Network Working Group Paul E. Jones
Internet Draft Gonzalo Salgueiro
Intended status: Standards Track Cisco Systems
Expires: April 22, 2013 Joseph Smarr
Google
October 22, 2012
WebFinger
draft-ietf-appsawg-webfinger-02.txt
Abstract
This specification defines the WebFinger protocol. WebFinger may be
used to discover information about people on the Internet, such as a
person's personal profile address, identity service, telephone
number, or preferred avatar. WebFinger may also be used to discover
information about objects on the network, such as the amount of toner
in a printer or the physical location of a server.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 22, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction...................................................2
2. Terminology....................................................3
3. Overview.......................................................3
4. Example Uses of WebFinger......................................4
4.1. Locating a User's Blog....................................4
4.2. Simplifying the Login Process.............................7
4.3. Retrieving Device Information.............................8
5. WebFinger Protocol.............................................8
5.1. Performing a WebFinger Query..............................9
5.2. The Web Host Metadata "resource" Parameter...............10
5.3. The Web Host Metadata "rel" Parameter....................12
5.4. WebFinger and URIs.......................................14
6. The "acct" Link Relation......................................14
6.1. Purpose for the "acct" Link Relation.....................14
6.2. Example Message Exchange Using the "acct" Link Relation..15
7. Cross-Origin Resource Sharing (CORS)..........................16
8. Controlling Access to Information.............................17
9. Hosted and Distributed WebFinger Services.....................17
9.1. Hosting the Entire Domain................................17
9.2. Distributed WebFinger Services...........................18
10. Web Host Metadata Interoperability Considerations............20
11. Security Considerations......................................20
12. IANA Considerations..........................................21
12.1. Registration of the "acct" Link Relation Type...........21
13. Acknowledgments..............................................21
14. References...................................................21
14.1. Normative References....................................21
14.2. Informative References..................................22
APPENDIX A: XRD Usage (Non-normative)............................24
A.1. How XRD Documents are Requested via WebFinger............24
A.2. WebFinger Example using XRDs.............................24
A.3. Security Considerations Related to XRDs..................25
Author's Addresses...............................................26
1. Introduction
There is a utility found on UNIX systems called "finger" [14] that
allows a person to access information about another person or entity
that has a UNIX account. The information queried might be on the
same computer or a computer anywhere in the world. What is returned
via "finger" is simply a plain text file that contains unstructured
information provided by the queried user, stored in a file named
.plan in the user's home directory.
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WebFinger borrows the concept of the legacy finger protocol, but
introduces a very different approach to sharing information. Rather
than return a simple unstructured text file, Webfinger uses
structured documents that contain link relations. These link
relations point to information and might return properties related to
information a user or entity on the Internet wishes to expose. For a
person, the kinds of information that might be exposed include a
personal profile address, identity service, telephone number, or
preferred avatar. WebFinger may also be used to discover information
about objects on the network, such as the amount of toner in a
printer or the physical location of a server.
Information returned via WebFinger might be for direct human
consumption (e.g., another user's phone number) or it might be used
by systems to help carry out some operation (e.g., facilitate logging
into a web site by determining a user's identity service).
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
WebFinger makes heavy use of "Link Relations". Briefly, a Link
Relation is an attribute and value pair used on the Internet wherein
the attribute identifies the type of link to which the associated
value refers. In Hypertext Transfer Protocol (HTTP) [2] and Web
Linking [4], the attribute is a "rel" and the value is an "href".
3. Overview
WebFinger enables the discovery of information about accounts,
devices, and other entities that are associated with a host.
Discover involves two distinct steps that may be optimized as a
single step, as will be explained later. The first step is to query
the host to find out how to discover information about accounts,
devices, and other entities associated with that host. The second
step is to query explicitly for a specific resource (e.g., user
account) to discover a set of link relations that point to resource-
specific information about the entity being queried.
This protocol makes heavy use of well-known URIs as defined in RFC
5785 [3] and "Link Relations" as defined in RFC 5988 [4]. Further,
the protocol builds on RFC 6415 [11], which provides the foundation
for the procedures described in this document.
Briefly, a link is a typed connection between two web resources that
are identified by Internationalized Resource Identifiers (IRIs) [13];
this connection consists of a context IRI, a link relation type, a
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target IRI, and optionally some target attributes, resulting in
statements of the form "{context IRI} has a {relation type} resource
at {target IRI}, which has {target attributes}". When used in the
Link HTTP header, the context IRI is the IRI of the requested
resource, the relation type is the value of the "rel" parameter, the
target IRI is URI-Reference contained in the Link header, and the
target attributes are the parameters such as "hreflang", "media",
"title", "title*", "type", and any other link-extension parameters.
The framework for WebFinger consists of several building blocks:
1. To query the host, one requests a web host metadata document
located at the well-known URI /.well-known/host-meta or /.well-
known/host-meta.json (referred to as the host-meta resources) at
the host.
