Network Working Group M. Wahl
Request for Comments: 2251 Critical Angle Inc.
Category: Standards Track T. Howes
Netscape Communications Corp.
S. Kille
Isode Limited
December 1997
Lightweight Directory Access Protocol (v3)
1. 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 (1997). All Rights Reserved.
IESG Note
This document describes a directory access protocol that provides
both read and update access. Update access requires secure
authentication, but this document does not mandate implementation of
any satisfactory authentication mechanisms.
In accordance with RFC 2026, section 4.4.1, this specification is
being approved by IESG as a Proposed Standard despite this
limitation, for the following reasons:
a. to encourage implementation and interoperability testing of
these protocols (with or without update access) before they
are deployed, and
b. to encourage deployment and use of these protocols in read-only
applications. (e.g. applications where LDAPv3 is used as
a query language for directories which are updated by some
secure mechanism other than LDAP), and
c. to avoid delaying the advancement and deployment of other Internet
standards-track protocols which require the ability to query, but
not update, LDAPv3 directory servers.
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RFC 2251 LDAPv3 December 1997
Readers are hereby warned that until mandatory authentication
mechanisms are standardized, clients and servers written according to
this specification which make use of update functionality are
UNLIKELY TO INTEROPERATE, or MAY INTEROPERATE ONLY IF AUTHENTICATION
IS REDUCED TO AN UNACCEPTABLY WEAK LEVEL.
Implementors are hereby discouraged from deploying LDAPv3 clients or
servers which implement the update functionality, until a Proposed
Standard for mandatory authentication in LDAPv3 has been approved and
published as an RFC.
Table of Contents
1. Status of this Memo .................................... 1
Copyright Notice ....................................... 1
IESG Note .............................................. 1
2. Abstract ............................................... 3
3. Models ................................................. 4
3.1. Protocol Model ........................................ 4
3.2. Data Model ............................................ 5
3.2.1. Attributes of Entries ............................... 5
3.2.2. Subschema Entries and Subentries .................... 7
3.3. Relationship to X.500 ................................. 8
3.4. Server-specific Data Requirements ..................... 8
4. Elements of Protocol ................................... 9
4.1. Common Elements ....................................... 9
4.1.1. Message Envelope .................................... 9
4.1.1.1. Message ID ........................................ 11
4.1.2. String Types ........................................ 11
4.1.3. Distinguished Name and Relative Distinguished Name .. 11
4.1.4. Attribute Type ...................................... 12
4.1.5. Attribute Description ............................... 13
4.1.5.1. Binary Option ..................................... 14
4.1.6. Attribute Value ..................................... 14
4.1.7. Attribute Value Assertion ........................... 15
4.1.8. Attribute ........................................... 15
4.1.9. Matching Rule Identifier ............................ 15
4.1.10. Result Message ..................................... 16
4.1.11. Referral ........................................... 18
4.1.12. Controls ........................................... 19
4.2. Bind Operation ........................................ 20
4.2.1. Sequencing of the Bind Request ...................... 21
4.2.2. Authentication and Other Security Services .......... 22
4.2.3. Bind Response ....................................... 23
4.3. Unbind Operation ...................................... 24
4.4. Unsolicited Notification .............................. 24
4.4.1. Notice of Disconnection ............................. 24
4.5. Search Operation ...................................... 25
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4.5.1. Search Request ...................................... 25
4.5.2. Search Result ....................................... 29
4.5.3. Continuation References in the Search Result ........ 31
4.5.3.1. Example ........................................... 31
4.6. Modify Operation ...................................... 32
4.7. Add Operation ......................................... 34
4.8. Delete Operation ...................................... 35
4.9. Modify DN Operation ................................... 36
4.10. Compare Operation .................................... 37
4.11. Abandon Operation .................................... 38
4.12. Extended Operation ................................... 38
5. Protocol Element Encodings and Transfer ................ 39
5.1. Mapping Onto BER-based Transport Services ............. 39
5.2. Transfer Protocols .................................... 40
5.2.1. Transmission Control Protocol (TCP) ................. 40
6. Implementation Guidelines .............................. 40
6.1. Server Implementations ................................ 40
6.2. Client Implementations ................................ 40
7. Security Considerations ................................ 41
8. Acknowledgements ....................................... 41
9. Bibliography ........................................... 41
10. Authors’ Addresses ..................................... 42
Appendix A - Complete ASN.1 Definition ..................... 44
Full Copyright Statement ................................... 50
2. Abstract
The protocol described in this document is designed to provide access
to directories supporting the X.500 models, while not incurring the
resource requirements of the X.500 Directory Access Protocol (DAP).
This protocol is specifically targeted at management applications and
browser applications that provide read/write interactive access to
directories. When used with a directory supporting the X.500
protocols, it is intended to be a complement to the X.500 DAP.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", and "MAY" in this document
are to be interpreted as described in RFC 2119 [10].
Key aspects of this version of LDAP are:
- All protocol elements of LDAPv2 (RFC 1777) are supported. The
protocol is carried directly over TCP or other transport, bypassing
much of the session/presentation overhead of X.500 DAP.
- Most protocol data elements can be encoded as ordinary strings
(e.g., Distinguished Names).
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- Referrals to other servers may be returned.
- SASL mechanisms may be used with LDAP to provide association
security services.
- Attribute values and Distinguished Names have been
internationalized through the use of the ISO 10646 character set.
