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Versions: (draft-jones-appsawg-webfinger) 00 01 02 03 04 05 06 07 08 09 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: October 16, 2013                                   Joseph Smarr
                                                                  Google
                                                          April 16, 2013


                                 WebFinger
                    draft-ietf-appsawg-webfinger-13.txt


Abstract

   This specification defines the WebFinger protocol, which can be used
   to discover information about people or other entities on the
   Internet using standard HTTP methods.  WebFinger discovers
   information for a URI that might not be usable as a locator
   otherwise, such as account or email URIs.

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 October 16, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.




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Table of Contents

   1. Introduction...................................................2
   2. Terminology....................................................3
   3. Example Uses of WebFinger......................................3
      3.1. Locating a User's Blog....................................3
      3.2. Identity Provider Discovery for OpenID Connect............5
      3.3. Auto-Configuration of Email Clients.......................6
      3.4. Retrieving Device Information.............................7
   4. WebFinger Protocol.............................................8
      4.1. Constructing the Query Component of the Request URI.......9
      4.2. Performing a WebFinger Query..............................9
      4.3. The "rel" Parameter......................................10
      4.4. The JSON Resource Descriptor (JRD).......................11
         4.4.1. subject.............................................12
         4.4.2. aliases.............................................12
         4.4.3. properties..........................................12
         4.4.4. links...............................................12
      4.5. WebFinger and URIs.......................................14
   5. Cross-Origin Resource Sharing (CORS)..........................15
   6. Access Control................................................15
   7. Hosted WebFinger Services.....................................16
   8. Security Considerations.......................................17
      8.1. Transport-Related Issues.................................17
      8.2. User Privacy Considerations..............................17
      8.3. Abuse Potential..........................................18
      8.4. Information Reliability..................................19
   9. IANA Considerations...........................................19
      9.1. Well-Known URI...........................................19
      9.2. JSON Resource Descriptor (JRD) Media Type................20
   10. Acknowledgments..............................................21
   11. References...................................................21
      11.1. Normative References....................................21
      11.2. Informative References..................................22
   Author's Addresses...............................................23

1. Introduction

   WebFinger is used to discover information about people or other
   entities on the Internet that are identified by a URI [6] or IRI [7]
   using standard Hypertext Transfer Protocol (HTTP) [2] methods over a
   secure transport [14].  A WebFinger resource returns a JavaScript
   Object Notation (JSON) [5] object describing the entity that is
   queried.  The JSON object is referred to as the JSON Resource
   Descriptor (JRD).

   For a person, the kinds of information that might be discoverable via
   WebFinger include a personal profile address, identity service,
   telephone number, or preferred avatar.  For other entities on the
   Internet, a WebFinger resource might return JRDs containing link
   relations [10] that enable a client to discover, for example, the



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   that a printer can print in color on A4 paper, the physical location
   of a server, or other static information.

   Information returned via WebFinger might be for direct human
   consumption (e.g., looking up someone'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).
   The information is intended to be static in nature and, as such,
   WebFinger is not intended to be used to return dynamic information
   like the temperature of a CPU or the current toner level in a laser
   printer.

   The WebFinger protocol is designed to be used across many
   applications.  Applications that wish to utilize WebFinger will need
   to specify properties, titles, and link relation types that are
   appropriate for the application.  Further, applications will need to
   define the appropriate URI scheme to utilize for the query target.

   Use of WebFinger is illustrated in the examples in Section 3 and
   described more formally in Section 4.

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".  A Link Relation is an
   attribute-and-value pair in which the attribute identifies the type
   of relationship between the linked entity or resource and the
   information specified in the value.  In Web Linking [4], the link
   relation is represented using an HTTP entity-header of "Link", where
   the "rel" attribute specifies the type of relationship and the "href"
   attribute specifies the information that is linked to the entity or
   resource.  In WebFinger, the same concept is represented using a JSON
   array of "links" objects, where each member named "rel" specifies the
   type of relationship and each member named "href" specifies the
   information that is linked to the entity or resource.  Note that
   WebFinger narrows the scope of a link relation beyond what is defined
   for Web Linking by stipulating that the value of the "rel" member
   needs to be either a single IANA-registered link relation type [10]
   or a URI [6].

3. Example Uses of WebFinger

   This non-normative section shows a few sample uses of WebFinger.

