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Lightweight Directory Access Protocol (v3): Extension for Transport Layer Security

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2830.
Authors RL Bob Morgan , Mark Wahl , Jeff Hodges
Last updated 2013-03-02 (Latest revision 1999-06-22)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
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IESG IESG state Became RFC 2830 (Proposed Standard)
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LDAPEXT Working Group                            Jeff Hodges, Stanford
INTERNET-DRAFT                     RL "Bob" Morgan, Univ of Washington
Intended Category:                                 Mark Wahl, Innosoft
  Standards Track                                           June, 1999

              Lightweight Directory Access Protocol (v3):
                 Extension for Transport Layer Security

                        Status of this Document

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other groups
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time.  It is inappropriate to use Internet- Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at

Comments and suggestions on this document are encouraged.  Comments on
this document should be sent to the LDAPEXT working group discussion

This document expires in December 1999.

1.  Abstract

This document defines the "Start Transport Layer Security (TLS) Opera-
tion" for LDAP [LDAPv3, TLS]. This operation provides for TLS establish-
ment in an LDAP association and is defined in terms of an LDAP extended

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2.  Conventions Used in this Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in [ReqsKeywords].

3.  The Start TLS Request

This section describes the Start TLS extended request and extended
response themselves: how to form the request, the form of the response,
and enumerates the various result codes the client MUST be prepared to

The section following this one then describes how to sequence an overall
Start TLS Operation.

3.1.  Requesting TLS Establishment

A client may perform a Start TLS operation by transmitting an LDAP PDU
containing an ExtendedRequest [LDAPv3] specifying the OID for the Start
TLS operation:

An LDAP ExtendedRequest is defined as follows:

     ExtendedRequest ::= [APPLICATION 23] SEQUENCE {
             requestName             [0] LDAPOID,
             requestValue            [1] OCTET STRING OPTIONAL }

A Start TLS extended request is formed by setting the requestName field
to the OID string given above.  The requestValue field is absent.  The
client MUST NOT send any PDUs on this connection following this request
until it receives a Start TLS extended response.

When a Start TLS extended request is made, the server MUST return an
LDAP PDU containing a Start TLS extended response.  An LDAP Exten-
dedResponse is defined as follows:

     ExtendedResponse ::= [APPLICATION 24] SEQUENCE {
             COMPONENTS OF LDAPResult,
             responseName     [10] LDAPOID OPTIONAL,
             response         [11] OCTET STRING OPTIONAL }

A Start TLS extended response MUST contain a responseName field which
MUST be set to the same string as that in the responseName field present
in the Start TLS extended request. The response field is absent. The
server MUST set the resultCode field to either success or one of the

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other values outlined in section 3.3.

3.2.  "Success" Response

If the ExtendedResponse contains a resultCode of success, this indicates
that the server is willing and able to negotiate TLS. Refer to section
4, below, for details.

3.3.  Response other than "success"

If the ExtendedResponse contains a resultCode other than success, this
indicates that the server is unwilling or unable to negotiate TLS.

If the Start TLS extended request was not successful, the resultCode
will be one of:

     operationsError    (operations sequencing incorrect; e.g. TLS already

     protocolError      (TLS not supported or incorrect PDU structure)

     referral           (this server doesn't do TLS, try this one)

     unavailable        (e.g. some major problem with TLS, or server is
                         shutting down)

The server MUST return operationsError if the client violates any of the
Start TLS extended operation sequencing requirements described in sec-
tion 4, below.

If the server does not support TLS (whether by design or by current con-
figuration), it MUST set the resultCode to protocolError (see section
4.1.1 of [LDAPv3]), or to referral. The server MUST include an actual
referral value in the LDAP Result if it returns a resultCode of refer-
ral. The client's current session is unaffected if the server does not
support TLS. The client MAY proceed with any LDAP operation, or it MAY
close the connection.

The server MUST return unavailable if it supports TLS but cannot estab-
lish a TLS connection for some reason, e.g. the certificate server not
responding, it cannot contact its TLS implementation, or if the server
is in process of shutting down. The client MAY retry the StartTLS opera-
tion, or it MAY proceed with any other LDAP operation, or it MAY close
the connection.

