RFC 3744
Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol
Network Working Group G. Clemm Request for Comments: 3744 IBM Category: Standards Track J. Reschke greenbytes E. Sedlar Oracle Corporation J. Whitehead U.C. Santa Cruz May 2004 Web Distributed Authoring and Versioning (WebDAV) Access Control Protocol Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved.Abstract
This document specifies a set of methods, headers, message bodies, properties, and reports that define Access Control extensions to the WebDAV Distributed Authoring Protocol. This protocol permits a client to read and modify access control lists that instruct a server whether to allow or deny operations upon a resource (such as HyperText Transfer Protocol (HTTP) method invocations) by a given principal. A lightweight representation of principals as Web resources supports integration of a wide range of user management repositories. Search operations allow discovery and manipulation of principals using human names.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1. Terms. . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Notational Conventions . . . . . . . . . . . . . . . . . 8 2. Principals . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3. Privileges . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. DAV:read Privilege . . . . . . . . . . . . . . . . . . . 10 3.2. DAV:write Privilege. . . . . . . . . . . . . . . . . . . 10 3.3. DAV:write-properties Privilege . . . . . . . . . . . . . 10 3.4. DAV:write-content Privilege. . . . . . . . . . . . . . . 11 3.5. DAV:unlock Privilege . . . . . . . . . . . . . . . . . . 11 3.6. DAV:read-acl Privilege . . . . . . . . . . . . . . . . . 11 3.7. DAV:read-current-user-privilege-set Privilege. . . . . . 12 3.8. DAV:write-acl Privilege. . . . . . . . . . . . . . . . . 12 3.9. DAV:bind Privilege . . . . . . . . . . . . . . . . . . . 12 3.10. DAV:unbind Privilege . . . . . . . . . . . . . . . . . . 12 3.11. DAV:all Privilege. . . . . . . . . . . . . . . . . . . . 13 3.12. Aggregation of Predefined Privileges . . . . . . . . . . 13 4. Principal Properties . . . . . . . . . . . . . . . . . . . . . 13 4.1. DAV:alternate-URI-set. . . . . . . . . . . . . . . . . . 14 4.2. DAV:principal-URL. . . . . . . . . . . . . . . . . . . . 14 4.3. DAV:group-member-set . . . . . . . . . . . . . . . . . . 14 4.4. DAV:group-membership . . . . . . . . . . . . . . . . . . 14 5. Access Control Properties. . . . . . . . . . . . . . . . . . . 15 5.1. DAV:owner. . . . . . . . . . . . . . . . . . . . . . . . 15 5.1.1. Example: Retrieving DAV:owner . . . . . . . . . . 15 5.1.2. Example: An Attempt to Set DAV:owner. . . . . . . 16 5.2. DAV:group. . . . . . . . . . . . . . . . . . . . . . . . 18 5.3. DAV:supported-privilege-set. . . . . . . . . . . . . . . 18 5.3.1. Example: Retrieving a List of Privileges Supported on a Resource . . . . . . . . . . . . . 19 5.4. DAV:current-user-privilege-set . . . . . . . . . . . . . 21 5.4.1. Example: Retrieving the User's Current Set of Assigned Privileges . . . . . . . . . . . . . . . 22 5.5. DAV:acl. . . . . . . . . . . . . . . . . . . . . . . . . 23 5.5.1. ACE Principal . . . . . . . . . . . . . . . . . . 23 5.5.2. ACE Grant and Deny. . . . . . . . . . . . . . . . 25 5.5.3. ACE Protection. . . . . . . . . . . . . . . . . . 25 5.5.4. ACE Inheritance . . . . . . . . . . . . . . . . . 25 5.5.5. Example: Retrieving a Resource's Access Control List. . . . . . . . . . . . . . . . . . . . . . . 25 5.6. DAV:acl-restrictions . . . . . . . . . . . . . . . . . . 27 5.6.1. DAV:grant-only. . . . . . . . . . . . . . . . . . 27 5.6.2. DAV:no-invert ACE Constraint. . . . . . . . . . . 28 5.6.3. DAV:deny-before-grant . . . . . . . . . . . . . . 28 5.6.4. Required Principals . . . . . . . . . . . . . . . 28 5.6.5. Example: Retrieving DAV:acl-restrictions. . . . . 28
5.7. DAV:inherited-acl-set. . . . . . . . . . . . . . . . . . 29
5.8. DAV:principal-collection-set . . . . . . . . . . . . . . 30
5.8.1. Example: Retrieving DAV:principal-collection-set. 30
5.9. Example: PROPFIND to retrieve access control properties. 32
6. ACL Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 36
7. Access Control and existing methods. . . . . . . . . . . . . . 37
7.1. Any HTTP method. . . . . . . . . . . . . . . . . . . . . 37
7.1.1. Error Handling. . . . . . . . . . . . . . . . . . 37
7.2. OPTIONS. . . . . . . . . . . . . . . . . . . . . . . . . 38
7.2.1. Example - OPTIONS . . . . . . . . . . . . . . . . 39