2. The web server at the host returns a JavaScript Object Notation
(JSON) [5] Resource Descriptor (JRD) or an Extensible Resource
Descriptor (XRD) [10] document, including a Link-based Resource
Descriptor Document (LRDD) link relation.
3. To discover information about accounts, devices, or other entities
associated with the host, one requests the actual Link-based
Resource Descriptor Document associated with a particular URI at
the host (e.g., an "acct" URI, "http" URI, or "mailto" URI).
4. The web server at the host returns a JRD or XRD document for the
requested URI, which includes link relations pointing to resources
that contain more detailed information about the entity.
This model is illustrated in the examples in Section 4, then
described more formally in Section 5. Steps 2 and 3 above can be
accomplished simultaneously by utilizing the "resource" parameter
defined in Section 5.2.
4. Example Uses of WebFinger
In this section, we describe just a few sample uses for WebFinger and
show what the protocol looks like. This is not an exhaustive list of
possible uses and the entire section should be considered non-
normative. The list of potential use cases is virtually unlimited
since a user can share any kind of machine-consumable information via
WebFinger.
All of the following examples utilize JRDs, as that is the only
mandatory format required to be supported by WebFinger servers. For
completeness, an example utilizing XRDs is presented in Appendix A.
4.1. Locating a User's Blog
Assume you receive an email from Bob and he refers to something he
posted on his blog, but you do not know where Bob's blog is located.
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It would be simple to discover the address of Bob's blog if he makes
that information available via WebFinger.
Let's assume your email client discovers that blog automatically for
you. After receiving the message from Bob (bob@example.com), your
email client performs the following steps behind the scenes.
First, your email client tries to get the host metadata information
for the host example.com. It does this by issuing the following
HTTPS query to example.com:
GET /.well-known/host-meta.json HTTP/1.1
Host: example.com
The server replies with a JRD document:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"links" :
[
{
"rel" : "lrdd",
"type" : "application/json",
"template" : "https://example.com/lrdd/?f=json&uri={uri}"
}
]
}
The client then processes the received JRD in accordance with the Web
Host Metadata procedures. The client will see the LRDD link relation
and issue a query with the user's account URI [6] or other URI that
serves as an alias for the account. (The account URI is discussed in
Section 4.2.) The query might look like this:
GET /lrdd/?f=json&uri=acct%3Abob%40example.com HTTP/1.1
Host: example.com
The server might then respond with a message like this:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"expires" : "2012-10-12T20:56:11Z",
"subject" : "acct:bob@example.com",
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"aliases" :
[
"http://www.example.com/~bob/"
],
"links" :
[
{
"rel" : "http://webfinger.net/rel/avatar",
"href" : "http://www.example.com/~bob/bob.jpg"
},
{
"rel" : "http://webfinger.net/rel/profile-page",
"href" : "http://www.example.com/~bob/"
},
{
"rel" : "http://packetizer.com/rel/blog",
"href" : "http://blogs.example.com/bob/"
},
{
"rel" : "vcard",
"href" : "http://www.example.com/~bob/bob.vcf"
}
]
}
The email client might take note of the "blog" link relation in the
above JRD document that refers to Bob's blog. This URL would then be
presented to you so that you could then visit his blog. The email
client might also note that Bob has published an avatar link relation
and use that picture to represent Bob inside the email client.
Lastly, the client might consider the vcard [16] link relation in
order to update contact information for Bob.
Note in the above example that an alias is provided that can also be
used to return information about the user's account. Had the "http:"
URI shown as an alias been used to query for information about Bob,
the query would have appeared as:
GET /lrdd/?uri=http%3A%2F%2Fwww.example.com%2F~bob%2F HTTP/1.1
Host: example.com
The response would have been substantially the same, with the subject
and alias information changed as necessary. Other information, such
as the expiration time might also change, but the set of link
relations and properties would be the same with either response.
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4.2. Simplifying the Login Process
OpenID (http://www.openid.net) is great for allowing users to log
into a web site, though one criticism is that it is challenging for
users to remember the URI they are assigned. WebFinger can help
address this issue by allowing users to use user@domain-style
addresses. Using a user's account URI, a web site can perform a
query to discover the associated OpenID identifier for a user.
Let's assume Carol is trying to use OpenID to log into a blog. The
blog server might issue the following query to discover the OpenID
identity provider URL for Carol and to get Carol's avatar. In this
example, we utilize the "rel" and "resource" parameters as described
in sections 5.2 and 5.3:
GET /.well-known/host-meta.json?\
rel=avatar%20\
http%3A%3F%3Fspecs.openid.net%3Fauth%3F2.0%3Fprovider&\
resource=acct%3Acarol%40example.com HTTP/1.1
Host: example.com
The server might return a response like this:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"subject" : "acct:carol@example.com",
"links" :
[
{
"rel" : "http://webfinger.net/rel/avatar",
"href" : "http://example.com/~alice/alice.jpg"
},
{
"rel" : "http://specs.openid.net/auth/2.0/provider",
"href" : "https://openid.example.com/carol"
}
]
}
At this point, the blog server knows that Carol's OpenID identifier
is https://openid.example.com/carol and could then proceed with the
login process as usual. Her avatar can also be displayed for the
benefit of other users on the blog.
