INTERNET-DRAFT P. J. Leach
Expires: April 1998 Y. Y. Goland
Standards Track Microsoft Corporation
WebDAV Working Group November 10, 1997
WebDAV ACL Protocol
draft-ietf-webdav-acl-00.txt
Status of this Memo
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Abstract
This memo specifies the format and manipulation mechanisms for
access control lists (ACLs) for WebDAV objects.
Contents
Status of this Memo................................................1
Abstract...........................................................1
Contents...........................................................1
1. Introduction....................................................2
2. Principals Identifiers..........................................2
3. Granting and Denying Rights.....................................3
4. ACL Inheritance.................................................4
5. Properties and ACLs.............................................4
6. Rights Definitions..............................................5
7. Default Principal Types.........................................7
8. ACL Method......................................................7
9. Examples.......................................................11
10.Authors' Addresses.............................................13
11.Bibliography...................................................14
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1. Introduction
The basic model for access control, informally expressed, is that
who you are determines how you can access 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
"how" is determined by an "access control list" (ACL) associated
with a resource.
An ACL contains Access Control Elements (ACE). An ACE specifies a
set of principals, a set of granted rights, and a set of denied
rights.
Rights may be generic, such as "read", "write", or "delete", or they
might be specific to the kind of resource protected by that ACL,
such as (perhaps) "send-to", "unsubscribe", and "administer" for
mailing lists.
When a resource is created it inherits a set of default ACL
properties from a designated resource, referred to as an ACL source.
The inheritance can be "static", so that subsequent changes to the
ACL source do not effect the new resources ACL properties; or it can
be "dynamic", so that subsequent changes are reflected in the new
resource's ACL properties.
Properties on a resource, by default, dynamically inherit from the
ACL on the resource. In other words, the resource is the ACL source
for the properties. However individual properties can be given their
own ACL.
2. Principals Identifiers
A principal identifier can identify a single principal or a compound
principal. A single principal identifier refers to a single
principal, such as a person or a program. A compound principal
identifier specifies one or more principals. A compound principal
may not necessarily just be a list of other principals. It may in
fact be a program that accepts a principal identifier as input and
output true or false to indicate membership.
This specification relies upon the underlying authentication
mechanism(s) to provide the syntax of principal identifiers. Thus,
for the purposes of this specification, principal identifiers are
opaque.
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3. Granting and Denying Rights
An ACL can both grant and deny rights. The reason both types of
grants are required is because of compound principals.
The consequence of the existence of compound principals is that
there are times when a compound principal may be granted a right but
a particular member of the compound principal may need to be denied
access. In order to make this possible an ACL must be able to list
principals both allowed and denied a right.
By default all rights for a principal MUST be denied. Rights MAY
only be granted to a principal by an explicit listing of that
principal in a "grant" section of an ACL.
Additionally it is possible for access rights to collide in scope.
For example, a container may have an access right which specifies
the ability of principals to delete the children of that container.
This would affect a principal's ability to use the DELETE method.
However a particular internal child may have granted access rights
to DELETE. As such, the two rights could collide.
The following rules, processed in order, MUST be used to resolve
scope conflicts between rights.
1) In a conflict between a right granted by a parent and a right
granted by a child, the right specified by the child MUST override.
2) In a conflict between a right granted or denied to a compound
principal and a right granted or denied to a member of the compound
principal, the reference to the member of the compound principal
MUST override.
Note that rule 2 is conceptually identical to rule 1. The concept
represented by rules 1 and 2, stated generally, is that a specific
references always overrides a more general reference.
3.1. Examples
The following examples demonstrate a situation where the specified
conflict resolution rule would be applied.
3.1.1. Rule 1
A resource specifies that principal A is granted the right to delete
the resource. A property on the resource specifies that principal A
is denied the right to delete the property. The conflict is resolved
by rule 1, the resource is the parent and the property is the child.
As such the child's declaration overrides and principal A is denied
the right to delete the resource.
