Using POST to add Members to Web Distributed Authoring and Versioning (WebDAV) Collections

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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 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.¶

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.¶

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 an internal one like a UUID or a primary key).¶

This specification defines a discovery mechanism through which servers can advertise support for POST requests with the aforementioned "add collection member" semantics.¶

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: ¶

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.]
Element ordering is irrelevant.
Extension elements/attributes (elements/attributes not already defined as valid child elements) may be added anywhere, except when explicitly stated otherwise.

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 ↑ I ↓~~different URI for that purpose~~separate URI. 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 I (see Section 3 of [RFC4918] for the definition of "internal member"). 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).¶

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.¶

3.2.2. Example

>>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>

>>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:¶

<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.¶

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 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 ¶

not following p:add-member URIs to different servers, and/or
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.¶

Appendix A. Change Log (to be removed by RFC Editor before publication)

A.1. since draft-reschke-webdav-post-00

Added Acknowledgements.¶

Added and resolved issues "acl", "forbidden-put", "member-uri-content", "post-error-semantics", "property-trust", "rational-server-uri", ""remote-uri", "uri-format" and "uri-uniqueness".¶

↑ I

A.2. since draft-reschke-webdav-post-01

Add and resolve issue "containment".

Update draft-ietf-vcarddav-carddav reference.

Index

A C D L M P R X

A
- Add-Member URI 3.1
C
- Condition Names
  - p:allow-client-defined-URI (pre) 4.1
- COPY method
  - Additional Preconditions 4.1
D
- draft-ietf-vcarddav-carddav 1, 10.2
- draft-nottingham-http-link-header 3.2.3, 10.1
- draft-reschke-http-addmember 1, 10.2
L
- LOCK method
  - Additional Preconditions 4.1
M
- MKCOL method
  - Additional Preconditions 4.1
- MOVE method
  - Additional Preconditions 4.1
P
- p:allow-client-defined-URI 4.1
- PUT method
  - Additional Preconditions 4.1
R
- RFC2119 2, 10.1
- RFC2616 3.1, 3.1, 10.1
  - Section 3.2.2 3.1
  - Section 14.30 3.1
- RFC3253 3.2.1, 10.1
  - Section 3.1.4 3.2.1
- RFC3470 8, 10.2
  - Section 7 8
- RFC3744 5, 10.1
  - Section 3.9 5
- RFC4791 1, 10.2
  - Section 5.3.2 1
- RFC4918 1, 2, 3.1, 3.2.1, 3.2.1, 4, 6, 8, 10.1
  - Section 3 3.1
  - Section 9.1 3.2.1
  - Section 9.5 1
  - Section 14.7 3.2.1
  - Section 16 4
  - Section 20 6, 8
- RFC5023 1, 3.3, 10.2
  - Section 9.2 1
  - Section 9.7 3.3
X
- XML 2, 10.1

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I edit (type: edit, status: open)
julian.reschke@greenbytes.de	2008-09-22	Umbrella issue for editorial fixes/enhancements.
Associated changes in this document: 1, 3, 3.1, A.

I containment (type: change, status: closed)
julian.reschke@greenbytes.de	2008-11-20	There seems to be some confusion about whether a POST as described here would allow a server to store the resource into a different folder. That is not the intent, e.g., it doesn't change WebDAV's containment model to the one used in AtomPub. Is a clarification needed?
	2008-11-30	Resolution: Clarify "internal member" by pointing to RFC 4918, Section 3.
Associated changes in this document: 3.1.

Using POST to add Members to Web Distributed Authoring and Versioning (WebDAV) Collections

Status of this Memo

Abstract

Editorial Note (To be removed by RFC Editor before publication)

1. Introduction

2. Terminology

3. Protocol Extension

3.1. Definition of 'Add-Member' URI

3.2. Discovery

3.2.1. p:add-member Property

3.2.2. Example

3.2.3. add-member Link Relation

3.3. Relation to AtomPub's 'Slug' Header

3.4. Example Operation

4. Additional Semantics for existing Methods

4.1. Additional Preconditions

4.2. Example: Failed PUT Request

5. Relationship to WebDAV Access Control Protocol

6. Internationalization Considerations

7. IANA Considerations

8. Security Considerations

9. Acknowledgements

10. References

10.1. Normative References

10.2. Informative References

Appendix A. Change Log (to be removed by RFC Editor before publication)

A.1. since draft-reschke-webdav-post-00

A.2. since draft-reschke-webdav-post-01

Index

Author's Address

Full Copyright Statement

Intellectual Property

Id	Type	Status	Date	Raised By
containment	change	closed	2008-11-20	julian.reschke@greenbytes.de
edit	edit	open	2008-09-22	julian.reschke@greenbytes.de