INTERNET-DRAFT Kurt D. Zeilenga
Intended Category: Experimental OpenLDAP Foundation
Expires in six months 28 October 2005
LDAP Turn Operation
<draft-zeilenga-ldap-turn-03.txt>
1. Status of this Memo
This document is intended to be, after appropriate review and
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Abstract
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This specification describes a Lightweight Directory Access Protocol
(LDAP) extended operation to reverse (or "turn") the roles of client
and server for subsequent protocol exchanges in the session, or to
enable each peer to act as both client and server with respect to the
other.
1. Background and Intent of Use
The Lightweight Directory Access Protocol (LDAP) [Roadmap][Protocol]
is a client-server protocol which typically operates over reliable
octet-stream transports such as the Transport Control Protocol (TCP).
Generally, the client initiates the stream by connecting to the
server's listener at some well-known address.
There are cases where it is desirable for the server to initiate the
stream. While it certainly is possible to write a technical
specification detailing how to implement server-initiated LDAP
sessions, this would require the design of new authentication and
other security mechanisms to support server-initiated LDAP sessions.
Instead, this document introduces an operation, the Turn operation,
which may be used to reverse the client-servers roles of the protocol
peers. This allows the initiating protocol peer to become server
(after the reversal).
As an additional feature, the Turn operation may be used to allow both
peers to act in both roles. This is useful where both peers are
directory servers that desire to request, as LDAP clients, operations
be performed by the other. This may be useful in replicated and/or
distributed environments.
This operation is intended to be used between protocol peers which
have established a mutual agreement, by means outside of the protocol,
which requires reversal of client-server roles, or allows both peers
to act both as client and server.
1.1 Terminology
Protocol elements are described using ASN.1 [X.680] with implicit
tags. The term "BER-encoded" means the element is to be encoded using
the Basic Encoding Rules [X.690] under the restrictions detailed in
Section 5.2 of [Protocol].
2. Turn Operation
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The Turn operation is defined as a LDAP Extended Operation [Protocol,
Section 4.12] identified by the IANA-ASSIGNED-OID. The function of
the Turn Operation is to request that the client-server roles be
reversed, or, optionally to request that both protocol peers to be
able to act both as client and server in respect to the other.
2.1. Turn Request
The Turn request is an ExtendedRequest with the requestName field
containing the IANA-ASSIGNED-OID and a requestValue field is a
BER-encoded turnValue:
turnValue ::= SEQUENCE {
mutual BOOLEAN DEFAULT FALSE,
identifier LDAPString
}
A TRUE mutual field value indicates a request to allow both peers to
act both as client and server. A FALSE mutual field value indicates a
request to reserve the client and server roles.
The value of the identifier field is a locally-defined policy
identifier (typically associated with a mutual agreement for which
this turn is be executed as part of).
2.2. Turn Response
A Turn response is an ExtendedResponse where the responseName and
responseValue fields are absent. A resultCode of success is returned
if and only if the responder is willing and able to turn the session
as requested. Otherwise, a different resultCode is returned.
3. Authentication
This extension's authentication model assumes separate authentication
of the peers in each of their roles. A separate Bind exchange is
expected between the peers in their new roles to establish identities
in these roles.
Upon completion of the Turn, the responding peer in its new client
role has an anonymous association at the initiating peer in its new
server role. If the turn was mutual, the authentication association
of the initiating peer in its pre-existing client role is left intact
at the responding peer in its pre-existing server role. If the turn
was not mutual, this association is void.
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The responding peer may establish its identity in its client role by
requesting and successfully completing a Bind operation.
The remainder of this section discuss some authentication scenarios.
In the protocol exchange illustrations, A refers to the initiating
peer (the original client) and B refers to the responding peer (the
original server).