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4.3. Retrieving Device Information
While the examples thus far have been focused on information about
humans, WebFinger does not limit queries to only those that use the
account URI scheme. Any URI scheme that contains host information
MAY be used with WebFinger. Let's suppose there are devices on the
network like printers and you would like to check the current toner
level for a particular printer identified via the URI like
device:p1.example.com. While the "device" URI scheme is not
presently specified, we use it here only for illustrative purposes.
Following the procedures similar to those above, a query may be
issued to get link relations specific to this URI like this:
GET /.well-known/host-meta.json?resource=\
device%3Ap1.example.com HTTP/1.1
Host: example.com
The link relations that are returned may be quite different than
those for user accounts. Perhaps we may see a response like this:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"subject" : "device:p1.example.com",
"links" :
[
{
"rel" : "tipsi",
"href" : "http://192.168.1.5/npap/"
}
]
}
While this example is entirely fictitious, you can imagine that
perhaps the Transport Independent, Printer/System Interface [18] may
be enhanced with a web interface that allows a device that
understands the TIP/SI web interface specification to query the
printer for toner levels.
5. WebFinger Protocol
WebFinger does not actually introduce a new protocol, per se.
Rather, it builds upon the existing Web Host Metadata specification
and leverages the Cross-Origin Resource Sharing (CORS) [9]
specification.
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While WebFinger strives to maintain backward-compatibility with RFC
6415, this specification introduces a fundamental change in
requirements. Specifically, support for server-side production of
JSON Resource Descriptor (JRD) documents is mandatory and support for
server-side production Extensible Resource Descriptor (XRD) documents
is optional. Please refer to Section 10 for interoperability
considerations.
5.1. Performing a WebFinger Query
The first step a client performs in executing a WebFinger query is to
query for the host metadata using HTTPS or HTTP. The procedures are
defined in the Web Host Metadata specification. It is strongly
RECOMMENDED that WebFinger servers return content using secure
(HTTPS) connections. Clients MUST first attempt queries using HTTPS
before attempting a query using HTTP.
WebFinger clients MUST locate the LRDD link relation and perform a
query for that link relation, if present. All other link templates
found must be processed to form a complete resource descriptor. The
processing rules in Section 4.2 of RFC 6415 MUST be followed.
WebFinger servers MAY accept requests for both JRD and XRD documents,
but MUST support requests for JRD documents. For interoperability
with RFC 6415 implementations, the default representation returned by
a server via the resource at /.well-known/host-meta MUST be an XRD
document if XRD is supported by the server and a JRD document is not
explicitly requested by the client. The default format returned via
the resource /.well-known/host-meta.json MUST be a JRD document.
As per RFC 6415, a JRD document MUST be returned by the WebFinger
server if the client explicitly requests it by querying /.well-
known/host-meta.json or by querying /.well-known/host-meta and
including an "Accept" header in the HTTP request with a type of
"application/json" [5]. Additionally, the server MUST return a JRD
document if it does not support production of XRD documents (or any
other format requested by the client). Servers MUST indicate the
type of document returned using the "Content-Type" header in the HTTP
response.
To avoid the possibility of receiving the wrong document format,
WebFinger clients SHOULD submit queries to the server via the /.well-
known/host-meta.json resource.
If the client requests a JRD document when querying for host
metadata, the WebFinger server MUST assume that the client will want
a JRD document when querying the LRDD resource. Thus when the
WebFinger server returns a JRD document containing host metadata that
contains an LRDD link relation, it MUST include a URI for the LRDD
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resource(s) that will return a JRD document. Likewise, if a client
requests an XRD document when querying the host metadata resource,
the server MUST, unless unable due to external factors, return LRDD
link relations that would return XRD documents.
It is important to note that unless the "resource" parameter is used
as per section 5.2, it is the responsibility of the client to process
each of the LRDD link relations as per Section 4.2 of RFC 6415 if a
server returns multiple LRDD link relations. Multiple LRDD link
relations in a server response do not represent alternative URIs for
the same LRDD document.
If the client queries the LRDD resource and provides a URI for which
the server has no information, the server MUST return a 404 status
code. Likewise, any query to a URI in the resource descriptor that
is unknown to the server MUST result in the server returning a 404
status code.
WebFinger servers MAY include cache validators in a response to
enable conditional requests by clients and/or expiration times as per
RFC 2616 section 13.
