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Network Working Group K. Zeilenga
Request for Comments: 4512 OpenLDAP Foundation
Obsoletes: 2251, 2252, 2256, 3674 June 2006
Category: Standards Track
Lightweight Directory Access Protocol (LDAP):
Directory Information Models
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 (2006).
Abstract
The Lightweight Directory Access Protocol (LDAP) is an Internet
protocol for accessing distributed directory services that act in
accordance with X.500 data and service models. This document
describes the X.500 Directory Information Models, as used in LDAP.
Zeilenga Standards Track [Page 1]
RFC 4512 LDAP Models June 2006
Table of Contents
1. Introduction ....................................................3
1.1. Relationship to Other LDAP Specifications ..................3
1.2. Relationship to X.501 ......................................4
1.3. Conventions ................................................4
1.4. Common ABNF Productions ....................................4
2. Model of Directory User Information .............................6
2.1. The Directory Information Tree .............................7
2.2. Structure of an Entry ......................................7
2.3. Naming of Entries ..........................................8
2.4. Object Classes .............................................9
2.5. Attribute Descriptions ....................................12
2.6. Alias Entries .............................................16
3. Directory Administrative and Operational Information ...........17
3.1. Subtrees ..................................................17
3.2. Subentries ................................................18
3.3. The 'objectClass' attribute ...............................18
3.4. Operational Attributes ....................................19
4. Directory Schema ...............................................22
4.1. Schema Definitions ........................................23
4.2. Subschema Subentries ......................................32
4.3. 'extensibleObject' object class ...........................35
4.4. Subschema Discovery .......................................35
5. DSA (Server) Informational Model ...............................36
5.1. Server-Specific Data Requirements .........................36
6. Other Considerations ...........................................40
6.1. Preservation of User Information ..........................40
6.2. Short Names ...............................................41
6.3. Cache and Shadowing .......................................41
7. Implementation Guidelines ......................................42
7.1. Server Guidelines .........................................42
7.2. Client Guidelines .........................................42
8. Security Considerations ........................................43
9. IANA Considerations ............................................43
10. Acknowledgements ..............................................44
11. Normative References ..........................................45
Appendix A. Changes ...............................................47
A.1. Changes to RFC 2251 .......................................47
A.2. Changes to RFC 2252 .......................................49
A.3. Changes to RFC 2256 .......................................50
A.4. Changes to RFC 3674 .......................................51
Zeilenga Standards Track [Page 2]
RFC 4512 LDAP Models June 2006
1. Introduction
This document discusses the X.500 Directory Information Models
[X.501], as used by the Lightweight Directory Access Protocol (LDAP)
[RFC4510].
The Directory is "a collection of open systems cooperating to provide
directory services" [X.500]. The information held in the Directory
is collectively known as the Directory Information Base (DIB). A
Directory user, which may be a human or other entity, accesses the
Directory through a client (or Directory User Agent (DUA)). The
client, on behalf of the directory user, interacts with one or more
servers (or Directory System Agents (DSA)). A server holds a
fragment of the DIB.
The DIB contains two classes of information:
1) user information (e.g., information provided and administrated
by users). Section 2 describes the Model of User Information.
2) administrative and operational information (e.g., information
used to administer and/or operate the directory). Section 3
describes the model of Directory Administrative and Operational
Information.
These two models, referred to as the generic Directory Information
Models, describe how information is represented in the Directory.
These generic models provide a framework for other information
models. Section 4 discusses the subschema information model and
subschema discovery. Section 5 discusses the DSA (Server)
Informational Model.
Other X.500 information models (such as access control distribution
knowledge and replication knowledge information models) may be
adapted for use in LDAP. Specification of how these models apply to
LDAP is left to future documents.
1.1. Relationship to Other LDAP Specifications
This document is a integral part of the LDAP technical specification
[RFC4510], which obsoletes the previously defined LDAP technical
specification, RFC 3377, in its entirety.
This document obsoletes RFC 2251, Sections 3.2 and 3.4, as well as
portions of Sections 4 and 6. Appendix A.1 summarizes changes to
these sections. The remainder of RFC 2251 is obsoleted by the
[RFC4511], [RFC4513], and [RFC4510] documents.
Zeilenga Standards Track [Page 3]
RFC 4512 LDAP Models June 2006
This document obsoletes RFC 2252, Sections 4, 5, and 7. Appendix A.2
summarizes changes to these sections. The remainder of RFC 2252 is
obsoleted by [RFC4517].
This document obsoletes RFC 2256, Sections 5.1, 5.2, 7.1, and 7.2.
Appendix A.3 summarizes changes to these sections. The remainder of
RFC 2256 is obsoleted by [RFC4519] and [RFC4517].
This document obsoletes RFC 3674 in its entirety. Appendix A.4
summarizes changes since RFC 3674.
1.2. Relationship to X.501
This document includes material, with and without adaptation, from
[X.501] as necessary to describe this protocol. These adaptations
(and any other differences herein) apply to this protocol, and only
this protocol.
1.3. Conventions
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 BCP 14 [RFC2119].
Schema definitions are provided using LDAP description formats (as
defined in Section 4.1). Definitions provided here are formatted
(line wrapped) for readability. Matching rules and LDAP syntaxes
referenced in these definitions are specified in [RFC4517].
1.4. Common ABNF Productions
A number of syntaxes in this document are described using Augmented
Backus-Naur Form (ABNF) [RFC4234]. These syntaxes (as well as a
number of syntaxes defined in other documents) rely on the following
common productions:
keystring = leadkeychar *keychar
leadkeychar = ALPHA
keychar = ALPHA / DIGIT / HYPHEN
number = DIGIT / ( LDIGIT 1*DIGIT )
ALPHA = %x41-5A / %x61-7A ; "A"-"Z" / "a"-"z"
DIGIT = %x30 / LDIGIT ; "0"-"9"
LDIGIT = %x31-39 ; "1"-"9"
HEX = DIGIT / %x41-46 / %x61-66 ; "0"-"9" / "A"-"F" / "a"-"f"
SP = 1*SPACE ; one or more " "
WSP = 0*SPACE ; zero or more " "
Zeilenga Standards Track [Page 4]
RFC 4512 LDAP Models June 2006
NULL = %x00 ; null (0)
SPACE = %x20 ; space (" ")
DQUOTE = %x22 ; quote (""")
SHARP = %x23 ; octothorpe (or sharp sign) ("#")
DOLLAR = %x24 ; dollar sign ("$")
SQUOTE = %x27 ; single quote ("'")
LPAREN = %x28 ; left paren ("(")
RPAREN = %x29 ; right paren (")")
PLUS = %x2B ; plus sign ("+")
COMMA = %x2C ; comma (",")
HYPHEN = %x2D ; hyphen ("-")
DOT = %x2E ; period (".")
SEMI = %x3B ; semicolon (";")
LANGLE = %x3C ; left angle bracket ("<")
EQUALS = %x3D ; equals sign ("=")
RANGLE = %x3E ; right angle bracket (">")
ESC = %x5C ; backslash ("\")
USCORE = %x5F ; underscore ("_")
LCURLY = %x7B ; left curly brace "{"
RCURLY = %x7D ; right curly brace "}"
; Any UTF-8 [RFC3629] encoded Unicode [Unicode] character
UTF8 = UTF1 / UTFMB
UTFMB = UTF2 / UTF3 / UTF4
UTF0 = %x80-BF
UTF1 = %x00-7F
UTF2 = %xC2-DF UTF0
UTF3 = %xE0 %xA0-BF UTF0 / %xE1-EC 2(UTF0) /
%xED %x80-9F UTF0 / %xEE-EF 2(UTF0)
UTF4 = %xF0 %x90-BF 2(UTF0) / %xF1-F3 3(UTF0) /
%xF4 %x80-8F 2(UTF0)
OCTET = %x00-FF ; Any octet (8-bit data unit)
Object identifiers (OIDs) [X.680] are represented in LDAP using a
dot-decimal format conforming to the ABNF:
numericoid = number 1*( DOT number )
Short names, also known as descriptors, are used as more readable
aliases for object identifiers. Short names are case insensitive and
conform to the ABNF:
descr = keystring
Zeilenga Standards Track [Page 5]
RFC 4512 LDAP Models June 2006
Where either an object identifier or a short name may be specified,
the following production is used:
oid = descr / numericoid
While the <descr> form is generally preferred when the usage is
restricted to short names referring to object identifiers that
identify like kinds of objects (e.g., attribute type descriptions,
matching rule descriptions, object class descriptions), the
<numericoid> form should be used when the object identifiers may
identify multiple kinds of objects or when an unambiguous short name
(descriptor) is not available.
Implementations SHOULD treat short names (descriptors) used in an
ambiguous manner (as discussed above) as unrecognized.
Short Names (descriptors) are discussed further in Section 6.2.
2. Model of Directory User Information
As [X.501] states:
The purpose of the Directory is to hold, and provide access to,
information about objects of interest (objects) in some 'world'.
An object can be anything which is identifiable (can be named).
An object class is an identified family of objects, or conceivable
objects, which share certain characteristics. Every object
belongs to at least one class. An object class may be a subclass
of other object classes, in which case the members of the former
class, the subclass, are also considered to be members of the
latter classes, the superclasses. There may be subclasses of
subclasses, etc., to an arbitrary depth.
A directory entry, a named collection of information, is the basic
unit of information held in the Directory. There are multiple kinds
of directory entries.
An object entry represents a particular object. An alias entry
provides alternative naming. A subentry holds administrative and/or
operational information.
The set of entries representing the DIB are organized hierarchically
in a tree structure known as the Directory Information Tree (DIT).
Section 2.1 describes the Directory Information Tree.
Section 2.2 discusses the structure of entries.
Section 2.3 discusses naming of entries.
Zeilenga Standards Track [Page 6]
RFC 4512 LDAP Models June 2006
Section 2.4 discusses object classes.
Section 2.5 discusses attribute descriptions.
Section 2.6 discusses alias entries.
2.1. The Directory Information Tree
As noted above, the DIB is composed of a set of entries organized
hierarchically in a tree structure known as the Directory Information
Tree (DIT); specifically, a tree where vertices are the entries.
The arcs between vertices define relations between entries. If an
arc exists from X to Y, then the entry at X is the immediate superior
of Y, and Y is the immediate subordinate of X. An entry's superiors
are the entry's immediate superior and its superiors. An entry's
subordinates are all of its immediate subordinates and their
subordinates.
