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)
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.
Wahl, et. al. Standards Track [Page 1]
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 .............................................. 12. Abstract ............................................... 33. Models ................................................. 43.1. Protocol Model ........................................ 43.2. Data Model ............................................ 53.2.1. Attributes of Entries ............................... 53.2.2. Subschema Entries and Subentries .................... 73.3. Relationship to X.500 ................................. 83.4. Server-specific Data Requirements ..................... 84. Elements of Protocol ................................... 94.1. Common Elements ....................................... 94.1.1. Message Envelope .................................... 94.1.1.1. Message ID ........................................ 114.1.2. String Types ........................................ 114.1.3. Distinguished Name and Relative Distinguished Name .. 11
4.1.4. Attribute Type ...................................... 124.1.5. Attribute Description ............................... 134.1.5.1. Binary Option ..................................... 144.1.6. Attribute Value ..................................... 144.1.7. Attribute Value Assertion ........................... 154.1.8. Attribute ........................................... 154.1.9. Matching Rule Identifier ............................ 154.1.10. Result Message ..................................... 164.1.11. Referral ........................................... 184.1.12. Controls ........................................... 194.2. Bind Operation ........................................ 204.2.1. Sequencing of the Bind Request ...................... 214.2.2. Authentication and Other Security Services .......... 224.2.3. Bind Response ....................................... 234.3. Unbind Operation ...................................... 244.4. Unsolicited Notification .............................. 244.4.1. Notice of Disconnection ............................. 244.5. Search Operation ...................................... 25
Wahl, et. al. Standards Track [Page 2]
RFC 2251 LDAPv3 December 1997
4.5.1. Search Request ...................................... 254.5.2. Search Result ....................................... 294.5.3. Continuation References in the Search Result ........ 314.5.3.1. Example ........................................... 314.6. Modify Operation ...................................... 324.7. Add Operation ......................................... 344.8. Delete Operation ...................................... 354.9. Modify DN Operation ................................... 364.10. Compare Operation .................................... 374.11. Abandon Operation .................................... 384.12. Extended Operation ................................... 385. Protocol Element Encodings and Transfer ................ 395.1. Mapping Onto BER-based Transport Services ............. 395.2. Transfer Protocols .................................... 405.2.1. Transmission Control Protocol (TCP) ................. 406. Implementation Guidelines .............................. 406.1. Server Implementations ................................ 406.2. Client Implementations ................................ 407. Security Considerations ................................ 418. Acknowledgements ....................................... 419. Bibliography ........................................... 4110. Authors' Addresses ..................................... 42
Appendix A - Complete ASN.1 Definition ..................... 44
Full Copyright Statement ................................... 50
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).
Wahl, et. al. Standards Track [Page 3]
RFC 2251 LDAPv3 December 1997
- 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.
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.
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.
Wahl, et. al. Standards Track [Page 4]
RFC 2251 LDAPv3 December 1997
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.
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).
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
Wahl, et. al. Standards Track [Page 5]
RFC 2251 LDAPv3 December 1997
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:
Wahl, et. al. Standards Track [Page 6]
RFC 2251 LDAPv3 December 1997
- 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.
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.
Wahl, et. al. Standards Track [Page 7]
RFC 2251 LDAPv3 December 1997
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.)
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.
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.
Wahl, et. al. Standards Track [Page 8]
RFC 2251 LDAPv3 December 1997
- 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.
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.
This section describes the LDAPMessage envelope PDU (Protocol Data
Unit) format, as well as data type definitions which are used in the
protocol operations.
For the purposes of protocol exchanges, all protocol operations are
encapsulated in a common envelope, the LDAPMessage, which is defined
as follows:
LDAPMessage ::= SEQUENCE {
Wahl, et. al. Standards Track [Page 9]
RFC 2251 LDAPv3 December 1997
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.
Wahl, et. al. Standards Track [Page 10]
RFC 2251 LDAPv3 December 1997
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.
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
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
Wahl, et. al. Standards Track [Page 11]
RFC 2251 LDAPv3 December 1997
<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.
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.