- The protocol can be extended to support new operations, and
controls may be used to extend existing operations.
- Schema is published in the directory for use by clients.
3. Models
Interest in X.500 [1] directory technologies in the Internet has led
to efforts to reduce the high cost of entry associated with use of
these technologies. This document continues the efforts to define
directory protocol alternatives, updating the LDAP [2] protocol
specification.
3.1. Protocol Model
The general model adopted by this protocol is one of clients
performing protocol operations against servers. In this model, a
client transmits a protocol request describing the operation to be
performed to a server. The server is then responsible for performing
the necessary operation(s) in the directory. Upon completion of the
operation(s), the server returns a response containing any results or
errors to the requesting client.
In keeping with the goal of easing the costs associated with use of
the directory, it is an objective of this protocol to minimize the
complexity of clients so as to facilitate widespread deployment of
applications capable of using the directory.
Note that although servers are required to return responses whenever
such responses are defined in the protocol, there is no requirement
for synchronous behavior on the part of either clients or servers.
Requests and responses for multiple operations may be exchanged
between a client and server in any order, provided the client
eventually receives a response for every request that requires one.
In LDAP versions 1 and 2, no provision was made for protocol servers
returning referrals to clients. However, for improved performance
and distribution this version of the protocol permits servers to
return to clients referrals to other servers. This allows servers to
offload the work of contacting other servers to progress operations.
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Note that the core protocol operations defined in this document can
be mapped to a strict subset of the X.500(1997) directory abstract
service, so it can be cleanly provided by the DAP. However there is
not a one-to-one mapping between LDAP protocol operations and DAP
operations: server implementations acting as a gateway to X.500
directories may need to make multiple DAP requests.
3.2. Data Model
This section provides a brief introduction to the X.500 data model,
as used by LDAP.
The LDAP protocol assumes there are one or more servers which jointly
provide access to a Directory Information Tree (DIT). The tree is
made up of entries. Entries have names: one or more attribute values
from the entry form its relative distinguished name (RDN), which MUST
be unique among all its siblings. The concatenation of the relative
distinguished names of the sequence of entries from a particular
entry to an immediate subordinate of the root of the tree forms that
entry’s Distinguished Name (DN), which is unique in the tree. An
example of a Distinguished Name is
CN=Steve Kille, O=Isode Limited, C=GB
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 which perform caching or shadowing MUST ensure that they do
not violate any access control constraints placed on the data by the
originating server.
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 which are mastered by
different servers, is termed a naming context. The root of the DIT
is a DSA-specific Entry (DSE) and not part of any naming context:
each server has different attribute values in the root DSE. (DSA is
an X.500 term for the directory server).
3.2.1. Attributes of Entries
Entries consist of a set of attributes. An attribute is a type with
one or more associated values. The attribute type is identified by a
short descriptive name and an OID (object identifier). The attribute
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type governs whether there can be more than one value of an attribute
of that type in an entry, the syntax to which the values must
conform, the kinds of matching which can be performed on values of
that attribute, and other functions.
An example of an attribute is "mail". There may be one or more values
of this attribute, they must be IA5 (ASCII) strings, and they are
case insensitive (e.g. "foo@bar.com" will match "FOO@BAR.COM").
Schema is the collection of attribute type definitions, object class
definitions and other information which a server uses to determine
how to match a filter or attribute value assertion (in a compare
operation) against the attributes of an entry, and whether to permit
add and modify operations. The definition of schema for use with
LDAP is given in [5] and [6]. Additional schema elements may be
defined in other documents.
Each entry MUST have an objectClass attribute. The objectClass
attribute specifies the object classes of an entry, which along with
the system and user schema determine the permitted attributes of an
entry. Values of this attribute may be modified by clients, but the
objectClass attribute cannot be removed. Servers may restrict the
modifications of this attribute to prevent the basic structural class
of the entry from being changed (e.g. 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 are implicitly added
as well if not already present, and the client must supply values for
any mandatory attributes of new superclasses.
Some attributes, termed operational attributes, are used by servers
for administering the directory system itself. They are not returned
in search results unless explicitly requested by name. Attributes
which are not operational, such as "mail", will have their schema and
syntax constraints enforced by servers, but servers will generally
not make use of their values.
Servers MUST NOT permit clients to add attributes to an entry unless
those attributes are permitted by the object class definitions, the
schema controlling that entry (specified in the subschema - see
below), or are operational attributes known to that server and used
for administrative purposes. Note that there is a particular
objectClass ’extensibleObject’ defined in [5] which permits all user
attributes to be present in an entry.
Entries MAY contain, among others, the following operational
attributes, defined in [5]. These attributes are maintained
automatically by the server and are not modifiable by clients:
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- 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.
- modifyTimestamp: the time this entry was last modified.
- subschemaSubentry: the Distinguished Name of the subschema entry
(or subentry) which controls the schema for this entry.
3.2.2. Subschema Entries and Subentries
Subschema entries are used for administering information about the
directory schema, in particular the object classes and attribute
types supported by directory servers. A single subschema entry
contains all schema definitions used by entries in a particular part
of the directory tree.
Servers which follow X.500(93) models SHOULD implement subschema
using the X.500 subschema mechanisms, and so these subschemas are not
ordinary entries. LDAP clients SHOULD NOT assume that servers
implement any of the other aspects of X.500 subschema. A server
which masters entries and permits clients to modify these entries
MUST implement and provide access to these subschema entries, so that
its clients may discover the attributes and object classes which are
permitted to be present. It is strongly recommended that all other
servers implement this as well.
