Network Working Group J. Whitehead
Request for Comments: 4437 U.C. Santa Cruz
Category: Experimental G. Clemm
IBM
J. Reschke, Ed.
greenbytes
March 2006
Web Distributed Authoring and Versioning (WebDAV)
Redirect Reference Resources
Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification defines an extension to Web Distributed Authoring
and Versioning (WebDAV) to allow clients to author HTTP redirect
reference resources whose default response is an HTTP/1.1 3xx
(Redirection) status code. A redirect reference makes it possible to
access the target resourced indirectly through any URI mapped to the
redirect reference resource. This specification does not address
remapping of trees of resources or regular expression based
redirections. There are no integrity guarantees associated with
redirect reference resources. Other mechanisms can also be used to
achieve the same functionality as this specification. This
specification allows operators to experiment with this mechanism and
develop experience on what is the best approach to the problem.
Table of Contents
1. Introduction ....................................................3
2. Notational Conventions ..........................................4
3. Terminology .....................................................4
4. Overview of Redirect Reference Resources ........................5
5. Operations on Redirect Reference Resources ......................6
6. MKREDIRECTREF Method ............................................7
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6.1. Example: Creating a Redirect Reference Resource
with MKREDIRECTREF .........................................8
7. UPDATEREDIRECTREF Method ........................................9
7.1. Example: Updating a Redirect Reference Resource with
UPDATEREDIRECTREF .........................................10
8. Operations on Collections That Contain Redirect
Reference Resources ............................................11
8.1. Example: PROPFIND on a Collection with Redirect
Reference .................................................11
8.2. Example: PROPFIND with Apply-To-Redirect-Ref on a
Collection with Redirect Reference Resources ..............13
9. Operations on Targets of Redirect Reference Resources ..........15
10. Relative References in DAV:reftarget ..........................15
10.1. Example: Resolving a Relative Reference in a
Multi-Status Response.....................................16
11. Redirect References to Collections ............................17
12. Headers .......................................................18
12.1. Redirect-Ref Response Header .............................18
12.2. Apply-To-Redirect-Ref Request Header .....................19
13. Redirect Reference Resource Properties ........................19
13.1. DAV:redirect-lifetime (protected) ........................19
13.2. DAV:reftarget (protected) ................................19
14. XML Elements ..................................................19
14.1. redirectref XML Element ..................................19
15. Extensions to the DAV:response XML Element for Multi-Status
Responses .....................................................20
16. Capability Discovery ..........................................20
16.1. Example: Discovery of Support for Redirect
Reference Resources ......................................20
17. Security Considerations .......................................21
17.1. Privacy Concerns .........................................21
17.2. Redirect Loops ...........................................21
17.3. Redirect Reference Resources and Denial of Service .......21
17.4. Revealing Private Locations ..............................22
18. Internationalization Considerations ...........................22
19. IANA Considerations ...........................................22
19.1. HTTP headers .............................................22
19.1.1. Redirect-Ref ......................................22
19.1.2. Apply-To-Redirect-Ref .............................23
20. Contributors ..................................................23
21. Acknowledgements ..............................................23
22. Normative References ..........................................23
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1. Introduction
This specification extends the Web Distributed Authoring Protocol
(WebDAV) to enable clients to create new access paths to existing
resources. This capability is useful for several reasons.
WebDAV makes it possible to organize HTTP resources into hierarchies,
placing them into groupings, known as collections, that are more
easily browsed and manipulated than a single flat collection.
However, hierarchies require categorization decisions that locate
resources at a single location in the hierarchy, a drawback when a
resource has multiple valid categories. For example, in a hierarchy
of vehicle descriptions containing collections for cars and boats, a
description of a combination car/boat vehicle could belong in either
collection. Ideally, the description should be accessible from both.
Allowing clients to create new URIs that access the existing resource
lets them put that resource into multiple collections.
Hierarchies also make resource sharing more difficult, since
resources that have utility across many collections are still forced
into a single collection. For example, the mathematics department at
one university might create a collection of information on fractals
that contains bindings to some local resources, but also provides
access to some resources at other universities. For many reasons, it
may be undesirable to make physical copies of the shared resources:
to conserve disk space, to respect copyright constraints, or to make
any changes in the shared resources visible automatically. Being
able to create new access paths to existing resources in other
collections or even on other unrelated systems is useful for this
sort of case.