Similarly, the superior/subordinate relationship between object
entries can be used to derive a relation between the objects they
represent. DIT structure rules can be used to govern relationships
between objects.
Note: An entry's immediate superior is also known as the entry's
parent, and an entry's immediate subordinate is also known as
the entry's child. Entries that have the same parent are known
as siblings.
2.2. Structure of an Entry
An entry consists of a set of attributes that hold information about
the object that the entry represents. Some attributes represent user
information and are called user attributes. Other attributes
represent operational and/or administrative information and are
called operational attributes.
An attribute is an attribute description (a type and zero or more
options) with one or more associated values. An attribute is often
referred to by its attribute description. For example, the
'givenName' attribute is the attribute that consists of the attribute
description 'givenName' (the 'givenName' attribute type [RFC4519] and
zero options) and one or more associated values.
The attribute type governs whether the attribute can have multiple
values, the syntax and matching rules used to construct and compare
values of that attribute, and other functions. Options indicate
subtypes and other functions.
Attribute values conform to the defined syntax of the attribute type.
Zeilenga Standards Track [Page 7]
RFC 4512 LDAP Models June 2006
No two values of an attribute may be equivalent. Two values are
considered equivalent if and only if they would match according to
the equality matching rule of the attribute type. Or, if the
attribute type is defined with no equality matching rule, two values
are equivalent if and only if they are identical. (See 2.5.1 for
other restrictions.)
For example, a 'givenName' attribute can have more than one value,
they must be Directory Strings, and they are case insensitive. A
'givenName' attribute cannot hold both "John" and "JOHN", as these
are equivalent values per the equality matching rule of the attribute
type.
Additionally, no attribute is to have a value that is not equivalent
to itself. For example, the 'givenName' attribute cannot have as a
value a directory string that includes the REPLACEMENT CHARACTER
(U+FFFD) code point, as matching involving that directory string is
Undefined per this attribute's equality matching rule.
When an attribute is used for naming of the entry, one and only one
value of the attribute is used in forming the Relative Distinguished
Name. This value is known as a distinguished value.
2.3. Naming of Entries
2.3.1. Relative Distinguished Names
Each entry is named relative to its immediate superior. This
relative name, known as its Relative Distinguished Name (RDN)
[X.501], is composed of an unordered set of one or more attribute
value assertions (AVA) consisting of an attribute description with
zero options and an attribute value. These AVAs are chosen to match
attribute values (each a distinguished value) of the entry.
An entry's relative distinguished name must be unique among all
immediate subordinates of the entry's immediate superior (i.e., all
siblings).
The following are examples of string representations of RDNs
[RFC4514]:
UID=12345
OU=Engineering
CN=Kurt Zeilenga+L=Redwood Shores
The last is an example of a multi-valued RDN; that is, an RDN
composed of multiple AVAs.
Zeilenga Standards Track [Page 8]
RFC 4512 LDAP Models June 2006
2.3.2. Distinguished Names
An entry's fully qualified name, known as its Distinguished Name (DN)
[X.501], is the concatenation of its RDN and its immediate superior's
DN. A Distinguished Name unambiguously refers to an entry in the
tree. The following are examples of string representations of DNs
[RFC4514]:
UID=nobody@example.com,DC=example,DC=com
CN=John Smith,OU=Sales,O=ACME Limited,L=Moab,ST=Utah,C=US
2.3.3. Alias Names
An alias, or alias name, is "an name for an object, provided by the
use of alias entries" [X.501]. Alias entries are described in
Section 2.6.
2.4. Object Classes
An object class is "an identified family of objects (or conceivable
objects) that share certain characteristics" [X.501].
As defined in [X.501]:
Object classes are used in the Directory for a number of purposes:
- describing and categorizing objects and the entries that
correspond to these objects;
- where appropriate, controlling the operation of the Directory;
- regulating, in conjunction with DIT structure rule
specifications, the position of entries in the DIT;
- regulating, in conjunction with DIT content rule
specifications, the attributes that are contained in entries;
- identifying classes of entry that are to be associated with a
particular policy by the appropriate administrative authority.
An object class (a subclass) may be derived from an object class
(its direct superclass) which is itself derived from an even more
generic object class. For structural object classes, this process
stops at the most generic object class, 'top' (defined in Section
2.4.1). An ordered set of superclasses up to the most superior
object class of an object class is its superclass chain.
Zeilenga Standards Track [Page 9]
RFC 4512 LDAP Models June 2006
An object class may be derived from two or more direct
superclasses (superclasses not part of the same superclass chain).
This feature of subclassing is termed multiple inheritance.
Each object class identifies the set of attributes required to be
present in entries belonging to the class and the set of attributes
allowed to be present in entries belonging to the class. As an entry
of a class must meet the requirements of each class it belongs to, it
can be said that an object class inherits the sets of allowed and
required attributes from its superclasses. A subclass can identify
an attribute allowed by its superclass as being required. If an
attribute is a member of both sets, it is required to be present.
Each object class is defined to be one of three kinds of object
classes: Abstract, Structural, or Auxiliary.
Each object class is identified by an object identifier (OID) and,
optionally, one or more short names (descriptors).
2.4.1. Abstract Object Classes
An abstract object class, as the name implies, provides a base of
characteristics from which other object classes can be defined to
inherit from. An entry cannot belong to an abstract object class
unless it belongs to a structural or auxiliary class that inherits
from that abstract class.
Abstract object classes cannot derive from structural or auxiliary
object classes.
All structural object classes derive (directly or indirectly) from
the 'top' abstract object class. Auxiliary object classes do not
necessarily derive from 'top'.
The following is the object class definition (see Section 4.1.1) for
the 'top' object class:
( 2.5.6.0 NAME 'top' ABSTRACT MUST objectClass )
All entries belong to the 'top' abstract object class.
Zeilenga Standards Track [Page 10]
RFC 4512 LDAP Models June 2006
2.4.2. Structural Object Classes
As stated in [X.501]:
An object class defined for use in the structural specification of
the DIT is termed a structural object class. Structural object
classes are used in the definition of the structure of the names
of the objects for compliant entries.
An object or alias entry is characterized by precisely one
structural object class superclass chain which has a single
structural object class as the most subordinate object class.
This structural object class is referred to as the structural
object class of the entry.
Structural object classes are related to associated entries:
- an entry conforming to a structural object class shall
represent the real-world object constrained by the object
class;
- DIT structure rules only refer to structural object classes;
the structural object class of an entry is used to specify the
position of the entry in the DIT;
- the structural object class of an entry is used, along with an
associated DIT content rule, to control the content of an
entry.
The structural object class of an entry shall not be changed.
Each structural object class is a (direct or indirect) subclass of
the 'top' abstract object class.
Structural object classes cannot subclass auxiliary object classes.
Each entry is said to belong to its structural object class as well
as all classes in its structural object class's superclass chain.
2.4.3. Auxiliary Object Classes
Auxiliary object classes are used to augment the characteristics of
entries. They are commonly used to augment the sets of attributes
required and allowed to be present in an entry. They can be used to
describe entries or classes of entries.
Auxiliary object classes cannot subclass structural object classes.
Zeilenga Standards Track [Page 11]
RFC 4512 LDAP Models June 2006
An entry can belong to any subset of the set of auxiliary object
classes allowed by the DIT content rule associated with the
structural object class of the entry. If no DIT content rule is
associated with the structural object class of the entry, the entry
cannot belong to any auxiliary object class.
The set of auxiliary object classes that an entry belongs to can
change over time.
2.5. Attribute Descriptions
An attribute description is composed of an attribute type (see
Section 2.5.1) and a set of zero or more attribute options (see
Section 2.5.2).
An attribute description is represented by the ABNF:
attributedescription = attributetype options
attributetype = oid
options = *( SEMI option )
option = 1*keychar
where <attributetype> identifies the attribute type and each <option>
identifies an attribute option. Both <attributetype> and <option>
productions are case insensitive. The order in which <option>s
appear is irrelevant. That is, any two <attributedescription>s that
consist of the same <attributetype> and same set of <option>s are
equivalent.
Examples of valid attribute descriptions:
2.5.4.0
cn;lang-de;lang-en
owner
An attribute description with an unrecognized attribute type is to be
treated as unrecognized. Servers SHALL treat an attribute
description with an unrecognized attribute option as unrecognized.
Clients MAY treat an unrecognized attribute option as a tagging
option (see Section 2.5.2.1).
All attributes of an entry must have distinct attribute descriptions.
2.5.1. Attribute Types
An attribute type governs whether the attribute can have multiple
values, the syntax and matching rules used to construct and compare
values of that attribute, and other functions.
Zeilenga Standards Track [Page 12]
RFC 4512 LDAP Models June 2006
If no equality matching is specified for the attribute type:
- the attribute (of the type) cannot be used for naming;
- when adding the attribute (or replacing all values), no two
values may be equivalent (see 2.2);
- individual values of a multi-valued attribute are not to be
independently added or deleted;
- attribute value assertions (such as matching in search filters
and comparisons) using values of such a type cannot be
performed.
Otherwise, the specified equality matching rule is to be used to
evaluate attribute value assertions concerning the attribute type.
The specified equality rule is to be transitive and commutative.
The attribute type indicates whether the attribute is a user
attribute or an operational attribute. If operational, the attribute
type indicates the operational usage and whether or not the attribute
is modifiable by users. Operational attributes are discussed in
Section 3.4.
An attribute type (a subtype) may derive from a more generic
attribute type (a direct supertype). The following restrictions
apply to subtyping:
- a subtype must have the same usage as its direct supertype,
- a subtype's syntax must be the same, or a refinement of, its
supertype's syntax, and
- a subtype must be collective [RFC3671] if its supertype is
collective.
An attribute description consisting of a subtype and no options is
said to be the direct description subtype of the attribute
description consisting of the subtype's direct supertype and no
options.
Each attribute type is identified by an object identifier (OID) and,
optionally, one or more short names (descriptors).
2.5.2. Attribute Options
There are multiple kinds of attribute description options. The LDAP
technical specification details one kind: tagging options.
Not all options can be associated with attributes held in the
directory. Tagging options can be.
Zeilenga Standards Track [Page 13]
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Not all options can be used in conjunction with all attribute types.