The following four attributes MUST be present in all subschema
entries:
- cn: this attribute MUST be used to form the RDN of the subschema
entry.
- objectClass: the attribute MUST have at least the values "top" and
"subschema".
- objectClasses: each value of this attribute specifies an object
class known to the server.
- attributeTypes: each value of this attribute specifies an attribute
type known to the server.
These are defined in [5]. Other attributes MAY be present in
subschema entries, to reflect additional supported capabilities.
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These include matchingRules, matchingRuleUse, dITStructureRules,
dITContentRules, nameForms and ldapSyntaxes.
Servers SHOULD provide the attributes createTimestamp and
modifyTimestamp in subschema entries, in order to allow clients to
maintain their caches of schema information.
Clients MUST only retrieve attributes from a subschema entry by
requesting a base object search of the entry, where the search filter
is "(objectClass=subschema)". (This will allow LDAPv3 servers which
gateway to X.500(93) to detect that subentry information is being
requested.)
3.3. Relationship to X.500
This document defines LDAP in terms of X.500 as an X.500 access
mechanism. An LDAP server MUST act in accordance with the
X.500(1993) series of ITU recommendations when providing the service.
However, it is not required that an LDAP server make use of any X.500
protocols in providing this service, e.g. LDAP can be mapped onto any
other directory system so long as the X.500 data and service model as
used in LDAP is not violated in the LDAP interface.
3.4. Server-specific Data Requirements
An LDAP server MUST 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 (DSA-Specific Entry),
which is named with the zero-length LDAPDN. These attributes are
retrievable if a client performs a base object search of the root
with filter "(objectClass=*)", however they are subject to access
control restrictions. The root DSE MUST NOT be included if the
client performs a subtree search starting from the root.
Servers may allow clients to modify these attributes.
The following attributes of the root DSE are defined in section 5 of
[5]. Additional attributes may be defined in other documents.
- namingContexts: naming contexts held in the server. Naming contexts
are defined in section 17 of X.501 [6].
- subschemaSubentry: subschema entries (or subentries) known by this
server.
- altServer: alternative servers in case this one is later
unavailable.
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- supportedExtension: list of supported extended operations.
- supportedControl: list of supported controls.
- supportedSASLMechanisms: list of supported SASL security features.
- supportedLDAPVersion: LDAP versions implemented by the server.
If the server does not master entries and does not know the locations
of schema information, the subschemaSubentry attribute is not present
in the root DSE. If the server masters directory entries under one
or more schema rules, there may be any number of values of the
subschemaSubentry attribute in the root DSE.
4. Elements of Protocol
The LDAP protocol is described using Abstract Syntax Notation 1
(ASN.1) [3], and is typically transferred using a subset of ASN.1
Basic Encoding Rules [11]. In order to support future extensions to
this protocol, clients and servers MUST ignore elements of SEQUENCE
encodings whose tags they do not recognize.
Note that unlike X.500, each change to the LDAP protocol other than
through the extension mechanisms will have a different version
number. A client will indicate the version it supports as part of
the bind request, described in section 4.2. If a client has not sent
a bind, the server MUST assume that version 3 is supported in the
client (since version 2 required that the client bind first).
Clients may determine the protocol version a server supports by
reading the supportedLDAPVersion attribute from the root DSE. Servers
which implement version 3 or later versions MUST provide this
attribute. Servers which only implement version 2 may not provide
this attribute.
4.1. Common Elements
This section describes the LDAPMessage envelope PDU (Protocol Data
Unit) format, as well as data type definitions which are used in the
protocol operations.
4.1.1. Message Envelope
For the purposes of protocol exchanges, all protocol operations are
encapsulated in a common envelope, the LDAPMessage, which is defined
as follows:
LDAPMessage ::= SEQUENCE {
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messageID MessageID,
protocolOp CHOICE {
bindRequest BindRequest,
bindResponse BindResponse,
unbindRequest UnbindRequest,
searchRequest SearchRequest,
searchResEntry SearchResultEntry,
searchResDone SearchResultDone,
searchResRef SearchResultReference,
modifyRequest ModifyRequest,
modifyResponse ModifyResponse,
addRequest AddRequest,
addResponse AddResponse,
delRequest DelRequest,
delResponse DelResponse,
modDNRequest ModifyDNRequest,
modDNResponse ModifyDNResponse,
compareRequest CompareRequest,
compareResponse CompareResponse,
abandonRequest AbandonRequest,
extendedReq ExtendedRequest,
extendedResp ExtendedResponse },
controls [0] Controls OPTIONAL }
MessageID ::= INTEGER (0 .. maxInt)
maxInt INTEGER ::= 2147483647 -- (2^^31 - 1) --
The function of the LDAPMessage is to provide an envelope containing
common fields required in all protocol exchanges. At this time the
only common fields are the message ID and the controls.
If the server receives a PDU from the client in which the LDAPMessage
SEQUENCE tag cannot be recognized, the messageID cannot be parsed,
the tag of the protocolOp is not recognized as a request, or the
encoding structures or lengths of data fields are found to be
incorrect, then the server MUST return the notice of disconnection
described in section 4.4.1, with resultCode protocolError, and
immediately close the connection. In other cases that the server
cannot parse the request received by the client, the server MUST
return an appropriate response to the request, with the resultCode
set to protocolError.