The redirect reference resources defined here provide a mechanism for
creating alternative access paths to existing resources. A redirect
reference resource is a resource in one collection whose purpose is
to redirect requests to another resource (its target), possibly in a
different collection. In this way, it allows clients to submit
requests to the target resource from another collection. It
redirects most requests to the target resource using an HTTP status
code from the 3xx range (Redirection), thereby providing a form of
mediated access to the target resource.
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A redirect reference is a resource with properties but with no body
of its own. Properties of a redirect reference resource can contain
information such as who created the reference, when, and why. Since
redirect reference resources are implemented using HTTP 3xx
responses, it generally takes two round trips to submit a request to
the intended resource. Redirect references work equally well for
local resources and for resources that reside on a different system
from the reference.
The remainder of this document is structured as follows: Section 3
defines terms that will be used throughout the specification.
Section 4 provides an overview of redirect reference resources.
Section 5 defines the semantics of existing methods when applied to
redirect reference resources. Section 6 discusses how to create a
redirect reference resource, and Section 7 discusses updating
redirect references. Section 8 discusses their semantics when
applied to collections that contain redirect reference resources.
Sections 9 through 11 discuss several other issues raised by the
existence of redirect reference resources. Sections 12 through 15
define the new headers, properties, and XML elements required to
support redirect reference resources. Section 16 discusses
capability discovery. Sections 17 through 19 present the security,
internationalization, and IANA concerns raised by this specification.
The remaining sections provide a variety of supporting information.
2. Notational Conventions
Since this document describes a set of extensions to the WebDAV
Distributed Authoring Protocol [RFC2518], itself an extension to the
HTTP/1.1 protocol, the augmented BNF used here to describe protocol
elements is exactly the same as described in Section 2.1 of
[RFC2616]. Since this augmented BNF uses the basic production rules
provided in Section 2.2 of [RFC2616], these rules apply to this
document as well.
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in [RFC2119].
3. Terminology
The terminology used here follows and extends that in the WebDAV
Distributed Authoring Protocol specification [RFC2518]. Definitions
of the terms resource, Uniform Resource Identifier (URI), and Uniform
Resource Locator (URL) are provided in [RFC3986].
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Redirect Reference Resource
A resource created to redirect all requests made to it, using an
HTTP status code from the 3xx range, to a defined target resource.
Non-Reference Resource
A resource that is not a reference to another resource.
Target Resource
The resource to which requests are redirected by a redirect
reference resource. A target resource can be anything that can be
identified by an absolute URI (see [RFC3986], "absolute-URI").
This document uses the terms "precondition", "postcondition", and
"protected property" as defined in [RFC3253]. Servers MUST report
pre-/postcondition failures as described in Section 1.6 of this
document.
4. Overview of Redirect Reference Resources
For all operations submitted to a redirect reference resource, the
default response is a 302 (Found), accompanied by the Redirect-Ref
header (defined in Section 12.1, below) and the Location header
([RFC2616], Section 14.30) set to the URI of the target resource.
With this information, the client can resubmit the request to the URI
of the target resource.
A redirect reference resource never automatically forwards requests
to its target resource. Redirect resources bring the same benefits
as links in HTML documents. They can be created and maintained
without the involvement or even knowledge of their target resource.
This reduces the cost of linking between resources.
If the client is aware that it is operating on a redirect reference
resource, it can resolve the reference by retrieving the reference
resource's DAV:reftarget property (defined in Section 13.2, below),
whose value contains the URI of the target resource. It can then
submit requests to the target resource.
A redirect reference resource is a new type of resource. To
distinguish redirect reference resources from non-reference
resources, a new value of the DAV:resourcetype property (defined in
[RFC2518]), DAV:redirectref, is defined in Section 14.1, below.
Since a redirect reference resource is a resource, methods can be
applied to the reference resource as well as to its target resource.
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The Apply-To-Redirect-Ref request header (defined in Section 12.2,
below) is provided so that referencing-aware clients can control
whether an operation is applied to the redirect reference resource or
standard HTTP/WebDAV behaviour (redirection with a 3xx status code)
should occur. The Apply-To-Redirect-Ref header can be used with most
requests to redirect reference resources. This header is
particularly useful with PROPFIND, to retrieve the reference
resource's own properties.
Implementation Note: Operations on the target of a redirect reference
usually do not affect the redirect reference itself. However,
clients should not rely on this behaviour (for instance, some servers
may update redirect references as a result of namespace operations on
the reference's target).