5.2. The Web Host Metadata "resource" Parameter
In addition to the traditional processing logic for processing host
metadata information, WebFinger defines the "resource" parameter for
querying for host metadata and returning all of the link relations
from LRDD and other resource-specific link templates in a single
response. This parameter essentially pushes the work to the server
to form a complete resource descriptor for the specified resource.
WebFinger servers compliant with this specification MUST support for
the "resource" parameter. Note that an RFC 6415-compliant server
might not implement the "resource" parameter, though the server would
respond to queries from the client as described in RFC 6415. Thus,
WebFinger clients MUST check the server response to ensure that the
"resource" parameter is supported as explained below.
To utilize the host-meta "resource" parameter, a WebFinger client
issues a request to /.well-known/host-meta.json (RECOMMENDED) or
/.well-known/host-meta as usual, but then appends a "resource"
parameter as shown in this example:
GET /.well-known/host-meta.json?resource=\
acct%3Abob%40example.com HTTP/1.1
Host: example.com
When processing this request, the WebFinger server MUST
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* Return a 404 status code if the URI provided in the resource
parameter is unknown to the server; and
* Set the "Subject" returned in the response to the value of the
"resource" parameter if the URI provided in the resource
parameter is known to the server; and
* Collect and expand all resource-specific link relations,
including those returned by querying for any LRDD link
relations, discard any host-wide link relations, and return a
complete resource descriptor following the processing rules in
Section 4.2 of RFC 6415; and
It is not the responsibility of the WebFinger server to verify, for
example, that a URI pointing to a person's avatar is a valid URI. If
the WebFinger server needs to query an LRDD resource to collect
additional resource-specific information, any errors (e.g., 500 or
404) MUST be ignored by the server. When a request for an LRDD
fails, the server MUST NOT attempt to augment missing resource
information or return a "template" type link relation to a client
that utilizes the "resource" parameter. Note that a WebFinger server
might be implemented such that all LRDD resource-specific information
can be resolved without issuing HTTP requests. How a WebFinger
server collects and expands such resource-specific link relations is
an implementation matter.
The WebFinger client MUST verify support for the "resource" parameter
by checking the value of the Subject returned in the response. If
the Subject matches the value of the "resource" parameter, then the
"resource" parameter is supported by the server. The Subject would
be absent if the "resource" parameter is not supported.
For illustrative purposes, the following is an example usage of the
"resource" parameter that aligns with the example in Section 1.1.1 of
RFC 6415. The WebFinger client would issue this request:
GET /.well-known/host-meta.json?resource=\
http%3A%2F%2Fexample.com%2Fxy HTTP/1.1
Host: example.com
Note: The "\" character shown above and used throughout this document
indicates that the line breaks at this point and continues on the
next line. The content of the next line should be concatenated to
the previous line without any whitespace characters, replacing the
"\" character. This is shown only to avoid line wrapping in this
document.
The WebFinger server would reply with this response:
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HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"subject" : "http://example.com/xy",
"properties" :
{
"http://spec.example.net/color" : "red"
},
"links" :
[
{
"rel" : "hub",
"href" : "http://example.com/hub"
},
{
"rel" : "hub",
"href" : "http://example.com/another/hub"
},
{
"rel" : "author",
"href" : "http://example.com/john"
},
{
"rel" : "author",
"href" : "http://example.com/author?\
q=http%3A%2F%2Fexample.com%2Fxy"
}
]
}
5.3. The Web Host Metadata "rel" Parameter
WebFinger also defines the "rel" parameter for use when querying for
host metadata or resource-specific information. It is used to return
a subset of the information that would otherwise be returned without
the "rel" parameter. When the "rel" parameter is used, only the link
relations that match the space-separated list of link relations
provided via "rel" are included in the list of links returned in the
resource descriptor. All other information normally present in a
resource descriptor is present in the resource descriptor, even when
"rel" is employed.
The purpose of the "rel" parameter is to return a subset of
resource's link relations. It is not intended to reduce the work
required of a server to produce a response. That said, use of the
parameter might reduce processing requirements on either the client
or server, and it might also reduce the bandwidth required to convey
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the partial resource descriptor, especially if there are numerous
link relation values to convey for a given resource.
Support for the "rel" parameter is OPTIONAL, but support is
RECOMMENDED for the host-meta resources and LRDD resources.
For illustrative purposes, the following is an example usage of the
"rel" parameter that aligns with the example in Section 1.1.1 of RFC
6415. The WebFinger client would issue this request to receive links
that are of the type "hub" and "copyright":
GET /.well-known/host-meta.json?resource=\
http%3A%2F%2Fexample.com%2Fxy&rel=hub%20copyright HTTP/1.1
Host: example.com
The WebFinger server would reply with this response:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"subject" : "http://example.com/xy",
"properties" :
{
"http://spec.example.net/color" : "red"
},
"links" :
[
{
"rel" : "hub",
"href" : "http://example.com/hub"
},
{
"rel" : "hub",
"href" : "http://example.com/another/hub"
}
]
}
Note that in this example, the "author" links are removed, though all
other content is present. Since there were no "copyright" links,
none are returned.