3.1. 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 made
   that information available via WebFinger.

   Assume your email client can discover the blog for you.  After
   receiving the message from Bob (bob@example.com), your email client
   performs a WebFinger query either automatically or at your command.
   (Please refer to Section 8.2 for user privacy considerations and
   Section 8.3 for abuse considerations, particularly when considering
   any kind of automated query feature.)  It does so by issuing the
   following HTTPS [14] query to example.com:

     GET /.well-known/webfinger?
                           resource=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/jrd+json

     {
       "subject" : "acct:bob@example.com",
       "aliases" :
       [
         "http://www.example.com/~bob/"
       ],
       "properties" :
       {
           "http://example.com/ns/role/" : "employee"
       },
       "links" :
       [
         {
           "rel" : "http://webfinger.example/rel/avatar",
           "type" : "image/jpeg",
           "href" : "http://www.example.com/~bob/bob.jpg"
         },
         {
           "rel" : "http://webfinger.example/rel/profile-page",
           "href" : "http://www.example.com/~bob/"
         },
         {
           "rel" : "http://webfinger.example/rel/blog",
           "type" : "text/html",
           "href" : "http://blogs.example.com/bob/",
           "titles" :
           {
               "en-us" : "The Magical World of Bob",
               "fr" : "Le Monde Magique de Bob"
           }
         },


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         {
           "rel" : "http://webfinger.example/rel/businesscard",
           "href" : "https://www.example.com/~bob/bob.vcf"
         }
       ]
     }

   Note the assumption made in the above example is that there is an
   "acct" URI for the given "mailto" URI.  This may not always be the
   case.

   The email client would take note of the link relation in the above
   JRD that refers to Bob's blog.  The blog's URI 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 automatically retrieve the data located at
   the URI specified by the "businesscard" link relation (which might be
   a vcard [16]) to update the information about Bob in its internal
   address book.

   In the above example, an "acct" URI [8] is used in the query, though
   any valid alias for the user might also be used.  See Section 4.5 for
   more information on WebFinger and URIs.

   An alias is a URI that is different from the "subject" URI, yet
   identifies the same entity.  In the above example, there is one
   "http" alias returned, though there might have been more than one.
   Had the "http:" URI shown as an alias been used to query for
   information about Bob, the query would have appeared as:

     GET /.well-known/webfinger?
              resource=http%3A%2F%2Fwww.example.com%2F~bob%2F HTTP/1.1
     Host: www.example.com

   Note that the host queried in this example is different than for the
   acct URI example, since the URI refers to a different host.  Either
   this host would provide a response, or it would redirect the client
   to another host (e.g., redirect back to example.com).  Either way,
   the response would have been substantially the same, with the subject
   and alias information changed as necessary.

3.2. Identity Provider Discovery for OpenID Connect

   Suppose Carol wishes to authenticate with a web site she visits using
   OpenID Connect [18].  She would provide the web site with her OpenID
   Connect identifier, say carol@example.com.  The visited web site
   would perform a WebFinger query looking for the OpenID Connect
   Provider.  Since the site is interested in only one particular link
   relation, the WebFinger resource might utilize the "rel" parameter as
   described in Section 4.3:



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     GET /.well-known/webfinger?
            resource=acct%3Acarol%40example.com&
            rel=http%3A%2F%2Fopenid.net%2Fspecs%2Fconnect%2F1.0%2Fissuer
            HTTP/1.1
     Host: example.com

   The server might respond like this:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/jrd+json

     {
       "subject" : "acct:carol@example.com",
       "links" :
       [
         {
           "rel" : "http://openid.net/specs/connect/1.0/issuer",
           "href" : "https://openid.example.com"
         }
       ]
     }

   Since the "rel" parameter only serves to filter the link relations
   returned by the resource, other name/value pairs in the response,
   including any aliases or properties, would be returned.  Also, since
   support for the "rel" parameter is not guaranteed, the client must
   not assume the "links" array will contain only the requested link
   relation.