Wahl, et. al. Standards Track [Page 12]
RFC 2251 LDAPv3 December 1997
Some attribute type names which are used in this version of LDAP are
described in [5]. Servers may implement additional attribute types.
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
Wahl, et. al. Standards Track [Page 13]
RFC 2251 LDAPv3 December 1997
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".
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.
Wahl, et. al. Standards Track [Page 14]
RFC 2251 LDAPv3 December 1997
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.
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.
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.
Wahl, et. al. Standards Track [Page 15]
RFC 2251 LDAPv3 December 1997
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].
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 --
Wahl, et. al. Standards Track [Page 16]
RFC 2251 LDAPv3 December 1997
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,
Wahl, et. al. Standards Track [Page 17]
RFC 2251 LDAPv3 December 1997
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.
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
Wahl, et. al. Standards Track [Page 18]
RFC 2251 LDAPv3 December 1997
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.
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.
Wahl, et. al. Standards Track [Page 19]
RFC 2251 LDAPv3 December 1997
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.
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-
Wahl, et. al. Standards Track [Page 20]
RFC 2251 LDAPv3 December 1997
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.
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.
Wahl, et. al. Standards Track [Page 21]
RFC 2251 LDAPv3 December 1997
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.
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.
Wahl, et. al. Standards Track [Page 22]
RFC 2251 LDAPv3 December 1997
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.)
Wahl, et. al. Standards Track [Page 23]
RFC 2251 LDAPv3 December 1997
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.
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.
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.
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
Wahl, et. al. Standards Track [Page 24]
RFC 2251 LDAPv3 December 1997
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.
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.
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),
Wahl, et. al. Standards Track [Page 25]
RFC 2251 LDAPv3 December 1997
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;
Wahl, et. al. Standards Track [Page 26]
RFC 2251 LDAPv3 December 1997
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.
- typesOnly: An indicator as to whether search results will contain
both attribute types and values, or just attribute types. Setting
this field to TRUE causes only attribute types (no values) to be
returned. Setting this field to FALSE causes both attribute types
and values to be returned.
- filter: A filter that defines the conditions that must be fulfilled
in order for the search to match a given entry.
The 'and', 'or' and 'not' choices can be used to form combinations of
filters. At least one filter element MUST be present in an 'and' or
'or' choice. The others match against individual attribute values of
entries in the scope of the search. (Implementor's note: the 'not'
filter is an example of a tagged choice in an implicitly-tagged
module. In BER this is treated as if the tag was explicit.)
A server MUST evaluate filters according to the three-valued logic
of X.511(93) section 7.8.1. In summary, a filter is evaluated to
either "TRUE", "FALSE" or "Undefined". If the filter evaluates
to TRUE for a particular entry, then the attributes of that entry
are returned as part of the search result (subject to any applicable
access control restrictions). If the filter evaluates to FALSE or
Undefined, then the entry is ignored for the search.
Wahl, et. al. Standards Track [Page 27]
RFC 2251 LDAPv3 December 1997
A filter of the "and" choice is TRUE if all the filters in the SET
OF evaluate to TRUE, FALSE if at least one filter is FALSE, and
otherwise Undefined. A filter of the "or" choice is FALSE if all
of the filters in the SET OF evaluate to FALSE, TRUE if at least
one filter is TRUE, and Undefined otherwise. A filter of the "not"
choice is TRUE if the filter being negated is FALSE, FALSE if it is
TRUE, and Undefined if it is Undefined.
The present match evaluates to TRUE where there is an attribute or
subtype of the specified attribute description present in an entry,
and FALSE otherwise (including a presence test with an unrecognized
attribute description.)
The extensibleMatch is new in this version of LDAP. If the
matchingRule field is absent, the type field MUST be present, and
the equality match is performed for that type. If the type field is
absent and matchingRule is present, the matchValue is compared
against all attributes in an entry which support that matchingRule,
and the matchingRule determines the syntax for the assertion value
(the filter item evaluates to TRUE if it matches with at least
one attribute in the entry, FALSE if it does not match any attribute
in the entry, and Undefined if the matchingRule is not recognized
or the assertionValue cannot be parsed.) If the type field is
present and matchingRule is present, the matchingRule MUST be one
permitted for use with that type, otherwise the filter item is
undefined. If the dnAttributes field is set to TRUE, the match is
applied against all the attributes in an entry's distinguished name
as well, and also evaluates to TRUE if there is at least one
attribute in the distinguished name for which the filter item
evaluates to TRUE. (Editors note: The dnAttributes field is present
so that there does not need to be multiple versions of generic
matching rules such as for word matching, one to apply to entries
and another to apply to entries and dn attributes as well).