If the client receives a PDU from the server which cannot be parsed,
the client may discard the PDU, or may abruptly close the connection.
The ASN.1 type Controls is defined in section 4.1.12.
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4.1.1.1. Message ID
All LDAPMessage envelopes encapsulating responses contain the
messageID value of the corresponding request LDAPMessage.
The message ID of a request MUST have a value different from the
values of any other requests outstanding in the LDAP session of which
this message is a part.
A client MUST NOT send a second request with the same message ID as
an earlier request on the same connection if the client has not
received the final response from the earlier request. Otherwise the
behavior is undefined. Typical clients increment a counter for each
request.
A client MUST NOT reuse the message id of an abandonRequest or of the
abandoned operation until it has received a response from the server
for another request invoked subsequent to the abandonRequest, as the
abandonRequest itself does not have a response.
4.1.2. String Types
The LDAPString is a notational convenience to indicate that, although
strings of LDAPString type encode as OCTET STRING types, the ISO
10646 [13] character set (a superset of Unicode) is used, encoded
following the UTF-8 algorithm [14]. Note that in the UTF-8 algorithm
characters which are the same as ASCII (0x0000 through 0x007F) are
represented as that same ASCII character in a single byte. The other
byte values are used to form a variable-length encoding of an
arbitrary character.
LDAPString ::= OCTET STRING
The LDAPOID is a notational convenience to indicate that the
permitted value of this string is a (UTF-8 encoded) dotted-decimal
representation of an OBJECT IDENTIFIER.
LDAPOID ::= OCTET STRING
For example,
1.3.6.1.4.1.1466.1.2.3
4.1.3. Distinguished Name and Relative Distinguished Name
An LDAPDN and a RelativeLDAPDN are respectively defined to be the
representation of a Distinguished Name and a Relative Distinguished
Name after encoding according to the specification in [4], such that
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<distinguished-name> ::= <name>
<relative-distinguished-name> ::= <name-component>
where <name> and <name-component> are as defined in [4].
LDAPDN ::= LDAPString
RelativeLDAPDN ::= LDAPString
Only Attribute Types can be present in a relative distinguished name
component; the options of Attribute Descriptions (next section) MUST
NOT be used in specifying distinguished names.
4.1.4. Attribute Type
An AttributeType takes on as its value the textual string associated
with that AttributeType in its specification.
AttributeType ::= LDAPString
Each attribute type has a unique OBJECT IDENTIFIER which has been
assigned to it. This identifier may be written as decimal digits
with components separated by periods, e.g. "2.5.4.10".
A specification may also assign one or more textual names for an
attribute type. These names MUST begin with a letter, and only
contain ASCII letters, digit characters and hyphens. They are case
insensitive. (These ASCII characters are identical to ISO 10646
characters whose UTF-8 encoding is a single byte between 0x00 and
0x7F.)
If the server has a textual name for an attribute type, it MUST use a
textual name for attributes returned in search results. The dotted-
decimal OBJECT IDENTIFIER is only used if there is no textual name
for an attribute type.
Attribute type textual names are non-unique, as two different
specifications (neither in standards track RFCs) may choose the same
name.
A server which masters or shadows entries SHOULD list all the
attribute types it supports in the subschema entries, using the
attributeTypes attribute. Servers which support an open-ended set of
attributes SHOULD include at least the attributeTypes value for the
’objectClass’ attribute. Clients MAY retrieve the attributeTypes
value from subschema entries in order to obtain the OBJECT IDENTIFIER
and other information associated with attribute types.
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Some attribute type names which are used in this version of LDAP are
described in [5]. Servers may implement additional attribute types.
4.1.5. Attribute Description
An AttributeDescription is a superset of the definition of the
AttributeType. It has the same ASN.1 definition, but allows
additional options to be specified. They are also case insensitive.
AttributeDescription ::= LDAPString
A value of AttributeDescription is based on the following BNF:
<AttributeDescription> ::= <AttributeType> [ ";" <options> ]
<options> ::= <option> | <option> ";" <options>
<option> ::= <opt-char> <opt-char>*
<opt-char> ::= ASCII-equivalent letters, numbers and hyphen
Examples of valid AttributeDescription:
cn
userCertificate;binary
One option, "binary", is defined in this document. Additional
options may be defined in IETF standards-track and experimental RFCs.
Options beginning with "x-" are reserved for private experiments.
Any option could be associated with any AttributeType, although not
all combinations may be supported by a server.
An AttributeDescription with one or more options is treated as a
subtype of the attribute type without any options. Options present
in an AttributeDescription are never mutually exclusive.
Implementations MUST generate the <options> list sorted in ascending
order, and servers MUST treat any two AttributeDescription with the
same AttributeType and options as equivalent. A server will treat an
AttributeDescription with any options it does not implement as an
unrecognized attribute type.
The data type "AttributeDescriptionList" describes a list of 0 or
more attribute types. (A list of zero elements has special
significance in the Search request.)
AttributeDescriptionList ::= SEQUENCE OF
AttributeDescription
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4.1.5.1. Binary Option
If the "binary" option is present in an AttributeDescription, it
overrides any string-based encoding representation defined for that
attribute in [5]. Instead the attribute is to be transferred as a
binary value encoded using the Basic Encoding Rules [11]. The syntax
of the binary value is an ASN.1 data type definition which is
referenced by the "SYNTAX" part of the attribute type definition.
