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Versions: (draft-jones-appsawg-webfinger) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 7033

Network Working Group                                      Paul E. Jones
Internet Draft                                         Gonzalo Salgueiro
Intended status: Standards Track                           Cisco Systems
Expires: January 3, 2013                                    Joseph Smarr
                                                                  Google
                                                            July 3, 2012



                                 WebFinger
                    draft-ietf-appsawg-webfinger-00.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 learn
   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 January 3, 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. Retrieving a Person's Contact Information.................7
      4.3. Simplifying the Login Process.............................8
      4.4. Retrieving Device Information.............................9
   5. WebFinger Protocol............................................10
      5.1. Performing a WebFinger Query.............................10
      5.2. The Web Host Metadata "resource" Parameter...............11
      5.3. The Web Host Metadata "rel" Parameter....................13
      5.4. WebFinger and URIs.......................................14
   6. The "acct" Link Relation......................................15
      6.1. Purpose for the "acct" Link Relation.....................15
      6.2. Example Message Exchange Using the "acct" Link Relation..16
   7. Cross-Origin Resource Sharing (CORS)..........................17
   8. Controlling Access to Information.............................17
   9. Implementation Notes (Non-Normative)..........................18
   10. Security Considerations......................................18
   11. IANA Considerations..........................................19
      11.1. Registration of the "acct" Link Relation Type...........19
   12. Acknowledgments..............................................19
   13. References...................................................19
      13.1. Normative References....................................19
      13.2. Informative References..................................20
   Author's Addresses...............................................21

1. Introduction

   There is a utility found on UNIX systems called "finger" [14] that
   allows a person to access information about another person.  The
   information being queried might be on a computer anywhere in the
   world.  The information returned via "finger" is simply a plain text
   file that contains unstructured information provided by the queried
   user.

   WebFinger borrows the concept of the legacy finger protocol, but
   introduces a very different approach to sharing information.  Rather
   than returning a simple unstructured text file, Webfinger uses
   structured documents that contain link relations.  These link
   relations point to information a user or entity on the Internet
   wishes to expose.  For a person, the kinds of information that might


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   be exposed include a personal profile address, identity service,
   telephone number, or preferred avatar.  WebFinger may also be used to
   learn 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 identification 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 Error! Reference source not found., 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 web-accessible
   domains.  In essence, there are two steps to discovering such
   information:

   1. By querying the domain itself, one can find out how to discover
      information about accounts, devices, and other associated with
      that domain.
   2. By then querying an entity at the domain, one will find links to
      more detailed information, which can then be queried individually.

   To enable such functionality, WebFinger makes heavy use of well-known
   URIs as defined in RFC 5785 [3] and "Link Relations" as defined in
   RFC 5988 [3].  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 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



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   "hreflang", "media", "title", "title*", "type", and any other link-
   extension parameters.

   Thus the framework for WebFinger consists of several building blocks:

   1. To query the domain, one requests a web host metadata file [11]
      located at a well-known URI of /.well-known/host-meta at the
      domain of interest.
   2. The web server at the domain returns an Extensible Resource
      Descriptor (XRD) or a JavaScript Object Notation (JSON) Resource
      Descriptor (JRD) document, including a Link-based Resource
      Descriptor Document (LRDD) link relation.
   3. To discover information about accounts, devices, or other entities
      associated with the domain, one requests the actual Link-based
      Resource Descriptor Document associated with a particular URI at
      the domain (e.g., an 'acct' URI, 'http' URI', or 'mailto' URI).
   4. The web server at the domain returns an XRD or JRD document about
      the requested URI, which includes specialized link relations
      pointing to resources that contain more detailed information about
      the entity.

   This model is illustrated in the examples under Section 4, then
   described more formally under Section 5.  Note that steps 2 and 3
   above may 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.

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.
   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, it tries to get the host metadata [11] information for the
   domain example.com.  It does this by issuing the following HTTPS
   query to example.com:


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

   The server replies with an XRD [10] document:

     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">
       <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 [11] 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>2012-03-13T20:56:11Z</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.




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   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.

   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 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.
   Let's assume, though, that for the above query the client requested a
   JRD representation for the resource rather than an XRD
   representation.  In that case, the response would have been:

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

     {
       "expires" : "2012-03-13T20:56:11Z",
       "subject" : "http://www.example.com/~bob/",
       "aliases" :
       [
         "acct:bob@example.com"
       ],
       "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/"
         }
       ]
     }




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4.2. Retrieving a Person's Contact Information

   Assume you have Alice in your address book, but her phone number
   appears to be invalid.  You could use WebFinger to find her current
   phone number and update your address book.

   Let's assume you have a web-based address book that you wish to
   update.  When you instruct the address book to pull Alice's current
   contact information, the address book might issue a query like this
   to get host metadata information for example.com:

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

   Note the address book is looking for a JSON [5] representation,
   whereas we used XML in the previous example.