7.3. MOVE . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7.4. COPY . . . . . . . . . . . . . . . . . . . . . . . . . . 39
7.5. LOCK . . . . . . . . . . . . . . . . . . . . . . . . . . 39
8. Access Control Methods . . . . . . . . . . . . . . . . . . . . 40
8.1. ACL. . . . . . . . . . . . . . . . . . . . . . . . . . . 40
8.1.1. ACL Preconditions . . . . . . . . . . . . . . . . 40
8.1.2. Example: the ACL method . . . . . . . . . . . . . 42
8.1.3. Example: ACL method failure due to protected
ACE conflict. . . . . . . . . . . . . . . . . . . 43
8.1.4. Example: ACL method failure due to an
inherited ACE conflict. . . . . . . . . . . . . . 44
8.1.5. Example: ACL method failure due to an attempt
to set grant and deny in a single ACE . . . . . . 45
9. Access Control Reports . . . . . . . . . . . . . . . . . . . . 46
9.1. REPORT Method. . . . . . . . . . . . . . . . . . . . . . 46
9.2. DAV:acl-principal-prop-set Report. . . . . . . . . . . . 47
9.2.1. Example: DAV:acl-principal-prop-set Report. . . . 48
9.3. DAV:principal-match REPORT . . . . . . . . . . . . . . . 49
9.3.1. Example: DAV:principal-match REPORT . . . . . . . 50
9.4. DAV:principal-property-search REPORT . . . . . . . . . . 51
9.4.1. Matching. . . . . . . . . . . . . . . . . . . . . 53
9.4.2. Example: successful DAV:principal-property-search
REPORT. . . . . . . . . . . . . . . . . . . . . . 54
9.5. DAV:principal-search-property-set REPORT . . . . . . . . 56
9.5.1. Example: DAV:principal-search-property-set
REPORT. . . . . . . . . . . . . . . . . . . . . . 58
10. XML Processing . . . . . . . . . . . . . . . . . . . . . . . . 59
11. Internationalization Considerations. . . . . . . . . . . . . . 59
12. Security Considerations. . . . . . . . . . . . . . . . . . . . 60
12.1. Increased Risk of Compromised Users. . . . . . . . . . . 60
12.2. Risks of the DAV:read-acl and
DAV:current-user-privilege-set Privileges. . . . . . . . 60
12.3. No Foreknowledge of Initial ACL. . . . . . . . . . . . . 61
13. Authentication . . . . . . . . . . . . . . . . . . . . . . . . 61
14. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 62
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 62
16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 62 16.1. Normative References . . . . . . . . . . . . . . . . . . 62 16.2. Informative References . . . . . . . . . . . . . . . . . 63 Appendices A. WebDAV XML Document Type Definition Addendum . . . . . . . . . 64 B. WebDAV Method Privilege Table (Normative). . . . . . . . . . . 67 Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 71 Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 721. Introduction
The goal of the WebDAV access control extensions is to provide an interoperable mechanism for handling discretionary access control for content and metadata managed by WebDAV servers. WebDAV access control can be implemented on content repositories with security as simple as that of a UNIX file system, as well as more sophisticated models. The underlying principle of access control is that who you are determines what operations you can perform on a resource. The "who you are" is defined by a "principal" identifier; users, client software, servers, and groups of the previous have principal identifiers. The "operations you can perform" are determined by a single "access control list" (ACL) associated with a resource. An ACL contains a set of "access control entries" (ACEs), where each ACE specifies a principal and a set of privileges that are either granted or denied to that principal. When a principal submits an operation (such as an HTTP or WebDAV method) to a resource for execution, the server evaluates the ACEs in the ACL to determine if the principal has permission for that operation. Since every ACE contains the identifier of a principal, client software operated by a human must provide a mechanism for selecting this principal. This specification uses http(s) scheme URLs to identify principals, which are represented as WebDAV-capable resources. There is no guarantee that the URLs identifying principals will be meaningful to a human. For example, http://www.example.com/u/256432 and http://www.example.com/people/Greg.Stein are both valid URLs that could be used to identify the same principal. To remedy this, every principal resource has the DAV:displayname property containing a human-readable name for the principal. Since a principal can be identified by multiple URLs, it raises the problem of determining exactly which principal is being referenced in a given ACE. It is impossible for a client to determine that an ACE granting the read privilege to http://www.example.com/people/ Greg.Stein also affects the principal at http://www.example.com/u/ 256432. That is, a client has no mechanism for determining that two