4.  Sequencing of the Start TLS Operation

This section describes the overall procedures clients and servers MUST

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follow for TLS establishment. These procedures take into consideration
various aspects of the overall security of the LDAP association includ-
ing discovery of resultant security level and assertion of the client's
authorization identity.

Note that the precise effects, on a client's authorzation identity, of
establishing TLS on an LDAP association are described in detail in sec-
tion 7.

4.1.  Requesting to Start TLS on an LDAP Association

The client MAY send the Start TLS extended request at any time after
establishing an LDAP association, except that in the following cases the
client MUST NOT send a Start TLS extended request:

     - if TLS is currently established on the connection, or
     - during a multi-stage SASL negotiation, or
     - if there are any LDAP operations outstanding on the connection.

The result of violating any of these requirements is a resultCode of
operationsError, as described above in section 3.3.

The client MAY have already perfomed a Bind operation when it sends a
Start TLS request, or the client might have not yet bound.

If the client did not establish a TLS connection before sending any
other requests, and the server requires the client to establish a TLS
connection before performing a particular request, the server MUST
reject that request with a confidentialityRequired or strongAuthRequired
result. The client MAY send a Start TLS extended request, or it MAY
choose to close the connection.

4.2.  Starting TLS

The server will return an extended response with the resultCode of suc-
cess if it is willing and able to negotiate TLS.  It will return other
resultCodes, documented above, if it is unable.

In the successful case, the client, which has ceased to transfer LDAP
requests on the connection, MUST either begin a TLS negotiation or close
the connection. The client will send PDUs in the TLS Record Protocol
directly over the underlying transport connection to the server to ini-
tiate TLS negotiation [TLS].

4.3.  TLS Version Negotiation

Negotiating the version of TLS or SSL to be used is a part of the TLS
Handshake Protocol, as documented in [TLS]. Please refer to that

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document for details.

4.4.  Discovery of Resultant Security Level

After a TLS connection is established on an LDAP association, both par-
ties MUST individually decide whether or not to continue based on the
privacy level achieved. Ascertaining the TLS connection's privacy level
is implementation dependent, and accomplished by communicating with
one's respective local TLS implementation.

If the client or server decides that the level of authentication or
privacy is not high enough for it to continue, it SHOULD gracefully
close the TLS connection immediately after the TLS negotiation has com-
pleted (see sections 5 and 7.2, below).

The client MAY attempt to Start TLS again, or MAY send an unbind
request, or send any other LDAP request.

4.5.  Assertion of Client's Authorization Identity

The client MAY, upon receipt of a Start TLS extended response indicating
success, assert that a specific authorization identity be utilized in
determining the client's authorization status. The client accomplishes
this via an LDAP Bind request specifying a SASL mechanism of "EXTERNAL"
[SASL]. See section 7, below.

4.6.  Server Identity Check

The client MUST check its understanding of the server's hostname against
the server's identity as presented in the server's Certificate message,
in order to prevent man-in-the-middle attacks.

If a subjectAltName extension of type dNSName is present, it SHOULD be
used as the source of the server's identity.

Matching is performed according to these rules:

   - The client MUST use the server hostname it used to open
     the LDAP connection as the value to compare against the
     server name as expressed in the server's certificate.
     The client MUST NOT use the server's canonical DNS name or
     any other derived form of name.

   - If a subjectAltName extension of type dNSName is present
     in the certificate, it SHOULD be used as the source of the
     server's identity.

   - Matching is case-insensitive.

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   - The "*" wildcard character is allowed.
     - If present, it applies only to the left-most name component.

E.g. * would match,, etc. but not
If more than one identity of a given type is present in the certificate
(e.g. more than one dNSName name), a match in any one of the set is con-
sidered acceptable.

If the hostname does not match the dNSName-based identity in the certi-
ficate per the above check, user-oriented clients SHOULD either notify
the user (clients MAY give the user the opportunity to continue with the
connection in any case) or terminate the connection and indicate that
the server's identity is suspect. Automated clients SHOULD close the
connection, returning and/or logging an error indicating hat the
server's identity is suspect.

