Network Working Group J. Reschke
Internet-Draft greenbytes
Intended status: Standards Track October 2, 2008
Expires: April 5, 2009
Using POST to add Members to Web Distributed Authoring and Versioning
(WebDAV) Collections
draft-reschke-webdav-post-01
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Abstract
The Hypertext Transfer Protocol (HTTP) Extensions for the Web
Distributed Authoring and Versioning (WebDAV) do not define the
behavior for the "POST" method when applied to collections, as the
base specification (HTTP) leaves implementers lots of freedom for the
semantics of "POST".
This has lead to a situation where many WebDAV servers do not
implement POST for collections at all, although it is well suited to
be used for the purpose of adding new members to a collection, where
the server remains in control of the newly assigned URL. As a matter
of fact, the Atom Publishing Protocol (AtomPub) uses POST exactly for
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that purpose. On the other hand, WebDAV-based protocols such as the
Calendar Extensions to WebDAV (CalDAV) frequently require clients to
pick a unique URL, although the server could easily perform that
task.
This specification defines a discovery mechanism through which
servers can advertise support for POST requests with the
aforementioned "add collection member" semantics.
Editorial Note (To be removed by RFC Editor before publication)
Please send comments to the Distributed Authoring and Versioning
(WebDAV) working group at <mailto:w3c-dist-auth@w3.org>, which may be
joined by sending a message with subject "subscribe" to
<mailto:w3c-dist-auth-request@w3.org>. Discussions of the WEBDAV
working group are archived at
<http://lists.w3.org/Archives/Public/w3c-dist-auth/>.
Note that although discussion takes place on the WebDAV working
group's mailing list, this is not a working group document.
XML versions, latest edits and the issues list for this document are
available from
<http://greenbytes.de/tech/webdav/#draft-reschke-webdav-post>.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol Extension . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Definition of 'Add-Member' URI . . . . . . . . . . . . . . 6
3.2. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.1. p:add-member Property . . . . . . . . . . . . . . . . 7
3.2.2. Example . . . . . . . . . . . . . . . . . . . . . . . 7
3.2.3. add-member Link Relation . . . . . . . . . . . . . . . 8
3.3. Relation to AtomPub's 'Slug' Header . . . . . . . . . . . 9
3.4. Example Operation . . . . . . . . . . . . . . . . . . . . 9
4. Additional Semantics for existing Methods . . . . . . . . . . 9
4.1. Additional Preconditions . . . . . . . . . . . . . . . . . 10
4.2. Example: Failed PUT Request . . . . . . . . . . . . . . . 10
5. Relationship to WebDAV Access Control Protocol . . . . . . . . 10
6. Internationalization Considerations . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 13
A.1. since draft-reschke-webdav-post-00 . . . . . . . . . . . . 13
Appendix B. Resolved issues (to be removed by RFC Editor
before publication) . . . . . . . . . . . . . . . . . 13
B.1. rational-server-uri . . . . . . . . . . . . . . . . . . . 13
B.2. remote-uri . . . . . . . . . . . . . . . . . . . . . . . . 14
B.3. post-error-semantics . . . . . . . . . . . . . . . . . . . 14
B.4. uri-uniqueness . . . . . . . . . . . . . . . . . . . . . . 14
B.5. uri-format . . . . . . . . . . . . . . . . . . . . . . . . 15
B.6. property-trust . . . . . . . . . . . . . . . . . . . . . . 15
B.7. forbidden-put . . . . . . . . . . . . . . . . . . . . . . 15
B.8. acl . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
B.9. member-uri-content . . . . . . . . . . . . . . . . . . . . 16
Appendix C. Open issues (to be removed by RFC Editor prior to
publication) . . . . . . . . . . . . . . . . . . . . 17
C.1. edit . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
Intellectual Property and Copyright Statements . . . . . . . . . . 19
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1. Introduction
The Hypertext Transfer Protocol (HTTP) Extensions for the Web
Distributed Authoring and Versioning (WebDAV) ([RFC4918], Section
9.5) do not define the behavior for the "POST" method when applied to
collections, as the base specification (HTTP) leaves implementers
lots of freedom for the semantics of "POST":
9.5 POST for Collections
Since by definition the actual function performed by POST is
determined by the server and often depends on the particular
resource, the behavior of POST when applied to collections cannot
be meaningfully modified because it is largely undefined. Thus,
the semantics of POST are unmodified when applied to a collection.