URLs identify the same principal resource. As a result, this specification requires clients to use just one of the many possible URLs for a principal when creating ACEs. A client can discover which URL to use by retrieving the DAV:principal-URL property (Section 4.2) from a principal resource. No matter which of the principal's URLs is used with PROPFIND, the property always returns the same URL. With a system having hundreds to thousands of principals, the problem arises of how to allow a human operator of client software to select just one of these principals. One approach is to use broad collection hierarchies to spread the principals over a large number of collections, yielding few principals per collection. An example of this is a two level hierarchy with the first level containing 36 collections (a-z, 0-9), and the second level being another 36, creating collections /a/a/, /a/b/, ..., /a/z/, such that a principal with last name "Stein" would appear at /s/t/Stein. In effect, this pre-computes a common query, search on last name, and encodes it into a hierarchy. The drawback with this scheme is that it handles only a small set of predefined queries, and drilling down through the collection hierarchy adds unnecessary steps (navigate down/up) when the user already knows the principal's name. While organizing principal URLs into a hierarchy is a valid namespace organization, users should not be forced to navigate this hierarchy to select a principal. This specification provides the capability to perform substring searches over a small set of properties on the resources representing principals. This permits searches based on last name, first name, user name, job title, etc. Two separate searches are supported, both via the REPORT method, one to search principal resources (DAV:principal-property-search, Section 9.4), the other to determine which properties may be searched at all (DAV:principal-search- property-set, Section 9.5). Once a principal has been identified in an ACE, a server evaluating that ACE must know the identity of the principal making a protocol request, and must validate that that principal is who they claim to be, a process known as authentication. This specification intentionally omits discussion of authentication, as the HTTP protocol already has a number of authentication mechanisms [RFC2617]. Some authentication mechanism (such as HTTP Digest Authentication, which all WebDAV compliant implementations are required to support) must be available to validate the identity of a principal.
The following issues are out of scope for this document:
o Access control that applies only to a particular property on a
resource (excepting the access control properties DAV:acl and
DAV:current-user-privilege-set), rather than the entire resource,
o Role-based security (where a role can be seen as a dynamically
defined group of principals),
o Specification of the ways an ACL on a resource is initialized,
o Specification of an ACL that applies globally to all resources,
rather than to a particular resource.
o Creation and maintenance of resources representing people or
computational agents (principals), and groups of these.
This specification is organized as follows. Section 1.1 defines key
concepts used throughout the specification, and is followed by a more
in-depth discussion of principals (Section 2), and privileges
(Section 3). Properties defined on principals are specified in
Section 4, and access control properties for content resources are
specified in Section 5. The ways ACLs are to be evaluated is
described in Section 6. Client discovery of access control
capability using OPTIONS is described in Section 7.2. Interactions
between access control functionality and existing HTTP and WebDAV
methods are described in the remainder of Section 7. The access
control setting method, ACL, is specified in Section 8. Four reports
that provide limited server-side searching capabilities are described
in Section 9. Sections on XML processing (Section 10),
Internationalization considerations (Section 11), security
considerations (Section 12), and authentication (Section 13) round
out the specification. An appendix (Appendix A) provides an XML
Document Type Definition (DTD) for the XML elements defined in the
specification.