A filter item evaluates to Undefined when the server would not
be able to determine whether the assertion value matches an
entry. If an attribute description in an equalityMatch, substrings,
greaterOrEqual, lessOrEqual, approxMatch or extensibleMatch
filter is not recognized by the server, a matching rule id in the
extensibleMatch is not recognized by the server, the assertion
value cannot be parsed, or the type of filtering requested is not
implemented, then the filter is Undefined. Thus for example if a
server did not recognize the attribute type shoeSize, a filter of
(shoeSize=*) would evaluate to FALSE, and the filters (shoeSize=12),
(shoeSize>=12) and (shoeSize<=12) would evaluate to Undefined.
Wahl, et. al. Standards Track [Page 28]
RFC 2251 LDAPv3 December 1997
Servers MUST NOT return errors if attribute descriptions or matching
rule ids are not recognized, or assertion values cannot be parsed.
More details of filter processing are given in section 7.8 of X.511
[8].
- attributes: A list of the attributes to be returned from each entry
which matches the search filter. There are two special values which
may be used: an empty list with no attributes, and the attribute
description string "*". Both of these signify that all user
attributes are to be returned. (The "*" allows the client to
request all user attributes in addition to specific operational
attributes).
Attributes MUST be named at most once in the list, and are returned
at most once in an entry. If there are attribute descriptions in
the list which are not recognized, they are ignored by the server.
If the client does not want any attributes returned, it can specify
a list containing only the attribute with OID "1.1". This OID was
chosen arbitrarily and does not correspond to any attribute in use.
Client implementors should note that even if all user attributes are
requested, some attributes of the entry may not be included in
search results due to access control or other restrictions.
Furthermore, servers will not return operational attributes, such
as objectClasses or attributeTypes, unless they are listed by name,
since there may be extremely large number of values for certain
operational attributes. (A list of operational attributes for use
in LDAP is given in [5].)
Note that an X.500 "list"-like operation can be emulated by the client
requesting a one-level LDAP search operation with a filter checking
for the existence of the objectClass attribute, and that an X.500
"read"-like operation can be emulated by a base object LDAP search
operation with the same filter. A server which provides a gateway to
X.500 is not required to use the Read or List operations, although it
may choose to do so, and if it does must provide the same semantics
as the X.500 search operation.
The results of the search attempted by the server upon receipt of a
Search Request are returned in Search Responses, which are LDAP
messages containing either SearchResultEntry, SearchResultReference,
ExtendedResponse or SearchResultDone data types.
SearchResultEntry ::= [APPLICATION 4] SEQUENCE {
objectName LDAPDN,
Wahl, et. al. Standards Track [Page 29]
RFC 2251 LDAPv3 December 1997
attributes PartialAttributeList }
PartialAttributeList ::= SEQUENCE OF SEQUENCE {
type AttributeDescription,
vals SET OF AttributeValue }
-- implementors should note that the PartialAttributeList may
-- have zero elements (if none of the attributes of that entry
-- were requested, or could be returned), and that the vals set
-- may also have zero elements (if types only was requested, or
-- all values were excluded from the result.)
SearchResultReference ::= [APPLICATION 19] SEQUENCE OF LDAPURL
-- at least one LDAPURL element must be present
SearchResultDone ::= [APPLICATION 5] LDAPResult
Upon receipt of a Search Request, a server will perform the necessary
search of the DIT.