In such cases, the attribute description is to be treated as
unrecognized.
An attribute description that contains mutually exclusive options
shall be treated as unrecognized. That is, "cn;x-bar;x-foo", where
"x-foo" and "x-bar" are mutually exclusive, is to be treated as
unrecognized.
Other kinds of options may be specified in future documents. These
documents must detail how new kinds of options they define relate to
tagging options. In particular, these documents must detail whether
or not new kinds of options can be associated with attributes held in
the directory, how new kinds of options affect transfer of attribute
values, and how new kinds of options are treated in attribute
description hierarchies.
Options are represented as short, case-insensitive textual strings
conforming to the <option> production defined in Section 2.5 of this
document.
Procedures for registering options are detailed in BCP 64, RFC 4520
[RFC4520].
2.5.2.1. Tagging Options
Attributes held in the directory can have attribute descriptions with
any number of tagging options. Tagging options are never mutually
exclusive.
An attribute description with N tagging options is a direct
(description) subtype of all attribute descriptions of the same
attribute type and all but one of the N options. If the attribute
type has a supertype, then the attribute description is also a direct
(description) subtype of the attribute description of the supertype
and the N tagging options. That is, 'cn;lang-de;lang-en' is a direct
(description) subtype of 'cn;lang-de', 'cn;lang-en', and
'name;lang-de;lang-en' ('cn' is a subtype of 'name'; both are defined
in [RFC4519]).
2.5.3. Attribute Description Hierarchies
An attribute description can be the direct subtype of zero or more
other attribute descriptions as indicated by attribute type subtyping
(as described in Section 2.5.1) or attribute tagging option subtyping
(as described in Section 2.5.2.1). These subtyping relationships are
used to form hierarchies of attribute descriptions and attributes.
Zeilenga Standards Track [Page 14]
RFC 4512 LDAP Models June 2006
As adapted from [X.501]:
Attribute hierarchies allow access to the DIB with varying degrees
of granularity. This is achieved by allowing the value components
of attributes to be accessed by using either their specific
attribute description (a direct reference to the attribute) or a
more generic attribute description (an indirect reference).
Semantically related attributes may be placed in a hierarchical
relationship, the more specialized being placed subordinate to the
more generalized. Searching for or retrieving attributes and
their values is made easier by quoting the more generalized
attribute description; a filter item so specified is evaluated for
the more specialized descriptions as well as for the quoted
description.
Where subordinate specialized descriptions are selected to be
returned as part of a search result these descriptions shall be
returned if available. Where the more general descriptions are
selected to be returned as part of a search result both the
general and the specialized descriptions shall be returned, if
available. An attribute value shall always be returned as a value
of its own attribute description.
All of the attribute descriptions in an attribute hierarchy are
treated as distinct and unrelated descriptions for user
modification of entry content.
An attribute value stored in an object or alias entry is of
precisely one attribute description. The description is indicated
when the value is originally added to the entry.
For the purpose of subschema administration of the entry, a
specification that an attribute is required is fulfilled if the entry
contains a value of an attribute description belonging to an
attribute hierarchy where the attribute type of that description is
the same as the required attribute's type. That is, a "MUST name"
specification is fulfilled by 'name' or 'name;x-tag-option', but is
not fulfilled by 'CN' or 'CN;x-tag-option' (even though 'CN' is a
subtype of 'name'). Likewise, an entry may contain a value of an
attribute description belonging to an attribute hierarchy where the
attribute type of that description is either explicitly included in
the definition of an object class to which the entry belongs or
allowed by the DIT content rule applicable to that entry. That is,
'name' and 'name;x-tag-option' are allowed by "MAY name" (or by "MUST
name"), but 'CN' and 'CN;x-tag-option' are not allowed by "MAY name"
(or by "MUST name").
Zeilenga Standards Track [Page 15]
RFC 4512 LDAP Models June 2006
For the purposes of other policy administration, unless stated
otherwise in the specification of the particular administrative
model, all of the attribute descriptions in an attribute hierarchy
are treated as distinct and unrelated descriptions.
2.6. Alias Entries
As adapted from [X.501]:
An alias, or an alias name, for an object is an alternative name
for an object or object entry which is provided by the use of
alias entries.
Each alias entry contains, within the 'aliasedObjectName'
attribute (known as the 'aliasedEntryName' attribute in X.500), a
name of some object. The distinguished name of the alias entry is
thus also a name for this object.
NOTE - The name within the 'aliasedObjectName' is said to be
pointed to by the alias. It does not have to be the
distinguished name of any entry.
The conversion of an alias name to an object name is termed
(alias) dereferencing and comprises the systematic replacement of
alias names, where found within a purported name, by the value of
the corresponding 'aliasedObjectName' attribute. The process may
require the examination of more than one alias entry.
Any particular entry in the DIT may have zero or more alias names.
It therefore follows that several alias entries may point to the
same entry. An alias entry may point to an entry that is not a
leaf entry and may point to another alias entry.
An alias entry shall have no subordinates, so that an alias entry
is always a leaf entry.
Every alias entry shall belong to the 'alias' object class.
An entry with the 'alias' object class must also belong to an object
class (or classes), or be governed by a DIT content rule, which
allows suitable naming attributes to be present.
Example:
dn: cn=bar,dc=example,dc=com
objectClass: top
objectClass: alias
objectClass: extensibleObject
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cn: bar
aliasedObjectName: cn=foo,dc=example,dc=com
2.6.1. 'alias' Object Class
Alias entries belong to the 'alias' object class.
( 2.5.6.1 NAME 'alias'
SUP top STRUCTURAL
MUST aliasedObjectName )
2.6.2. 'aliasedObjectName' Attribute Type
The 'aliasedObjectName' attribute holds the name of the entry an
alias points to. The 'aliasedObjectName' attribute is known as the
'aliasedEntryName' attribute in X.500.
( 2.5.4.1 NAME 'aliasedObjectName'
EQUALITY distinguishedNameMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
SINGLE-VALUE )
The 'distinguishedNameMatch' matching rule and the DistinguishedName
(1.3.6.1.4.1.1466.115.121.1.12) syntax are defined in [RFC4517].
3. Directory Administrative and Operational Information
This section discusses select aspects of the X.500 Directory
Administrative and Operational Information model [X.501]. LDAP
implementations MAY support other aspects of this model.
3.1. Subtrees
As defined in [X.501]:
A subtree is a collection of object and alias entries situated at
the vertices of a tree. Subtrees do not contain subentries. The
prefix sub, in subtree, emphasizes that the base (or root) vertex
of this tree is usually subordinate to the root of the DIT.
A subtree begins at some vertex and extends to some identifiable
lower boundary, possibly extending to leaves. A subtree is always
defined within a context which implicitly bounds the subtree. For
example, the vertex and lower boundaries of a subtree defining a
replicated area are bounded by a naming context.
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3.2. Subentries
A subentry is a "special sort of entry, known by the Directory, used
to hold information associated with a subtree or subtree refinement"
[X.501]. Subentries are used in Directory to hold for administrative
and operational purposes as defined in [X.501]. Their use in LDAP is
detailed in [RFC3672].
The term "(sub)entry" in this specification indicates that servers
implementing X.500(93) models are, in accordance with X.500(93) as
described in [RFC3672], to use a subentry and that other servers are
to use an object entry belonging to the appropriate auxiliary class
normally used with the subentry (e.g., 'subschema' for subschema
subentries) to mimic the subentry. This object entry's RDN SHALL be
formed from a value of the 'cn' (commonName) attribute [RFC4519] (as
all subentries are named with 'cn').
3.3. The 'objectClass' attribute
Each entry in the DIT has an 'objectClass' attribute.
( 2.5.4.0 NAME 'objectClass'
EQUALITY objectIdentifierMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.38 )
The 'objectIdentifierMatch' matching rule and the OBJECT IDENTIFIER
(1.3.6.1.4.1.1466.115.121.1.38) syntax are defined in [RFC4517].
The 'objectClass' attribute specifies the object classes of an entry,
which (among other things) are used in conjunction with the
controlling schema to determine the permitted attributes of an entry.
Values of this attribute can be modified by clients, but the
'objectClass' attribute cannot be removed.
Servers that follow X.500(93) models SHALL restrict modifications of
this attribute to prevent the basic structural class of the entry
from being changed. That is, one cannot change a 'person' into a
'country'.
When creating an entry or adding an 'objectClass' value to an entry,
all superclasses of the named classes SHALL be implicitly added as
well if not already present. That is, if the auxiliary class 'x-a'
is a subclass of the class 'x-b', adding 'x-a' to 'objectClass'
causes 'x-b' to be implicitly added (if is not already present).
Servers SHALL restrict modifications of this attribute to prevent
superclasses of remaining 'objectClass' values from being deleted.
That is, if the auxiliary class 'x-a' is a subclass of the auxiliary
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class 'x-b' and the 'objectClass' attribute contains 'x-a' and 'x-b',
an attempt to delete only 'x-b' from the 'objectClass' attribute is
an error.
3.4. Operational Attributes
Some attributes, termed operational attributes, are used or
maintained by servers for administrative and operational purposes.
As stated in [X.501]: "There are three varieties of operational
attributes: Directory operational attributes, DSA-shared operational
attributes, and DSA-specific operational attributes".
A directory operational attribute is used to represent operational
and/or administrative information in the Directory Information Model.
This includes operational attributes maintained by the server (e.g.,
'createTimestamp') as well as operational attributes that hold values
administrated by the user (e.g., 'ditContentRules').
A DSA-shared operational attribute is used to represent information
of the DSA Information Model that is shared between DSAs.
A DSA-specific operational attribute is used to represent information
of the DSA Information Model that is specific to the DSA (though, in
some cases, may be derived from information shared between DSAs;
e.g., 'namingContexts').
The DSA Information Model operational attributes are detailed in
[X.501].
Operational attributes are not normally visible. They are not
returned in search results unless explicitly requested by name.
Not all operational attributes are user modifiable.
Entries may contain, among others, the following operational
attributes:
- creatorsName: the Distinguished Name of the user who added this
entry to the directory,
- createTimestamp: the time this entry was added to the directory,
- modifiersName: the Distinguished Name of the user who last
modified this entry, and
- modifyTimestamp: the time this entry was last modified.