3.3. Auto-Configuration of Email Clients

   WebFinger could be used to auto-provision an email client with basic
   configuration data.  Suppose that sue@example.com wants to configure
   her email client.  Her email client might issue the following query:

     GET /.well-known/webfinger?
              resource=mailto%3Asue%40example.com HTTP/1.1
     Host: example.com

   The returned resource representation would contain entries for the
   various protocols, transport options, and security options.  If there
   are multiple options, the resource representation might include a
   link relation for each of the valid options, and the client or Sue
   might select which option to choose.  Since JRDs list link relations
   in a specific order, then the most-preferred choices could be
   presented first.  Consider this response:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/jrd+json



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     {
       "subject" : "mailto:sue@example.com",
       "links" :
       [
         {
           "rel" : "http://webfinger.example/rel/smtp-server",
           "properties" :
           {
             "http://webfinger.example/email/host" : "smtp.example.com",
             "http://webfinger.example/email/port" : "587",
             "http://webfinger.example/email/login-required" : "yes",
             "http://webfinger.example/email/transport" : "starttls"
           }
         },
         {
           "rel" : "http://webfinger.example/rel/imap-server",
           "properties" :
           {
             "http://webfinger.example/email/host" : "imap.example.com",
             "http://webfinger.example/email/port" : "993",
             "http://webfinger.example/email/transport" : "ssl"
           }
         }
       ]
     }

   In this example, you can see that the WebFinger resource
   representation advertises an SMTP service and an IMAP service.  In
   this example, the "href" entries associated with the link relation
   are absent.  This is valid when there is no additional reference that
   needs to be made.

3.4. Retrieving Device Information

   As another example, suppose there are printers on the network and you
   would like to check a particular printer identified by the URI
   device:p1.example.com to see if it can print in color on A4 paper.
   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:

     GET /.well-known/webfinger?
                       resource=device%3Ap1.example.com HTTP/1.1
     Host: p1.example.com

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




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     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/jrd+json

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

   While this example is fictitious, you can imagine that perhaps the
   Transport Independent, Printer/System Interface [17] may be enhanced
   with a web interface enabling a device that understands the TIP/SI
   web interface specification to query for printer capabilities.

4. WebFinger Protocol

   The WebFinger protocol is used to request information about an entity
   identified by a query target (a URI).  The client can optionally
   specify one or more link relation types for which it would like to
   receive information.

   A WebFinger request is an HTTPS request to a WebFinger resource.  A
   WebFinger resource is a well-known URI [3] using the HTTPS scheme,
   constructed along with the required query target and optional link
   relation types.  WebFinger resources MUST NOT be served with any
   other URI scheme (such as HTTP).

   A WebFinger resource is always given a query target, which is another
   URI that identifies the entity whose information is sought.  GET
   requests to a WebFinger resource convey the query target in the
   "resource" parameter in the WebFinger URI's query string; see Section
   4.1 for details.

   The host to which a WebFinger query is issued is significant.  If the
   query target contains a "host" portion (Section 3.2.2 of RFC 3986),
   then the host to which the WebFinger query is issued MUST be the same
   as the "host" portion of the query target. If the query target does
   not contain a "host" portion, then the client MAY choose a host to
   which it directs the query based on local policy.

   The path component of a WebFinger URI MUST be the well-known path
   "/.well-known/webfinger".  A WebFinger URI MUST contain a query
   component that encodes the query target and optional link relation
   types as specified in Section 4.1.




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   The WebFinger resource returns a JSON Resource Descriptor (JRD) as
   the resource representation to convey information about an entity on
   the Internet.  Also, the Cross-Origin Resource Sharing (CORS) [9]
   specification is utilized to facilitate queries made via a web
   browser.

4.1. Constructing the Query Component of the Request URI

   A WebFinger URI MUST contain a query component (see Section 3.4 of
   RFC 3986).  The query component MUST contain a "resource" parameter
   and MAY contain one or more "rel" parameters.  The "resource"
   parameter MUST contain the query target (URI) and the "rel"
   parameters MUST contain encoded link relation types according to the
   encoding described in this section.

   To construct the query component, the client performs the following
   steps.  First, each parameter value is percent-encoded, as per
   Section 2.1 of RFC 3986.  The encoding is done to conform to the
   query production in Section 3.4 of that specification, with the
   addition that any instances of the "=" and "&" characters within the
   parameter values are also percent-encoded.  Next, the client
   constructs a string to be placed in the query component by
   concatenating the name of the first parameter together with an equal
   sign ("=") and the percent-encoded parameter value.  For any
   subsequent parameters, the client appends an ampersand ("&") to the
   string, the name of the next parameter, an equal sign, and the
   parameter value.  The client MUST NOT insert any spaces while
   constructing the string.  The order in which the client places each
   attribute-and-value pair within the query component does not matter
   in the interpretation of the query component.