If the LDAP session is operating over a connection-oriented transport
such as TCP, the server will return to the client a sequence of
responses in separate LDAP messages. There may be zero or more
responses containing SearchResultEntry, one for each entry found
during the search. There may also be zero or more responses
containing SearchResultReference, one for each area not explored by
this server during the search. The SearchResultEntry and
SearchResultReference PDUs may come in any order. Following all the
SearchResultReference responses and all SearchResultEntry responses
to be returned by the server, the server will return a response
containing the SearchResultDone, which contains an indication of
success, or detailing any errors that have occurred.
Each entry returned in a SearchResultEntry will contain all
attributes, complete with associated values if necessary, as
specified in the attributes field of the Search Request. Return of
attributes is subject to access control and other administrative
policy. Some attributes may be returned in binary format (indicated
by the AttributeDescription in the response having the binary option
present).
Some attributes may be constructed by the server and appear in a
SearchResultEntry attribute list, although they are not stored
attributes of an entry. Clients MUST NOT assume that all attributes
can be modified, even if permitted by access control.
LDAPMessage responses of the ExtendedResponse form are reserved for
returning information associated with a control requested by the
client. These may be defined in future versions of this document.
Wahl, et. al. Standards Track [Page 30]
RFC 2251 LDAPv3 December 1997
If the server was able to locate the entry referred to by the
baseObject but was unable to search all the entries in the scope at
and under the baseObject, the server may return one or more
SearchResultReference, each containing a reference to another set of
servers for continuing the operation. A server MUST NOT return any
SearchResultReference if it has not located the baseObject and
thus has not searched any entries; in this case it would return a
SearchResultDone containing a referral resultCode.
In the absence of indexing information provided to a server from
servers holding subordinate naming contexts, SearchResultReference
responses are not affected by search filters and are always returned
when in scope.
The SearchResultReference is of the same data type as the Referral.
URLs for servers implementing the LDAP protocol are written according
to [9]. The <dn> part MUST be present in the URL, with the new target
object name. The client MUST use this name in its next request.
Some servers (e.g. part of a distributed index exchange system) may
provide a different filter in the URLs of the SearchResultReference.
If the filter part of the URL is present in an LDAP URL, the client
MUST use the new filter in its next request to progress the search,
and if the filter part is absent the client will use again the same
filter. Other aspects of the new search request may be the same or
different as the search which generated the continuation references.
Other kinds of URLs may be returned so long as the operation could be
performed using that protocol.
The name of an unexplored subtree in a SearchResultReference need not
be subordinate to the base object.
In order to complete the search, the client MUST issue a new search
operation for each SearchResultReference that is returned. Note that
the abandon operation described in section 4.11 applies only to a
particular operation sent on a connection between a client and server,
and if the client has multiple outstanding search operations to
different servers, it MUST abandon each operation individually.
For example, suppose the contacted server (hosta) holds the entry
"O=MNN,C=WW" and the entry "CN=Manager,O=MNN,C=WW". It knows that
either LDAP-capable servers (hostb) or (hostc) hold
"OU=People,O=MNN,C=WW" (one is the master and the other server a
Wahl, et. al. Standards Track [Page 31]
RFC 2251 LDAPv3 December 1997
shadow), and that LDAP-capable server (hostd) holds the subtree
"OU=Roles,O=MNN,C=WW". If a subtree search of "O=MNN,C=WW" is
requested to the contacted server, it may return the following:
SearchResultEntry for O=MNN,C=WW
SearchResultEntry for CN=Manager,O=MNN,C=WW
SearchResultReference {
ldap://hostb/OU=People,O=MNN,C=WW
ldap://hostc/OU=People,O=MNN,C=WW
}
SearchResultReference {
ldap://hostd/OU=Roles,O=MNN,C=WW
}
SearchResultDone (success)
Client implementors should note that when following a
SearchResultReference, additional SearchResultReference may be
generated. Continuing the example, if the client contacted the
server (hostb) and issued the search for the subtree
"OU=People,O=MNN,C=WW", the server might respond as follows:
SearchResultEntry for OU=People,O=MNN,C=WW
SearchResultReference {
ldap://hoste/OU=Managers,OU=People,O=MNN,C=WW
}
SearchResultReference {
ldap://hostf/OU=Consultants,OU=People,O=MNN,C=WW
}
SearchResultDone (success)
If the contacted server does not hold the base object for the search,
then it will return a referral to the client. For example, if the
client requests a subtree search of "O=XYZ,C=US" to hosta, the server
may return only a SearchResultDone containing a referral.