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Servers SHOULD maintain the 'creatorsName', 'createTimestamp',
'modifiersName', and 'modifyTimestamp' attributes for all entries of
the DIT.
3.4.1. 'creatorsName'
This attribute appears in entries that were added using the protocol
(e.g., using the Add operation). The value is the distinguished name
of the creator.
( 2.5.18.3 NAME 'creatorsName'
EQUALITY distinguishedNameMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
SINGLE-VALUE NO-USER-MODIFICATION
USAGE directoryOperation )
The 'distinguishedNameMatch' matching rule and the DistinguishedName
(1.3.6.1.4.1.1466.115.121.1.12) syntax are defined in [RFC4517].
3.4.2. 'createTimestamp'
This attribute appears in entries that were added using the protocol
(e.g., using the Add operation). The value is the time the entry was
added.
( 2.5.18.1 NAME 'createTimestamp'
EQUALITY generalizedTimeMatch
ORDERING generalizedTimeOrderingMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.24
SINGLE-VALUE NO-USER-MODIFICATION
USAGE directoryOperation )
The 'generalizedTimeMatch' and 'generalizedTimeOrderingMatch'
matching rules and the GeneralizedTime
(1.3.6.1.4.1.1466.115.121.1.24) syntax are defined in [RFC4517].
3.4.3. 'modifiersName'
This attribute appears in entries that have been modified using the
protocol (e.g., using the Modify operation). The value is the
distinguished name of the last modifier.
( 2.5.18.4 NAME 'modifiersName'
EQUALITY distinguishedNameMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
SINGLE-VALUE NO-USER-MODIFICATION
USAGE directoryOperation )
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The 'distinguishedNameMatch' matching rule and the DistinguishedName
(1.3.6.1.4.1.1466.115.121.1.12) syntax are defined in [RFC4517].
3.4.4. 'modifyTimestamp'
This attribute appears in entries that have been modified using the
protocol (e.g., using the Modify operation). The value is the time
the entry was last modified.
( 2.5.18.2 NAME 'modifyTimestamp'
EQUALITY generalizedTimeMatch
ORDERING generalizedTimeOrderingMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.24
SINGLE-VALUE NO-USER-MODIFICATION
USAGE directoryOperation )
The 'generalizedTimeMatch' and 'generalizedTimeOrderingMatch'
matching rules and the GeneralizedTime
(1.3.6.1.4.1.1466.115.121.1.24) syntax are defined in [RFC4517].
3.4.5. 'structuralObjectClass'
This attribute indicates the structural object class of the entry.
( 2.5.21.9 NAME 'structuralObjectClass'
EQUALITY objectIdentifierMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.38
SINGLE-VALUE NO-USER-MODIFICATION
USAGE directoryOperation )
The 'objectIdentifierMatch' matching rule and OBJECT IDENTIFIER
(1.3.6.1.4.1.1466.115.121.1.38) syntax is defined in [RFC4517].
3.4.6. 'governingStructureRule'
This attribute indicates the structure rule governing the entry.
( 2.5.21.10 NAME 'governingStructureRule'
EQUALITY integerMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
SINGLE-VALUE NO-USER-MODIFICATION
USAGE directoryOperation )
The 'integerMatch' matching rule and INTEGER
(1.3.6.1.4.1.1466.115.121.1.27) syntax is defined in [RFC4517].
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4. Directory Schema
As defined in [X.501]:
The Directory Schema is a set of definitions and constraints
concerning the structure of the DIT, the possible ways entries are
named, the information that can be held in an entry, the
attributes used to represent that information and their
organization into hierarchies to facilitate search and retrieval
of the information and the ways in which values of attributes may
be matched in attribute value and matching rule assertions.
NOTE 1 - The schema enables the Directory system to, for example:
- prevent the creation of subordinate entries of the wrong
object-class (e.g., a country as a subordinate of a person);
- prevent the addition of attribute-types to an entry
inappropriate to the object-class (e.g., a serial number to a
person's entry);
- prevent the addition of an attribute value of a syntax not
matching that defined for the attribute-type (e.g., a printable
string to a bit string).
Formally, the Directory Schema comprises a set of:
a) Name Form definitions that define primitive naming relations
for structural object classes;
b) DIT Structure Rule definitions that define the names that
entries may have and the ways in which the entries may be
related to one another in the DIT;
c) DIT Content Rule definitions that extend the specification of
allowable attributes for entries beyond those indicated by the
structural object classes of the entries;
d) Object Class definitions that define the basic set of mandatory
and optional attributes that shall be present, and may be
present, respectively, in an entry of a given class, and which
indicate the kind of object class that is being defined;
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e) Attribute Type definitions that identify the object identifier
by which an attribute is known, its syntax, associated matching
rules, whether it is an operational attribute and if so its
type, whether it is a collective attribute, whether it is
permitted to have multiple values and whether or not it is
derived from another attribute type;
f) Matching Rule definitions that define matching rules.
And in LDAP:
g) LDAP Syntax definitions that define encodings used in LDAP.
4.1. Schema Definitions
Schema definitions in this section are described using ABNF and rely
on the common productions specified in Section 1.2 as well as these:
noidlen = numericoid [ LCURLY len RCURLY ]
len = number
oids = oid / ( LPAREN WSP oidlist WSP RPAREN )
oidlist = oid *( WSP DOLLAR WSP oid )
extensions = *( SP xstring SP qdstrings )
xstring = "X" HYPHEN 1*( ALPHA / HYPHEN / USCORE )
qdescrs = qdescr / ( LPAREN WSP qdescrlist WSP RPAREN )
qdescrlist = [ qdescr *( SP qdescr ) ]
qdescr = SQUOTE descr SQUOTE
qdstrings = qdstring / ( LPAREN WSP qdstringlist WSP RPAREN )
qdstringlist = [ qdstring *( SP qdstring ) ]
qdstring = SQUOTE dstring SQUOTE
dstring = 1*( QS / QQ / QUTF8 ) ; escaped UTF-8 string
QQ = ESC %x32 %x37 ; "\27"
QS = ESC %x35 ( %x43 / %x63 ) ; "\5C" / "\5c"
; Any UTF-8 encoded Unicode character
; except %x27 ("\'") and %x5C ("\")
QUTF8 = QUTF1 / UTFMB
; Any ASCII character except %x27 ("\'") and %x5C ("\")
QUTF1 = %x00-26 / %x28-5B / %x5D-7F
Schema definitions in this section also share a number of common
terms.
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The NAME field provides a set of short names (descriptors) that are
to be used as aliases for the OID.
The DESC field optionally allows a descriptive string to be provided
by the directory administrator and/or implementor. While
specifications may suggest a descriptive string, there is no
requirement that the suggested (or any) descriptive string be used.
The OBSOLETE field, if present, indicates the element is not active.
Implementors should note that future versions of this document may
expand these definitions to include additional terms. Terms whose
identifier begins with "X-" are reserved for private experiments and
are followed by <SP> and <qdstrings> tokens.
4.1.1. Object Class Definitions
Object Class definitions are written according to the ABNF:
ObjectClassDescription = LPAREN WSP
numericoid ; object identifier
[ SP "NAME" SP qdescrs ] ; short names (descriptors)
[ SP "DESC" SP qdstring ] ; description
[ SP "OBSOLETE" ] ; not active
[ SP "SUP" SP oids ] ; superior object classes
[ SP kind ] ; kind of class
[ SP "MUST" SP oids ] ; attribute types
[ SP "MAY" SP oids ] ; attribute types
extensions WSP RPAREN
kind = "ABSTRACT" / "STRUCTURAL" / "AUXILIARY"
where:
<numericoid> is object identifier assigned to this object class;
NAME <qdescrs> are short names (descriptors) identifying this
object class;
DESC <qdstring> is a short descriptive string;
OBSOLETE indicates this object class is not active;
SUP <oids> specifies the direct superclasses of this object class;
the kind of object class is indicated by one of ABSTRACT,
STRUCTURAL, or AUXILIARY (the default is STRUCTURAL);
MUST and MAY specify the sets of required and allowed attribute
types, respectively; and
<extensions> describe extensions.
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4.1.2. Attribute Types
Attribute Type definitions are written according to the ABNF:
AttributeTypeDescription = LPAREN WSP
numericoid ; object identifier
[ SP "NAME" SP qdescrs ] ; short names (descriptors)
[ SP "DESC" SP qdstring ] ; description
[ SP "OBSOLETE" ] ; not active
[ SP "SUP" SP oid ] ; supertype
[ SP "EQUALITY" SP oid ] ; equality matching rule
[ SP "ORDERING" SP oid ] ; ordering matching rule
[ SP "SUBSTR" SP oid ] ; substrings matching rule
[ SP "SYNTAX" SP noidlen ] ; value syntax
[ SP "SINGLE-VALUE" ] ; single-value
[ SP "COLLECTIVE" ] ; collective
[ SP "NO-USER-MODIFICATION" ] ; not user modifiable
[ SP "USAGE" SP usage ] ; usage
extensions WSP RPAREN ; extensions
usage = "userApplications" / ; user
"directoryOperation" / ; directory operational
"distributedOperation" / ; DSA-shared operational
"dSAOperation" ; DSA-specific operational
where:
<numericoid> is object identifier assigned to this attribute type;
NAME <qdescrs> are short names (descriptors) identifying this
attribute type;
DESC <qdstring> is a short descriptive string;
OBSOLETE indicates this attribute type is not active;
SUP oid specifies the direct supertype of this type;
EQUALITY, ORDERING, and SUBSTR provide the oid of the equality,
ordering, and substrings matching rules, respectively;
SYNTAX identifies value syntax by object identifier and may suggest
a minimum upper bound;
SINGLE-VALUE indicates attributes of this type are restricted to a
single value;
COLLECTIVE indicates this attribute type is collective
[X.501][RFC3671];
NO-USER-MODIFICATION indicates this attribute type is not user
modifiable;
USAGE indicates the application of this attribute type; and
<extensions> describe extensions.
Each attribute type description must contain at least one of the SUP
or SYNTAX fields. If no SYNTAX field is provided, the attribute type
description takes its value from the supertype.
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If SUP field is provided, the EQUALITY, ORDERING, and SUBSTRING
fields, if not specified, take their value from the supertype.
Usage of userApplications, the default, indicates that attributes of
this type represent user information. That is, they are user
attributes.