4.2. Performing a WebFinger Query

   A WebFinger client issues a query using the GET method to the well-
   known [3] resource identified by the URI whose path component is
   "/.well-known/webfinger" and whose query component MUST include the
   "resource" parameter exactly once and set to the value of the URI for
   which information is being sought.  If the "resource" parameter is
   absent or malformed, the WebFinger resource MUST indicate that the
   request is bad as per Section 10.4.1 of RFC 2616 [2].

   A client MUST query the WebFinger resource using HTTPS only.  If the
   client determines that the resource has an invalid certificate, the
   resource returns a 4xx or 5xx status code, or the HTTPS connection
   cannot be established for any reason, then the client MUST accept
   that the WebFinger query has failed and MUST NOT attempt to reissue
   the WebFinger request using HTTP over a non-secure connection.

   A WebFinger resource MUST return a JRD as the representation for the
   resource if the client requests no other supported format explicitly
   via the HTTP "Accept" header.  The client MAY include the "Accept"
   header to indicate a desired representation; representations other


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   than JRD might be defined in future specifications.  The WebFinger
   resource MUST silently ignore any requested representations that it
   does not understand and support.  The media type used for the JSON
   Resource Descriptor (JRD) is "application/jrd+json" (see Section
   9.2).

   A WebFinger resource MAY redirect the client; if it does, the
   redirection MUST only be to an "https" URI and the client MUST
   perform certificate validation again when redirected.

   A WebFinger resource can include cache validators in a response to
   enable conditional requests by the client and/or expiration times as
   per Section 13 of RFC 2616.

   A WebFinger client MAY utilize the HEAD method when querying a
   WebFinger resource.  Consequently, a WebFinger resource MUST support
   the receipt of the HEAD method.

4.3. The "rel" Parameter

   When issuing a request to a WebFinger resource, the client MAY
   utilize the "rel" parameter to request only a subset of the
   information that would otherwise be returned without the "rel"
   parameter.  When the "rel" parameter is used and accepted, only the
   link relation types that match the link relation types provided via
   the "rel" parameter are included in the array of links returned in
   the JRD.  If there are no matching link relation types defined for
   the resource, the "links" array in the JRD will either be absent or
   empty.  All other information present in a resource descriptor
   remains present, even when "rel" is employed.

   The "rel" parameter MAY be included multiple times in order to
   request multiple link relation types.

   The purpose of the "rel" parameter is to return a subset of "link
   relation objects" (see Section 4.4.4) that would otherwise be
   returned in the resource descriptor.  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" value.

   WebFinger resources SHOULD support the "rel" parameter.  If the
   resource does not support the "rel" parameter, it MUST ignore the
   parameter and process the request as if no "rel" parameter values
   were present.

   The following example presents the same example as found in Section
   3.1, but uses the "rel" parameter to select two link relations:

     GET /.well-known/webfinger?
            resource=acct%3Abob%40example.com&


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            rel=http%3A%2F%2Fwebfinger.example%2Frel%2Fprofile-page&
            rel=http://webfinger.example/rel/businesscard HTTP/1.1
     Host: example.com

   In this example, the client requests the link relations of type
   "http://webfinger.example/rel/profile-page" and
   "http://webfinger.example/rel/businesscard".  The server then
   responds with a message like this:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/jrd+json

     {
       "subject" : "acct:bob@example.com",
       "aliases" :
       [
         "http://www.example.com/~bob/"
       ],
       "properties" :
       {
           "http://example.com/ns/role/" : "employee"
       },
       "links" :
       [
         {
           "rel" : "http://webfinger.example/rel/profile-page",
           "href" : "http://www.example.com/~bob/"
         },
         {
           "rel" : "http://webfinger.example/rel/businesscard",
           "href" : "http://www.example.com/~bob/bob.vcf"
         }
       ]
     }

   As you can see in the response, the resource representation contains
   only the link relations requested by the client, but the other parts
   of the JRD are still present.