5. Operations on Redirect Reference Resources
Although non-referencing-aware clients cannot create reference
resources, they should be able to submit requests through the
reference resources created by reference-aware WebDAV clients. They
should be able to follow any references to their targets. To make
this possible, a server that receives any request made via a redirect
reference resource MUST return a 3xx range (Redirection) status code,
unless the request includes an Apply-To-Redirect-Ref header
specifying "T". The client and server MUST follow [RFC2616], Section
10.3, but with these additional rules:
o The Location response header MUST contain a URI (see [RFC3986],
Section 3) that identifies the target of the reference resource.
o The response MUST include the Redirect-Ref header. This header
allows reference-aware WebDAV clients to recognize the resource as
a reference resource and to understand the reason for the
redirection.
A reference-aware WebDAV client can, like a non-referencing client,
resubmit the request to the URI in the Location header in order to
operate on the target resource. Alternatively, it can resubmit the
request to the URI of the redirect reference resource with the
"Apply-To-Redirect-Ref: T" header in order to operate on the
reference resource itself. In this case, the request MUST be applied
to the reference resource itself, and a 3xx response MUST NOT be
returned.
As redirect references do not have bodies, GET and PUT requests with
"Apply-To-Redirect-Ref: T" MUST fail with status 403 (forbidden).
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6. MKREDIRECTREF Method
The MKREDIRECTREF method requests the creation of a redirect
reference resource.
If a MKREDIRECTREF request fails, the server state preceding the
request MUST be restored.
Responses from a MKREDIRECTREF request MUST NOT be cached, as
MKREDIRECTREF has non-idempotent and non-safe semantics (see
[RFC2616], Section 9.1).
Marshalling
The request body MUST be a DAV:mkredirectref XML element.
<!ELEMENT mkredirectref (reftarget, redirect-lifetime?)>
<!ELEMENT reftarget (href)>
<!ELEMENT redirect-lifetime (permanent | temporary)>
<!ELEMENT permanent EMPTY>
<!ELEMENT temporary EMPTY>
The DAV:href element is defined in [RFC2518] (Section 12.3) and
MUST contain either a URI or a relative-ref (see [RFC3986],
Sections 3 and 4.2).
If no DAV:redirect-lifetime element is specified, the server MUST
behave as if a value of DAV:temporary was specified.
If the request succeeds, the server MUST return 201 (Created)
status.
If a response body for a successful request is included, it MUST
be a DAV:mkredirectref-response XML element. Note that this
document does not define any elements for the MKREDIRECTREF
response body, but the DAV:mkredirectref-response element is
defined to ensure interoperability between future extensions that
do define elements for the response body.
<!ELEMENT mkredirectref-response ANY>
Preconditions
(DAV:resource-must-be-null): A resource MUST NOT exist at the
Request-URI.
(DAV:parent-resource-must-be-non-null): The Request-URI minus the
last past segment MUST identify a collection.
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(DAV:name-allowed): The last segment of the Request-URI is
available for use as a resource name.
(DAV:locked-update-allowed): If the collection identified by the
Request-URI minus the last path segment is write-locked, then the
appropriate token MUST be specified in an If request header.
(DAV:redirect-lifetime-supported): If the request body contains a
DAV:redirect-lifetime element, the server MUST support the
specified lifetime. Support for DAV:temporary is REQUIRED, while
support for DAV:permanent is OPTIONAL.
(DAV:legal-reftarget): The specified is a legal URI or relative-
ref.
Postconditions
(DAV:new-redirectref): a new redirect reference resource is
created whose DAV:reftarget property has the value specified in
the request body.
6.1. Example: Creating a Redirect Reference Resource with MKREDIRECTREF
>> Request:
MKREDIRECTREF /~whitehead/dav/spec08.ref HTTP/1.1
Host: www.example.com
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<D:mkredirectref xmlns:D="DAV:">
<D:reftarget>
<D:href>/i-d/draft-webdav-protocol-08.txt</D:href>
</D:reftarget>
</D:mkredirectref>
>> Response:
HTTP/1.1 201 Created
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This request resulted in the creation of a new redirect reference
resource at http://www.example.com/~whitehead/dav/spec08.ref, which
points to the resource identified by the DAV:reftarget property. In
this example, the target resource is identified by the URI
http://www.example.com/i-d/draft-webdav-protocol-08.txt. The
redirect reference resource's DAV:resourcetype property is set to
DAV:redirectref, and its DAV:redirect-lifetime property has the value
DAV:temporary.