SearchResultDone (referral) {
ldap://hostg/
}
The Modify Operation allows a client to request that a modification
of an entry be performed on its behalf by a server. The Modify
Request is defined as follows:
ModifyRequest ::= [APPLICATION 6] SEQUENCE {
object LDAPDN,
modification SEQUENCE OF SEQUENCE {
Wahl, et. al. Standards Track [Page 32]
RFC 2251 LDAPv3 December 1997
operation ENUMERATED {
add (0),
delete (1),
replace (2) },
modification AttributeTypeAndValues } }
AttributeTypeAndValues ::= SEQUENCE {
type AttributeDescription,
vals SET OF AttributeValue }
Parameters of the Modify Request are:
- object: The object to be modified. The value of this field contains
the DN of the entry to be modified. The server will not perform
any alias dereferencing in determining the object to be modified.
- modification: A list of modifications to be performed on the entry.
The entire list of entry modifications MUST be performed
in the order they are listed, as a single atomic operation. While
individual modifications may violate the directory schema, the
resulting entry after the entire list of modifications is performed
MUST conform to the requirements of the directory schema. The
values that may be taken on by the 'operation' field in each
modification construct have the following semantics respectively:
add: add values listed to the given attribute, creating
the attribute if necessary;
delete: delete values listed from the given attribute,
removing the entire attribute if no values are listed, or
if all current values of the attribute are listed for
deletion;
replace: replace all existing values of the given attribute
with the new values listed, creating the attribute if it
did not already exist. A replace with no value will delete
the entire attribute if it exists, and is ignored if the
attribute does not exist.
The result of the modify attempted by the server upon receipt of a
Modify Request is returned in a Modify Response, defined as follows:
ModifyResponse ::= [APPLICATION 7] LDAPResult
Upon receipt of a Modify Request, a server will perform the necessary
modifications to the DIT.
Wahl, et. al. Standards Track [Page 33]
RFC 2251 LDAPv3 December 1997
The server will return to the client a single Modify Response
indicating either the successful completion of the DIT modification,
or the reason that the modification failed. Note that due to the
requirement for atomicity in applying the list of modifications in
the Modify Request, the client may expect that no modifications of
the DIT have been performed if the Modify Response received indicates
any sort of error, and that all requested modifications have been
performed if the Modify Response indicates successful completion of
the Modify Operation. If the connection fails, whether the
modification occurred or not is indeterminate.
The Modify Operation cannot be used to remove from an entry any of
its distinguished values, those values which form the entry's
relative distinguished name. An attempt to do so will result in the
server returning the error notAllowedOnRDN. The Modify DN Operation
described in section 4.9 is used to rename an entry.
If an equality match filter has not been defined for an attribute type,
clients MUST NOT attempt to delete individual values of that attribute
from an entry using the "delete" form of a modification, and MUST
instead use the "replace" form.
Note that due to the simplifications made in LDAP, there is not a
direct mapping of the modifications in an LDAP ModifyRequest onto the
EntryModifications of a DAP ModifyEntry operation, and different
implementations of LDAP-DAP gateways may use different means of
representing the change. If successful, the final effect of the
operations on the entry MUST be identical.
The Add Operation allows a client to request the addition of an entry
into the directory. The Add Request is defined as follows:
AddRequest ::= [APPLICATION 8] SEQUENCE {
entry LDAPDN,
attributes AttributeList }
AttributeList ::= SEQUENCE OF SEQUENCE {
type AttributeDescription,
vals SET OF AttributeValue }
Parameters of the Add Request are:
- entry: the Distinguished Name of the entry to be added. Note that
the server will not dereference any aliases in locating the entry
to be added.
Wahl, et. al. Standards Track [Page 34]
RFC 2251 LDAPv3 December 1997
- attributes: the list of attributes that make up the content of the
entry being added. Clients MUST include distinguished values
(those forming the entry's own RDN) in this list, the objectClass
attribute, and values of any mandatory attributes of the listed
object classes. Clients MUST NOT supply the createTimestamp or
creatorsName attributes, since these will be generated
automatically by the server.