The presence or absence of the "binary" option only affects the
transfer of attribute values in protocol; servers store any
particular attribute in a single format. If a client requests that a
server return an attribute in the binary format, but the server
cannot generate that format, the server MUST treat this attribute
type as an unrecognized attribute type. Similarly, clients MUST NOT
expect servers to return an attribute in binary format if the client
requested that attribute by name without the binary option.
This option is intended to be used with attributes whose syntax is a
complex ASN.1 data type, and the structure of values of that type is
needed by clients. Examples of this kind of syntax are "Certificate"
and "CertificateList".
4.1.6. Attribute Value
A field of type AttributeValue takes on as its value either a string
encoding of a AttributeValue data type, or an OCTET STRING containing
an encoded binary value, depending on whether the "binary" option is
present in the companion AttributeDescription to this AttributeValue.
The definition of string encodings for different syntaxes and types
may be found in other documents, and in particular [5].
AttributeValue ::= OCTET STRING
Note that there is no defined limit on the size of this encoding;
thus protocol values may include multi-megabyte attributes (e.g.
photographs).
Attributes may be defined which have arbitrary and non-printable
syntax. Implementations MUST NEITHER simply display nor attempt to
decode as ASN.1 a value if its syntax is not known. The
implementation may attempt to discover the subschema of the source
entry, and retrieve the values of attributeTypes from it.
Clients MUST NOT send attribute values in a request which are not
valid according to the syntax defined for the attributes.
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4.1.7. Attribute Value Assertion
The AttributeValueAssertion type definition is similar to the one in
the X.500 directory standards. It contains an attribute description
and a matching rule assertion value suitable for that type.
AttributeValueAssertion ::= SEQUENCE {
attributeDesc AttributeDescription,
assertionValue AssertionValue }
AssertionValue ::= OCTET STRING
If the "binary" option is present in attributeDesc, this signals to
the server that the assertionValue is a binary encoding of the
assertion value.
For all the string-valued user attributes described in [5], the
assertion value syntax is the same as the value syntax. Clients may
use attribute values as assertion values in compare requests and
search filters.
Note however that the assertion syntax may be different from the
value syntax for other attributes or for non-equality matching rules.
These may have an assertion syntax which contains only part of the
value. See section 20.2.1.8 of X.501 [6] for examples.
4.1.8. Attribute
An attribute consists of a type and one or more values of that type.
(Though attributes MUST have at least one value when stored, due to
access control restrictions the set may be empty when transferred in
protocol. This is described in section 4.5.2, concerning the
PartialAttributeList type.)
Attribute ::= SEQUENCE {
type AttributeDescription,
vals SET OF AttributeValue }
Each attribute value is distinct in the set (no duplicates). The
order of attribute values within the vals set is undefined and
implementation-dependent, and MUST NOT be relied upon.
4.1.9. Matching Rule Identifier
A matching rule is a means of expressing how a server should compare
an AssertionValue received in a search filter with an abstract data
value. The matching rule defines the syntax of the assertion value
and the process to be performed in the server.
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An X.501(1993) Matching Rule is identified in the LDAP protocol by
the printable representation of its OBJECT IDENTIFIER, either as one
of the strings given in [5], or as decimal digits with components
separated by periods, e.g. "caseIgnoreIA5Match" or
"1.3.6.1.4.1.453.33.33".
MatchingRuleId ::= LDAPString
Servers which support matching rules for use in the extensibleMatch
search filter MUST list the matching rules they implement in
subschema entries, using the matchingRules attributes. The server
SHOULD also list there, using the matchingRuleUse attribute, the
attribute types with which each matching rule can be used. More
information is given in section 4.4 of [5].
4.1.10. Result Message
The LDAPResult is the construct used in this protocol to return
success or failure indications from servers to clients. In response
to various requests servers will return responses containing fields
of type LDAPResult to indicate the final status of a protocol
operation request.
LDAPResult ::= SEQUENCE {
resultCode ENUMERATED {
success (0),
operationsError (1),
protocolError (2),
timeLimitExceeded (3),
sizeLimitExceeded (4),
compareFalse (5),
compareTrue (6),
authMethodNotSupported (7),
strongAuthRequired (8),
-- 9 reserved --
referral (10), -- new
adminLimitExceeded (11), -- new
unavailableCriticalExtension (12), -- new
confidentialityRequired (13), -- new
saslBindInProgress (14), -- new
noSuchAttribute (16),
undefinedAttributeType (17),
inappropriateMatching (18),
constraintViolation (19),
attributeOrValueExists (20),
invalidAttributeSyntax (21),
-- 22-31 unused --
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noSuchObject (32),
aliasProblem (33),
invalidDNSyntax (34),
-- 35 reserved for undefined isLeaf --
aliasDereferencingProblem (36),
-- 37-47 unused --
inappropriateAuthentication (48),
invalidCredentials (49),
insufficientAccessRights (50),
busy (51),
unavailable (52),
unwillingToPerform (53),
loopDetect (54),
-- 55-63 unused --
namingViolation (64),
objectClassViolation (65),
notAllowedOnNonLeaf (66),
notAllowedOnRDN (67),
entryAlreadyExists (68),
objectClassModsProhibited (69),
-- 70 reserved for CLDAP --
affectsMultipleDSAs (71), -- new
-- 72-79 unused --
other (80) },
-- 81-90 reserved for APIs --
matchedDN LDAPDN,
errorMessage LDAPString,
referral [3] Referral OPTIONAL }
All the result codes with the exception of success, compareFalse and
compareTrue are to be treated as meaning the operation could not be
completed in its entirety.