7. UPDATEREDIRECTREF Method
The UPDATEREDIRECTREF method requests the update of a redirect
reference resource.
If a UPDATEREDIRECTREF request fails, the server state preceding the
request MUST be restored.
Responses from a UPDATEREDIRECTREF request MUST NOT be cached, as
UPDATEREDIRECTREF has non-safe semantics (see [RFC2616], Section
9.1).
Marshalling
The request body MUST be a DAV:updateredirectref XML element.
<!ELEMENT updateredirectref (reftarget?, redirect-lifetime?)>
See Section 6 for a definition of DAV:reftarget and DAV:redirect-
lifetime.
If no DAV:reftarget element is specified, the server MUST NOT
change the target of the redirect reference.
If no DAV:redirect-lifetime element is specified, the server MUST
NOT change the lifetime of the redirect reference.
If a response body for a successful request is included, it MUST
be a DAV:updateredirectref-response XML element. Note that this
document does not define any elements for the UPDATEREDIRECTREF
response body, but the DAV:updateredirectref-response element is
defined to ensure interoperability between future extensions that
do define elements for the response body.
<!ELEMENT updateredirectref-response ANY>
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Preconditions
(DAV:locked-update-allowed): if the resource is write-locked, then
the appropriate token MUST be specified in an If request header.
(DAV:must-be-redirectref): the resource identified by the
Request-URI must be a redirect reference resource as defined by
this specification.
(DAV:redirect-lifetime-supported): see Section 6.
(DAV:redirect-lifetime-update-supported): servers MAY support
changing the DAV:redirect-lifetime property; if they don't, this
condition code can be used to signal failure.
(DAV:legal-reftarget): see Section 6.
Postconditions
(DAV:redirectref-updated): the DAV:reftarget and DAV:redirect-
lifetime properties of the redirect reference have been updated
accordingly.
7.1. Example: Updating a Redirect Reference Resource with
UPDATEREDIRECTREF
>> Request:
UPDATEREDIRECTREF /~whitehead/dav/spec08.ref HTTP/1.1
Host: www.example.com
Apply-To-Redirect-Ref: T
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<D:updateredirectref xmlns:D="DAV:">
<D:reftarget>
<D:href>/i-d/draft-webdav-protocol-08b.txt</D:href>
</D:reftarget>
</D:updateredirectref>
>> Response:
HTTP/1.1 200 OK
This request has updated the redirect reference's DAV:reftarget
property to "/i-d/draft-webdav-protocol-08b.txt" and has not changed
the DAV:redirect-lifetime value. Note that the "Apply-To-Redirect-
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Ref" request header must be used; otherwise, the request would result
in a redirect (3xx) response status.
8. Operations on Collections That Contain Redirect Reference Resources
According to [RFC2518], Section 9.2, methods that have defined
interactions with the "Depth" request header should apply all other
request headers to each resource in scope. However, applying this
principle to the "Apply-To-Redirect-Ref" header uniformly would make
it impractical to implement this specification on top of existing
servers and also would result in unexpected server behaviour for
clients that do not take the existence of redirect references into
account. On the other hand, the definition of the "Depth" header
allows alternate behaviours to be explicitly defined.
For this reason, this specification defines the interaction between
"Depth" and "Apply-To-Redirect-Ref" request headers on a case-by-case
basis and also provides a default for methods not mentioned here that
do not specify the behaviour themselves.
+-------------+-----------------+------------------+-----------+
| method name | defined in | supported depths | behaviour |
+-------------+-----------------+------------------+-----------+
| COPY | [RFC2518], 8.9 | 0, infinity | "T" |
| DELETE | [RFC2518], 8.7 | infinity | "T" |
| LOCK | [RFC2518], 8.11 | 0, infinity | "T" |
| MOVE | [RFC2518], 8.10 | 0, infinity | "T" |
| PROPFIND | [RFC2518], 8.2 | 0, 1, infinity | inherit |
| REPORT | [RFC3253], 3.6 | 0, 1, infinity | inherit |
| default | | | "T" |
+-------------+-----------------+------------------+-----------+
When the behaviour is defined to be "inherit", the method should
follow RFC2518's default behaviour for "Depth" operations, which
means applying the value given for "Apply-To-Redirect-Ref" to each
resource in scope. On the other hand, when it is defined to be "T",
the method should behave as if a "Apply-To-Redirect-Ref: T" header
was specified for each operation on child resources. The latter
ensures that "Depth: infinity" operations will not fail unexpectedly
just because there was a redirect reference resource in scope.
