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
Jones, et al. Expires January 3, 2013 [Page 1]
Internet-Draft WebFinger July 2012
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
Jones, et al. Expires January 3, 2013 [Page 2]
Internet-Draft WebFinger July 2012
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
Jones, et al. Expires January 3, 2013 [Page 3]
Internet-Draft WebFinger July 2012
"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:
Jones, et al. Expires January 3, 2013 [Page 4]
Internet-Draft WebFinger July 2012
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.
Jones, et al. Expires January 3, 2013 [Page 5]
Internet-Draft WebFinger July 2012
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/"
}
]
}
Jones, et al. Expires January 3, 2013 [Page 6]
Internet-Draft WebFinger July 2012
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
{
Jones, et al. Expires January 3, 2013 [Page 7]
Internet-Draft WebFinger July 2012
"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}"
}
Jones, et al. Expires January 3, 2013 [Page 8]
Internet-Draft WebFinger July 2012
]
}
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:
Jones, et al. Expires January 3, 2013 [Page 9]
Internet-Draft WebFinger July 2012
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
Jones, et al. Expires January 3, 2013 [Page 10]
Internet-Draft WebFinger July 2012
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
Jones, et al. Expires January 3, 2013 [Page 11]
Internet-Draft WebFinger July 2012
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"
},
Jones, et al. Expires January 3, 2013 [Page 12]
Internet-Draft WebFinger July 2012
{
"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
Jones, et al. Expires January 3, 2013 [Page 13]
Internet-Draft WebFinger July 2012
{
"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
Jones, et al. Expires January 3, 2013 [Page 14]
Internet-Draft WebFinger July 2012
"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.
Jones, et al. Expires January 3, 2013 [Page 15]
Internet-Draft WebFinger July 2012
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.
Jones, et al. Expires January 3, 2013 [Page 16]
Internet-Draft WebFinger July 2012
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.
Jones, et al. Expires January 3, 2013 [Page 17]
Internet-Draft WebFinger July 2012
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
Jones, et al. Expires January 3, 2013 [Page 18]
Internet-Draft WebFinger July 2012
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.
Jones, et al. Expires January 3, 2013 [Page 19]
Internet-Draft WebFinger July 2012
[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.
Jones, et al. Expires January 3, 2013 [Page 20]
Internet-Draft WebFinger July 2012
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
Jones, et al. Expires January 3, 2013 [Page 21]