1.1. Terms
This document uses the terms defined in HTTP [RFC2616] and WebDAV
[RFC2518]. In addition, the following terms are defined:
principal
A "principal" is a distinct human or computational actor that
initiates access to network resources. In this protocol, a
principal is an HTTP resource that represents such an actor.
group
A "group" is a principal that represents a set of other
principals.
privilege
A "privilege" controls access to a particular set of HTTP
operations on a resource.
aggregate privilege
An "aggregate privilege" is a privilege that contains a set of
other privileges.
abstract privilege
The modifier "abstract", when applied to a privilege on a
resource, means the privilege cannot be set in an access control
element (ACE) on that resource.
access control list (ACL)
An "ACL" is a list of access control elements that define access
control to a particular resource.
access control element (ACE)
An "ACE" either grants or denies a particular set of (non-
abstract) privileges for a particular principal.
inherited ACE
An "inherited ACE" is an ACE that is dynamically shared from the
ACL of another resource. When a shared ACE changes on the primary
resource, it is also changed on inheriting resources.
protected property
A "protected property" is one whose value cannot be updated except
by a method explicitly defined as updating that specific property.
In particular, a protected property cannot be updated with a
PROPPATCH request.
1.2. Notational Conventions
The augmented BNF used by this document to describe protocol elements is described in Section 2.1 of [RFC2616]. Because this augmented BNF uses the basic production rules provided in Section 2.2 of [RFC2616], those rules apply to this document as well. 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]. Definitions of XML elements in this document use XML element type declarations (as found in XML Document Type Declarations), described in Section 3.2 of [REC-XML]. When an XML element type in the "DAV:" namespace is referenced in this document outside of the context of an XML fragment, the string "DAV:" will be prefixed to the element name.2. Principals
A principal is a network resource that represents a distinct human or computational actor that initiates access to network resources. Users and groups are represented as principals in many implementations; other types of principals are also possible. A URI of any scheme MAY be used to identify a principal resource. However, servers implementing this specification MUST expose principal resources at an http(s) URL, which is a privileged scheme that points to resources that have additional properties, as described in Section 4. So, a principal resource can have multiple URIs, one of which has to be an http(s) scheme URL. Although an implementation SHOULD support PROPFIND and MAY support PROPPATCH to access and modify information about a principal, it is not required to do so. A principal resource may be a group, where a group is a principal that represents a set of other principals, called the members of the group. If a person or computational agent matches a principal resource that is a member of a group, they also match the group. Membership in a group is recursive, so if a principal is a member of group GRPA, and GRPA is a member of group GRPB, then the principal is also a member of GRPB.3. Privileges
Ability to perform a given method on a resource MUST be controlled by one or more privileges. Authors of protocol extensions that define new HTTP methods SHOULD specify which privileges (by defining new privileges, or mapping to ones below) are required to perform the method. A principal with no privileges to a resource MUST be denied any HTTP access to that resource, unless the principal matches an ACE
constructed using the DAV:all, DAV:authenticated, or DAV:unauthenticated pseudo-principals (see Section 5.5.1). Servers MUST report a 403 "Forbidden" error if access is denied, except in the case where the privilege restricts the ability to know the resource exists, in which case 404 "Not Found" may be returned. Privileges may be containers of other privileges, in which case they are termed "aggregate privileges". If a principal is granted or denied an aggregate privilege, it is semantically equivalent to granting or denying each of the aggregated privileges individually. For example, an implementation may define add-member and remove- member privileges that control the ability to add and remove a member of a group. Since these privileges control the ability to update the state of a group, these privileges would be aggregated by the DAV:write privilege on a group, and granting the DAV:write privilege on a group would also grant the add-member and remove-member privileges. Privileges may be declared to be "abstract" for a given resource, in which case they cannot be set in an ACE on that resource. Aggregate and non-aggregate privileges are both capable of being abstract. Abstract privileges are useful for modeling privileges that otherwise would not be exposed via the protocol. Abstract privileges also provide server implementations with flexibility in implementing the privileges defined in this specification. For example, if a server is incapable of separating the read resource capability from the read ACL capability, it