD documents.

   Given that some servers might implement only RFC 6415 and other
   servers might implement only the minimum required set of features
   defined for WebFinger, all clients should take care to ensure to
   request a resource descriptor in the appropriate format.  If a client
   wishes to receive only JRDs, for example, it SHOULD issue a request
   to /.well-known/host-meta.json, but MAY issue a request to /.well-
   known/host-meta and include the "Accept" header with the type
   "application/json".

   Further, clients MUST ensure that the response returned from the
   server contains the correct format.  RFC 6415-compliant servers might
   return an XRD document, regardless of what is requested by the
   client.

   Lastly, RFC 6415 did not require clients to follow 301, 302, or 307
   redirection requests, but WebFinger clients MUST re-issue requests
   when redirected using any of those HTTP status codes.

11. Security Considerations

   All of the security considerations applicable to Web Host Metadata
   and Cross-Origin Resource Sharing [9] are also applicable to this
   specification.  Of particular importance is the recommended use of
   HTTPS to ensure that information is not modified during transit.
   Clients SHOULD verify that the certificate used on an HTTPS
   connection is valid.

   Service providers and users should be aware that placing information
   on the Internet accessible through WebFinger means that any user can
   access that information.  While WebFinger can be an extremely useful
   tool for allowing quick and easy access to one's avatar, blog, or
   other personal information, users should understand the risks, too.
   If one does not wish to share certain information with the world, do
   not allow that information to be freely accessible through WebFinger.

   The aforementioned word of caution is perhaps worth emphasizing again
   with respect to dynamic information one might wish to share, such as
   the current location of a user.  WebFinger can be a powerful tool
   used to assemble information about a person all in one place, but
   service providers and users should be mindful of the nature of that
   information shared and the fact that it might be available for the

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   entire world to see.  Sharing location information, for example,
   would potentially put a person in danger from any individual who
   might seek to inflict harm on that person.

   The easy access to user information via WebFinger was a design goal
   of the protocol, not a limitation.  If one wishes to limit access to
   information available via WebFinger, such as a WebFinger server for
   use inside a corporate network, the network administrator must take
   measures necessary to limit access from outside the network.  Using
   standard methods for securing web resources, network administrators
   do have the ability to control access to resources that might return
   sensitive information.  Further, WebFinger servers can be employed in
   such a way as to require authentication and prevent disclosure of
   information to unauthorized entities.

12. IANA Considerations

   RFC Editor: Please replace QQQQ in the following two sub-sections
   with a reference to this RFC.

12.1. Registration of the "acct" Link Relation Type

     Relation Name: acct

     Description: A link relation that refers to a user's WebFinger
     account identifier.

     Reference: RFC QQQQ

     Notes:

     Application Data:

13. Acknowledgments

   The authors would like to acknowledge Eran Hammer-Lahav, Blaine Cook,
   Brad Fitzpatrick, Laurent-Walter Goix, Joe Clarke, Mike Jones, and
   Peter Saint-Andre for their invaluable input.

14. References

14.1. Normative References

   [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [2]   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.

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

   [4]   Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [5]   Crockford, D., "The application/json Media Type for
         JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [6]   Berners-Lee, T., Fielding, R., and Masinter, L., "Uniform
         Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
         January 2005.

   [7]   Saint-Andre, P., "The 'acct' URI Scheme", draft-ietf-appsawg-
         acct-uri-00, August 2012.

   [8]   Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto' URI
         Scheme", RFC 6068, October 2010.

   [9]   Van Kesteren, A., "Cross-Origin Resource Sharing", W3C CORS
         http://www.w3.org/TR/cors/, July 2010.

   [10]  Hammer-Lahav, E. and W. Norris, "Extensible Resource Descriptor
         (XRD) Version 1.0", http://docs.oasis-
         open.org/xri/xrd/v1.0/xrd-1.0.html.

   [11]  Hammer-Lahav, E. and Cook, B., "Web Host Metadata", RFC 6415,
         October 2011.

   [12]  American National Standards Institute, "Coded Character Set -
         7-bit American Standard Code for Information Interchange", ANSI
         X3.4, 1986.

   [13]  Duerst, M., "Internationalized Resource Identifiers (IRIs)",
         RFC 3987, January 2005.

14.2. Informative References

   [14]  Zimmerman, D., "The Finger User Information Protocol", RFC
         1288, December 1991.

   [15]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
         Registration Procedures for New URI Schemes", BCP 35, RFC 4395,
         February 2006.

   [16]  Perreault, S., "vCard Format Specification", RFC 6350, August
         2011.