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Note, however, that if other properties do not deny principal A the
right to delete them then principal A could delete all properties
but the one mentioned above and could PUT an empty body to the
resource. However it could not successfully execute a DELETE on the
resource, as this would have the effect of deleting the property
along with the resource.
3.1.2. Rule 2
A resource specifies that principal A is denied the read right. The
same resource also specifies that principal B is granted the read
right. Principal A, however, is a compound principal of which
Principal B is a member. Rule 1 does not apply because the rights in
question are defined on the same resource. The conflict is resolved
by rule 2 as the conflict is between a compound principal and one of
its members. In that case principal B has the right to read the
resource.
4. ACL Inheritance
When a new resource is created it may inherit its ACL from its
containing resource. If no inheritance method is specified then the
resource has no ACL. Note, however, that the owner value is
automatically set when a resource is created, so even without
inheritance, there will always be an owner.
An inherited ACL MUST be applied to the resource before it is
available for manipulation. Thus, if inheritance is used, the
resource will never be in a state where it does not have access
control protection.
Inheritance can either be static or dynamic.
Static inheritance means that the ACL specified by the parent will
be used to define the ACL for the child. Any subsequent changes made
to the parent will not cause the child's ACL to be altered.
Dynamic inheritance means that the ACL specified by the parent is
used to define the ACL for the child but any changes on the parent's
ACL MUST automatically be made to any inheriting children. The child
is still allowed to define its own ACL values that MUST override any
conflicting inherited ACL.
5. Properties and ACLs
Properties MAY have their own ACL independent of the associated
resource. By default a property's ACL MUST be dynamically inherited
from the associated resource. Note that properties can only inherit
from their associated resource.
It is legal for a property to carry a setting for what sort of
inheritance its children will have. Currently this value has no
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meaning as properties can not have children, but it is expected in
the future that hierarchical properties will be adopted, so this
setting will then have meaning. For now compliant resources MUST
record this value but do not have to do anything with it.
For purposes of applying scoping conflict resolution rules the
resource is the parent and the property is the child.
Compliant resources are not required to support setting ACLs
directly on properties.
6. Rights Definitions
The following define a variety of rights. A compliant resource MUST
support all of the rights contained herein.
6.1. all Right
Name: all
Namespace: http://www.ietf.org/standards/acl/
Purpose: The all right provides all rights.
Values: None
Description: In a conflict between the "all" right and other
rights, the "all" right is considered a parent and the other rights
a "child." Thus one ACE could provide the ALL right for a particular
principal but another ACE in the same ACL could deny the same
principal a particular right. The conflict would be resolved by
denying the specified principal the more specific right.
6.2. read Right
Name: read
Namespace: http://www.ietf.org/standards/acl/
Purpose: The read right provides and restricts access to
information regarding the state of the resource, including the
resource's properties. Effected methods are GET, INDEX, and
PROPFIND. OPTIONS is not covered by a Read ACL as it reflects
capabilities rather than state.
Values: None
6.3. write Right
Name: write
Namespace: http://www.ietf.org/standards/acl/
Purpose: The Write right affects the same methods as the
Write Lock. Please refer to [WEBDAV] section 5.3 for the list of
affected methods. Note however, that a write lock is a different
mechanism than a write access change, although they affect the same
methods, they have independent methods to set them and independent
error codes.
Values: None
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6.4. delete Right
Name: delete
Namespace: http://www.ietf.org/standards/acl/
Purpose: The delete right controls access to the DELETE
method. This method does not affect the ability to remove
properties.
Values: None
6.5. createchild Right
Name: createchild
Namespace: http://www.ietf.org/standards/acl/
Purpose: This right controls the ability to PUT internal
members of a collection and ADDREF external members of a collection.
This ACL has no affect if set on non-collections.
Values: None
6.6. deletechild Right
Name: deletechild
Namespace: http://www.ietf.org/standards/acl/
Purpose: The deletechild right controls the ability to the
DELETE internal members of a collection and DELREF external members
of a collection. This ACL has no affect if set on non-collections.