A usage of directoryOperation, distributedOperation, or dSAOperation
indicates that attributes of this type represent operational and/or
administrative information. That is, they are operational
attributes.
directoryOperation usage indicates that the attribute of this type is
a directory operational attribute. distributedOperation usage
indicates that the attribute of this type is a DSA-shared usage
operational attribute. dSAOperation usage indicates that the
attribute of this type is a DSA-specific operational attribute.
COLLECTIVE requires usage userApplications. Use of collective
attribute types in LDAP is discussed in [RFC3671].
NO-USER-MODIFICATION requires an operational usage.
Note that the <AttributeTypeDescription> does not list the matching
rules that can be used with that attribute type in an extensibleMatch
search filter [RFC4511]. This is done using the 'matchingRuleUse'
attribute described in Section 4.1.4.
This document refines the schema description of X.501 by requiring
that the SYNTAX field in an <AttributeTypeDescription> be a string
representation of an object identifier for the LDAP string syntax
definition, with an optional indication of the suggested minimum
bound of a value of this attribute.
A suggested minimum upper bound on the number of characters in a
value with a string-based syntax, or the number of bytes in a value
for all other syntaxes, may be indicated by appending this bound
count inside of curly braces following the syntax's OBJECT IDENTIFIER
in an Attribute Type Description. This bound is not part of the
syntax name itself. For instance, "1.3.6.4.1.1466.0{64}" suggests
that server implementations should allow a string to be 64 characters
long, although they may allow longer strings. Note that a single
character of the Directory String syntax may be encoded in more than
one octet since UTF-8 [RFC3629] is a variable-length encoding.
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4.1.3. Matching Rules
Matching rules are used in performance of attribute value assertions,
such as in performance of a Compare operation. They are also used in
evaluating search filters, determining which individual values are to
be added or deleted during performance of a Modify operation, and in
comparing distinguished names.
Each matching rule is identified by an object identifier (OID) and,
optionally, one or more short names (descriptors).
Matching rule definitions are written according to the ABNF:
MatchingRuleDescription = LPAREN WSP
numericoid ; object identifier
[ SP "NAME" SP qdescrs ] ; short names (descriptors)
[ SP "DESC" SP qdstring ] ; description
[ SP "OBSOLETE" ] ; not active
SP "SYNTAX" SP numericoid ; assertion syntax
extensions WSP RPAREN ; extensions
where:
<numericoid> is object identifier assigned to this matching rule;
NAME <qdescrs> are short names (descriptors) identifying this
matching rule;
DESC <qdstring> is a short descriptive string;
OBSOLETE indicates this matching rule is not active;
SYNTAX identifies the assertion syntax (the syntax of the assertion
value) by object identifier; and
<extensions> describe extensions.
4.1.4. Matching Rule Uses
A matching rule use lists the attribute types that are suitable for
use with an extensibleMatch search filter.
Matching rule use descriptions are written according to the following
ABNF:
MatchingRuleUseDescription = LPAREN WSP
numericoid ; object identifier
[ SP "NAME" SP qdescrs ] ; short names (descriptors)
[ SP "DESC" SP qdstring ] ; description
[ SP "OBSOLETE" ] ; not active
SP "APPLIES" SP oids ; attribute types
extensions WSP RPAREN ; extensions
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where:
<numericoid> is the object identifier of the matching rule
associated with this matching rule use description;
NAME <qdescrs> are short names (descriptors) identifying this
matching rule use;
DESC <qdstring> is a short descriptive string;
OBSOLETE indicates this matching rule use is not active;
APPLIES provides a list of attribute types the matching rule
applies to; and
<extensions> describe extensions.
4.1.5. LDAP Syntaxes
LDAP Syntaxes of (attribute and assertion) values are described in
terms of ASN.1 [X.680] and, optionally, have an octet string encoding
known as the LDAP-specific encoding. Commonly, the LDAP-specific
encoding is constrained to a string of Unicode [Unicode] characters
in UTF-8 [RFC3629] form.
Each LDAP syntax is identified by an object identifier (OID).
LDAP syntax definitions are written according to the ABNF:
SyntaxDescription = LPAREN WSP
numericoid ; object identifier
[ SP "DESC" SP qdstring ] ; description
extensions WSP RPAREN ; extensions
where:
<numericoid> is the object identifier assigned to this LDAP syntax;
DESC <qdstring> is a short descriptive string; and
<extensions> describe extensions.
4.1.6. DIT Content Rules
A DIT content rule is a "rule governing the content of entries of a
particular structural object class" [X.501].
For DIT entries of a particular structural object class, a DIT
content rule specifies which auxiliary object classes the entries are
allowed to belong to and which additional attributes (by type) are
required, allowed, or not allowed to appear in the entries.
The list of precluded attributes cannot include any attribute listed
as mandatory in the rule, the structural object class, or any of the
allowed auxiliary object classes.
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Each content rule is identified by the object identifier, as well as
any short names (descriptors), of the structural object class it
applies to.
An entry may only belong to auxiliary object classes listed in the
governing content rule.
An entry must contain all attributes required by the object classes
the entry belongs to as well as all attributes required by the
governing content rule.
An entry may contain any non-precluded attributes allowed by the
object classes the entry belongs to as well as all attributes allowed
by the governing content rule.
An entry cannot include any attribute precluded by the governing
content rule.
An entry is governed by (if present and active in the subschema) the
DIT content rule that applies to the structural object class of the
entry (see Section 2.4.2). If no active rule is present for the
entry's structural object class, the entry's content is governed by
the structural object class (and possibly other aspects of user and
system schema). DIT content rules for superclasses of the structural
object class of an entry are not applicable to that entry.
DIT content rule descriptions are written according to the ABNF:
DITContentRuleDescription = LPAREN WSP
numericoid ; object identifier
[ SP "NAME" SP qdescrs ] ; short names (descriptors)
[ SP "DESC" SP qdstring ] ; description
[ SP "OBSOLETE" ] ; not active
[ SP "AUX" SP oids ] ; auxiliary object classes
[ SP "MUST" SP oids ] ; attribute types
[ SP "MAY" SP oids ] ; attribute types
[ SP "NOT" SP oids ] ; attribute types
extensions WSP RPAREN ; extensions
where:
<numericoid> is the object identifier of the structural object
class associated with this DIT content rule;
NAME <qdescrs> are short names (descriptors) identifying this DIT
content rule;
DESC <qdstring> is a short descriptive string;
OBSOLETE indicates this DIT content rule use is not active;
AUX specifies a list of auxiliary object classes that entries
subject to this DIT content rule may belong to;
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MUST, MAY, and NOT specify lists of attribute types that are
required, allowed, or precluded, respectively, from appearing
in entries subject to this DIT content rule; and
<extensions> describe extensions.
4.1.7. DIT Structure Rules and Name Forms
It is sometimes desirable to regulate where object and alias entries
can be placed in the DIT and how they can be named based upon their
structural object class.
4.1.7.1. DIT Structure Rules
A DIT structure rule is a "rule governing the structure of the DIT by
specifying a permitted superior to subordinate entry relationship. A
structure rule relates a name form, and therefore a structural object
class, to superior structure rules. This permits entries of the
structural object class identified by the name form to exist in the
DIT as subordinates to entries governed by the indicated superior
structure rules" [X.501].
DIT structure rule descriptions are written according to the ABNF:
DITStructureRuleDescription = LPAREN WSP
ruleid ; rule identifier
[ SP "NAME" SP qdescrs ] ; short names (descriptors)
[ SP "DESC" SP qdstring ] ; description
[ SP "OBSOLETE" ] ; not active
SP "FORM" SP oid ; NameForm
[ SP "SUP" ruleids ] ; superior rules
extensions WSP RPAREN ; extensions
ruleids = ruleid / ( LPAREN WSP ruleidlist WSP RPAREN )
ruleidlist = ruleid *( SP ruleid )
ruleid = number
where:
<ruleid> is the rule identifier of this DIT structure rule;
NAME <qdescrs> are short names (descriptors) identifying this DIT
structure rule;
DESC <qdstring> is a short descriptive string;
OBSOLETE indicates this DIT structure rule use is not active;
FORM is specifies the name form associated with this DIT structure
rule;
SUP identifies superior rules (by rule id); and
<extensions> describe extensions.
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If no superior rules are identified, the DIT structure rule applies
to an autonomous administrative point (e.g., the root vertex of the
subtree controlled by the subschema) [X.501].
4.1.7.2. Name Forms
A name form "specifies a permissible RDN for entries of a particular
structural object class. A name form identifies a named object class
and one or more attribute types to be used for naming (i.e., for the
RDN). Name forms are primitive pieces of specification used in the
definition of DIT structure rules" [X.501].
Each name form indicates the structural object class to be named, a
set of required attribute types, and a set of allowed attribute
types. A particular attribute type cannot be in both sets.
Entries governed by the form must be named using a value from each
required attribute type and zero or more values from the allowed
attribute types.
Each name form is identified by an object identifier (OID) and,
optionally, one or more short names (descriptors).
Name form descriptions are written according to the ABNF:
NameFormDescription = LPAREN WSP
numericoid ; object identifier
[ SP "NAME" SP qdescrs ] ; short names (descriptors)
[ SP "DESC" SP qdstring ] ; description
[ SP "OBSOLETE" ] ; not active
SP "OC" SP oid ; structural object class
SP "MUST" SP oids ; attribute types
[ SP "MAY" SP oids ] ; attribute types
extensions WSP RPAREN ; extensions
where:
<numericoid> is object identifier that identifies this name form;
NAME <qdescrs> are short names (descriptors) identifying this name
form;
DESC <qdstring> is a short descriptive string;
OBSOLETE indicates this name form is not active;
OC identifies the structural object class this rule applies to,
MUST and MAY specify the sets of required and allowed,
respectively, naming attributes for this name form; and
<extensions> describe extensions.
All attribute types in the required ("MUST") and allowed ("MAY")
lists shall be different.
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4.2. Subschema Subentries
Subschema (sub)entries are used for administering information about
the directory schema. A single subschema (sub)entry contains all
schema definitions (see Section 4.1) used by entries in a particular
part of the directory tree.
Servers that follow X.500(93) models SHOULD implement subschema using
the X.500 subschema mechanisms (as detailed in Section 12 of
[X.501]), so these are not ordinary object entries but subentries
(see Section 3.2). LDAP clients SHOULD NOT assume that servers
implement any of the other aspects of X.500 subschema.