8.1. Example: PROPFIND on a Collection with Redirect Reference
Resources
Suppose a PROPFIND request with Depth: infinity is submitted to the
following collection, with the members shown here:
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/MyCollection/
(non-reference resource) diary.html
(redirect reference resource) nunavut
>> Request:
PROPFIND /MyCollection/ HTTP/1.1
Host: example.com
Depth: infinity
Apply-To-Redirect-Ref: F
Content-Type: text/xml
Content-Length: xxxx
<?xml version="1.0" ?>
<D:propfind xmlns:D="DAV: ">
<D:prop xmlns:J="http://example.com/jsprops/">
<D:resourcetype/>
<J:keywords/>
</D:prop>
</D:propfind>
>> Response:
HTTP/1.1 207 Multi-Status
Content-Type: text/xml
Content-Length: xxxx
<?xml version="1.0" ?>
<D:multistatus xmlns:D="DAV:" xmlns:J="http://example.com/jsprops/">
<D:response>
<D:href>/MyCollection/</D:href>
<D:propstat>
<D:prop>
<D:resourcetype><D:collection/></D:resourcetype>
<J:keywords>diary, interests, hobbies</J:keywords>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>/MyCollection/diary.html</D:href>
<D:propstat>
<D:prop>
<D:resourcetype/>
<J:keywords>diary, travel, family, history</J:keywords>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
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</D:response>
<D:response>
<D:href>/MyCollection/nunavut</D:href>
<D:status>HTTP/1.1 302 Found</D:status>
<D:location>
<D:href>http://example.ca/art/inuit/</D:href>
</D:location>
</D:response>
</D:multistatus>
In this example, the Depth header is set to infinity, and the Apply-
To-Redirect-Ref header is set to "F". The collection contains one
URI that identifies a redirect reference resource. The response
element for the redirect reference resource has a status of 302
(Found) and includes a DAV:location extension element to allow
clients to retrieve the properties of its target resource. (The
response element for the redirect reference resource does not include
the requested properties. The client can submit another PROPFIND
request to the URI in the DAV:location pseudo-property to retrieve
those properties.)
8.2. Example: PROPFIND with Apply-To-Redirect-Ref on a Collection with
Redirect Reference Resources
Suppose a PROPFIND request with "Apply-To-Redirect-Ref: T" and Depth:
infinity is submitted to the following collection, with the members
shown here:
/MyCollection/
(non-reference resource) diary.html
(redirect reference resource) nunavut
>> Request:
PROPFIND /MyCollection/ HTTP/1.1
Host: example.com
Depth: infinity
Apply-To-Redirect-Ref: T
Content-Type: text/xml
Content-Length: xxxx
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<?xml version="1.0" ?>
<D:propfind xmlns:D="DAV:">
<D:prop>
<D:resourcetype/>
<D:reftarget/>
<D:redirect-lifetime/>
</D:prop>
</D:propfind>
>> Response:
HTTP/1.1 207 Multi-Status
Content-Type: text/xml
Content-Length: xxxx
<?xml version="1.0" ?>
<D:multistatus xmlns:D="DAV:">
<D:response>
<D:href>/MyCollection/</D:href>
<D:propstat>
<D:prop>
<D:resourcetype><D:collection/></D:resourcetype>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
<D:propstat>
<D:prop>
<D:reftarget/>
<D:redirect-lifetime/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>/MyCollection/diary.html</D:href>
<D:propstat>
<D:prop>
<D:resourcetype/>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
<D:propstat>
<D:prop>
<D:reftarget/>
<D:redirect-lifetime/>
</D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
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</D:response>
<D:response>
<D:href>/MyCollection/nunavut</D:href>
<D:propstat>
<D:prop>
<D:resourcetype><D:redirectref/></D:resourcetype>
<D:reftarget>
<D:href>http://example.ca/art/inuit/</D:href>
</D:reftarget>
<D:redirect-lifetime><D:temporary/></D:redirect-lifetime>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
</D:multistatus>
Since the "Apply-To-Redirect-Ref: T" header is present, the response
shows the properties of the redirect reference resource in the
collection rather than reporting a 302 status.