This has lead to a situation where many WebDAV servers do not
implement POST for collections at all, although it is well suited to
be used for the purpose of adding new members to a collection, where
the server remains in control of the newly assigned URL. As a matter
of fact, the Atom Publishing Protocol (AtomPub) uses POST exactly for
that purpose ([RFC5023], Section 9.2):
9.2 Creating Resources with POST
To add members to a Collection, clients send POST requests to the
URI of the Collection.
On the other hand, WebDAV-based protocols such as Calendaring
Extensions to WebDAV (CalDAV) frequently require clients to pick a
unique URL, although the server could easily perform that task
([RFC4791], Section 5.3.2):
5.3.2 Creating Calendar Object Resources
...
When servers create new resources, it's not hard for the server to
choose an unmapped URI. It's slightly tougher for clients,
because a client might not want to examine all resources in the
collection and might not want to lock the entire collection to
ensure that a new resource isn't created with a name collision.
(...)
As a matter of fact, letting the server choose the member URI not
only is a simplification for certain types of clients, but can also
reduce the complexity of the server (in that it doesn't need to
persist an additional client-supplied identifier where it already has
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an internal one like a UUID or a primary key).
Note: the vCard Extensions to WebDAV (CardDAV)
([draft-ietf-vcarddav-carddav]) suffer from the same issue, and
may be able to take advantage of this specification.
This specification defines a discovery mechanism through which
servers can advertise support for POST requests with the
aforementioned "add collection member" semantics.
Note: the author previously proposed a new HTTP method for exactly
this purpose ([draft-reschke-http-addmember]), but quite a few
reviewers pointed out that this would duplicate the original
semantics of POST. Thus this proposal that avoids adding a new
HTTP method.
2. Terminology
The terminology used here follows that in the WebDAV specification
([RFC4918]).
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 [RFC2119].
This document uses XML DTD fragments ([XML]) as a purely notational
convention. In particular:
o Element names use the namespace
"http://purl.org/NET/webdav/post#". When an XML element type in
this namespace is referenced in this document outside of the
context of an XML fragment, the string "p:" will be prefixed to
the element name. [[ns: With sufficient support from the WebDAV
WG's former mailing list and/or the IESG Applications Area the
property defined later on could potentially be moved into the DAV:
namespace.]]
o Element ordering is irrelevant.
o Extension elements/attributes (elements/attributes not already
defined as valid child elements) may be added anywhere, except
when explicitly stated otherwise.
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3. Protocol Extension
Due to the reasons stated in Section 1, clients can not rely on a
specific server behavior when POST is applied to a collection. This
problem is addressed by this specification by allowing servers to
advertise a URI that has the desired "add member" semantics.
Note that servers that already use POST for a different purpose can
just expose a different URI for that purpose. Other servers can just
advertise the collection's own URI, thus avoiding minting another URI
for a limited purpose.
3.1. Definition of 'Add-Member' URI
The "Add-Member" URI of a WebDAV collection is a URI that will accept
HTTP POST requests, and will interpret these as requests to store the
enclosed entity as a new internal member of the collection. It MUST
identify a resource on the same server as the WebDAV collection (the
host and port components ([RFC2616], Section 3.2.2) of the URIs must
match).
If there are pre-conditions related to creating a resource in the
collection using a PUT request, then those same pre-conditions apply
to the new POST request behavior, and the same HTTP response body
will returned on failure.
The URI of the newly created resource is returned in the HTTP
Location response header ([RFC2616], Section 14.30).
Note: the fact that a server advertises an "Add-Member" URI does
not imply any special semantics of the collection itself. For
instance, member URIs assigned by the server are not necessarily
unique over time (a member URI may be assigned again to a new
resource when it previously was removed).
Note: the "Add-Member" URI can be the identical to the
collection's URI (in which case the server just advertises the
fact that POST to the WebDAV collection's URI is supported as
defined within this specification). But it can also be different
from it, in which case it doesn't need to have any relation to the
collection's URI.
Given a collection URI of
/docs/collection/
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all of the URIs below might occur as "Add-Member" URIs:
/docs/collection/
/docs/collection/;post
/docs/collection;post/
/docs/collection/&post
/post-service?path=/collection/
The remainder of the document uses the same format just for
reasons of consistency; any other HTTP URI would do as well.
3.2. Discovery
3.2.1. p:add-member Property
The p:add-member property is defined on WebDAV collections, and
contains the "Add-Member" URI for that collection (embedded inside a
DAV:href element).
<!ELEMENT p:add-member (href)>
<!-- href: defined in [RFC4918], Section 14.7 -->
A PROPFIND/allprop request SHOULD NOT return this property (see
[RFC4918], Section 9.1). Servers MUST implement the DAV:supported-
live-property-set property defined in Section 3.1.4 of [RFC3253], and
report the property p:add-member as a supported live property.