   The server might reply with something like this:

     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/?format=json&uri={uri}"
         }
       ]
     }

   The client processes the response as described in RFC 6415 [11].  It
   will process the LRDD link relation using Alice's account URI by
   issuing this query:

     GET /lrdd/?format=json&uri=acct%3Aalice%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

     {


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       "expires" : "2012-03-13T20:56:11Z",
       "subject" : "acct:alice@example.com",
       "links" :
       [
         {
           "rel" : "http://webfinger.net/rel/avatar",
           "href" : "http://example.com/~alice/alice.jpg"
         },
         {
           "rel" : "vcard",
           "href" : "http://example.com/~alice/alice.vcf"
         }
       ]
     }

   With this response, the address book might see the vcard [16] link
   relation and use that file to offer you updated contact information.

4.3. 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 get the host metadata
   information:

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

   The response that comes back is similar to the previous example:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8
     {
       "expires" : "2012-03-13T20:56:11Z",
       "links" :
       [
         {
           "rel" : "lrdd",
           "type" : "application/json",
           "template" :
             "https://example.com/lrdd/?format=json&uri={uri}"
         }


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       ]
     }

   The blog server processes the response as described in RFC 6415.  It
   will process the LRDD link relation using Carol's account URI by
   issuing this query:

     GET /lrdd/?format=json&uri=acct%3Acarol%40example.com HTTP/1.1

   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.

4.4. 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 domain 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 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:




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     GET /lrdd/?format=json&uri=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 [11]
   specification and leverages the Cross-Origin Resource Sharing (CORS)
   [9] specification.

5.1. Performing a WebFinger Query

   The first step a client must perform 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 [11] specification.

   WebFinger clients MUST locate the LRDD link relation, if present, 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 MUST accept requests for both XRD [10] and JRD [11]
   documents.  The default representation returned by the server MUST be
   an XRD document, but a JRD document MUST be returned if the client


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   explicitly requests it by using /.well-known/host-meta.json or
   includes an Accept header in the HTTP request with a type of
   "application/json" [5].

   If the client requests a JRD document when querying for host
   metadata, the WebFinger server can assume that the client will want a
   JRD documents when querying the LRDD resource.  As such, when the
   WebFinger server returns a JRD document containing host metadata it
   should include a URI for an LRDD resource that can return a JRD
   document and MAY include a URI for an LRDD resource that will return
   an XRD 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 normal 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
   query.  This resource 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 as a means of improving performance and
   reducing client complexity.  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 or /.well-known/host-
   meta.json 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




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   Note that the "\" character shown above is to indicate that the line
   breaks at this point and continues on the next line.  This was shown
   only to avoid line wrapping in this document and is not a part of the
   HTTP protocol.

   When processing this request, the WebFinger server MUST

       *  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

   The WebFinger client can 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.

   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

   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"
         },


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         {
           "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.  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
   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 both the host-meta resource and the LRDD resource.

   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



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     {
       "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"
   is not defined for the resource, as a 404 status code is reserved for
   indicating that the resource itself (e.g., as indicated via the
   "resource" parameter) does not exist.

5.4. WebFinger and URIs

   Requests for both LRDD documents and hostmeta files 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 XRD
   or JRD template, for hostmeta this is the "resource" parameter).
   WebFinger itself is agnostic regarding the scheme of such a URI: it
   could be an "acct" URI as defined in the next section, an "http" or
   "https" URI, a "mailto" URI, or some other scheme.

   For resources associated with a user account at a domain, use of the
   "acct" URI scheme [7] is RECOMMENDED, since it explicitly identifies
   an account accessible via WebFinger.  Further, the "acct" URI scheme
   is not associated other protocols as, by way of example, the "mailto"
   URI scheme is associated with email.  Since not every domain offers
   email service, using the "mailto" URI scheme [8] is not ideal for
   identifying user accounts across all domains.  That said, use of the


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   "mailto" URI scheme would be ideal for use with WebFinger to discover
   mail server configuration information for a user, for example.

   A domain 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 XRD or
   JRD.

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 WebFinger server to reference one
   or more other user account URIs from within a user account.  The
   "acct" link relation is intended to allow a client to incorporate
   additional link relations by reference to produce a complete set of
   link relations for a user.  Any newly discovered link relations found
   by querying the referenced account SHOULD be merged into the resource
   descriptor document at the point where the "acct" link relation was
   inserted.

   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.

   Since an account may make a reference to one or more different
   accounts, WebFinger clients 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.




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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 to enable redirecting Alice when she queries for
   acct:bob@example.net.

   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

   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"
         }
       ]
     }

   Alice's WebFinger client could then perform 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.




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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, /.well-
   known/host-meta.json, and to the LRDD URI 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.

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 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.






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9. Implementation Notes (Non-Normative)

   A user should not be required to enter the "acct" URI scheme name
   along with his account identifier into any WebFinger client.  Rather,
   the WebFinger client should accept identifiers that are void of the
   "acct:" portion of the identifier.  Composing a properly formatted
   "acct" URI is the responsibility of the WebFinger client.

10. Security Considerations

   All of the security considerations applicable to Web Host Metadata
   [11] 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.

   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.

   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
   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



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   such a way as to require authentication and prevent disclosure of
   information to unauthorized entities.

11. IANA Considerations

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

11.1. 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:

12. 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.

13. References

13.1. 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.

   [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.


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   [7]   Saint-Andre, P., "The 'acct' URI Scheme", draft-saintandre-
         acct-uri-01, July 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.

13.2. 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.

   [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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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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