The entry named in the entry field of the AddRequest MUST NOT exist
for the AddRequest to succeed. The parent of the entry to be added
MUST exist. For example, if the client attempted to add
"CN=JS,O=Foo,C=US", the "O=Foo,C=US" entry did not exist, and the
"C=US" entry did exist, then the server would return the error
noSuchObject with the matchedDN field containing "C=US". If the
parent entry exists but is not in a naming context held by the
server, the server SHOULD return a referral to the server holding the
parent entry.
Servers implementations SHOULD NOT restrict where entries can be
located in the directory. Some servers MAY allow the administrator
to restrict the classes of entries which can be added to the
directory.
Upon receipt of an Add Request, a server will attempt to perform the
add requested. The result of the add attempt will be returned to the
client in the Add Response, defined as follows:
AddResponse ::= [APPLICATION 9] LDAPResult
A response of success indicates that the new entry is present in the
directory.
The Delete Operation allows a client to request the removal of an
entry from the directory. The Delete Request is defined as follows:
DelRequest ::= [APPLICATION 10] LDAPDN
The Delete Request consists of the Distinguished Name of the entry to
be deleted. Note that the server will not dereference aliases while
resolving the name of the target entry to be removed, and that only
leaf entries (those with no subordinate entries) can be deleted with
this operation.
The result of the delete attempted by the server upon receipt of a
Delete Request is returned in the Delete Response, defined as
follows:
Wahl, et. al. Standards Track [Page 35]
RFC 2251 LDAPv3 December 1997
DelResponse ::= [APPLICATION 11] LDAPResult
Upon receipt of a Delete Request, a server will attempt to perform
the entry removal requested. The result of the delete attempt will be
returned to the client in the Delete Response.
The Modify DN Operation allows a client to change the leftmost (least
significant) component of the name of an entry in the directory, or
to move a subtree of entries to a new location in the directory. The
Modify DN Request is defined as follows:
ModifyDNRequest ::= [APPLICATION 12] SEQUENCE {
entry LDAPDN,
newrdn RelativeLDAPDN,
deleteoldrdn BOOLEAN,
newSuperior [0] LDAPDN OPTIONAL }
Parameters of the Modify DN Request are:
- entry: the Distinguished Name of the entry to be changed. This
entry may or may not have subordinate entries.
- newrdn: the RDN that will form the leftmost component of the new
name of the entry.
- deleteoldrdn: a boolean parameter that controls whether the old RDN
attribute values are to be retained as attributes of the entry, or
deleted from the entry.
- newSuperior: if present, this is the Distinguished Name of the entry
which becomes the immediate superior of the existing entry.
The result of the name change attempted by the server upon receipt of
a Modify DN Request is returned in the Modify DN Response, defined
as follows:
ModifyDNResponse ::= [APPLICATION 13] LDAPResult
Upon receipt of a ModifyDNRequest, a server will attempt to
perform the name change. The result of the name change attempt will
be returned to the client in the Modify DN Response.
For example, if the entry named in the "entry" parameter was
"cn=John Smith,c=US", the newrdn parameter was "cn=John Cougar Smith",
and the newSuperior parameter was absent, then this operation would
Wahl, et. al. Standards Track [Page 36]
RFC 2251 LDAPv3 December 1997
attempt to rename the entry to be "cn=John Cougar Smith,c=US". If
there was already an entry with that name, the operation would fail
with error code entryAlreadyExists.
If the deleteoldrdn parameter is TRUE, the values forming the old
RDN are deleted from the entry. If the deleteoldrdn parameter is
FALSE, the values forming the old RDN will be retained as
non-distinguished attribute values of the entry. The server may
not perform the operation and return an error code if the setting of
the deleteoldrdn parameter would cause a schema inconsistency in the
entry.
Note that X.500 restricts the ModifyDN operation to only affect
entries that are contained within a single server. If the LDAP
server is mapped onto DAP, then this restriction will apply, and the
resultCode affectsMultipleDSAs will be returned if this error
occurred. In general clients MUST NOT expect to be able to perform
arbitrary movements of entries and subtrees between servers.