4.4. The JSON Resource Descriptor (JRD)

   The JSON Resource Descriptor (JRD), originally introduced in RFC 6415
   [19] and based on the Extensible Resource Descriptor (XRD) format
   [20], is a JSON object that comprises the following name/value pairs:

        o subject
        o aliases
        o properties
        o links




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   The member "subject" is a name/value pair whose value is a string,
   "aliases" is an array of strings, "properties" is an object
   comprising name/value pairs whose values are strings, and "links" is
   an array of objects that contain link relation information.

   When processing a JRD, the client MUST ignore any unknown member and
   not treat the presence of an unknown member as an error.

   Below, each of these members of the JRD is described in more detail.

4.4.1. subject

   The value of the "subject" member is a URI that identifies the entity
   that the JRD describes.

   The "subject" value returned by a WebFinger resource MAY differ from
   the value of the "resource" parameter used in the client's request.
   This might happen, for example, when the subject's identity changes
   (e.g., a user moves his or her account to another service) or when
   the resource prefers to express URIs in canonical form.

   The "subject" member MUST be present in the JRD.

4.4.2. aliases

   The "aliases" array is an array of zero or more URI strings that
   identify the same entity as the "subject" URI. Each URI must be an
   absolute URI.

   The "aliases" array is OPTIONAL in the JRD.

4.4.3. properties

   The "properties" object comprises zero or more name/value pairs whose
   names are absolute URIs and whose values are strings or null.
   Properties are used to convey additional information about the
   subject of the JRD.  As an example, consider this use of
   "properties":

     "properties" : { "http://webfinger.example/ns/name" : "Bob Smith" }

   The "properties" member is OPTIONAL in the JRD.

4.4.4. links

   The "links" array has any number of member objects, each of which
   represents a link [4]. Each of these link objects can have the
   following members:

        o rel
        o type
        o href


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        o titles
        o properties

   The "rel" and "href" members are strings representing the link's
   relation type and the target IRI, respectively. The context of the
   link is the "subject" (see Section 4.4.1).

   The "type" member is a string indicating what the media type of the
   result of dereferencing the link ought to be.

   The order of elements in the "links" array indicates an order of
   preference.  Thus, if there are two or more link relations having the
   same "rel" value, the first link relation would indicate the user's
   preferred link.

   The "links" array is OPTIONAL in the JRD.

   Below, each of the members of the objects found in the "links" array
   is described in more detail.  Each object in the "links" array,
   referred to as a "link relation object", is completely independent
   from any other object in the array; any requirement to include a
   given member in the link relation object refers only to that
   particular object.

4.4.4.1. rel

   The value of the "rel" member is a string that is either an absolute
   URI or a registered relation type [10] (see RFC 5988 [4]).  The value
   of the "rel" member MUST contain exactly one URI or registered
   relation type.  The URI or registered relation type identifies the
   type of the link relation.

   The other members of the object have meaning only once the type of
   link relation is understood.  In some instances, the link relation
   will have associated semantics enabling the client to query for other
   resources on the Internet.  In other instances, the link relation
   will have associated semantics enabling the client to utilize the
   other members of the link relation object without fetching additional
   external resources.

   URI link relation type values are compared using the "Simple String
   Comparison" algorithm of section 6.2.1 of RFC 3986 [6].

   The "rel" member MUST be present in the link relation object.

4.4.4.2. type

   The value of the "type" member is a string that indicates the media
   type [11] of the target resource (see RFC 6838 [12]).

   The "type" member is OPTIONAL in the link relation object.



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4.4.4.3. href

   The value of the "href" member is a string that contains a URI
   pointing to the target resource.

   The "href" member is OPTIONAL in the link relation object.

4.4.4.4.  titles

   The "titles" object comprises zero or more name/value pairs whose
   name is a language tag [13] or the string "default".  The string is
   human-readable and describes the link relation.  More than one title
   for the link relation MAY be provided for the benefit of users who
   utilize the link relation and, if used, a language identifier SHOULD
   be duly used as the name.  If the language is unknown or unspecified,
   then the name is "default".

   A JRD SHOULD NOT include more than one title identified with the same
   language tag (or "default") within the link relation object.  Meaning
   is undefined if a link relation object includes more than one title
   named with the same language tag (or "default"), though this MUST NOT
   be treated as an error.  A client MAY select whichever title or
   titles it wishes to utilize.

   Here is an example of the titles object:

     "titles" :
     {
       "en-us" : "The Magical World of Bob",
       "fr" : "Le Monde Magique de Bob"
     }

   The "titles" member is OPTIONAL in the link relation object.