can still model the DAV:read and DAV:read-acl privileges defined in this specification by declaring them abstract, and containing them within a non-abstract aggregate privilege (say, read-all) that holds DAV:read, and DAV:read-acl. In this way, it is possible to set the aggregate privilege, read-all, thus coupling the setting of DAV:read and DAV:read-acl, but it is not possible to set DAV:read, or DAV:read-acl individually. Since aggregate privileges can be abstract, it is also possible to use abstract privileges to group or organize non-abstract privileges. Privilege containment loops are not allowed; therefore, a privilege MUST NOT contain itself. For example, DAV:read cannot contain DAV:read. The set of privileges that apply to a particular resource may vary with the DAV:resourcetype of the resource, as well as between different server implementations. To promote interoperability, however, this specification defines a set of well-known privileges (e.g., DAV:read, DAV:write, DAV:read-acl, DAV:write-acl, DAV:read- current-user-privilege-set, and DAV:all), which can at least be used to classify the other privileges defined on a particular resource. The access permissions on null resources (defined in [RFC2518], Section 3) are solely those they inherit (if any), and they are not discoverable (i.e., the access control properties specified in
Section 5 are not defined on null resources). On the transition from null to stateful resource, the initial access control list is set by the server's default ACL value policy (if any). Server implementations MAY define new privileges beyond those defined in this specification. Privileges defined by individual implementations MUST NOT use the DAV: namespace, and instead should use a namespace that they control, such as an http scheme URL.3.1. DAV:read Privilege
The read privilege controls methods that return information about the state of the resource, including the resource's properties. Affected methods include GET and PROPFIND. Any implementation-defined privilege that also controls access to GET and PROPFIND must be aggregated under DAV:read - if an ACL grants access to DAV:read, the client may expect that no other privilege needs to be granted to have access to GET and PROPFIND. Additionally, the read privilege MUST control the OPTIONS method. <!ELEMENT read EMPTY>3.2. DAV:write Privilege
The write privilege controls methods that lock a resource or modify the content, dead properties, or (in the case of a collection) membership of the resource, such as PUT and PROPPATCH. Note that state modification is also controlled via locking (see section 5.3 of [RFC2518]), so effective write access requires that both write privileges and write locking requirements are satisfied. Any implementation-defined privilege that also controls access to methods modifying content, dead properties or collection membership must be aggregated under DAV:write, e.g., if an ACL grants access to DAV:write, the client may expect that no other privilege needs to be granted to have access to PUT and PROPPATCH. <!ELEMENT write EMPTY>3.3. DAV:write-properties Privilege
The DAV:write-properties privilege controls methods that modify the dead properties of the resource, such as PROPPATCH. Whether this privilege may be used to control access to any live properties is determined by the implementation. Any implementation-defined privilege that also controls access to methods modifying dead properties must be aggregated under DAV:write-properties - e.g., if
an ACL grants access to DAV:write-properties, the client can safely expect that no other privilege needs to be granted to have access to PROPPATCH. <!ELEMENT write-properties EMPTY>3.4. DAV:write-content Privilege
The DAV:write-content privilege controls methods that modify the content of an existing resource, such as PUT. Any implementation- defined privilege that also controls access to content must be aggregated under DAV:write-content - e.g., if an ACL grants access to DAV:write-content, the client can safely expect that no other privilege needs to be granted to have access to PUT. Note that PUT - when applied to an unmapped URI - creates a new resource and therefore is controlled by the DAV:bind privilege on the parent collection. <!ELEMENT write-content EMPTY>3.5. DAV:unlock Privilege
The DAV:unlock privilege controls the use of the UNLOCK method by a principal other than the lock owner (the principal that created a lock can always perform an UNLOCK). While the set of users who may lock a resource is most commonly the same set of users who may modify a resource, servers may allow various kinds of administrators to unlock resources locked by others. Any privilege controlling access by non-lock owners to UNLOCK MUST be aggregated under DAV:unlock. A lock owner can always remove a lock by issuing an UNLOCK with the correct lock token and authentication credentials. That is, even if a principal does not have DAV:unlock privilege, they can still remove locks they own. Principals other than the lock owner can remove a lock only if they have DAV:unlock privilege and they issue an UNLOCK with the correct lock token. Lock timeout is not affected by the DAV:unlock privilege. <!ELEMENT unlock EMPTY>3.6. DAV:read-acl Privilege