The Compare Operation allows a client to compare an assertion
provided with an entry in the directory. The Compare Request is
defined as follows:
CompareRequest ::= [APPLICATION 14] SEQUENCE {
entry LDAPDN,
ava AttributeValueAssertion }
Parameters of the Compare Request are:
- entry: the name of the entry to be compared with.
- ava: the assertion with which an attribute in the entry is to be
compared.
The result of the compare attempted by the server upon receipt of a
Compare Request is returned in the Compare Response, defined as
follows:
CompareResponse ::= [APPLICATION 15] LDAPResult
Upon receipt of a Compare Request, a server will attempt to perform
the requested comparison. The result of the comparison will be
returned to the client in the Compare Response. Note that errors and
the result of comparison are all returned in the same construct.
Wahl, et. al. Standards Track [Page 37]
RFC 2251 LDAPv3 December 1997
Note that some directory systems may establish access controls which
permit the values of certain attributes (such as userPassword) to be
compared but not read. In a search result, it may be that an
attribute of that type would be returned, but with an empty set of
values.
The function of the Abandon Operation is to allow a client to request
that the server abandon an outstanding operation. The Abandon
Request is defined as follows:
AbandonRequest ::= [APPLICATION 16] MessageID
The MessageID MUST be that of a an operation which was requested
earlier in this connection.
(The abandon request itself has its own message id. This is distinct
from the id of the earlier operation being abandoned.)
There is no response defined in the Abandon Operation. Upon
transmission of an Abandon Operation, a client may expect that the
operation identified by the Message ID in the Abandon Request has
been abandoned. In the event that a server receives an Abandon
Request on a Search Operation in the midst of transmitting responses
to the search, that server MUST cease transmitting entry responses to
the abandoned request immediately, and MUST NOT send the
SearchResponseDone. Of course, the server MUST ensure that only
properly encoded LDAPMessage PDUs are transmitted.
Clients MUST NOT send abandon requests for the same operation
multiple times, and MUST also be prepared to receive results from
operations it has abandoned (since these may have been in transit
when the abandon was requested).
Servers MUST discard abandon requests for message IDs they do not
recognize, for operations which cannot be abandoned, and for
operations which have already been abandoned.
An extension mechanism has been added in this version of LDAP, in
order to allow additional operations to be defined for services not
available elsewhere in this protocol, for instance digitally signed
operations and results.
Wahl, et. al. Standards Track [Page 38]
RFC 2251 LDAPv3 December 1997
The extended operation allows clients to make requests and receive
responses with predefined syntaxes and semantics. These may be
defined in RFCs or be private to particular implementations. Each
request MUST have a unique OBJECT IDENTIFIER assigned to it.
ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
requestName [0] LDAPOID,
requestValue [1] OCTET STRING OPTIONAL }
The requestName is a dotted-decimal representation of the OBJECT
IDENTIFIER corresponding to the request. The requestValue is
information in a form defined by that request, encapsulated inside an
OCTET STRING.
The server will respond to this with an LDAPMessage containing the
ExtendedResponse.
ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
COMPONENTS OF LDAPResult,
responseName [10] LDAPOID OPTIONAL,
response [11] OCTET STRING OPTIONAL }
If the server does not recognize the request name, it MUST return
only the response fields from LDAPResult, containing the
protocolError result code.
The protocol elements of LDAP are encoded for exchange using the
Basic Encoding Rules (BER) [11] of ASN.1 [3]. However, due to the
high overhead involved in using certain elements of the BER, the
following additional restrictions are placed on BER-encodings of LDAP
protocol elements:
(1) Only the definite form of length encoding will be used.
(2) OCTET STRING values will be encoded in the primitive form only.
(3) If the value of a BOOLEAN type is true, the encoding MUST have
its contents octets set to hex "FF".
Wahl, et. al. Standards Track [Page 39]
RFC 2251 LDAPv3 December 1997
(4) If a value of a type is its default value, it MUST be absent.
Only some BOOLEAN and INTEGER types have default values in this
protocol definition.