9. Operations on Targets of Redirect Reference Resources
Operations on targets of redirect reference resources have no effect
on the reference resource.
10. Relative References in DAV:reftarget
The URI in the href in a DAV:reftarget property MAY be a relative
reference. In this case, the base URI to be used for resolving it to
absolute form is the URI used in the HTTP message to identify the
redirect reference resource to which the DAV:reftarget property
belongs.
When DAV:reftarget appears in the context of a Multi-Status response,
it is in a DAV:response element that contains a single DAV:href
element. The value of this DAV:href element serves as the base URI
for resolving a relative reference in DAV:reftarget. The value of
DAV:href may itself be relative, in which case it must be resolved
first in order to serve as the base URI for the relative reference in
DAV:reftarget. If the DAV:href element is relative, its base URI is
constructed from the scheme component "http", the value of the Host
header in the request, and the Request-URI.
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10.1. Example: Resolving a Relative Reference in a Multi-Status
Response
>> Request:
PROPFIND /geog/ HTTP/1.1
Host: example.com
Apply-To-Redirect-Ref: T
Depth: 1
Content-Type: text/xml
Content-Length: nnn
<?xml version="1.0" ?>
<D:propfind xmlns:D="DAV:">
<D:prop>
<D:resourcetype/>
<D:reftarget/>
</D:prop>
</D:propfind>
>> Response:
HTTP/1.1 207 Multi-Status
Content-Type: text/xml
Content-Length: nnn
<?xml version="1/0" ?>
<D:multistatus xmlns:D="DAV:">
<D:response>
<D:href>/geog/</D:href>
<D:propstat>
<D:prop>
<D:resourcetype><D:collection/></D:resourcetype>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
<D:propstat>
<D:prop><D:reftarget/></D:prop>
<D:status>HTTP/1.1 404 Not Found</D:status>
</D:propstat>
</D:response>
<D:response>
<D:href>/geog/stats.html</D:href>
<D:propstat>
<D:prop>
<D:resourcetype><D:redirectref/></D:resourcetype>
<D:reftarget>
<D:href>statistics/population/1997.html</D:href>
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</D:reftarget>
</D:prop>
<D:status>HTTP/1.1 200 OK</D:status>
</D:propstat>
</D:response>
</D:multistatus>
In this example, the relative reference
"statistics/population/1997.html" is returned as the value of the
DAV:reftarget property for the reference resource identified by href
/geog/stats.html. The href is itself a relative reference, which
resolves to http://example.com/geog/stats.html. This is the base URI
for resolving the relative reference in reftarget. The absolute URI
of reftarget is
http://example.com/geog/statistics/population/1997.html.
11. Redirect References to Collections
In a Request-URI /segment1/segment2/segment3, any of the three
segments may identify a redirect reference resource. (See [RFC3986],
Section 3.3, for definitions of "path" and "segment".) If any
segment in a Request-URI identifies a redirect reference resource,
the response SHOULD be a 3xx. The value of the Location header in
the response is as follows:
The leftmost path segment of the Request-URI that identifies a
redirect reference resource, together with all path segments and
separators to the left of it, is replaced by the value of the
redirect reference resource's DAV:reftarget property (resolved to an
absolute URI). The remainder of the Request-URI is concatenated to
this path.
Note: If the DAV:reftarget property ends with a "/" and the remainder
of the Request-URI is non-empty (and therefore must begin with a
"/"), the final "/" in the DAV:reftarget property is dropped before
the remainder of the Request-URI is appended.
Consider Request-URI /x/y/z.html. Suppose that /x/ is a redirect
reference resource, whose target resource is collection /a/, which
contains redirect reference resource y whose target resource is
collection /b/, which contains redirect reference resource z.html,
whose target resource is /c/d.html.
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/x/y/z.html
|
| /x -> /a
|
v
/a/y/z.html
|
| /a/y -> /b
|
v
/b/z.html
|
| /b/z.html -> /c/d.html
|
v
/c/d.html
In this case, the client must follow up three separate 3xx responses
before finally reaching the target resource. The server responds to
the initial request with a 3xx with Location: /a/y/z.html, and the
client resubmits the request to /a/y/z.html. The server responds to
this request with a 3xx with Location: /b/z.html, and the client
resubmits the request to /b/z.html. The server responds to this
request with a 3xx with Location: /c/d.html, and the client resubmits
the request to /c/d.html. This final request succeeds.