Values: None
6.7. writeowner Right
Name: writeowner
Namespace: http://www.ietf.org/standards/acl/
Purpose: The writeowner right controls the ability to change
the value of the owner right.
Values: None
6.8. readacl Right
Name: readacl
Namespace: http://www.ietf.org/standards/acl/
Purpose: The readacl right controls the ability to read the
ACL property.
Values: None
6.9. writeacl Right
Name: writeacl
Namespace: http://www.ietf.org/standards/acl/
Purpose: The writeacl right controls the ability to alter
the ACL property.
Values: None
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7. Default Principal Types
The following two XML elements are defined principal types.
7.1. allprincipals XML Element
Name: allprincipals
Namespace: http://www.ietf.org/standards/acl/
Purpose: The allprincipals XML element represents all
principals. It is used to specify rights belonging to all
principals, regardless of authentication.
Values: None
7.2. allauthprincipals XML Element
Name: allauthprincipals
Namespace: http://www.ietf.org/standards/acl/
Purpose: The allauthprincipals XML element represents all
authenticated principals. It is used to specify rights belonging to
all authenticated principals.
Values: None
8. ACL Method
The ACL Method serves two distinct purposes. Its request body is
used to define alterations to the ACL of the resource and its
properties. The response contains the ACL for the associated
resource and its properties.
The Request-URI of the ACL method identifies the resource whose ACL
information is to be retrieved and possibly altered.
Change requests through the ACL method MUST be atomic, additionally
changes are all or nothing. If any part of the change request fails
then all changes MUST fail.
8.1. Request
The request may contain up to four XML elements, owner,
aclinheritance, and ACL. The presence of an element, except as
otherwise specified, in the request body causes the associated value
to change.
The presence of an empty ACL causes all ACEs in the ACL, including
ACEs for associated properties, to be deleted.
If an ACE is specified in a request it completely replaces the ACE
currently use for the same principal, if it exists. If an ACE is
submitted with empty grant and deny lists then the ACE is deleted.
It is a syntax error for two ACEs to reference the same principal.
Additionally, although an ACE can be submitted which references
multiple principals, this is primarily a convenience feature.
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Strictly speaking, what the user has done is specify an ACE for
every principal specified. The same logic applies to results that
aggregate principals into a single ACE.
It is illegal to delete any value, ACE, owner, aclinheritance, etc.
with a redundant value. For example, if one ACE grants all
principals read rights and another ACE grants a single principal
read rights, both ACEs MUST be maintained. The reason being that in
the future all principals may have their read rights removed but the
single principal will retain the read right because the more
specific ACE will override the more general ACE. Additionally if the
currently inherited value of owner is "someuser" and the principal
explicitly requests that the owner by set to "someuser", the
information MUST be recorded on the resource. That way if owner on
the source ACL is changed, the proper value as requested by the
client will remain on the inheriting ACL.
An empty request body will cause no change to the ACL or associated
values.
8.2. Response
If the request body was empty or if the changes were successful a
200 Success response MUST be returned with a response body
containing the owner, aclinheritance, and the acl XML elements with
values for all properties.
[Ed Note - I am not dealing with error conditions yet as the error
reporting format is going to have to be pretty complex and I want to
get buy off on the ACL model and the request format before I write
up a couple of pages on how to do errors for that format.]
8.3. acl XML element
Name: acl
Namespace: http://www.ietf.org/standards/acl/
Purpose: Defines an ACL.
Parent: Any
Values= (inheritance [owner] [aclinheritance] *ACE
*property)
Description: An empty ACL element will delete all ACEs contained
in an ACL, including the ACEs of any properties. Note, however, that
there MUST always be an ACL value defined on a resource, even if it
is empty.
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8.4. owner XML Element
Name: owner
Namespace: http://www.ietf.org/standards/acl/
Purpose: Specifies the owner of the resource.
Parent= acl
Values= Principal
Description: The owner XML element specifies the principal who
owns the resource. The default value for owner MUST be the principal
who created the resource. The owner always retains the right to
alter the ACL. So, for example, an owner who was not granted the
right to read the resource could not read the resource. However the
owner could alter the ACL so as to grant the read right to
themselves. A principal MUST have the writeowner right to change the
owner property's value. An empty Owner element submitted in a
request will not cause a change in the Onwer value. Owner MUST
always have a value. All compliant resources MUST support the owner
value.