4.7.  Refresh of Server Capabilities Information

The client MUST refresh any cached server capabilities information (e.g.
from the server's root DSE; see section 3.4 of [LDAPv3]) upon TLS ses-
sion establishment. This is necessary to protect against active-
intermediary attacks which may have altered any server capabilities
information retrieved prior to TLS establishment. The server MAY adver-
tise different capabilities after TLS establishment.

5.  Closing a TLS Connection

5.1.  Graceful Closure

Either the client or server MAY terminate the TLS connection on an LDAP
association by sending a TLS closure alert. This will leave the LDAP
association intact.

Before closing a TLS connection, the client MUST either wait for any
outstanding LDAP operations to complete, or explicitly abandon them

After the initiator of a close has sent a closure alert, it MUST discard
any TLS messages until it has received an alert from the other party.
It will cease to send TLS Record Protocol PDUs, and following the
reciept of the alert, MAY send and receive LDAP PDUs.

The other party, if it receives a closure alert, MUST immediately
transmit a TLS closure alert.  It will subequently cease to send TLS
Record Protocol PDUs, and MAY send and receive LDAP PDUs.

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5.2.  Abrupt Closure

Either the client or server MAY abruptly close the entire LDAP associa-
tion and any TLS connection established on it by dropping the underlying
TCP connection. A server MAY beforehand send the client a Notice of
Disconnection [LDAPv3] in this case.

6.  Authentication and Authorization:  Definitions and Concepts

This section defines basic terms, concepts, and interrelationships
regarding authentication, authorization, credentials, and identity.
These concepts are used in describing how various security approaches
are utilized in client authentication and authorization.

6.1.  Access Control Policy

An access control policy is a set of rules defining the protection of
resources, generally in terms of the capabilities of persons or other
entities accessing those resources.  A common expression of an access
control policy is an access control list.  Security objects and mechan-
isms, such as those described here, enable the expression of access con-
trol policies and their enforcement.  Access control policies are typi-
cally expressed in terms of access control attributes as described

7.  Effects of TLS on a Client's Authorization Identity

This section describes the effects on a client's authorization identity
brought about by establishing TLS on an LDAP association. The default
effects are described first, and next the facilities for client asser-
tion of authorization identity are discussed including error conditions.
Lastly, the effects of closing the TLS connection are described.

Authorization identities and related concepts are defined in [AuthMeth].

7.1.  TLS Connection Establishment Effects

7.1.1.  Default Effects

Upon establishment of the TLS connection onto the LDAP association, any
previously established authentication and authorization identities MUST
remain in force, including anonymous state. This holds even in the case
where the server requests client authentication via TLS -- e.g. requests
the client to supply its certificate during TLS negotiation (see [TLS]).

7.1.2.  Client Assertion of Authorization Identity

A client MAY either implicitly request that its LDAP authorization

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identity be derived from its authenticated TLS credentials or it MAY
explicitly provide an authorization identity and assert that it be used
in combination with its authenticated TLS credentials. The former is
known as an implicit assertion, and the latter as an explicit assertion.  Implicit Assertion

An implicit authorization identity assertion is accomplished after TLS
establishment by invoking a Bind request of the SASL form using the
"EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL NOT include the
optional credentials octet string (found within the SaslCredentials
sequence in the Bind Request). The server will derive the client's
authorization identity from the authentication identity supplied in the
client's TLS credentials (typically a public key certificate) according
to local policy. The underlying mechanics of how this is accomplished
are implementation specific.  Explicit Assertion

An explicit authorization identity assertion is accomplished after TLS
establishment by invoking a Bind request of the SASL form using the
"EXTERNAL" mechanism name [SASL, LDAPv3] that SHALL include the creden-
tials octet string. This string MUST be constructed as documented in
section 11 of [AuthMeth].  Error Conditions

For either form of assertion, the server MUST verify that the client's
authentication identity as supplied in its TLS credentials is permitted
to be mapped to the asserted authorization identity. The server MUST
reject the Bind operation with an invalidCredentials resultCode in the
Bind response if the client is not so authorized. The LDAP association's
authentication identity and authorization identity (if any) which were
in effect after TLS establishment but prior to making the Bind request,
MUST remain in force.