In the event that a client requests links for link relations that are
not defined for the specified resource, a resource descriptor MUST be
returned, void of any links. When a JRD is returned, the "links"
array MAY be either absent or empty. The server MUST NOT return a
404 status code when a particular link relation specified via "rel"
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is not defined for the resource, as a 404 status code is reserved for
indicating that the resource itself (e.g., either /.well-known/host-
meta.json or the resource indicated via the "resource" parameter)
does not exist.
5.4. WebFinger and URIs
Requests for both LRDD documents and host metadata can include a
parameter specifying the URI of an account, device, or other entity
(for LRDD this is the "uri" parameter as defined by the operative JRD
or XRD template and for host metadata this is the "resource"
parameter). WebFinger itself is agnostic regarding the scheme of
such a URI: it could be an "acct" URI [7], an "http" or "https" URI,
a "mailto" URI, or some other scheme.
For resources associated with a user account at a host, use of the
"acct" URI scheme is RECOMMENDED, since it explicitly identifies an
account accessible via WebFinger. Further, the "acct" URI scheme is
not associated with other protocols as, by way of example, the
"mailto" URI scheme is associated with email. Since not every host
offers email service, using the "mailto" URI scheme [8] is not ideal
for identifying user accounts on all hosts. That said, use of the
"mailto" URI scheme would be ideal for use with WebFinger to discover
mail server configuration information for a user, for example.
A host MAY utilize one or more URIs that serve as aliases for the
user's account, such as URIs that use the "http" URI scheme [2]. A
WebFinger server MUST return substantially the same response to both
an "acct" URI and any alias URI for the account, including the same
set of link relations and properties. In addition, the server SHOULD
include the entire list aliases for the user's account in the JRD or
XRD returned when querying the LRDD resource or when utilizing the
"resource" parameter.
6. The "acct" Link Relation
6.1. Purpose for the "acct" Link Relation
Users of some services might have an "acct" URI that looks
significantly different from his or her email address, perhaps using
an entirely different domain name. It is also possible for a user to
have multiple accounts that a user wants to have cross-referenced
from another account. To address both of these needs, this
specification defines the "acct" link relation.
The "acct" link relation allows a resource descriptor to reference
one or more other user account URIs. The "acct" link relation is
intended to allow a client to incorporate additional link relations
by reference so that it might utilize a more complete set of link
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relations for a user. For example, a user acct:bob@example.com might
wish to allow a client to discover additional information about him
by including an "acct" link relation with the URI
acct:bob@example.net.
Note that the "acct" link relation does not replace the use of
standard HTTP 3xx response codes to indicate the new temporary or
permanent location of a user account. If a user account is moved to
a different location, then a 3xx response code SHOULD be used. Also,
the "acct" link relation does not replace Link-based Resource
Descriptor Documents (LRDDs). A WebFinger server might return
multiple LRDD link relations for a user, each of which perhaps
containing link relations that are to be merged to form a complete
resource descriptor. The "acct" link relation is different in that
it would refer to an entirely different, separate resource
descriptor. Further, only a client would act consider the "acct"
link relations as it performs queries, not the WebFinger server.
Since an account may make a reference to one or more different
accounts, WebFinger clients that support automatic processing of the
"acct" link relations MUST take steps to avoid loops wherein two
account URIs, directly or indirectly, refer the client to each other.
There are no limits on the number of "acct" link relations that might
be returned in a WebFinger query.
An "acct" link relation used within the context of a WebFinger query
for a user's account MUST NOT return "acct" link relations for
another user.
Client-side consideration of the "acct" link relation is OPTIONAL and
WebFinger server MUST NOT assume a client will perform additional
processing in response to receiving an "acct" link relation.
6.2. Example Message Exchange Using the "acct" Link Relation
Consider the following non-normative example.
Suppose Alice receives an email from bob@example.net. While Bob's
email identifier might be in the example.net domain, he holds a user
account in the example.com domain and another account in the
example.org domain. His email provider may provide WebFinger
services, but is unable to serve information from other domains.
Suppose Alice's client issues the following request:
GET /.well-known/host-meta.json?resource=\
acct%3Abob%40example.net HTTP/1.1
Host: example.net
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The response that Alice's client receives back might be:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"subject" : "acct:bob@example.net",
"links" :
[
{
"rel" : "acct",
"href" : "acct:bob@example.com"
},
{
"rel" : "acct",
"href" : "acct:bob@example.org"
},
{
"rel" : "acct",
"href" : "mailto:bob@example.net"
}
]
}
While these link relations provide Alice with very little
information, Alice's WebFinger client could then perform subsequent
queries against the URIs acct:bob@example.com, acct:bob@example.org,
and mailto:bob@example.net in order to get the information Alice is
seeking.