3.1. Use with TLS and Simple Authentication
A->B: StartTLS Request
B->A: StartTLS(success) Response
A->B: Bind(Simple(cn=B,dc=example,dc=net,B's secret)) Request
B->A: Bind(success) Response
A->B: Turn(TRUE,"XXYYZ") Request
B->A: Turn(success) Response
A->B: Bind(Simple(DN/Password)) Request
B->A: Bind(Simple(cn=A,dc=example,dc=net,A's secret)) Request
A->B: Bind(success) Response
In this scenario, TLS (Transport Layer Security) [TLS] is started and
the initiating peer (the original client) establishes its identity
with the responding peer prior to the Turn using the the DN/password
mechanism of the Simple method of the Bind operation. After the turn,
the responding peer in its new client role establishes its identity
with the initiating peer in its new server role.
3.2. Use with TLS and SASL EXTERNAL
A->B: StartTLS Request
B->A: StartTLS(success) Response
A->B: Bind(SASL(EXTERNAL)) Request
B->A: Bind(success) Response
A->B: Turn(TRUE,"XXYYZ") Request
B->A: Turn(success) Response
B->A: Bind(SASL(EXTERNAL)) Request
A->B: Bind(success) Response
In this scenario, TLS is started prior with each peer providing a
valid certificate and the initiating peer (the original client)
establishes its identity through the use of the EXTERNAL mechanism of
the SASL (Simple Authentication and Security Layer) [SASL] method of
the Bind operation prior to the Turn. After the turn, the responding
peer in its new client role establishes its identity with the
initiating peer in its new server role.
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3.3. Use of mutual authentication and SASL EXTERNAL
A number of SASL mechanisms, such as GSSAPI [GSSAPI] and DIGEST-MD5
[DIGEST-MD5], support mutual authentication. The initiating peer, it
its new server role, may use the identity of the responding peer
established by a prior authentication exchange, as its source for
"external" identity in subsequent EXTERNAL exchange.
A->B: Bind(SASL(GSSAPI)) Request
<intermediate messages>
B->A: Bind(success) Response
A->B: Turn(TRUE,"XXYYZ") Request
B->A: Turn(success) Response
B->A: Bind(SASL(EXTERNAL)) Request
A->B: Bind(success) Response
In this scenario, a GSSAPI mutual-authentication exchange is completed
between the initiating peer (the original client) and the the
responding server (the original server) prior to the turn. After the
turn, the responding peer in its new client role requests the
initiating peer utilize an "external" identity to establish its LDAP
authorization identity.
4. TLS and SASL security layers
As described in [Protocol], LDAP supports both Transport Layer
Security (TLS) [TLS] and Simple Authentication and Security Layer
(SASL) [SASL] security frameworks. The following table illustrates
the relationship between the LDAP message layer, SASL layer, TLS
layer, and transport connection within an LDAP session.
+----------------------+
| LDAP message layer |
+----------------------+ > LDAP PDUs
+----------------------+ < data
| SASL layer |
+----------------------+ > SASL-protected data
+----------------------+ < data
| TLS layer |
Application +----------------------+ > TLS-protected data
------------+----------------------+ < data
Transport | transport connection |
+----------------------+
This extension does not alter this relationship, nor does it remove
the general restriction against multiple TLS layers, nor does it
remove the general restriction against multiple SASL layers.
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As specified in [Protocol], the StartTLS operation is used to initiate
negotiation of a TLS layer. If a TLS is already installed, the
StartTLS operation must fail. Upon establishment of the TLS layer,
regardless of which peer issued the request to start TLS, the peer
which initiated the LDAP session (the original client) performs the
"server identity check" as described in Section 3.1.5 of [AuthMeth]
treating itself as the "client" and its peer as the "server".
As specified in [SASL], newly negotiated SASL security layer replace
the installed SASL security layer. Though the client/server roles in
LDAP, and hence SASL, may be reversed in subsequent exchanges, only
one SASL security layer may be installed at any instance.
5. Security Considerations
Implementors should be aware that the reversing of client/server roles
and/or allowing both peers to act as client and server likely
introduces security considerations not foreseen by the authors of this
document. In particular, the security implications of the design
choices made in the authentication and data security models for this
extension (discussed in sections 3 and 4, respectively) are not fully
studied. It is hoped that experimentation with this extension will
lead to better understanding of the security implications of these
models and other aspects of this extension, and that appropriate
considerations will be documented in a future document. The following
security considerations are apparent at this time.