Note: The behaviour described above may have a very serious impact
on the efficiency of mapping Request-URIs to resources in HTTP
request processing. Therefore, servers MAY respond with a 404
status code if the cost of checking all leading path segments for
redirect references seems prohibitive.
12. Headers
12.1. Redirect-Ref Response Header
Redirect-Ref = "Redirect-Ref:" (URI | relative-ref)
; URI: see [RFC3986], Section 3
; relative-ref: see [RFC3986], Section 4.2
The Redirect-Ref header is used in all 3xx responses from redirect
reference resources. The value is the link target as specified
during redirect reference resource creation.
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12.2. Apply-To-Redirect-Ref Request Header
Apply-To-Redirect-Ref = "Apply-To-Redirect-Ref" ":" ("T" | "F")
The optional Apply-To-Redirect-Ref header can be used on any request
to a redirect reference resource. When it is present and set to "T",
the request MUST be applied to the reference resource itself, and a
3xx response MUST NOT be returned.
If the Apply-To-Redirect-Ref header is used on a request to any other
sort of resource besides a redirect reference resource, the server
MUST ignore it.
13. Redirect Reference Resource Properties
The properties defined below are REQUIRED on redirect reference
resources. A PROPFIND/allprop request SHOULD NOT return any of the
properties defined in this document.
13.1. DAV:redirect-lifetime (protected)
This property provides information about the lifetime of a redirect.
It can be either DAV:permanent (HTTP status 301) or DAV:temporary
(HTTP status 302). Future protocols may define additional values.
<!ELEMENT redirect-lifetime (permanent | temporary)>
<!ELEMENT permanent EMPTY>
<!ELEMENT temporary EMPTY>
13.2. DAV:reftarget (protected)
This property provides an efficient way for clients to discover the
URI of the target resource. This is a read-only property after its
initial creation. Its value can only be set in a MKREDIRECTREF
request. The value is a DAV:href element containing the URI of the
target resource.
<!ELEMENT reftarget href >
14. XML Elements
14.1. redirectref XML Element
Name: redirectref
Namespace: DAV:
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Purpose: Used as the value of the DAV:resourcetype property to
specify that the resource type is a redirect reference resource.
<!ELEMENT redirectref EMPTY >
15. Extensions to the DAV:response XML Element for Multi-Status
Responses
As described in Section 8, the DAV:location element may be returned
in the DAV:response element of a 207 Multi-Status response, to allow
clients to resubmit their requests to the target resource of a
redirect reference resource.
Consequently, the definition of the DAV:response XML element changes
to the following:
<!ELEMENT response (href, ((href*, status)|(propstat+)),
responsedescription?, location?) >
<!ELEMENT location (href) >
16. Capability Discovery
Sections 9.1 and 15 of [RFC2518] describe the use of compliance
classes with the DAV header in responses to OPTIONS, to indicate
which parts of the WebDAV Distributed Authoring protocols the
resource supports. This specification defines an OPTIONAL extension
to [RFC2518]. It defines a new compliance class, called
redirectrefs, for use with the DAV header in responses to OPTIONS
requests. If a resource does support redirect references, its
response to an OPTIONS request may indicate that it does, by listing
the new redirectrefs compliance class in the DAV header and by
listing the MKREDIRECTREF method as one it supports.
When responding to an OPTIONS request, any type of resource can
include redirectrefs in the value of the DAV header. Doing so
indicates that the server permits a redirect reference resource at
the Request-URI.
16.1. Example: Discovery of Support for Redirect Reference Resources
>> Request:
OPTIONS /somecollection/someresource HTTP/1.1
Host: example.org
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>> Response:
HTTP/1.1 200 OK
Allow: OPTIONS, GET, HEAD, POST, PUT, DELETE, TRACE, COPY, MOVE
Allow: MKCOL, PROPFIND, PROPPATCH, LOCK, UNLOCK, MKREDIRECTREF
DAV: 1, 2, redirectrefs
The DAV header in the response indicates that the resource
/somecollection/someresource is level 1 and level 2 compliant, as
defined in [RFC2518]. In addition, /somecollection/someresource
supports redirect reference resources. The Allow header indicates
that MKREDIRECTREF requests can be submitted to
/somecollection/someresource.
17. Security Considerations
This section is provided to make applications that implement this
protocol aware of the security implications of this protocol.