Additionally, with either form of assertion, if a TLS session has not
been established between the client and server prior to making the SASL
EXTERNAL Bind request and there is no other external source of authenti-
cation credentials (e.g.  IP-level security RFC 1825), or if, during the
process of establishing the TLS session, the server did not request the
client's authentication credentials, the SASL EXTERNAL bind MUST fail
with a result code of inappropriateAuthentication. Any authentication
identity and authorization identity, as well as TLS connection, which
were in effect prior to making the Bind request, MUST remain in force.

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7.2.  TLS Connection Closure Effects

Closure of the TLS connection MUST cause the LDAP association to move to
an anonymous authentication and authorization state regardless of the
state established over TLS and regardless of the authentication and
authorization state prior to TLS connection establishment.

8.  Security Considerations

The goals of using the TLS protocol with LDAP are to ensure connection
confidentiality and integrity, and to optionally provide for authentica-
tion. TLS expressly provides these capabilities, as described in [TLS].

All security gained via use of the Start TLS operation is gained by the
use of TLS itself. The Start TLS operation, on its own, does not provide
any additional security.

The use of TLS does not provide or ensure for confidentiality and/or
non-repudiation of the data housed by an LDAP-based directory server.
Nor does it secure the data from inspection by the server administra-
tors.  Once established, TLS only provides for and ensures confidential-
ity and integrity of the operations and data in transit over the LDAP
association, and only if the implementations on the client and server
support and negotiate it.

The level of security provided though the use of TLS depends directly on
both the quality of the TLS implementation used and the style of usage
of that implementation. Additionally, an active-intermediary attacker
can remove the Start TLS extended operation from the supportedExtension
attribute of the root DSE. Therefore, both parties SHOULD independently
ascertain and consent to the security level achieved once TLS is esta-
blished and before begining use of the TLS connection. For example, the
security level of the TLS connection might have been negotiated down to

Clients SHOULD either warn the user when the security level achieved
does not provide confidentiality and/or integrity protection, or be con-
figurable to refuse to proceed without an acceptable level of security.

Client and server implementors SHOULD take measures to ensure proper
protection of credentials and other confidential data where such meas-
ures are not otherwise provided by the TLS implementation.

Server implementors SHOULD allow for server administrators to elect
whether and when connection confidentiality and/or integrity is
required, as well as elect whether and when client authentication via
TLS is required.

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9.  Acknowledgements

The authors thank Tim Howes, Paul Hoffman, John Kristian, Shirish Rai,
Jonathan Trostle, and Harald Alvestrand for their contributions to this

10.  References

[AuthMeth]     M. Wahl, H. Alvestrand, J. Hodges, R. Morgan. "Authenti-
               cation Methods for LDAP".  INTERNET-DRAFT, Work In Pro-
               gress. draft-ietf-ldapext-authmeth-04.txt

[LDAPv3]       M. Wahl, S. Kille and T. Howes. "Lightweight Directory
               Access Protocol (v3)". RFC 2251.

[ReqsKeywords] Scott Bradner. "Key Words for use in RFCs to Indicate
               Requirement Levels". RFC 2119.

[SASL]         J. Myers. "Simple Authentication and Security Layer
               (SASL)". RFC 2222.

[TLS]          Tim Dierks, C. Allen. "The TLS Protocol Version 1.0". RFC

11.  Authors' Addresses

   Jeff Hodges
   Computing & Communication Services
   Stanford University
   Pine Hall
   241 Panama Street
   Stanford, CA 94305-4122

   Phone: +1-650-723-2452

   RL "Bob" Morgan
   Networks and Distributed Computing
   University of Washington
   Seattle, WA

   Phone: +1-206-221-3307

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   Mark Wahl
   Innosoft International, Inc.
   8911 Capital of Texas Hwy, Suite 4140
   Austin, TX 78759

   Phone: +1 626 919 3600

12.  Intellectual Property Rights Notices

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The IETF invites any interested party to bring to its attention any
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   Copyright (C) The Internet Society (1998).  All Rights Reserved.

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   revoked by the Internet Society or its successors or assigns.

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