8.5. aclinheritance XML Element
Name: aclinheritance
Namespace: http://www.ietf.org/standards/acl/
Parent= acl
Purpose: The aclinheritance XML element identifies the type
of inheritance to be used with children of the associated resource.
The AclInheritance value MUST default to Dynamic. An empty
AclInheritance submitted in a request will not cause a change in the
AclInheritance value. This element has no meaning on non-
collections. However, collections MUST provide this property.
Values= ("Dynamic" | "Static" | "None" | Extension)
;Extension is defined, somewhere in DAV. URL is defined, someplace,
somewhere.
Description: Although this element has no meaning when defined
on a property, resources MUST record its value.
8.6. inheritance XML Element
Name: inheritance
Namespace: http://www.ietf.org/standards/acl/
Purpose: Defines the inheritance used for a particular ACL.
Parent: acl
Values= ("Dynamic" | "None" | Extension)
Description: Specifies if the ACL is inheriting its value
dynamically or not at all. Static is not an option since static
inheritance can only occur when the ACL was created and so was
controlled by the ACL source.
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8.7. principal XML Element
Name: principal
Namespace: http://www.ietf.org/standards/acl/
Purpose: To identify a principal.
Parent: any
Values= *cdata
8.8. ace XML Element
Name: ace
Namespace: http://www.ietf.org/standards/acl/
Purpose: Contains access right information associated with
one or more principals.
Parent: acl
Values= Principal Allow Deny
Description: A principal MUST NOT be directly referred to in
more than one ACE on a resource. That is, each principal has a
particular ACE which specifies all of its directly granted rights.
Thus specifying two ACEs which directly reference the same principal
in a request is a syntax error.
8.9. grant XML Element
Name: grant
Namespace: http://www.ietf.org/standards/acl/
Purpose: Identifies the rights the associated principals are
granted.
Parent: ACE
Values: Right Identifiers
8.10. deny XML Element
Name: deny
Namespace: http://www.ietf.org/standards/acl/
Purpose: Identifies the rights the associated principals are
denied.
Parent: ACE
Values: Right Identifiers
8.11. property XML Element
Name: property
Namespace: http://www.ietf.org/standards/acl/
Purpose: Provides ACL for properties.
Parent: ACL
Values= Prop ACL
Description: The properties in the Prop XML element MUST be
empty.
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9. Examples
9.1. Example 1 - Retrieving ACL information
ACL /top/container/ HTTP/1.1
Host: www.foo.bar
Content-Length: xxxx
Content-Type: text/xml
HTTP/1.1 200 Success
Content-Type: text/xml
Content-Length: xxxxx
<?namespace href = "http://www.ietf.org/standards/acl/" AS = "a"?>
<?namespace href = "http://www.ietf.org/standards/dav/" AS = "d"?>
<a:acl>
<a:inheritance>dynamic</a:inheritance>
<a:owner>someonesomewhere</a:owner>
<a:ace>
<a:principal>domain/joebob</a:principal>
<a:grant/>
<a:deny><a:all/></a:deny>
</a:ace>
<a:property>
<d:prop><d:creationdate></d:prop>
<a:acl>
<a:inheritance>dynamic</a:inheritance>
<a:owner>blah</a:owner>
<a:ace>
<a:principal>domain/joebob</a:principal>
<a:grant/><a:all/></a:grant>
<a:deny/>
</a:ace>
</a:acl>
</a:property>
<a:property>
<d:prop>
<d:displayname><d:get-content-language><d:get-content-
length><d:get-content-type><d:get-etag><d:get-
lastmodified><d:index-content-language><d:index-content-
length><d:index-etag><d:index-last-modified><d:resourcetype>
</d:prop>
<a:acl>
<a:inheritance>dynamic</a:inheritance>
</a:acl>
</a:property>
</a:acl>
The request was empty so no changes will be made, rather the
response will just contain all the relevant values. The resource
gets its own ACL dynamically from its parent, top. However this
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resource does override the inherited ACL. Specifically, it defines
its owner, someonesomewhere, rather than inheriting it. However, the
absence of an aclinheritance element indicates that the resource
inherits that value. Additional the principal domain/joebob is
denied all rights. So regardless of what rights domain/joebob may
have been granted in top's ACL, all those rights are denied in
relation to top/container. While the ACL for creationdate is also
inherited it has its own owner, blah, and has an additional ACE for
joebob. All the rest of the properties have their ACLs inherited
from the resource. Therefore the denial of all rights to
domain/joebob would also apply to the resource's properties but
creationdate..