All of the security considerations of HTTP/1.1 and the WebDAV
Distributed Authoring Protocol specification also apply to this
protocol specification. In addition, redirect reference resources
introduce several new security concerns and increase the risk of some
existing threats. These issues are detailed below.
17.1. Privacy Concerns
By creating redirect reference resources on a trusted server, it is
possible for a hostile agent to induce users to send private
information to a target on an unrelated system. This risk is
mitigated somewhat, since clients are required to notify the user of
the redirection for any request other than GET or HEAD. (See
[RFC2616], Section 10.3.3, 302 Found.)
17.2. Redirect Loops
Although redirect loops were already possible in HTTP 1.1, the
introduction of the MKREDIRECTREF method creates a new avenue for
clients to create loops accidentally or maliciously. If the
reference resource and its target are on the same server, the server
may be able to detect MKREDIRECTREF requests that would create loops.
See also [RFC2616], Section 10.3, "Redirection 3xx."
17.3. Redirect Reference Resources and Denial of Service
Denial of service attacks were already possible by posting URLs that
were intended for limited use at heavily used Web sites. The
introduction of MKREDIRECTREF creates a new avenue for similar denial
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of service attacks. Clients can now create redirect reference
resources at heavily used sites to target locations that were not
designed for heavy usage.
17.4. Revealing Private Locations
There are several ways that redirect reference resources may reveal
information about collection structures. First, the DAV:reftarget
property of every redirect reference resource contains the URI of the
target resource. Anyone who has access to the reference resource can
discover the collection path that leads to the target resource. The
owner of the target resource may have wanted to limit knowledge of
this collection structure.
Sufficiently powerful access control mechanisms can control this risk
to some extent. Property-level access control could prevent users
from examining the DAV:reftarget property. (The Location header
returned in responses to requests on redirect reference resources
reveals the same information, however.)
This risk is no greater than the similar risk posed by HTML links.
18. Internationalization Considerations
All internationalization considerations mentioned in [RFC2518] also
apply to this document.
19. IANA Considerations
All IANA considerations mentioned in [RFC2518] also apply to this
document.
19.1. HTTP headers
This document specifies the two new HTTP headers listed below.
19.1.1. Redirect-Ref
Header field name: Redirect-Ref
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document: this specification (Section 12.1)
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19.1.2 Apply-To-Redirect-Ref
Header field name: Apply-To-Redirect-Ref
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document: this specification (Section 12.2)
20. Contributors
Many thanks to Jason Crawford, Jim Davis, Chuck Fay, and Judith
Slein, who can take credit for big parts of the original design of
this specification.
21. Acknowledgements
This document has benefited from thoughtful discussion by Jim Amsden,
Peter Carlson, Steve Carter, Tyson Chihaya, Ken Coar, Ellis Cohen,
Bruce Cragun, Spencer Dawkins, Mark Day, Rajiv Dulepet, David Durand,
Lisa Dusseault, Stefan Eissing, Roy Fielding, Yaron Goland, Fred
Hitt, Alex Hopmann, James Hunt, Marcus Jager, Chris Kaler, Manoj
Kasichainula, Rohit Khare, Daniel LaLiberte, Steve Martin, Larry
Masinter, Jeff McAffer, Joe Orton, Surendra Koduru Reddy, Juergen
Reuter, Max Rible, Sam Ruby, Bradley Sergeant, Nick Shelness, John
Stracke, John Tigue, John Turner, Kevin Wiggen, and others.
22. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2518] Goland, Y., Whitehead, E., Faizi, A., Carter, S., and D.
Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, February 1999.
[RFC2616] 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.
[RFC3253] Clemm, G., Amsden, J., Ellison, T., Kaler, C., and J.
Whitehead, "Versioning Extensions to WebDAV (Web
Distributed Authoring and Versioning)", RFC 3253, March
2002.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
Authors' Addresses
Jim Whitehead
UC Santa Cruz, Dept. of Computer Science
1156 High Street
Santa Cruz, CA 95064
US
EMail: ejw@cse.ucsc.edu
Geoff Clemm
IBM
20 Maguire Road
Lexington, MA 02421
US
EMail: geoffrey.clemm@us.ibm.com
Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Phone: +49 251 2807760
Fax: +49 251 2807761
EMail: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
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Copies of IPR disclosures made to the IETF Secretariat and any
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this standard. Please address the information to the IETF at
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Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
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