>>Request
PROPFIND /collection/ HTTP/1.1
Host: example.com
Content-Type: application/xml; charset="utf-8"
Content-Length: 163
<?xml version="1.0" encoding="utf-8" ?>
<propfind xmlns="DAV:" xmlns:p="http://purl.org/NET/webdav/post#">
<prop>
<p:add-member/>
</prop>
</propfind>
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>>Response
HTTP/1.1 207 Multi-Status
Content-Type: application/xml; charset="utf-8"
Content-Length: 385
<?xml version="1.0" encoding="utf-8" ?>
<multistatus xmlns="DAV:" xmlns:p="http://purl.org/NET/webdav/post#">
<response>
<href>/collection/</href>
<propstat>
<prop>
<p:add-member>
<href>/collection;add-member/</href>
</p:add-member>
</prop>
<status>HTTP/1.1 200 OK</status>
</propstat>
</response>
</multistatus>
Note that in this case, the server has minted a separate URI for the
purpose of adding new content.
3.2.3. add-member Link Relation
There may be cases in which it is more efficient to expose the Add-
Member URI in HTTP headers or in (X)HTML content. For this use case,
this specification defines a new link relation with the name:
http://purl.org/NET/webdav/post#add-member
It can be used both in HTTP Link headers (see
[draft-nottingham-http-link-header]):
>>Request
HEAD /collection/ HTTP/1.1
Host: example.com
>>Response
HTTP/1.1 200 OK
Link: </collection;add-member/>;
rel="http://purl.org/NET/webdav/post#add-member"
...and in (X)HTML content:
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<title>Contents of /collection/</title>
<link rel="http://purl.org/NET/webdav/post#add-member"
href="/collection;add-member/" />
</head
<body>...</body>
</html>
3.3. Relation to AtomPub's 'Slug' Header
In the AtomPub protocol, clients can use the entity header "Slug" to
suggest parts of the URI to be created (see [RFC5023], Section 9.7).
Note that servers are free to ignore this suggestion, or to use
whatever algorithm that makes sense to generate the new URI.
The same applies to the extension defined here: clients can use the
"Slug" header as by its definition of a generic HTTP header. Servers
should process it exactly the way defined by AtomPub.
3.4. Example Operation
>>Request
POST /collection;add-member/ HTTP/1.1
Host: example.com
Content-Type: text/plain
Slug: Sample Text
Content-Length: 12
Sample text.
>>Response
HTTP/1.1 201 Created
Location: http://example.com/collection/sample%20text
4. Additional Semantics for existing Methods
One important use case for this specification are collections that
act as WebDAV collections for the purpose of read access (PROPFIND
Depth 1/Infinity), but which only support internal member URIs
assigned by the server. These collections will not allow a client to
create a new member using methods like PUT, MKCOL, LOCK, COPY or
MOVE. Therefore, this specification defines a new precondition name
([RFC4918], Section 16) that can be used to provide the client with
additional information about why exactly the request failed.
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4.1. Additional Preconditions
(p:allow-client-defined-URI): the server allows clients to specify
the last path segment for newly created resources.
The precondition element MAY contain a p:add-member-uri XML element
specifying the "Add-Member" URI associated with the collection, on
which the creation of a new child resource was attempted:
<!ELEMENT p:allow-client-defined-uri (p:add-member?)>
4.2. Example: Failed PUT Request
In this example, the client tries to use PUT to create a new internal
member of /collection/.
>>Request
PUT /collection/new.txt HTTP/1.1
Host: example.com
Content-Type: text/plain
Content-Length: 12
Sample text.
>>Response
HTTP/1.1 405 Method Not Allowed
Allow: GET, HEAD, TRACE, PROPFIND, COPY, MOVE
Content-Type: application/xml; charset=UTF-8
Content-Length: 223
<error xmlns="DAV:" xmlns:p="http://purl.org/NET/webdav/post#">
<p:allow-client-defined-uri>
<p:add-member>
<href>/collection;add-member/</href>
</p:add-member>
</p:allow-client-defined-uri>
</error>
The request fails with a 405 (Method Not Allowed) status, but also
provides the reason, and a pointer to the "Add-Member" URI in the
response body.
5. Relationship to WebDAV Access Control Protocol
The WebDAV ACL specification requires the DAV:bind privilege to be
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granted on a collection for the client to able add new collection
members ([RFC3744], Section 3.9). Consistent with that, a server
MUST reject a POST request to the Add-Member URI of a collection
unless the authenticated principal is granted DAV:bind privilege on
the associated WebDAV collection resource.
6. Internationalization Considerations
This document does not introduce any new internationalization
considerations beyond those discussed in Section 20 of [RFC4918].