4.4.4.5. properties

   The "properties" object within the link relation object comprises
   zero or more name/value pairs whose names are absolute URIs and whose
   values are strings or null.  Properties are used to convey additional
   information about the link relation.  As an example, consider this
   use of "properties":

     "properties" : { "http://webfinger.example/mail/port" : "993" }

   The "properties" member is OPTIONAL in the link relation object.

4.5. WebFinger and URIs

   WebFinger requests include a "resource" parameter (see Section 4.1)
   specifying the URI of an account, device, or other entity.  WebFinger
   is neutral regarding the scheme of such a URI: it could be an "acct"



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   URI [7], an "http" or "https" URI, a "mailto" URI [21], or some other
   scheme.

   To perform a WebFinger lookup on an account specific to the host
   being queried, use of the "acct" URI scheme is recommended, since it
   explicitly identifies a generic user account that is not necessarily
   bound to a specific protocol.  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 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.

5. Cross-Origin Resource Sharing (CORS)

   WebFinger resources might not be accessible from a web browser due to
   "Same-Origin" policies.  The current best practice is to make
   resources available to browsers through Cross-Origin Resource Sharing
   (CORS) [9], and servers MUST include the Access-Control-Allow-Origin
   HTTP header in responses.  Servers SHOULD support the least
   restrictive setting by allowing any domain access to the WebFinger
   resource:

      Access-Control-Allow-Origin: *

   There are cases where defaulting to the least restrictive setting is
   not appropriate, for example a server on an intranet that provides
   sensitive company information SHOULD NOT allow CORS requests from any
   domain, as that could allow leaking of that sensitive information.  A
   server that wishes to restrict access to information from external
   entities SHOULD use a more restrictive Access-Control-Allow-Origin
   header.

6. Access Control

   As with all web resources, access to the WebFinger resource could
   require authentication.  Further, failure to provide required
   credentials might result in the server forbidding access or providing
   a different response than had the client authenticated with the
   server.

   Likewise, a WebFinger resource 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 resource
   representation might point to web resources that impose access
   restrictions.  For example, the aforementioned corporate server may


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   provide both internal and external entities with URIs to employee
   pictures, but further authentication might be required in order for
   the client to access the picture resources if the request comes from
   outside the corporate network.

   The decisions made with respect to what set of link relations a
   WebFinger resource 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.

7. Hosted WebFinger Services

   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 for email.  In the case of
   email, MX records identify mail servers for a domain.  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
   queries to its domain to allow for hosted WebFinger services.

   When a query is issued to the WebFinger resource, the web server MUST
   return a response with a redirection status code that includes a
   Location header pointing to the location of the hosted WebFinger
   service URI.  This WebFinger service URI does not need to point to
   the well-known WebFinger location on the hosting service provider
   server.

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

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

   The server might respond with this:

     HTTP/1.1 307 Temporary Redirect
     Access-Control-Allow-Origin: *
     Location: https://wf.example.net/example.com/webfinger?
                   resource=acct%3Aalice%40example.com




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   The client can then follow the redirection, re-issuing the request to
   the URI provided in the Location header.  Note that the server will
   include any required URI parameters in the Location header value,
   which could be different than the URI parameters the client
   originally used.

8. Security Considerations

8.1. Transport-Related Issues

   Since this specification utilizes Cross-Origin Resource Sharing
   (CORS) [9], all of the security considerations applicable to CORS are
   also applicable to this specification.

   The use of HTTPS is REQUIRED to ensure that information is not
   modified during transit.  It should be appreciated that in
   environments where a web server is normally available, there exists
   the possibility that a compromised network might have its WebFinger
   resource operating on HTTPS replaced with one operating only over
   HTTP.  As such, clients MUST NOT issue queries over a non-secure
   connection.

   Clients MUST verify that the certificate used on an HTTPS connection
   is valid (as defined in [14]) and accept a response only if the
   certificate is valid.

8.2. User Privacy Considerations

   Service providers and users should be aware that placing information
   on the Internet means that any user can access that information and
   WebFinger can be used to make it even easier to discover that
   information.  While WebFinger can be an extremely useful tool for
   discovering one's avatar, blog, or other personal data, 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 on the Internet or discoverable via WebFinger.