These restrictions do not apply to ASN.1 types encapsulated inside of
OCTET STRING values, such as attribute values, unless otherwise
noted.
The LDAPMessage PDUs are mapped directly onto the TCP bytestream. It
is recommended that server implementations running over the TCP MAY
provide a protocol listener on the assigned port, 389. Servers may
instead provide a listener on a different port number. Clients MUST
support contacting servers on any valid TCP port.
The server MUST be capable of recognizing all the mandatory attribute
type names and implement the syntaxes specified in [5]. Servers MAY
also recognize additional attribute type names.
Clients which request referrals MUST ensure that they do not loop
between servers. They MUST NOT repeatedly contact the same server for
the same request with the same target entry name, scope and filter.
Some clients may be using a counter that is incremented each time
referral handling occurs for an operation, and these kinds of clients
MUST be able to handle a DIT with at least ten layers of naming
contexts between the root and a leaf entry.
In the absence of prior agreements with servers, clients SHOULD NOT
assume that servers support any particular schemas beyond those
referenced in section 6.1. Different schemas can have different
attribute types with the same names. The client can retrieve the
subschema entries referenced by the subschemaSubentry attribute in
the server's root DSE or in entries held by the server.
Wahl, et. al. Standards Track [Page 40]
RFC 2251 LDAPv3 December 1997
When used with a connection-oriented transport, this version of the
protocol provides facilities for the LDAP v2 authentication
mechanism, simple authentication using a cleartext password, as well
as any SASL mechanism [12]. SASL allows for integrity and privacy
services to be negotiated.
It is also permitted that the server can return its credentials to
the client, if it chooses to do so.
Use of cleartext password is strongly discouraged where the
underlying transport service cannot guarantee confidentiality and may
result in disclosure of the password to unauthorized parties.
When used with SASL, it should be noted that the name field of the
BindRequest is not protected against modification. Thus if the
distinguished name of the client (an LDAPDN) is agreed through the
negotiation of the credentials, it takes precedence over any value in
the unprotected name field.
Implementations which cache attributes and entries obtained via LDAP
MUST ensure that access controls are maintained if that information
is to be provided to multiple clients, since servers may have access
control policies which prevent the return of entries or attributes in
search results except to particular authenticated clients. For
example, caches could serve result information only to the client
whose request caused it to be cache.
This document is an update to RFC 1777, by Wengyik Yeong, Tim Howes,
and Steve Kille. Design ideas included in this document are based on
those discussed in ASID and other IETF Working Groups. The
contributions of individuals in these working groups is gratefully
acknowledged.
[1] ITU-T Rec. X.500, "The Directory: Overview of Concepts, Models
and Service", 1993.
[2] Yeong, W., Howes, T., and S. Kille, "Lightweight Directory Access
Protocol", RFC 1777, March 1995.
[3] ITU-T Rec. X.680, "Abstract Syntax Notation One (ASN.1) -
Specification of Basic Notation", 1994.
Wahl, et. al. Standards Track [Page 41]
RFC 2251 LDAPv3 December 1997
[4] Kille, S., Wahl, M., and T. Howes, "Lightweight Directory Access
Protocol (v3): UTF-8 String Representation of Distinguished
Names", RFC 2253, December 1997.
[5] Wahl, M., Coulbeck, A., Howes, T., and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax Definitions",
RFC 2252, December 1997.
[6] ITU-T Rec. X.501, "The Directory: Models", 1993.
[7] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
[8] ITU-T Rec. X.511, "The Directory: Abstract Service Definition",
1993.
[9] Howes, T., and M. Smith, "The LDAP URL Format", RFC 2255,
December 1997.
[10] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[11] ITU-T Rec. X.690, "Specification of ASN.1 encoding rules: Basic,
Canonical, and Distinguished Encoding Rules", 1994.
[12] Meyers, J., "Simple Authentication and Security Layer",
RFC 2222, October 1997.
[13] Universal Multiple-Octet Coded Character Set (UCS) -
Architecture and Basic Multilingual Plane, ISO/IEC 10646-1 :
1993.
[14] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
10646", RFC 2044, October 1996.