Servers MAY allow subschema modification. Procedures for subschema
modification are discussed in Section 14.5 of [X.501].
A server that masters entries and permits clients to modify these
entries SHALL implement and provide access to these subschema
(sub)entries including providing a 'subschemaSubentry' attribute in
each modifiable entry. This is so clients may discover the
attributes and object classes that are permitted to be present. It
is strongly RECOMMENDED that all other servers implement this as
well.
The value of the 'subschemaSubentry' attribute is the name of the
subschema (sub)entry holding the subschema controlling the entry.
( 2.5.18.10 NAME 'subschemaSubentry'
EQUALITY distinguishedNameMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
SINGLE-VALUE NO-USER-MODIFICATION
USAGE directoryOperation )
The 'distinguishedNameMatch' matching rule and the DistinguishedName
(1.3.6.1.4.1.1466.115.121.1.12) syntax are defined in [RFC4517].
Subschema is held in (sub)entries belonging to the subschema
auxiliary object class.
( 2.5.20.1 NAME 'subschema' AUXILIARY
MAY ( dITStructureRules $ nameForms $ ditContentRules $
objectClasses $ attributeTypes $ matchingRules $
matchingRuleUse ) )
The 'ldapSyntaxes' operational attribute may also be present in
subschema entries.
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Servers MAY provide additional attributes (described in other
documents) in subschema (sub)entries.
Servers SHOULD provide the attributes 'createTimestamp' and
'modifyTimestamp' in subschema (sub)entries, in order to allow
clients to maintain their caches of schema information.
The following subsections provide attribute type definitions for each
of schema definition attribute types.
4.2.1. 'objectClasses'
This attribute holds definitions of object classes.
( 2.5.21.6 NAME 'objectClasses'
EQUALITY objectIdentifierFirstComponentMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.37
USAGE directoryOperation )
The 'objectIdentifierFirstComponentMatch' matching rule and the
ObjectClassDescription (1.3.6.1.4.1.1466.115.121.1.37) syntax are
defined in [RFC4517].
4.2.2. 'attributeTypes'
This attribute holds definitions of attribute types.
( 2.5.21.5 NAME 'attributeTypes'
EQUALITY objectIdentifierFirstComponentMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.3
USAGE directoryOperation )
The 'objectIdentifierFirstComponentMatch' matching rule and the
AttributeTypeDescription (1.3.6.1.4.1.1466.115.121.1.3) syntax are
defined in [RFC4517].
4.2.3. 'matchingRules'
This attribute holds definitions of matching rules.
( 2.5.21.4 NAME 'matchingRules'
EQUALITY objectIdentifierFirstComponentMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.30
USAGE directoryOperation )
The 'objectIdentifierFirstComponentMatch' matching rule and the
MatchingRuleDescription (1.3.6.1.4.1.1466.115.121.1.30) syntax are
defined in [RFC4517].
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4.2.4 'matchingRuleUse'
This attribute holds definitions of matching rule uses.
( 2.5.21.8 NAME 'matchingRuleUse'
EQUALITY objectIdentifierFirstComponentMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.31
USAGE directoryOperation )
The 'objectIdentifierFirstComponentMatch' matching rule and the
MatchingRuleUseDescription (1.3.6.1.4.1.1466.115.121.1.31) syntax are
defined in [RFC4517].
4.2.5. 'ldapSyntaxes'
This attribute holds definitions of LDAP syntaxes.
( 1.3.6.1.4.1.1466.101.120.16 NAME 'ldapSyntaxes'
EQUALITY objectIdentifierFirstComponentMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.54
USAGE directoryOperation )
The 'objectIdentifierFirstComponentMatch' matching rule and the
SyntaxDescription (1.3.6.1.4.1.1466.115.121.1.54) syntax are defined
in [RFC4517].
4.2.6. 'dITContentRules'
This attribute lists DIT Content Rules that are present in the
subschema.
( 2.5.21.2 NAME 'dITContentRules'
EQUALITY objectIdentifierFirstComponentMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.16
USAGE directoryOperation )
The 'objectIdentifierFirstComponentMatch' matching rule and the
DITContentRuleDescription (1.3.6.1.4.1.1466.115.121.1.16) syntax are
defined in [RFC4517].
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4.2.7. 'dITStructureRules'
This attribute lists DIT Structure Rules that are present in the
subschema.
( 2.5.21.1 NAME 'dITStructureRules'
EQUALITY integerFirstComponentMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.17
USAGE directoryOperation )
The 'integerFirstComponentMatch' matching rule and the
DITStructureRuleDescription (1.3.6.1.4.1.1466.115.121.1.17) syntax
are defined in [RFC4517].
4.2.8 'nameForms'
This attribute lists Name Forms that are in force.
( 2.5.21.7 NAME 'nameForms'
EQUALITY objectIdentifierFirstComponentMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.35
USAGE directoryOperation )
The 'objectIdentifierFirstComponentMatch' matching rule and the
NameFormDescription (1.3.6.1.4.1.1466.115.121.1.35) syntax are
defined in [RFC4517].
4.3. 'extensibleObject' object class
The 'extensibleObject' auxiliary object class allows entries that
belong to it to hold any user attribute. The set of allowed
attribute types of this object class is implicitly the set of all
attribute types of userApplications usage.
( 1.3.6.1.4.1.1466.101.120.111 NAME 'extensibleObject'
SUP top AUXILIARY )
The mandatory attributes of the other object classes of this entry
are still required to be present, and any precluded attributes are
still not allowed to be present.
4.4. Subschema Discovery
To discover the DN of the subschema (sub)entry holding the subschema
controlling a particular entry, a client reads that entry's
'subschemaSubentry' operational attribute. To read schema attributes
from the subschema (sub)entry, clients MUST issue a Search operation
[RFC4511] where baseObject is the DN of the subschema (sub)entry,
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RFC 4512 LDAP Models June 2006
scope is baseObject, filter is "(objectClass=subschema)" [RFC4515],
and the attributes field lists the names of the desired schema
attributes (as they are operational). Note: the
"(objectClass=subschema)" filter allows LDAP servers that gateway to
X.500 to detect that subentry information is being requested.
Clients SHOULD NOT assume that a published subschema is complete,
that the server supports all of the schema elements it publishes, or
that the server does not support an unpublished element.
5. DSA (Server) Informational Model
The LDAP protocol assumes there are one or more servers that jointly
provide access to a Directory Information Tree (DIT). The server
holding the original information is called the "master" (for that
information). Servers that hold copies of the original information
are referred to as "shadowing" or "caching" servers.
As defined in [X.501]:
context prefix: The sequence of RDNs leading from the Root of the
DIT to the initial vertex of a naming context; corresponds to
the distinguished name of that vertex.
naming context: A subtree of entries held in a single master DSA.
That is, a naming context is the largest collection of entries,
starting at an entry that is mastered by a particular server, and
including all its subordinates and their subordinates, down to the
entries that are mastered by different servers. The context prefix
is the name of the initial entry.
The root of the DIT is a DSA-specific Entry (DSE) and not part of any
naming context (or any subtree); each server has different attribute
values in the root DSE.
5.1. Server-Specific Data Requirements
An LDAP server SHALL provide information about itself and other
information that is specific to each server. This is represented as
a group of attributes located in the root DSE, which is named with
the DN with zero RDNs (whose [RFC4514] representation is as the
zero-length string).
These attributes are retrievable, subject to access control and other
restrictions, if a client performs a Search operation [RFC4511] with
an empty baseObject, scope of baseObject, the filter
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"(objectClass=*)" [RFC4515], and the attributes field listing the
names of the desired attributes. It is noted that root DSE
attributes are operational and, like other operational attributes,
are not returned in search requests unless requested by name.
The root DSE SHALL NOT be included if the client performs a subtree
search starting from the root.
Servers may allow clients to modify attributes of the root DSE, where
appropriate.
The following attributes of the root DSE are defined below.
Additional attributes may be defined in other documents.
- altServer: alternative servers;
- namingContexts: naming contexts;
- supportedControl: recognized LDAP controls;
- supportedExtension: recognized LDAP extended operations;
- supportedFeatures: recognized LDAP features;
- supportedLDAPVersion: LDAP versions supported; and
- supportedSASLMechanisms: recognized Simple Authentication and
Security Layers (SASL) [RFC4422] mechanisms.
The values provided for these attributes may depend on session-
specific and other factors. For example, a server supporting the
SASL EXTERNAL mechanism might only list "EXTERNAL" when the client's
identity has been established by a lower level. See [RFC4513].
The root DSE may also include a 'subschemaSubentry' attribute. If it
does, the attribute refers to the subschema (sub)entry holding the
schema controlling the root DSE. Clients SHOULD NOT assume that this
subschema (sub)entry controls other entries held by the server.
General subschema discovery procedures are provided in Section 4.4.
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5.1.1. 'altServer'
The 'altServer' attribute lists URIs referring to alternative servers
that may be contacted when this server becomes unavailable. URIs for
servers implementing the LDAP are written according to [RFC4516].
Other kinds of URIs may be provided. If the server does not know of
any other servers that could be used, this attribute will be absent.
Clients may cache this information in case their preferred server
later becomes unavailable.
( 1.3.6.1.4.1.1466.101.120.6 NAME 'altServer'
SYNTAX 1.3.6.1.4.1.1466.115.121.1.26
USAGE dSAOperation )
The IA5String (1.3.6.1.4.1.1466.115.121.1.26) syntax is defined in
[RFC4517].
5.1.2. 'namingContexts'
The 'namingContexts' attribute lists the context prefixes of the
naming contexts the server masters or shadows (in part or in whole).
If the server is a first-level DSA [X.501], it should list (in
addition) an empty string (indicating the root of the DIT). If the
server does not master or shadow any information (e.g., it is an LDAP
gateway to a public X.500 directory) this attribute will be absent.
If the server believes it masters or shadows the entire directory,
the attribute will have a single value, and that value will be the
empty string (indicating the root of the DIT).
This attribute may be used, for example, to select a suitable entry
name for subsequent operations with this server.