7. Cross-Origin Resource Sharing (CORS)
WebFinger is most useful when it is accessible without restrictions
on the Internet, and that includes web browsers. Therefore,
WebFinger servers MUST support Cross-Origin Resource Sharing (CORS)
[9] when serving content intended for public consumption.
Specifically, all queries to /.well-known/host-meta.json, /.well-
known/host-meta, and to any LRDD URIs MUST include the following HTTP
header in the response:
Access-Control-Allow-Origin: *
Enterprise WebFinger servers that wish to restrict access to
information from external entities SHOULD use a more restrictive
Access-Control-Allow-Origin header and MAY exclude the header
entirely.
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8. Controlling Access to Information
As with all web resources, access to the Host Metadata resource and
the LRDD resource MAY require authentication. Further, failure to
provide required credentials MAY result in the server forbidding
access or providing a different response than had the client
authenticated with the server.
Likewise, a server MAY provide different responses to different
clients based on other factors, such as whether the client is inside
or outside a corporate network. As a concrete example, a query
performed on the internal corporate network might return link
relations to employee pictures whereas link relations for employee
pictures might not be provided to external entities.
Further, link relations provided in a WebFinger server response MAY
point to web resources that impose access restrictions. For example,
it is possible that the aforementioned corporate server may provide
both internal and external entities with URIs to employee pictures,
but further authentication MAY be required in order for the WebFinger
client to access those picture resources if the request comes from
outside the corporate network.
The decisions made with respect to what set of link relations a
WebFinger server provides to one client versus another and what
resources require further authentication, as well as the specific
authentication mechanisms employed, are outside the scope of this
document.
9. Hosted and Distributed WebFinger Services
9.1. Hosting the Entire Domain
As with most services provided on the Internet, it is possible for a
domain owner to utilize "hosted" WebFinger services. By way of
example, a domain owner might control most aspects of their domain,
but use a third-party hosting service email. In the case of email,
mail servers for a domain are identified by MX records. An MX record
points to the mail server to which mail for the domain should be
delivered. It does not matter to the sending mail server whether
those MX records point to a server in the destination domain or a
different domain.
Likewise, a domain owner might utilize the services of a third party
to provide WebFinger services on behalf of its users. Just as a
domain owner was required to insert MX records into DNS to allow for
hosted email serves, the domain owner is required to redirect HTTP(S)
queries to its domain to allow for hosted WebFinger services.
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When a query is issued to /.well-known/host-meta.json or /.well-
known/host-meta, the target domain's web server MUST return a 301,
302, or 307 response status code that includes a Location header
pointing to the location of the hosted WebFinger service URL. The
WebFinger service URL does not need to point to /.well-known/* on the
hosting service provider server. In fact, it should not, as that
location would be reserved for queries relating to the service
provider's domain. WebFinger clients MUST follow all 301, 302, or
307 redirection requests.
As an example, let's assume that example.com's WebFinger services are
hosted by example.net. Suppose a client issues a query for
acct:alice@example.com like this:
GET /.well-known/host-meta.json?
resource=acct%3Aalice%40example.com HTTP/1.1
Host: example.com
The server might respond with this:
HTTP/1.1 301 Moved Permanently
Location: http://wf.example.net/example.org/host-meta.json
The client should follow the request, re-issuing the request to the
URL provided in the Location header.
Note that both of the /.well-known/host-meta.json and /.well-
known/host-meta resources need to be considered when redirecting
request to third party service providers. Those URLs requests SHOULD
NOT be redirected to the same location and without any
differentiation, since the default format returned by host-meta.json
is a JRD and the default format returned by host-meta MAY be XRD.
Each resource is distinct and should be redirected separately and to
different service locations or differentiated with a URI parameter.
Since the "Referer" HTTP header field is not mandatory, service
providers cannot rely on that header to determine the URL of the
original request.
9.2. Distributed WebFinger Services
A domain owner may wish to manage only a part of its WebFinger
services and WebFinger service providers or the domain owner may wish
to distribute WebFinger services across a number of WebFinger service
locations. The key to enabling this type of distribution is
placement of resource-specific information in more than one LRDD
document, each document existing at different locations.
Assume that the company operating example.com manages its own
WebFinger services, but also wants to utilize the services of
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example.org to serve link relations related to some aspects of its
business. Suppose a client issued this request:
GET /.well-known/host-meta.json HTTP/1.1
Host: example.com
The server might reply with this JRD document:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"links" :
[
{
"rel" : "lrdd",
"type" : "application/json",
"template" : "https://example.com/lrdd/?f=json&uri={uri}"
},
{
"rel" : "lrdd",
"type" : "application/json",
"template" : "https://wf.example.org/lrdd/?f=json&uri={uri}"
}
]
}
This would indicate to the client that some of the resource-specific
information is found at example.com and some is found at example.org,
following those specific URLs. Observing the rules in Section 4.2 of
RFC 6415, the client would issue queries to both URLs and construct a
complete resource descriptor.