9.2. Example 2 - Setting ACLs
ACL /top/container.html HTTP/1.1
Host: www.foo.com
Content-Type: text/xml
Content-Length: xxxx
<?namespace href = "http://www.ietf.org/standards/dav/" AS = "D"?>
<?namespace href = "http://www.ietf.org/standards/acl/ AS = "A"?>
<a:acl>
<a:property>
<a:aclinheritance>none</a:aclinheritance>
<d:prop>
<d:creationdate/>
</d:prop>
<a:acl>
<a:ace>
<a:principal>domain/joebob</a:principal>
<a:grant/><a:deny/>
</a:ace>
</a:acl>
</a:property>
</a:acl>
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HTTP/1.1 200 Success
Content-Type: text/xml
Content-Length: xxxxx
<?namespace href = "http://www.ietf.org/standards/acl/" AS = "a"?>
<?namespace href = "http://www.ietf.org/standards/dav/" AS = "d"?>
<a:acl>
<a:inheritance>dynamic</a:inheritance>
<a:owner>someonesomewhere</a:owner>
<a:aclinheritance>none</a:aclinheritance>
<a:ace>
<a:principal>domain/joebob</a:principal>
<a:grant/>
<a:deny><a:all/></a:deny>
</a:ace>
<a:property>
<d:prop><d:creationdate></d:prop>
<a:acl>
<a:inheritance>dynamic</a:inheritance>
<a:owner>blah</a:owner>
</a:acl>
</a:property>
<a:property>
<d:prop>
<d:displayname><d:get-content-language><d:get-content-
length><d:get-content-type><d:get-etag><d:get-
lastmodified><d:index-content-language><d:index-content-
length><d:index-etag><d:index-last-modified><d:resourcetype>
</d:prop>
<a:acl>
<a:inheritance>dynamic</a:inheritance>
</a:acl>
</a:property>
</a:acl>
This example assumes the state left from example 1. In the request
the user asks that the aclinheritance value be set to none and that
the ACE on the property creationdate for the principal domain/joebob
be removed. Even if the inherited aclinheritance value is none, the
resource MUST still record the redundant value as the value on the
source ACL may change.
10. Authors' Addresses
Paul J. Leach
Yaron Y. Goland
Microsoft
1 Microsoft Way
Redmond, WA 98052
Phone: (425)936-4765
Email: {paulle, yarong}@microsoft.com
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11. Bibliography
[RFC2068] R. Fielding, J. Gettys, J. Mogul, H. Frystyk, T. Berners-
Lee, 'Hypertext Transfer Protocol -- HTTP/1.1', RFC 2068, January
1997, <URL:ftp://ds.internic.net/rfc/rfc2068.txt>
[WebDAV] Y. Goland, E. J. Whitehead, Jr., Asad Faizi, Stephen R.
Carter, Del Jensen 'Extensions for Distributed Authoring and
Versioning on the World Wide Web -- WEBDAV', October 1997, WORK
IN PROGRESS, <URL:ftp://ftp.ietf.org/internet-drafts/draft-ietf-
webdav-protocol-04.txt>
[XML] W3C, 'Extensible Markup Language - Part1. Syntax', March 1997
<URL:http://www.w3.org/pub/WWW/TR/WD-xml-lang.html>
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