Most of the result codes are based on problem indications from X.511
error data types. Result codes from 16 to 21 indicate an
AttributeProblem, codes 32, 33, 34 and 36 indicate a NameProblem,
codes 48, 49 and 50 indicate a SecurityProblem, codes 51 to 54
indicate a ServiceProblem, and codes 64 to 69 and 71 indicates an
UpdateProblem.
If a client receives a result code which is not listed above, it is
to be treated as an unknown error condition.
The errorMessage field of this construct may, at the server’s option,
be used to return a string containing a textual, human-readable
(terminal control and page formatting characters should be avoided)
error diagnostic. As this error diagnostic is not standardized,
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implementations MUST NOT rely on the values returned. If the server
chooses not to return a textual diagnostic, the errorMessage field of
the LDAPResult type MUST contain a zero length string.
For result codes of noSuchObject, aliasProblem, invalidDNSyntax and
aliasDereferencingProblem, the matchedDN field is set to the name of
the lowest entry (object or alias) in the directory that was matched.
If no aliases were dereferenced while attempting to locate the entry,
this will be a truncated form of the name provided, or if aliases
were dereferenced, of the resulting name, as defined in section 12.5
of X.511 [8]. The matchedDN field is to be set to a zero length
string with all other result codes.
4.1.11. Referral
The referral error indicates that the contacted server does not hold
the target entry of the request. The referral field is present in an
LDAPResult if the LDAPResult.resultCode field value is referral, and
absent with all other result codes. It contains a reference to
another server (or set of servers) which may be accessed via LDAP or
other protocols. Referrals can be returned in response to any
operation request (except unbind and abandon which do not have
responses). At least one URL MUST be present in the Referral.
The referral is not returned for a singleLevel or wholeSubtree search
in which the search scope spans multiple naming contexts, and several
different servers would need to be contacted to complete the
operation. Instead, continuation references, described in section
4.5.3, are returned.
Referral ::= SEQUENCE OF LDAPURL -- one or more
LDAPURL ::= LDAPString -- limited to characters permitted in URLs
If the client wishes to progress the operation, it MUST follow the
referral by contacting any one of servers. All the URLs MUST be
equally capable of being used to progress the operation. (The
mechanisms for how this is achieved by multiple servers are outside
the scope of this document.)
URLs for servers implementing the LDAP protocol are written according
to [9]. If an alias was dereferenced, the <dn> part of the URL MUST
be present, with the new target object name. If the <dn> part is
present, the client MUST use this name in its next request to
progress the operation, and if it is not present the client will use
the same name as in the original request. Some servers (e.g.
participating in distributed indexing) may provide a different filter
in a referral for a search operation. If the filter part of the URL
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is present in an LDAPURL, the client MUST use this filter in its next
request to progress this search, and if it is not present the client
MUST use the same filter as it used for that search. Other aspects
of the new request may be the same or different as the request which
generated the referral.
Note that UTF-8 characters appearing in a DN or search filter may not
be legal for URLs (e.g. spaces) and MUST be escaped using the %
method in RFC 1738 [7].
Other kinds of URLs may be returned, so long as the operation could
be performed using that protocol.
4.1.12. Controls
A control is a way to specify extension information. Controls which
are sent as part of a request apply only to that request and are not
saved.
Controls ::= SEQUENCE OF Control
Control ::= SEQUENCE {
controlType LDAPOID,
criticality BOOLEAN DEFAULT FALSE,
controlValue OCTET STRING OPTIONAL }
The controlType field MUST be a UTF-8 encoded dotted-decimal
representation of an OBJECT IDENTIFIER which uniquely identifies the
control. This prevents conflicts between control names.
The criticality field is either TRUE or FALSE.
If the server recognizes the control type and it is appropriate for
the operation, the server will make use of the control when
performing the operation.
If the server does not recognize the control type and the criticality
field is TRUE, the server MUST NOT perform the operation, and MUST
instead return the resultCode unsupportedCriticalExtension.
If the control is not appropriate for the operation and criticality
field is TRUE, the server MUST NOT perform the operation, and MUST
instead return the resultCode unsupportedCriticalExtension.
If the control is unrecognized or inappropriate but the criticality
field is FALSE, the server MUST ignore the control.
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The controlValue contains any information associated with the
control, and its format is defined for the control. The server MUST
be prepared to handle arbitrary contents of the controlValue octet
string, including zero bytes. It is absent only if there is no value
information which is associated with a control of its type.
This document does not define any controls. Controls may be defined
in other documents. The definition of a control consists of:
- the OBJECT IDENTIFIER assigned to the control,
- whether the control is always noncritical, always critical, or
critical at the client’s option,
- the format of the controlValue contents of the control.
Servers list the controls which they recognize in the
supportedControl attribute in the root DSE.
4.2. Bind Operation
The function of the Bind Operation is to allow authentication
information to be exchanged between the client and server.
The Bind Request is defined as follows:
BindRequest ::= [APPLICATION 0] SEQUENCE {
version INTEGER (1 .. 127),
name LDAPDN,
authentication AuthenticationChoice }
AuthenticationChoice ::= CHOICE {
simple [0] OCTET STRING,
-- 1 and 2 reserved
sasl [3] SaslCredentials }
SaslCredentials ::= SEQUENCE {
mechanism LDAPString,
credentials OCTET STRING OPTIONAL }
Parameters of the Bind Request are:
- version: A version number indicating the version of the protocol to
be used in this protocol session. This document describes version
3 of the LDAP protocol. Note that there is no version negotiation,
and the client just sets this parameter to the version it desires.