As discussed in Section 5.2, a client may issue a query like this to
the example.com domain:
GET /.well-known/host-meta.json?resource=\
acct%3Aalice%40example.com HTTP/1.1
Host: example.com
In that case, it would be the responsibility of the WebFinger server
at example.com to query the LRDD URL at example.org and then compose
a complete descriptor document. The client that uses the resource
parameter remains entirely oblivious to the fact that link relation
information is distributed across multiple servers or domains.
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10. Web Host Metadata Interoperability Considerations
As noted in Section 3, RFC 6415 required all servers to support the
production of Extensible Resource Documents (XRDs) and optionally
support the production of JSON Resource Documents (JRDs). This
specification reverses that requirement: WebFinger-compliant servers
MUST support JRD and MAY support XRD documents.
Given that some servers might implement only RFC 6415 and other
servers might implement only the minimum required set of features
defined for WebFinger, all clients should take care to ensure to
request a resource descriptor in the appropriate format. If a client
wishes to receive only JRDs, for example, it SHOULD issue a request
to /.well-known/host-meta.json, but MAY issue a request to /.well-
known/host-meta and include the "Accept" header with the type
"application/json".
Further, clients MUST ensure that the response returned from the
server contains the correct format. RFC 6415-compliant servers might
return an XRD document, regardless of what is requested by the
client.
Lastly, RFC 6415 did not require clients to follow 301, 302, or 307
redirection requests, but WebFinger clients MUST re-issue requests
when redirected using any of those HTTP status codes.
11. Security Considerations
All of the security considerations applicable to Web Host Metadata
and Cross-Origin Resource Sharing [9] are also applicable to this
specification. Of particular importance is the recommended use of
HTTPS to ensure that information is not modified during transit.
Clients SHOULD verify that the certificate used on an HTTPS
connection is valid.
Service providers and users should be aware that placing information
on the Internet accessible through WebFinger means that any user can
access that information. While WebFinger can be an extremely useful
tool for allowing quick and easy access to one's avatar, blog, or
other personal information, users should understand the risks, too.
If one does not wish to share certain information with the world, do
not allow that information to be freely accessible through WebFinger.
The aforementioned word of caution is perhaps worth emphasizing again
with respect to dynamic information one might wish to share, such as
the current location of a user. WebFinger can be a powerful tool
used to assemble information about a person all in one place, but
service providers and users should be mindful of the nature of that
information shared and the fact that it might be available for the
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entire world to see. Sharing location information, for example,
would potentially put a person in danger from any individual who
might seek to inflict harm on that person.
The easy access to user information via WebFinger was a design goal
of the protocol, not a limitation. If one wishes to limit access to
information available via WebFinger, such as a WebFinger server for
use inside a corporate network, the network administrator must take
measures necessary to limit access from outside the network. Using
standard methods for securing web resources, network administrators
do have the ability to control access to resources that might return
sensitive information. Further, WebFinger servers can be employed in
such a way as to require authentication and prevent disclosure of
information to unauthorized entities.
12. IANA Considerations
RFC Editor: Please replace QQQQ in the following two sub-sections
with a reference to this RFC.
12.1. Registration of the "acct" Link Relation Type
Relation Name: acct
Description: A link relation that refers to a user's WebFinger
account identifier.
Reference: RFC QQQQ
Notes:
Application Data:
13. Acknowledgments
The authors would like to acknowledge Eran Hammer-Lahav, Blaine Cook,
Brad Fitzpatrick, Laurent-Walter Goix, Joe Clarke, Mike Jones, and
Peter Saint-Andre for their invaluable input.
14. References
14.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
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[3] Nottingham, M., Hammer-Lahav, E., "Defining Well-Known Uniform
Resource Identifiers (URIs)", RFC 5785, April 2010.
[4] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[5] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[6] Berners-Lee, T., Fielding, R., and Masinter, L., "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[7] Saint-Andre, P., "The 'acct' URI Scheme", draft-ietf-appsawg-
acct-uri-00, August 2012.
[8] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto' URI
Scheme", RFC 6068, October 2010.
[9] Van Kesteren, A., "Cross-Origin Resource Sharing", W3C CORS
http://www.w3.org/TR/cors/, July 2010.
[10] Hammer-Lahav, E. and W. Norris, "Extensible Resource Descriptor
(XRD) Version 1.0", http://docs.oasis-
open.org/xri/xrd/v1.0/xrd-1.0.html.