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   [17]  Internet Assigned Numbers Authority (IANA) Registry, "Uniform
         Resource Identifier (URI) Schemes",
         <http://www.iana.org/assignments/uri-schemes.html>.

   [18]  "Transport Independent, Printer/System Interface", IEEE Std
         1284.1-1997, 1997.

   [19]  Hoffman, P., Yergeau, F., "UTF-16, an encoding of ISO 10646",
         RFC 2781, February 2000.

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APPENDIX A: XRD Usage (Non-normative)

A.1. How XRD Documents are Requested via WebFinger

   The framework for using XRD documents with WebFinger is as follows:

     1. WebFinger clients issue request for XRD documents by requesting
        the Web Host Metadata document located at the well-known URI
        /.well-known/host-meta at the host.
     2. The web server at the host returns an XRD document, including a
        Link-based Resource Descriptor Document (LRDD) link relation.
     3. To discover information about accounts, devices, or other
        entities associated with the host, a request is issued for the
        Link-based Resource Descriptor Document(s) associated with a
        particular URI at the host (e.g., an "acct" URI, "http" URI, or
        "mailto" URI).
     4. The web server at the host would return an XRD document about
        the requested URI, which included those resource-specific link
        relations pointing to resources that contain information about
        the entity.
     5. Following the procedures in Section 4.2 of RFC 6415, the client
        would assemble all of the resource-specific link relations from
        the host-meta resource and LRDD resource(s) into a complete
        resource descriptor.

   The LRDD resources return resource descriptor documents of the type
   "application/xrd+xml".

A.2. WebFinger Example using XRDs

   Section 4 introduces examples where JRD documents are returned to
   clients.  For completeness, this section shows an example where a
   client requests an XRD document.

   Recall the example from Section 4.1 where the email client tried to
   retrieve information about Bob to discover the URL for his blog.  If
   the client implemented support for XRD, it tries to get the host
   metadata information for the domain example.com in a similar way.  As
   with the original example, it issues the following HTTPS query to
   example.com:

     GET /.well-known/host-meta HTTP/1.1
     Host: example.com

   The server replies with an XRD document:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/xrd+xml; charset=UTF-8

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     <?xml version="1.0" encoding="UTF-8"?>
     <XRD xmlns="http://docs.oasis-open.org/ns/xri/xrd-1.0">
       <Link rel="lrdd"
             type="application/xrd+xml"
             template="https://example.com/lrdd/?uri={uri}"/>
     </XRD>

   The client then processes the received XRD in accordance with the Web
   Host Metadata procedures.  The client will see the LRDD link relation
   and issue a query with the user's account URI [6] or other URI that
   serves as an alias for the account.  (The account URI is discussed in
   Section 4.2.)  The query might look like this:

     GET /lrdd/?uri=acct%3Abob%40example.com HTTP/1.1
     Host: example.com

   The server might then respond with a message like this:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/xrd+xml; charset=UTF-8

     <?xml version="1.0" encoding="UTF-8"?>
     <XRD xmlns="http://docs.oasis-open.org/ns/xri/xrd-1.0">
       <Expires>2012-10-12T20:56:11Z</Expires>
       <Subject>acct:bob@example.com</Subject>
       <Alias>http://www.example.com/~bob/</Alias>
       <Link rel="http://webfinger.net/rel/avatar"
             href="http://www.example.com/~bob/bob.jpg"/>
       <Link rel="http://webfinger.net/rel/profile-page"
             href="http://www.example.com/~bob/"/>
       <Link rel="http://packetizer.com/rel/blog"
             href="http://blogs.example.com/bob/"/>
     </XRD>

   The email client might take note of the "blog" link relation in the
   above XRD document that refers to Bob's blog.  This URL would then be
   presented to you so that you could then visit his blog.

A.3. Security Considerations Related to XRDs

   When using HTTP to request an XRD document, WebFinger clients SHOULD
   verify the XRD document's signature, if present, to ensure that the
   XRD document has not been modified.  Additionally, WebFinger servers
   SHOULD include a signature for XRD documents served over HTTP.

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Author's Addresses

   Paul E. Jones
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709
   USA

   Phone: +1 919 476 2048
   Email: paulej@packetizer.com
   IM: xmpp:paulej@packetizer.com

   Gonzalo Salgueiro
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709
   USA

   Phone: +1 919 392 3266
   Email: gsalguei@cisco.com
   IM: xmpp:gsalguei@cisco.com

   Joseph Smarr
   Google

   Email: jsmarr@google.com

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