If the client requests protocol version 2, a server that supports
the version 2 protocol as described in [2] will not return any v3-
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specific protocol fields. (Note that not all LDAP servers will
support protocol version 2, since they may be unable to generate
the attribute syntaxes associated with version 2.)
- name: The name of the directory object that the client wishes to
bind as. This field may take on a null value (a zero length
string) for the purposes of anonymous binds, when authentication
has been performed at a lower layer, or when using SASL credentials
with a mechanism that includes the LDAPDN in the credentials.
- authentication: information used to authenticate the name, if any,
provided in the Bind Request.
Upon receipt of a Bind Request, a protocol server will authenticate
the requesting client, if necessary. The server will then return a
Bind Response to the client indicating the status of the
authentication.
Authorization is the use of this authentication information when
performing operations. Authorization MAY be affected by factors
outside of the LDAP Bind request, such as lower layer security
services.
4.2.1. Sequencing of the Bind Request
For some SASL authentication mechanisms, it may be necessary for the
client to invoke the BindRequest multiple times. If at any stage the
client wishes to abort the bind process it MAY unbind and then drop
the underlying connection. Clients MUST NOT invoke operations
between two Bind requests made as part of a multi-stage bind.
A client may abort a SASL bind negotiation by sending a BindRequest
with a different value in the mechanism field of SaslCredentials, or
an AuthenticationChoice other than sasl.
If the client sends a BindRequest with the sasl mechanism field as an
empty string, the server MUST return a BindResponse with
authMethodNotSupported as the resultCode. This will allow clients to
abort a negotiation if it wishes to try again with the same SASL
mechanism.
Unlike LDAP v2, the client need not send a Bind Request in the first
PDU of the connection. The client may request any operations and the
server MUST treat these as unauthenticated. If the server requires
that the client bind before browsing or modifying the directory, the
server MAY reject a request other than binding, unbinding or an
extended request with the "operationsError" result.
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If the client did not bind before sending a request and receives an
operationsError, it may then send a Bind Request. If this also fails
or the client chooses not to bind on the existing connection, it will
close the connection, reopen it and begin again by first sending a
PDU with a Bind Request. This will aid in interoperating with
servers implementing other versions of LDAP.
Clients MAY send multiple bind requests on a connection to change
their credentials. A subsequent bind process has the effect of
abandoning all operations outstanding on the connection. (This
simplifies server implementation.) Authentication from earlier binds
are subsequently ignored, and so if the bind fails, the connection
will be treated as anonymous. If a SASL transfer encryption or
integrity mechanism has been negotiated, and that mechanism does not
support the changing of credentials from one identity to another,
then the client MUST instead establish a new connection.
4.2.2. Authentication and Other Security Services
The simple authentication option provides minimal authentication
facilities, with the contents of the authentication field consisting
only of a cleartext password. Note that the use of cleartext
passwords is not recommended over open networks when there is no
authentication or encryption being performed by a lower layer; see
the "Security Considerations" section.
If no authentication is to be performed, then the simple
authentication option MUST be chosen, and the password be of zero
length. (This is often done by LDAPv2 clients.) Typically the DN is
also of zero length.
The sasl choice allows for any mechanism defined for use with SASL
[12]. The mechanism field contains the name of the mechanism. The
credentials field contains the arbitrary data used for
authentication, inside an OCTET STRING wrapper. Note that unlike
some Internet application protocols where SASL is used, LDAP is not
text-based, thus no base64 transformations are performed on the
credentials.
If any SASL-based integrity or confidentiality services are enabled,
they take effect following the transmission by the server and
reception by the client of the final BindResponse with resultCode
success.
The client can request that the server use authentication information
from a lower layer protocol by using the SASL EXTERNAL mechanism.
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4.2.3. Bind Response
The Bind Response is defined as follows.
BindResponse ::= [APPLICATION 1] SEQUENCE {
COMPONENTS OF LDAPResult,
serverSaslCreds [7] OCTET STRING OPTIONAL }
BindResponse consists simply of an indication from the server of he
status of the client’s request for authentication.
f the bind was successful, the resultCode will be success, therwise
it will be one of:
- operationsError: server encountered an internal error,
- protocolError: unrecognized version number or incorrect PDU
structure,
- authMethodNotSupported: unrecognized SASL mechanism name,
- strongAuthRequired: the server requires authentication be
performed with a SASL mechanism,
- referral: this server cannot accept this bind and the client
should try another,
- saslBindInProgress: the server requires the client to send a
new bind request, with the same sasl mechanism, to continue the
authentication process,
- inappropriateAuthentication: the server requires the client
which had attempted to bind anonymously or without supplying
credentials to provide some form of credentials,
- invalidCredentials: the wrong password was supplied or the SASL
credentials could not be processed,
- unavailable: the server is shutting down.
If the server does not support the client’s requested protocol
version, it MUST set the resultCode to protocolError.
If the client receives a BindResponse response where the resultCode
was protocolError, it MUST close the connection as the server will be
unwilling to accept further operations. (This is for compatibility
with earlier versions of LDAP, in which the bind was always the first
operation, and there was no negotiation.)