   Systems or services that expose personal data via WebFinger MUST
   provide an interface by which users can select which data elements
   are exposed through the WebFinger interface.  For example, social
   networking sites might allow users to mark certain data as "public"
   and then utilize that marking as a means of determining what
   information to expose via WebFinger.  The information published via
   WebFinger would thus comprise only the information marked as public
   by the user.  Further, the user has the ability to remove information
   from publication via WebFinger by removing this marking.

   WebFinger MUST NOT be used to provide any personal data to any party
   unless explicitly authorized by the person whose information is being
   shared.  Publishing one's personal data within an access-controlled
   or otherwise limited environment on the Internet does not equate to



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   providing implicit authorization of further publication of that data
   via WebFinger.

   The privacy and security concerns with publishing personal data via
   WebFinger are worth emphasizing again with respect to personal data
   that might reveal a user's current context (e.g., the user's
   location).  The power of WebFinger comes from providing a single
   place where others can find pointers to information about a person,
   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.

   Users should be aware of how easily personal data one might publish
   can be used in unintended ways.  In one study relevant to WebFinger-
   like services, Balduzzi et al. [22] took a large set of leaked email
   addresses and demonstrated a number of potential privacy concerns,
   including the ability to cross-correlate the same user's accounts
   over multiple social networks.  The authors also describe potential
   mitigation strategies.

   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 WebFinger resources for
   use inside a corporate network, the network administrator needs to
   take necessary measures 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, a server can be
   employed in such a way as to require authentication and prevent
   disclosure of information to unauthorized entities.

8.3. Abuse Potential

   Service providers should be mindful of the potential for abuse using
   WebFinger.

   As one example, one might query a WebFinger server only to discover
   whether a given URI is valid or not.  With such a query, the person
   may deduce that an email identifier is valid, for example.  Such an
   approach could help spammers maintain a current list of known email
   addresses and to discover new ones.

   WebFinger could be used to associate a name or other personal data
   with an email address, allowing spammers to craft more convincing
   email messages.  This might be of particular value in phishing
   attempts.

   It is RECOMMENDED that implementers of WebFinger server software take
   steps to mitigate abuse, including malicious over-use of the server
   and harvesting of user information.  Although there is no mechanism


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   that can guarantee that publicly-accessible WebFinger databases won't
   be harvested, rate-limiting by IP address will prevent or at least
   dramatically slow harvest by private individuals without access to
   botnets or other distributed systems.  The reason these mitigation
   strategies are not mandatory is that the correct choice of mitigation
   strategy (if any) depends greatly on the context.  Implementers
   should not construe this as meaning that they do not need to consider
   whether to use a mitigation strategy, and, if so, what strategy to
   use.

   WebFinger client developers should also be aware of potential abuse
   by spammers or those phishing for information about users.  As an
   example, suppose a mail client was configured to automatically
   perform a WebFinger query as discussed in the example in Section 3.1.
   If a spammer sent an email using a unique identifier in the 'From'
   header, then when the WF query was performed the spammer would be
   able to associate the request with a particular user's email address.
   This would provide information to the spammer, including the user's
   IP address, the fact the user just checked email, what kind of
   WebFinger client the user utilized, and so on.  For this reason, it
   is strongly advised that clients not perform WebFinger queries unless
   authorized by the user to do so.

8.4. Information Reliability

   A WebFinger resource has no means of ensuring that information
   provided by a user is accurate.  Likewise, neither the resource nor
   the client can be absolutely guaranteed that information has not been
   manipulated either at the server or along the communication path
   between the client and server.  Use of HTTPS helps to address some
   concerns with manipulation of information along the communication
   path, but it clearly cannot address issues where the resource
   provided incorrect information, either due to being provided false
   information or due to malicious behavior on the part of the server
   administrator.  As with any information service available on the
   Internet, users should be wary of information received from untrusted
   sources.

9. IANA Considerations

9.1. Well-Known URI

   This specification registers the "webfinger" well-known URI in the
   Well-Known URI Registry as defined by [3].

   URI suffix:  webfinger

   Change controller:  IETF

   Specification document(s):  RFC XXXX




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   Related information:  The query to the WebFinger resource will
   include one or more parameters in the query string; see Section 4.1
   of RFCXXXX.  Resources at this location are able to return a JSON
   Resource Descriptor (JRD) as described in Section 4.4 of RFCXXXX.