[11] Hammer-Lahav, E. and Cook, B., "Web Host Metadata", RFC 6415,
October 2011.
[12] American National Standards Institute, "Coded Character Set -
7-bit American Standard Code for Information Interchange", ANSI
X3.4, 1986.
[13] Duerst, M., "Internationalized Resource Identifiers (IRIs)",
RFC 3987, January 2005.
14.2. Informative References
[14] Zimmerman, D., "The Finger User Information Protocol", RFC
1288, December 1991.
[15] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", BCP 35, RFC 4395,
February 2006.
[16] Perreault, S., "vCard Format Specification", RFC 6350, August
2011.
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[17] Internet Assigned Numbers Authority (IANA) Registry, "Uniform
Resource Identifier (URI) Schemes",
<http://www.iana.org/assignments/uri-schemes.html>.
[18] "Transport Independent, Printer/System Interface", IEEE Std
1284.1-1997, 1997.
[19] Hoffman, P., Yergeau, F., "UTF-16, an encoding of ISO 10646",
RFC 2781, February 2000.
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APPENDIX A: XRD Usage (Non-normative)
A.1. How XRD Documents are Requested via WebFinger
The framework for using XRD documents with WebFinger is as follows:
1. WebFinger clients issue request for XRD documents by requesting
the Web Host Metadata document located at the well-known URI
/.well-known/host-meta at the host.
2. The web server at the host returns an XRD document, including a
Link-based Resource Descriptor Document (LRDD) link relation.
3. To discover information about accounts, devices, or other
entities associated with the host, a request is issued for the
Link-based Resource Descriptor Document(s) associated with a
particular URI at the host (e.g., an "acct" URI, "http" URI, or
"mailto" URI).
4. The web server at the host would return an XRD document about
the requested URI, which included those resource-specific link
relations pointing to resources that contain information about
the entity.
5. Following the procedures in Section 4.2 of RFC 6415, the client
would assemble all of the resource-specific link relations from
the host-meta resource and LRDD resource(s) into a complete
resource descriptor.
The LRDD resources return resource descriptor documents of the type
"application/xrd+xml".
A.2. WebFinger Example using XRDs
Section 4 introduces examples where JRD documents are returned to
clients. For completeness, this section shows an example where a
client requests an XRD document.
Recall the example from Section 4.1 where the email client tried to
retrieve information about Bob to discover the URL for his blog. If
the client implemented support for XRD, it tries to get the host
metadata information for the domain example.com in a similar way. As
with the original example, it issues the following HTTPS query to
example.com:
GET /.well-known/host-meta HTTP/1.1
Host: example.com
The server replies with an XRD document:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/xrd+xml; charset=UTF-8
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<?xml version="1.0" encoding="UTF-8"?>
<XRD xmlns="http://docs.oasis-open.org/ns/xri/xrd-1.0">
<Link rel="lrdd"
type="application/xrd+xml"
template="https://example.com/lrdd/?uri={uri}"/>
</XRD>
The client then processes the received XRD in accordance with the Web
Host Metadata procedures. The client will see the LRDD link relation
and issue a query with the user's account URI [6] or other URI that
serves as an alias for the account. (The account URI is discussed in
Section 4.2.) The query might look like this:
GET /lrdd/?uri=acct%3Abob%40example.com HTTP/1.1
Host: example.com
The server might then respond with a message like this:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/xrd+xml; charset=UTF-8
<?xml version="1.0" encoding="UTF-8"?>
<XRD xmlns="http://docs.oasis-open.org/ns/xri/xrd-1.0">
<Expires>2012-10-12T20:56:11Z</Expires>
<Subject>acct:bob@example.com</Subject>
<Alias>http://www.example.com/~bob/</Alias>
<Link rel="http://webfinger.net/rel/avatar"
href="http://www.example.com/~bob/bob.jpg"/>
<Link rel="http://webfinger.net/rel/profile-page"
href="http://www.example.com/~bob/"/>
<Link rel="http://packetizer.com/rel/blog"
href="http://blogs.example.com/bob/"/>
</XRD>
The email client might take note of the "blog" link relation in the
above XRD document that refers to Bob's blog. This URL would then be
presented to you so that you could then visit his blog.
A.3. Security Considerations Related to XRDs
When using HTTP to request an XRD document, WebFinger clients SHOULD
verify the XRD document's signature, if present, to ensure that the
XRD document has not been modified. Additionally, WebFinger servers
SHOULD include a signature for XRD documents served over HTTP.
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Author's Addresses
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Phone: +1 919 476 2048
Email: paulej@packetizer.com
IM: xmpp:paulej@packetizer.com
Gonzalo Salgueiro
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Phone: +1 919 392 3266
Email: gsalguei@cisco.com
IM: xmpp:gsalguei@cisco.com
Joseph Smarr
Google
Email: jsmarr@google.com
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