( 1.3.6.1.4.1.1466.101.120.5 NAME 'namingContexts'
SYNTAX 1.3.6.1.4.1.1466.115.121.1.12
USAGE dSAOperation )
The DistinguishedName (1.3.6.1.4.1.1466.115.121.1.12) syntax is
defined in [RFC4517].
5.1.3. 'supportedControl'
The 'supportedControl' attribute lists object identifiers identifying
the request controls [RFC4511] the server supports. If the server
does not support any request controls, this attribute will be absent.
Object identifiers identifying response controls need not be listed.
Procedures for registering object identifiers used to discovery of
protocol mechanisms are detailed in BCP 64, RFC 4520 [RFC4520].
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RFC 4512 LDAP Models June 2006
( 1.3.6.1.4.1.1466.101.120.13 NAME 'supportedControl'
SYNTAX 1.3.6.1.4.1.1466.115.121.1.38
USAGE dSAOperation )
The OBJECT IDENTIFIER (1.3.6.1.4.1.1466.115.121.1.38) syntax is
defined in [RFC4517].
5.1.4. 'supportedExtension'
The 'supportedExtension' attribute lists object identifiers
identifying the extended operations [RFC4511] that the server
supports. If the server does not support any extended operations,
this attribute will be absent.
An extended operation generally consists of an extended request and
an extended response but may also include other protocol data units
(such as intermediate responses). The object identifier assigned to
the extended request is used to identify the extended operation.
Other object identifiers used in the extended operation need not be
listed as values of this attribute.
Procedures for registering object identifiers used to discovery of
protocol mechanisms are detailed in BCP 64, RFC 4520 [RFC4520].
( 1.3.6.1.4.1.1466.101.120.7 NAME 'supportedExtension'
SYNTAX 1.3.6.1.4.1.1466.115.121.1.38
USAGE dSAOperation )
The OBJECT IDENTIFIER (1.3.6.1.4.1.1466.115.121.1.38) syntax is
defined in [RFC4517].
5.1.5. 'supportedFeatures'
The 'supportedFeatures' attribute lists object identifiers
identifying elective features that the server supports. If the
server does not support any discoverable elective features, this
attribute will be absent.
( 1.3.6.1.4.1.4203.1.3.5 NAME 'supportedFeatures'
EQUALITY objectIdentifierMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.38
USAGE dSAOperation )
Procedures for registering object identifiers used to discovery of
protocol mechanisms are detailed in BCP 64, RFC 4520 [RFC4520].
The OBJECT IDENTIFIER (1.3.6.1.4.1.1466.115.121.1.38) syntax and
objectIdentifierMatch matching rule are defined in [RFC4517].
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RFC 4512 LDAP Models June 2006
5.1.6. 'supportedLDAPVersion'
The 'supportedLDAPVersion' attribute lists the versions of LDAP that
the server supports.
( 1.3.6.1.4.1.1466.101.120.15 NAME 'supportedLDAPVersion'
SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
USAGE dSAOperation )
The INTEGER (1.3.6.1.4.1.1466.115.121.1.27) syntax is defined in
[RFC4517].
5.1.7. 'supportedSASLMechanisms'
The 'supportedSASLMechanisms' attribute lists the SASL mechanisms
[RFC4422] that the server recognizes and/or supports [RFC4513]. The
contents of this attribute may depend on the current session state.
If the server does not support any SASL mechanisms, this attribute
will not be present.
( 1.3.6.1.4.1.1466.101.120.14 NAME 'supportedSASLMechanisms'
SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
USAGE dSAOperation )
The Directory String (1.3.6.1.4.1.1466.115.121.1.15) syntax is
defined in [RFC4517].
6. Other Considerations
6.1. Preservation of User Information
Syntaxes may be defined that have specific value and/or value form
(representation) preservation requirements. For example, a syntax
containing digitally signed data can mandate that the server preserve
both the value and form of value presented to ensure that the
signature is not invalidated.
Where such requirements have not been explicitly stated, servers
SHOULD preserve the value of user information but MAY return the
value in a different form. And where a server is unable (or
unwilling) to preserve the value of user information, the server
SHALL ensure that an equivalent value (per Section 2.3) is returned.
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RFC 4512 LDAP Models June 2006
6.2. Short Names
Short names, also known as descriptors, are used as more readable
aliases for object identifiers and are used to identify various
schema elements. However, it is not expected that LDAP
implementations with human user interface would display these short
names (or the object identifiers they refer to) to the user.
Instead, they would most likely be performing translations (such as
expressing the short name in one of the local national languages).
For example, the short name "st" (stateOrProvinceName) might be
displayed to a German-speaking user as "Land".
The same short name might have different meaning in different
subschemas, and, within a particular subschema, the same short name
might refer to different object identifiers each identifying a
different kind of schema element.
Implementations MUST be prepared that the same short name might be
used in a subschema to refer to the different kinds of schema
elements. That is, there might be an object class 'x-fubar' and an
attribute type 'x-fubar' in a subschema.
Implementations MUST be prepared that the same short name might be
used in the different subschemas to refer to the different schema
elements. That is, there might be two matching rules 'x-fubar', each
in different subschemas.
Procedures for registering short names (descriptors) are detailed in
BCP 64, RFC 4520 [RFC4520].
6.3. Cache and Shadowing
Some servers may hold cache or shadow copies of entries, which can be
used to answer search and comparison queries, but will return
referrals or contact other servers if modification operations are
requested. Servers that perform shadowing or caching MUST ensure
that they do not violate any access control constraints placed on the
data by the originating server.
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RFC 4512 LDAP Models June 2006
7. Implementation Guidelines
7.1. Server Guidelines
Servers MUST recognize all names of attribute types and object
classes defined in this document but, unless stated otherwise, need
not support the associated functionality. Servers SHOULD recognize
all the names of attribute types and object classes defined in
Section 3 and 4, respectively, of [RFC4519].
Servers MUST ensure that entries conform to user and system schema
rules or other data model constraints.
Servers MAY support DIT Content Rules. Servers MAY support DIT
Structure Rules and Name Forms.
Servers MAY support alias entries.
Servers MAY support the 'extensibleObject' object class.
Servers MAY support subentries. If so, they MUST do so in accordance
with [RFC3672]. Servers that do not support subentries SHOULD use
object entries to mimic subentries as detailed in Section 3.2.
Servers MAY implement additional schema elements. Servers SHOULD
provide definitions of all schema elements they support in subschema
(sub)entries.
7.2. Client Guidelines
In the absence of prior agreements with servers, clients SHOULD NOT
assume that servers support any particular schema elements beyond
those referenced in Section 7.1. The client can retrieve subschema
information as described in Section 4.4.
Clients MUST NOT display or attempt to decode a value as ASN.1 if the
value's syntax is not known. Clients MUST NOT assume the LDAP-
specific string encoding is restricted to a UTF-8 encoded string of
Unicode characters or any particular subset of Unicode (such as a
printable subset) unless such restriction is explicitly stated.
Clients SHOULD NOT send attribute values in a request that are not
valid according to the syntax defined for the attributes.
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RFC 4512 LDAP Models June 2006
8. Security Considerations
Attributes of directory entries are used to provide descriptive
information about the real-world objects they represent, which can be
people, organizations, or devices. Most countries have privacy laws
regarding the publication of information about people.
General security considerations for accessing directory information
with LDAP are discussed in [RFC4511] and [RFC4513].
9. IANA Considerations
The Internet Assigned Numbers Authority (IANA) has updated the LDAP
descriptors registry as indicated in the following template:
Subject: Request for LDAP Descriptor Registration Update
Descriptor (short name): see comment
Object Identifier: see comment
Person & email address to contact for further information:
Kurt Zeilenga <kurt@OpenLDAP.org>
Usage: see comment
Specification: RFC 4512
Author/Change Controller: IESG
Comments:
The following descriptors (short names) has been added to
the registry.
NAME Type OID
------------------------ ---- -----------------
governingStructureRule A 2.5.21.10
structuralObjectClass A 2.5.21.9
The following descriptors (short names) have been updated to
refer to this RFC.
NAME Type OID
------------------------ ---- -----------------
alias O 2.5.6.1
aliasedObjectName A 2.5.4.1
altServer A 1.3.6.1.4.1.1466.101.120.6
attributeTypes A 2.5.21.5
createTimestamp A 2.5.18.1
creatorsName A 2.5.18.3
dITContentRules A 2.5.21.2
dITStructureRules A 2.5.21.1
extensibleObject O 1.3.6.1.4.1.1466.101.120.111
ldapSyntaxes A 1.3.6.1.4.1.1466.101.120.16
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RFC 4512 LDAP Models June 2006
matchingRuleUse A 2.5.21.8
matchingRules A 2.5.21.4
modifiersName A 2.5.18.4
modifyTimestamp A 2.5.18.2
nameForms A 2.5.21.7
namingContexts A 1.3.6.1.4.1.1466.101.120.5
objectClass A 2.5.4.0
objectClasses A 2.5.21.6
subschema O 2.5.20.1
subschemaSubentry A 2.5.18.10
supportedControl A 1.3.6.1.4.1.1466.101.120.13
supportedExtension A 1.3.6.1.4.1.1466.101.120.7
supportedFeatures A 1.3.6.1.4.1.4203.1.3.5
supportedLDAPVersion A 1.3.6.1.4.1.1466.101.120.15
supportedSASLMechanisms A 1.3.6.1.4.1.1466.101.120.14
top O 2.5.6.0
10. Acknowledgements
This document is based in part on RFC 2251 by M. Wahl, T. Howes, and
S. Kille; RFC 2252 by M. Wahl, A. Coulbeck, T. Howes, S. Kille; and
RFC 2556 by M. Wahl, all products of the IETF Access, Searching and
Indexing of Directories (ASID) Working Group. This document is also
based in part on "The Directory: Models" [X.501], a product of the
International Telephone Union (ITU). Additional text was borrowed
from RFC 2253 by M. Wahl, T. Howes, and S. Kille.
This document is a product of the IETF LDAP Revision (LDAPBIS)
Working Group.
Zeilenga Standards Track [Page 44]
RFC 4512 LDAP Models June 2006
11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3671] Zeilenga, K., "Collective Attributes in the Lightweight
Directory Access Protocol (LDAP)", RFC 3671, December
2003.
[RFC3672] Zeilenga, K., "Subentries in the Lightweight Directory
Access Protocol (LDAP)", RFC 3672, December 2003.
[RFC4234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
June 2006.