   [RFC EDITOR: Please replace "XXXX" references in this section and the
   following section with the number for this RFC.]

9.2. JSON Resource Descriptor (JRD) Media Type

   This specification registers the media type application/jrd+json for
   use with WebFinger in accordance with media type registration
   procedures defined in [12].

   Type name: application

   Subtype name: jrd+json

   Required parameters: N/A

   Optional parameters: N/A

     In particular, because RFC 4627 already defines the character
     encoding for JSON, no "charset" parameter is used.

   Encoding considerations: See RFC 6839, section 3.1.

   Security considerations:

     The JSON Resource Descriptor (JRD) is a JavaScript Object Notation
     (JSON) object.  It is a text format that must be parsed by entities
     that wish to utilize the format.  Depending on the language and
     mechanism used to parse a JSON object, it is possible for an
     attacker to inject behavior into a running program.  Therefore,
     care must be taken to properly parse a received JRD to ensure that
     only a valid JSON object is present and that no JavaScript or other
     code is injected or executed unexpectedly.

   Interoperability considerations:

     This media type is a JavaScript Object Notation (JSON) object and
     can be consumed by any software application that can consume JSON
     objects.

   Published specification: RFC XXXX

   Applications that use this media type:

     The JSON Resource Descriptor (JRD) is used by the WebFinger
     protocol (RFC XXXX) to enable the exchange of information between a
     client and a WebFinger resource over HTTPS.



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   Fragment identifier considerations:

     The syntax and semantics of fragment identifiers SHOULD be as
     specified for "application/json".  (At publication of this
     document, there is no fragment identification syntax defined for
     "application/json".)

   Additional information:

     Deprecated alias names for this type: N/A

     Magic number(s): N/A

     File extension(s): jrd

     Macintosh file type code(s): N/A

   Person & email address to contact for further information:

     Paul E. Jones <paulej@packetizer.com>

   Intended usage: COMMON

   Restrictions on usage: N/A

   Author: Paul E. Jones <paulej@packetizer.com>

   Change controller:

     IESG has change control over this registration.

   Provisional registration? (standards tree only): N/A

10. Acknowledgments

   This document has benefited from extensive discussion and review of
   many of the members of the APPSAWG working group. The authors would
   like to especially acknowledge the invaluable input of Eran Hammer-
   Lahav, Blaine Cook, Brad Fitzpatrick, Laurent-Walter Goix, Joe
   Clarke, Michael B. Jones, Peter Saint-Andre, Dick Hardt, Tim Bray,
   James Snell, Melvin Carvalho, Evan Prodromou, Mark Nottingham, Barry
   Leiba, Elf Pavlik, Bjoern Hoehrmann, SM, Joe Gregorio and others that
   we have undoubtedly, but inadvertently, missed.  Special thanks go to
   the chairs of APPSAWG, especially Salvatore Loreto for his assistance
   in shepherding this document.

11. References

11.1. Normative References

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


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   [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.

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

   [8]   Saint-Andre, P., "The 'acct' URI Scheme", draft-ietf-appsawg-
         acct-uri-03, February 2013.

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

   [10]  IANA, "Link Relations", http://www.iana.org/assignments/link-
         relations/.

   [11]  IANA, "MIME Media Types",
         http://www.iana.org/assignments/media-types/index.html.

   [12]  Freed, N., Klensin, J., Hansen, T., "Media Type Specifications
         and Registration Procedures", RFC 6838, January 2013.

   [13]  Phillips, A., Davis, M., "Tags for Identifying Languages", RFC
         5646, January 2009.

   [14]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [15]  Klyne, G., Newman, C., "Date and Time on the Internet:
         Timestamps", RFC 3339, July 2002.

11.2. Informative References

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

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





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   [18]  Sakimura, N., Bradley, J., Jones, M., de Medeiros, B.,
         Mortimore, C., and E. Jay, "OpenID Connect Messages 1.0",
         January 2013, http://openid.net/specs/openid-connect-messages-
         1_0.html.

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

   [20]  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.

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

   [22]  Balduzzi, Marco, et al., "Abusing social networks for automated
         user profiling", Recent Advances in Intrusion Detection,
         Springer Berlin Heidelberg, 2010,
         https://www.eurecom.fr/en/publication/3042/download/rs-publi-
         3042_1.pdf.



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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