Implementors should take special care to process LDAP, SASL, TLS, and
other events the appropriate roles for the peers. It is noted that
while the Turn reverses the client/server roles with LDAP, and in SASL
authentication exchanges, it does not reverse the roles within the TLS
layer or the transport connection.
The responding server (the original server) should restrict use of
this operation to authorized clients. Client knowledge of a valid
identifier should not be the sole factor in determining authorization
to turn.
Where the peers except to establish TLS, TLS should be started prior
to the Turn and any request to authenticate via the Bind operation.
LDAP security considerations [Protocol][AuthMeth] generally apply to
this extension.
6. IANA Considerations
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Registration of the following values [BCP64bis] is requested.
6.1. Object Identifier
It is requested that IANA assign an LDAP Object Identifier to identify
the LDAP Turn Operation as defined in this document.
Subject: Request for LDAP Object Identifier Registration
Person & email address to contact for further information:
Kurt Zeilenga <kurt@OpenLDAP.org>
Specification: RFC XXXX
Author/Change Controller: Author
Comments:
Identifies the LDAP Turn Operation
6.2. LDAP Protocol Mechanism
It is requested that IANA register the LDAP Protocol Mechanism
described in this document.
Subject: Request for LDAP Protocol Mechanism Registration
Object Identifier: IANA-ASSIGNED-OID
Description: LDAP Turn Operation
Person & email address to contact for further information:
Kurt Zeilenga <kurt@openldap.org>
Usage: Extended Operation
Specification: RFC XXXX
Author/Change Controller: Author
Comments: none
7. Author's Address
Kurt D. Zeilenga
OpenLDAP Foundation
Email: Kurt@OpenLDAP.org
8. References
[[Note to the RFC Editor: please replace the citation tags used in
referencing Internet-Drafts with tags of the form RFCnnnn where
possible.]]
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8.1. Normative References
[Roadmap] Zeilenga, K. (editor), "LDAP: Technical Specification
Road Map", draft-ietf-ldapbis-roadmap-xx.txt, a work in
progress.
[Protocol] Sermersheim, J. (editor), "LDAP: The Protocol",
draft-ietf-ldapbis-protocol-xx.txt, a work in progress.
[AuthMeth] Harrison, R. (editor), "LDAP: Authentication Methods and
Connection Level Security Mechanisms",
draft-ietf-ldapbis-authmeth-xx.txt, a work in progress.
[SASL] Melnikov, A. (Editor), "Simple Authentication and
Security Layer (SASL)",
draft-ietf-sasl-rfc2222bis-xx.txt, a work in progress.
[TLS] Dierks, T. and, E. Rescorla, "The TLS Protocol Version
1.1", draft-ietf-tls-rfc2246-bis-xx.txt, a work in
progress.
[X.680] International Telecommunication Union -
Telecommunication Standardization Sector, "Abstract
Syntax Notation One (ASN.1) - Specification of Basic
Notation", X.680(2002) (also ISO/IEC 8824-1:2002).
[X.690] International Telecommunication Union -
Telecommunication Standardization Sector, "Specification
of ASN.1 encoding rules: Basic Encoding Rules (BER),
Canonical Encoding Rules (CER), and Distinguished
Encoding Rules (DER)", X.690(2002) (also ISO/IEC
8825-1:2002).
8.2. Informative References
[BCP64bis] Zeilenga, K., "IANA Considerations for LDAP",
draft-ietf-ldapbis-bcp64-xx.txt, a work in progress.
[GSSAPI] Linn, J., "Generic Security Service
Application Program Interface, Version 2, Update 1", RFC
2743, January 2000.
[DIGEST-MD5] Leach, P., C. Newman, and A. Melnikov, "Using Digest
Authentication as a SASL Mechanism",
draft-ietf-sasl-rfc2831bis-xx.txt, a work in progress.
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