7. IANA Considerations
This specification does not require any actions from IANA.
8. Security Considerations
All security considerations connected to HTTP/WebDAV and XML apply
for this specification as well, namely, [RFC4918] (Section 20) and
[RFC3470] (Section 7).
Furthermore, servers should be aware that deriving the member path
from the data being stored in the resource could potentially expose
confidential information. This could even be the case when only a
hash code of the content is used.
In addition, on servers that do not support this specification, a
malevolent user could set the p:add-member URI as a custom property,
tricking other users to post content to an entirely different URI.
Clients can protect themselves against this scenario by
o not following p:add-member URIs to different servers, and/or
o verifying that the p:add-member property is indeed a live property
(this can be achieved by testing the DAV:supported-live-property-
set property, or by checking whether p:add-member is returned upon
PROPFIND/allprop)
9. Acknowledgements
This document has benefited from thoughtful discussion by Cyrus Daboo
and Bernard Desruissaux.
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10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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", RFC 3253,
March 2002.
[RFC3744] Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, "Web
Distributed Authoring and Versioning (WebDAV) Access
Control Protocol", RFC 3744, May 2004.
[RFC4918] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
[XML] Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fourth
Edition)", W3C REC-xml-20060816, August 2006,
<http://www.w3.org/TR/2006/REC-xml-20060816>.
[draft-nottingham-http-link-header]
Nottingham, M., "HTTP Header Linking",
draft-nottingham-http-link-header-02 (work in progress),
July 2008.
10.2. Informative References
[RFC3470] Hollenbeck, S., Rose, M., and L. Masinter, "Guidelines for
the Use of Extensible Markup Language (XML) within IETF
Protocols", RFC 3470, BCP 70, January 2003.
[RFC4791] Daboo, C., Desruisseaux, B., and L. Dusseault,
"Calendaring Extensions to WebDAV (CalDAV)", RFC 4791,
March 2007.
[RFC5023] Gregorio, J. and B. de hOra, "The Atom Publishing
Protocol", RFC 5023, October 2007.
[draft-ietf-vcarddav-carddav]
Daboo, C., "vCard Extensions to WebDAV (CardDAV)",
draft-ietf-vcarddav-carddav-01 (work in progress),
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July 2008.
[draft-reschke-http-addmember]
Reschke, J., "The HTTP ADDMEMBER Method",
draft-reschke-http-addmember-00 (work in progress),
February 2005.
Appendix A. Change Log (to be removed by RFC Editor before publication)
Added Acknowledgements.
Add and resolve issues "acl", "forbidden-put", "member-uri-content",
"post-error-semantics", "property-trust", "rational-server-uri",
""remote-uri", "uri-format" and "uri-uniqueness".
Appendix B. Resolved issues (to be removed by RFC Editor before
publication)
Issues that were either rejected or resolved in this version of this
document.
B.1. rational-server-uri
In Section 1:
Type: change
<http://lists.w3.org/Archives/Public/w3c-dist-auth/2008JulSep/
0060.html>
cyrus@daboo.name (2008-09-22): One thing this reminds me of: another
reason why this POST may be needed is if the server enforces a
particular naming scheme on members. e.g., a CalDAV server may
require that all member path segments match the UID in the calendar
data, or match a record-id in its data store etc. In this case the
add-member behavior is required. So its not just the case of "the
client doesn't care to name members itself", but also this other one.
Probably worth adding a comment on this.
Resolution (2009-09-23): Mention that this can simplify the server by
not having to persist a client-supplied name.
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B.2. remote-uri
In Section 3.1:
Type: change
julian.reschke@greenbytes.de (2008-09-30): Explicitly forbid Add-
Member URIs pointing to other servers, mainly for reasons of security
(DOS), but also for practical reasons (authentication).
Resolution (2008-10-01): Require that the "Add-Member" URI points to
the same server.
B.3. post-error-semantics
In Section 3.1:
Type: change
<http://lists.w3.org/Archives/Public/w3c-dist-auth/2008JulSep/
0058.html>
cyrus@daboo.name (2008-09-22): Something needs to be stated about
error handling: e.g., "If there are pre-conditions related to
creating a resource in the collection using a PUT request, then those
same pre-conditions apply to the new POST request behavior, and the
same DAV:error response body returned on failure". This would be a
"catch-all" to allow protocols such as CalDAV, which restrict certain
aspects of the data stored in collections, to simply return the same
pre-condition failure information for POST as for PUT.
Resolution (2009-09-23): Adopt most of the proposed text, except for
saying "HTTP response" to make it more generic (for instance, there
may be no DAV:error entity body involved).