[RFC4510] Zeilenga, K., Ed., "Lightweight Directory Access
Protocol (LDAP): Technical Specification Road Map", RFC
4510, June 2006.
[RFC4511] Sermersheim, J., Ed., "Lightweight Directory Access
Protocol (LDAP): The Protocol", RFC 4511, June 2006.
[RFC4513] Harrison, R., Ed., "Lightweight Directory Access
Protocol (LDAP): Authentication Methods and Security
Mechanisms", RFC 4513, June 2006.
[RFC4514] Zeilenga, K., Ed., "Lightweight Directory Access
Protocol (LDAP): String Representation of Distinguished
Names", RFC 4514, June 2006.
[RFC4515] Smith, M., Ed. and T. Howes, "Lightweight Directory
Access Protocol (LDAP): String Representation of Search
Filters", RFC 4515, June 2006.
[RFC4516] Smith, M., Ed. and T. Howes, "Lightweight Directory
Access Protocol (LDAP): Uniform Resource Locator", RFC
4516, June 2006.
[RFC4517] Legg, S., Ed., "Lightweight Directory Access Protocol
(LDAP): Syntaxes and Matching Rules", RFC 4517, June
2006.
Zeilenga Standards Track [Page 45]
RFC 4512 LDAP Models June 2006
[RFC4519] Sciberras, A., Ed., "Lightweight Directory Access
Protocol (LDAP): Schema for User Applications", RFC
4519, June 2006.
[RFC4520] Zeilenga, K., "Internet Assigned Numbers Authority
(IANA) Considerations for the Lightweight Directory
Access Protocol (LDAP)", BCP 64, RFC 4520, June 2006.
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0" is defined by "The Unicode Standard, Version
3.0" (Reading, MA, Addison-Wesley, 2000. ISBN 0-201-
61633-5), as amended by the "Unicode Standard Annex
#27: Unicode 3.1"
(http://www.unicode.org/reports/tr27/) and by the
"Unicode Standard Annex #28: Unicode 3.2"
(http://www.unicode.org/reports/tr28/).
[X.500] International Telecommunication Union -
Telecommunication Standardization Sector, "The
Directory -- Overview of concepts, models and
services," X.500(1993) (also ISO/IEC 9594-1:1994).
[X.501] International Telecommunication Union -
Telecommunication Standardization Sector, "The
Directory -- Models," X.501(1993) (also ISO/IEC 9594-
2:1994).
[X.680] International Telecommunication Union -
Telecommunication Standardization Sector, "Abstract
Syntax Notation One (ASN.1) - Specification of Basic
Notation", X.680(2002) (also ISO/IEC 8824-1:2002).
Zeilenga Standards Track [Page 46]
RFC 4512 LDAP Models June 2006
Appendix A. Changes
This appendix is non-normative.
This document amounts to nearly a complete rewrite of portions of RFC
2251, RFC 2252, and RFC 2256. This rewrite was undertaken to improve
overall clarity of technical specification. This appendix provides a
summary of substantive changes made to the portions of these
documents incorporated into this document. Readers should consult
[RFC4510], [RFC4511], [RFC4517], and [RFC4519] for summaries of
remaining portions of these documents.
A.1. Changes to RFC 2251
This document incorporates from RFC 2251, Sections 3.2 and 3.4, and
portions of Sections 4 and 6 as summarized below.
A.1.1. Section 3.2 of RFC 2251
Section 3.2 of RFC 2251 provided a brief introduction to the X.500
data model, as used by LDAP. The previous specification relied on
[X.501] but lacked clarity in how X.500 models are adapted for use by
LDAP. This document describes the X.500 data models, as used by
LDAP, in greater detail, especially in areas where adaptation is
needed.
Section 3.2.1 of RFC 2251 described an attribute as "a type with one
or more associated values". In LDAP, an attribute is better
described as an attribute description, a type with zero or more
options, and one or more associated values.
Section 3.2.2 of RFC 2251 mandated that subschema subentries contain
objectClasses and attributeTypes attributes, yet X.500(93) treats
these attributes as optional. While generally all implementations
that support X.500(93) subschema mechanisms will provide both of
these attributes, it is not absolutely required for interoperability
that all servers do. The mandate was removed for consistency with
X.500(93). The subschema discovery mechanism was also clarified to
indicate that subschema controlling an entry is obtained by reading
the (sub)entry referred to by that entry's 'subschemaSubentry'
attribute.
Zeilenga Standards Track [Page 47]
RFC 4512 LDAP Models June 2006
A.1.2. Section 3.4 of RFC 2251
Section 3.4 of RFC 2251 provided "Server-specific Data Requirements".
This material, with changes, was incorporated in Section 5.1 of this
document.
Changes:
- Clarify that attributes of the root DSE are subject to "other
restrictions" in addition to access controls.
- Clarify that only recognized extended requests need to be
enumerated 'supportedExtension'.
- Clarify that only recognized request controls need to be enumerated
'supportedControl'.
- Clarify that root DSE attributes are operational and, like other
operational attributes, will not be returned in search requests
unless requested by name.
- Clarify that not all root DSE attributes are user modifiable.
- Remove inconsistent text regarding handling of the
'subschemaSubentry' attribute within the root DSE. The previous
specification stated that the 'subschemaSubentry' attribute held in
the root DSE referred to "subschema entries (or subentries) known
by this server". This is inconsistent with the attribute's
intended use as well as its formal definition as a single valued
attribute [X.501]. It is also noted that a simple (possibly
incomplete) list of subschema (sub)entries is not terribly useful.
This document (in Section 5.1) specifies that the
'subschemaSubentry' attribute of the root DSE refers to the
subschema controlling the root DSE. It is noted that the general
subschema discovery mechanism remains available (see Section 4.4 of
this document).
A.1.3. Section 4 of RFC 2251
Portions of Section 4 of RFC 2251 detailing aspects of the
information model used by LDAP were incorporated in this document,
including:
- Restriction of distinguished values to attributes whose
descriptions have no options (from Section 4.1.3);
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RFC 4512 LDAP Models June 2006
- Data model aspects of Attribute Types (from Section 4.1.4),
Attribute Descriptions (from 4.1.5), Attribute (from 4.1.8),
Matching Rule Identifier (from 4.1.9); and
- User schema requirements (from Sections 4.1.6, 4.5.1, and 4.7).
Clarifications to these portions include:
- Subtyping and AttributeDescriptions with options.
A.1.4. Section 6 of RFC 2251
The Section 6.1 and the second paragraph of Section 6.2 of RFC 2251
where incorporated into this document.
A.2. Changes to RFC 2252
This document incorporates Sections 4, 5, and 7 from RFC 2252.
A.2.1. Section 4 of RFC 2252
The specification was updated to use Augmented BNF [RFC4234]. The
string representation of an OBJECT IDENTIFIER was tightened to
disallow leading zeros as described in RFC 2252.
The <descr> syntax was changed to disallow semicolon (U+003B)
characters in order to appear to be consistent its natural language
specification "descr is the syntactic representation of an object
descriptor, which consists of letters and digits, starting with a
letter". In a related change, the statement "an AttributeDescription
can be used as the value in a NAME part of an
AttributeTypeDescription" was deleted. RFC 2252 provided no
specification of the semantics of attribute options appearing in NAME
fields.
RFC 2252 stated that the <descr> form of <oid> SHOULD be preferred
over the <numericoid> form. However, <descr> form can be ambiguous.
To address this issue, the imperative was replaced with a statement
(in Section 1.4) that while the <descr> form is generally preferred,
<numericoid> should be used where an unambiguous <descr> is not
available. Additionally, an expanded discussion of descriptor issues
is in Section 6.2 ("Short Names").
The ABNF for a quoted string (qdstring) was updated to reflect
support for the escaping mechanism described in Section 4.3 of RFC
2252.
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RFC 4512 LDAP Models June 2006
A.2.2. Section 5 of RFC 2252
Definitions of operational attributes provided in Section 5 of RFC
2252 where incorporated into this document.
The 'namingContexts' description was clarified. A first-level DSA
should publish, in addition to other values, "" indicating the root
of the DIT.
The 'altServer' description was clarified. It may hold any URI.
The 'supportedExtension' description was clarified. A server need
only list the OBJECT IDENTIFIERs associated with the extended
requests of the extended operations it recognizes.
The 'supportedControl' description was clarified. A server need only
list the OBJECT IDENTIFIERs associated with the request controls it
recognizes.
Descriptions for the 'structuralObjectClass' and
'governingStructureRule' operational attribute types were added.
The attribute definition of 'subschemaSubentry' was corrected to list
the terms SINGLE-VALUE and NO-USER-MODIFICATION in proper order.
A.2.3. Section 7 of RFC 2252
Section 7 of RFC 2252 provides definitions of the 'subschema' and
'extensibleObject' object classes. These definitions where
integrated into Section 4.2 and Section 4.3 of this document,
respectively. Section 7 of RFC 2252 also contained the object class
implementation requirement. This was incorporated into Section 7 of
this document.
The specification of 'extensibleObject' was clarified regarding how
it interacts with precluded attributes.
A.3. Changes to RFC 2256
This document incorporates Sections 5.1, 5.2, 7.1, and 7.2 of RFC
2256.
Section 5.1 of RFC 2256 provided the definition of the 'objectClass'
attribute type. This was integrated into Section 2.4.1 of this
document. The statement "One of the values is either 'top' or
'alias'" was replaced with statement that one of the values is 'top'
as entries belonging to 'alias' also belong to 'top'.
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RFC 4512 LDAP Models June 2006
Section 5.2 of RFC 2256 provided the definition of the
'aliasedObjectName' attribute type. This was integrated into Section
2.6.2 of this document.
Section 7.1 of RFC 2256 provided the definition of the 'top' object
class. This was integrated into Section 2.4.1 of this document.
Section 7.2 of RFC 2256 provided the definition of the 'alias' object
class. This was integrated into Section 2.6.1 of this document.
A.4. Changes to RFC 3674
This document made no substantive change to the 'supportedFeatures'
technical specification provided in RFC 3674.
Editor's Address
Kurt D. Zeilenga
OpenLDAP Foundation
EMail: Kurt@OpenLDAP.org
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RFC 4512 LDAP Models June 2006
Full Copyright Statement
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contained in BCP 78, and except as set forth therein, the authors
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