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The serverSaslCreds are used as part of a SASL-defined bind mechanism
to allow the client to authenticate the server to which it is
communicating, or to perform "challenge-response" authentication. If
the client bound with the password choice, or the SASL mechanism does
not require the server to return information to the client, then this
field is not to be included in the result.
4.3. Unbind Operation
The function of the Unbind Operation is to terminate a protocol
session. The Unbind Operation is defined as follows:
UnbindRequest ::= [APPLICATION 2] NULL
The Unbind Operation has no response defined. Upon transmission of an
UnbindRequest, a protocol client may assume that the protocol session
is terminated. Upon receipt of an UnbindRequest, a protocol server
may assume that the requesting client has terminated the session and
that all outstanding requests may be discarded, and may close the
connection.
4.4. Unsolicited Notification
An unsolicited notification is an LDAPMessage sent from the server to
the client which is not in response to any LDAPMessage received by
the server. It is used to signal an extraordinary condition in the
server or in the connection between the client and the server. The
notification is of an advisory nature, and the server will not expect
any response to be returned from the client.
The unsolicited notification is structured as an LDAPMessage in which
the messageID is 0 and protocolOp is of the extendedResp form. The
responseName field of the ExtendedResponse is present. The LDAPOID
value MUST be unique for this notification, and not be used in any
other situation.
One unsolicited notification is defined in this document.
4.4.1. Notice of Disconnection
This notification may be used by the server to advise the client that
the server is about to close the connection due to an error
condition. Note that this notification is NOT a response to an
unbind requested by the client: the server MUST follow the procedures
of section 4.3. This notification is intended to assist clients in
distinguishing between an error condition and a transient network
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failure. As with a connection close due to network failure, the
client MUST NOT assume that any outstanding requests which modified
the directory have succeeded or failed.
The responseName is 1.3.6.1.4.1.1466.20036, the response field is
absent, and the resultCode is used to indicate the reason for the
disconnection.
The following resultCode values are to be used in this notification:
- protocolError: The server has received data from the client in
which
the LDAPMessage structure could not be parsed.
- strongAuthRequired: The server has detected that an established
underlying security association protecting communication between
the client and server has unexpectedly failed or been compromised.
- unavailable: This server will stop accepting new connections and
operations on all existing connections, and be unavailable for an
extended period of time. The client may make use of an alternative
server.
After sending this notice, the server MUST close the connection.
After receiving this notice, the client MUST NOT transmit any further
on the connection, and may abruptly close the connection.
4.5. Search Operation
The Search Operation allows a client to request that a search be
performed on its behalf by a server. This can be used to read
attributes from a single entry, from entries immediately below a
particular entry, or a whole subtree of entries.
4.5.1. Search Request
The Search Request is defined as follows:
SearchRequest ::= [APPLICATION 3] SEQUENCE {
baseObject LDAPDN,
scope ENUMERATED {
baseObject (0),
singleLevel (1),
wholeSubtree (2) },
derefAliases ENUMERATED {
neverDerefAliases (0),
derefInSearching (1),
derefFindingBaseObj (2),
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derefAlways (3) },
sizeLimit INTEGER (0 .. maxInt),
timeLimit INTEGER (0 .. maxInt),
typesOnly BOOLEAN,
filter Filter,
attributes AttributeDescriptionList }
Filter ::= CHOICE {
and [0] SET OF Filter,
or [1] SET OF Filter,
not [2] Filter,
equalityMatch [3] AttributeValueAssertion,
substrings [4] SubstringFilter,
greaterOrEqual [5] AttributeValueAssertion,
lessOrEqual [6] AttributeValueAssertion,
present [7] AttributeDescription,
approxMatch [8] AttributeValueAssertion,
extensibleMatch [9] MatchingRuleAssertion }
SubstringFilter ::= SEQUENCE {
type AttributeDescription,
-- at least one must be present
substrings SEQUENCE OF CHOICE {
initial [0] LDAPString,
any [1] LDAPString,
final [2] LDAPString } }
MatchingRuleAssertion ::= SEQUENCE {
matchingRule [1] MatchingRuleId OPTIONAL,
type [2] AttributeDescription OPTIONAL,
matchValue [3] AssertionValue,
dnAttributes [4] BOOLEAN DEFAULT FALSE }
Parameters of the Search Request are:
- baseObject: An LDAPDN that is the base object entry relative to
which the search is to be performed.
- scope: An indicator of the scope of the search to be performed. The
semantics of the possible values of this field are identical to the
semantics of the scope field in the X.511 Search Operation.
- derefAliases: An indicator as to how alias objects (as defined in
X.501) are to be handled in searching. The semantics of the
possible values of this field are:
neverDerefAliases: do not dereference aliases in searching
or in locating the base object of the search;
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derefInSearching: dereference aliases in subordinates of
the base object in searching, but not in locating the
base object of the search;
derefFindingBaseObj: dereference aliases in locating
the base object of the search, but not when searching
subordinates of the base object;
derefAlways: dereference aliases both in searching and in
locating the base object of the search.
- sizelimit: A sizelimit that restricts the maximum number of entries
to be returned as a result of the search. A value of 0 in this
field indicates that no client-requested sizelimit restrictions are
in effect for the search. Servers may enforce a maximum number of
entries to return.
- timelimit: A timelimit that restricts the maximum time (in seconds)
allowed for a search. A value of 0 in this field indicates that no
client-requested timelimit restrictions are in effect for the
search.
- 