B.4. uri-uniqueness
In Section 3.1:
Type: change
<http://lists.w3.org/Archives/Public/w3c-dist-auth/2008JulSep/
0058.html>
cyrus@daboo.name (2008-09-22): Is the server allowed to re-use new
member URIs? i.e. /a/b.txt exists at some point, then is deleted. Is
the server then allowed to use b.txt as a new member of /a/, or does
the new member path segment have to be unique for the entire
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existence of that collection? If the member path segment is expected
to be unique there should be some guidance to servers on how they
might implement that (uuid's for instance).
Resolution (2009-09-23): Clarified that this specification doesn't
make any additional requirements on the collection semantics.
B.5. uri-format
In Section 3.1:
Type: change
<http://lists.w3.org/Archives/Public/w3c-dist-auth/2008JulSep/
0063.html>
cyrus@daboo.name (2008-09-23): Another question: there is no
restriction on what p:add-member URI can be? e.g. if I have the
collection "/a/b/" can the p:add-member be another resource entirely,
e.g. "/a/use-c-to-create-in-b/"? If this is possible it should be
called out, as the behavior might be somewhat unexpected for clients.
It might even be the case that the p:add-member URI is on a different
server (e.g. new member items in a collection need "approval" from
some other service). The interaction with WebDAV ACL in this case
would need to be clear - i.e. what privileges are required on the
p:add-member URI?
Resolution (2008-09-30): Add a set of examples, and use
"/collection;add-member/" in subsequent examples.
B.6. property-trust
In Section 3.2.1:
Type: change
julian.reschke@greenbytes.de (2008-10-01): An attacker could set
p:add-member as dead property and thus trick clients into POSTing new
content somewhere else.
Resolution (2008-10-01): (1) Require server support for DAV:
supported-live-property-set; (2) mention issue in security
considerations.
B.7. forbidden-put
Type: change
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<http://lists.w3.org/Archives/Public/w3c-dist-auth/2008JulSep/
0063.html>
cyrus@daboo.name (2008-09-23): So one option for the server to
enforce its naming scheme would be to require POST by the client to
create new resources rather than allowing PUT/COPY/MOVE to do so.
However there is no way fort a client to discover that such a
restriction might be in place other than getting a 403. How about a
new pre-condition code for that so that the server can indicate "DAV:
only-post-allowed-to-create-new-members" to the client? With perhaps
a more compact name!
Resolution (2008-09-30): Mention the use case, define the
precondition, add example.
Type: change
<http://lists.w3.org/Archives/Public/w3c-dist-auth/2008JulSep/
0063.html>
cyrus@daboo.name (2008-09-23): This brings up another question:
presumably the DAV:bind privilege is required on the collection for
the new POST ;add-member behavior to be allowed (just as it would be
required for PUT creating a new member). I think we therefore need a
statement in Security Considerations: "When a server supports WebDAV
ACL [RFC3744], the DAV:bind privilege is required to be granted on
the collection resource in which the new member resource is being
created. If this privilege is denied or not present, the POST
request MUST fail."
Resolution (2008-09-25): Add "relation to ACL" section; required DAV:
bind privilege on associated WebDAV collection.
B.9. member-uri-content
In Section 8:
Type: change
<http://lists.w3.org/Archives/Public/w3c-dist-auth/2008JulSep/
0058.html>
cyrus@daboo.name (2008-09-22): Security consideration: "Servers MUST
NOT derive the member path segment from the data being stored in the
resource". This is important because you don't want server's leaking
information in the URI that would not otherwise be visible (e.g. a
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user can PROPFIND the members but cannot read the content of the
members - leaking content in the URI would be bad). Note this
impacts how the server generates the member path segment. e.g. an md5
hash of the data only may not be appropriate.
Resolution (2009-09-23): State the problem, but do not make a
requirement (for instance, the server could be entirely public in
which case this wouldn't be an issue at all).
Appendix C. Open issues (to be removed by RFC Editor prior to
publication)
Type: edit
julian.reschke@greenbytes.de (2008-09-22): Umbrella issue for
editorial fixes/enhancements.
Index
A
Add-Member URI 6
C
Condition Names
p:allow-client-defined-URI (pre) 10
COPY method
Additional Preconditions 10
L
LOCK method
Additional Preconditions 10
M
MKCOL method
Additional Preconditions 10
MOVE method
Additional Preconditions 10
P
p:allow-client-defined-URI 10
PUT method
Additional Preconditions 10
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Author's Address
Julian F. Reschke
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Phone: +49 251 2807760
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/
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Reschke Expires April 5, 2009 [Page 19]