The DAV:read-acl privilege controls the use of PROPFIND to retrieve the DAV:acl property of the resource. <!ELEMENT read-acl EMPTY>
3.7. DAV:read-current-user-privilege-set Privilege
The DAV:read-current-user-privilege-set privilege controls the use of PROPFIND to retrieve the DAV:current-user-privilege-set property of the resource. Clients are intended to use this property to visually indicate in their UI items that are dependent on the permissions of a resource, for example, by graying out resources that are not writable. This privilege is separate from DAV:read-acl because there is a need to allow most users access to the privileges permitted the current user (due to its use in creating the UI), while the full ACL contains information that may not be appropriate for the current authenticated user. As a result, the set of users who can view the full ACL is expected to be much smaller than those who can read the current user privilege set, and hence distinct privileges are needed for each. <!ELEMENT read-current-user-privilege-set EMPTY>3.8. DAV:write-acl Privilege
The DAV:write-acl privilege controls use of the ACL method to modify the DAV:acl property of the resource. <!ELEMENT write-acl EMPTY>3.9. DAV:bind Privilege
The DAV:bind privilege allows a method to add a new member URL to the specified collection (for example via PUT or MKCOL). It is ignored for resources that are not collections. <!ELEMENT bind EMPTY>3.10. DAV:unbind Privilege
The DAV:unbind privilege allows a method to remove a member URL from the specified collection (for example via DELETE or MOVE). It is ignored for resources that are not collections. <!ELEMENT unbind EMPTY>
3.11. DAV:all Privilege
DAV:all is an aggregate privilege that contains the entire set of privileges that can be applied to the resource. <!ELEMENT all EMPTY>3.12. Aggregation of Predefined Privileges
Server implementations are free to aggregate the predefined privileges (defined above in Sections 3.1-3.10) subject to the following limitations: DAV:read-acl MUST NOT contain DAV:read, DAV:write, DAV:write-acl, DAV:write-properties, DAV:write-content, or DAV:read-current-user- privilege-set. DAV:write-acl MUST NOT contain DAV:write, DAV:read, DAV:read-acl, or DAV:read-current-user-privilege-set. DAV:read-current-user-privilege-set MUST NOT contain DAV:write, DAV:read, DAV:read-acl, or DAV:write-acl. DAV:write MUST NOT contain DAV:read, DAV:read-acl, or DAV:read- current-user-privilege-set. DAV:read MUST NOT contain DAV:write, DAV:write-acl, DAV:write- properties, or DAV:write-content. DAV:write MUST contain DAV:bind, DAV:unbind, DAV:write-properties and DAV:write-content.4. Principal Properties
Principals are manifested to clients as a WebDAV resource, identified by a URL. A principal MUST have a non-empty DAV:displayname property (defined in Section 13.2 of [RFC2518]), and a DAV:resourcetype property (defined in Section 13.9 of [RFC2518]). Additionally, a principal MUST report the DAV:principal XML element in the value of the DAV:resourcetype property. The element type declaration for DAV:principal is: <!ELEMENT principal EMPTY>
This protocol defines the following additional properties for a principal. Since it can be expensive for a server to retrieve access control information, the name and value of these properties SHOULD NOT be returned by a PROPFIND allprop request (as defined in Section 12.14.1 of [RFC2518]).4.1. DAV:alternate-URI-set
This protected property, if non-empty, contains the URIs of network resources with additional descriptive information about the principal. This property identifies additional network resources (i.e., it contains one or more URIs) that may be consulted by a client to gain additional knowledge concerning a principal. One expected use for this property is the storage of an LDAP [RFC2255] scheme URL. A user-agent encountering an LDAP URL could use LDAP [RFC2251] to retrieve additional machine-readable directory information about the principal, and display that information in its user interface. Support for this property is REQUIRED, and the value is empty if no alternate URI exists for the principal. <!ELEMENT alternate-URI-set (href*)>4.2. DAV:principal-URL
A principal may have many URLs, but there must be one "principal URL" that clients can use to uniquely identify a principal. This protected property contains the URL that MUST be used to identify this principal in an ACL request. Support for this property is REQUIRED. <!ELEMENT principal-URL (href)>4.3. DAV:group-member-set
This property of a group principal identifies the principals that are direct members of this group. Since a group may be a member of another group, a group may also have indirect members (i.e., the members of its direct members). A URL in the DAV:group-member-set for a principal MUST be the DAV:principal-URL of that principal. <!ELEMENT group-member-set (href*)>4.4. DAV:group-membership
This protected property identifies the groups in which the principal is directly a member. Note that a server may allow a group to be a member of another group, in which case the DAV:group-membership of
those other groups would need to be queried in order to determine the groups in which the principal is indirectly a member. Support for this property is REQUIRED. <!ELEMENT group-membership (href*)>
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