Network Working Group                                             H. Chu
Internet-Draft                                               Symas Corp.
Intended status: Informational                         February 28, 2007
Expires: September 1, 2007

                     Using LDAP Over IPC Mechanisms

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   When both the LDAP client and server reside on the same machine,
   communication efficiency can be greatly improved using host- specific
   IPC mechanisms instead of a TCP session.  Such mechanisms can also
   implicitly provide the client's identity to the server for extremely
   lightweight authentication.  This document describes the
   implementation of LDAP over Unix IPC that has been in use in OpenLDAP
   since January 2000, including the URL format used to specify an IPC

Table of Contents

   1.          Introduction . . . . . . . . . . . . . . . . . . . . .  3
   2.          Conventions  . . . . . . . . . . . . . . . . . . . . .  4
   3.          Motivation . . . . . . . . . . . . . . . . . . . . . .  5
   4.          User-Visible Specification . . . . . . . . . . . . . .  6
   4.1.        URL Scheme . . . . . . . . . . . . . . . . . . . . . .  6
   5.          Implementation Details . . . . . . . . . . . . . . . .  7
   5.1.        Client Authentication  . . . . . . . . . . . . . . . .  7
   5.2.        Other Platforms  . . . . . . . . . . . . . . . . . . .  8
   6.          Security Considerations  . . . . . . . . . . . . . . .  9
   7.          References . . . . . . . . . . . . . . . . . . . . . . 10
   7.1.        Normative References . . . . . . . . . . . . . . . . . 10
   7.2.        Informative References . . . . . . . . . . . . . . . . 10
   Appendix A. IANA Considerations  . . . . . . . . . . . . . . . . . 11
               Author's Address . . . . . . . . . . . . . . . . . . . 12
               Intellectual Property and Copyright Statements . . . . 13

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1.  Introduction

   While LDAP is a distributed access protocol, it is common for clients
   to be deployed on the same machine that hosts the server.  Many
   applications are built on a tight integration of the client code and
   a co-resident server.  In these tightly integrated deployments, where
   no actual network traffic is involved in the communication, the use
   of TCP/IP is overkill.  Systems like Unix offer native IPC mechanisms
   that still provide the stream-oriented semantics of a TCP session,
   but with much greater efficiency.

   Since January 2000, OpenLDAP releases have provided the option to
   establish LDAP sessions over Unix Domain sockets as well as over
   TCP/IP.  Such sessions are inherently as secure as TCP loopback
   sessions, but they consume fewer system resources, are much faster to
   establish and tear down, and they also provide secure identification
   of the client without requiring any additional passwords or other

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2.  Conventions

   Imperative keywords defined in [RFC2119] are used in this document,
   and carry the meanings described there.

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3.  Motivation

   Many LDAP sessions consist of just one or two requests.  Connection
   setup and teardown can become a significant portion of the time
   needed to process these sessions.  Also under heavy load, the
   constraints of the 2MSL limit in TCP become a bottleneck.  For
   example, a modest single processor dual-core AMD64 server running
   OpenLDAP can handle over 32,000 authentication requests per second on
   100Mbps ethernet, with one connection per request.  Connected over a
   host's loopback interface, the rate is much higher, but connections
   get completely throttled in under one second, because all of the
   host's port numbers have been used up and are in TIME_WAIT state.  So
   even when the TCP processing overhead is insignificant, the
   constraints imposed in [RFC0793] create an artificial limit on the
   server's performance.  No such constraints exist when using IPC
   mechanisms instead of TCP.

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4.  User-Visible Specification

   The only change clients need to implement to use this feature is to
   use a special URL scheme instead of an ldap:// URL when specifying
   the target server.  Likewise, the server needs to include this URL in
   the list of addresses on which it will listen.

4.1.  URL Scheme

   The "ldapi:" URL scheme is used to denote an LDAP over IPC session.
   The address portion of the URL is the name of a Unix Domain socket,
   which is usually a fully qualified Unix filesystem pathname.  Slashes
   in the pathname must be percent-encoded as described in section 2.1
   of [RFC3986] since they do not represent URL path delimiters in this
   usage.  E.g., for a socket named "/var/run/ldapi" the server URL
   would be "ldapi://%26var%26run%26ldapi/".  In all other respects, an
   ldapi URL conforms to [RFC4516].

   If no specific address is supplied, a default address MAY be used
   implicitly.  In OpenLDAP the default address is a compile-time
   constant and its value is chosen by whoever built the software.

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5.  Implementation Details

   The basic transport uses a stream-oriented Unix Domain socket.  The
   semantics of communication over such a socket are essentially
   identical to using a TCP session.  Aside from the actual connection
   establishment, no special considerations are needed in the client,
   libraries, or server.

5.1.  Client Authentication

   Since their introduction in 4.2 BSD Unix, Unix Domain sockets have
   also allowed passing credentials from one process to another.  Modern
   systems may provide a server with easier means of obtaining the
   client's identity.  The OpenLDAP implementation exploits multiple
   methods to acquire the client's identity.  The discussion that
   follows is necessarily platform-specific.

   The OpenLDAP library provides a getpeereid() function to encapsulate
   all of the mechanisms used to acquire the identity.

   On FreeBSD and MacOSX the native getpeereid() is used.

   On modern Solaris systems the getpeerucred() system call is used.

   On systems like Linux that support the SO_PEERCRED option to
   getsockopt(), that option is used.

   On Unix systems lacking these explicit methods, descriptor passing is
   used.  In this case, the client must send a message containing the
   descriptor as its very first action immediately after the socket is
   connected.  The descriptor is attached to an LDAP Abandon Request
   [RFC4511] with message ID zero, whose parameter is also message ID
   zero.  This request is a pure no-op, and will be harmlessly ignored
   by any server that doesn't implement the protocol.

   For security reasons, the passed descriptor must be tightly
   controlled.  The client creates a pipe and sends the pipe descriptor
   in the message.  The server receives the descriptor and does an
   fstat() on it to determine the client's identity.  The received
   descriptor MUST be a pipe, and its permission bits MUST only allow
   access to its owner.  The owner uid and gid are then used as the
   client's identity.

   Note that these mechanisms are merely used to make the client's
   identity available to the server.  The server will not actually use
   the identity information unless the client performs a SASL Bind
   [RFC4513] using the EXTERNAL mechanism.  I.e., as with any normal
   LDAP session, the session remains in the anonymous state until the

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   client issues a Bind request.

5.2.  Other Platforms

   It is possible to implement the corresponding functionality on
   Microsoft Windows-based systems using Named Pipes, but thus far there
   has been no demand for it, so the implementation has not been
   written.  These are brief notes on the steps required for an

   The Pipe should be created in byte-read mode, and the client must
   specify SECURITY_IMPERSONATION access when it opens the pipe.  The
   server can then retrieve the client's identity using the
   GetNamedPipeHandleState() function.

   Since Windows socket handles are not interchangeable with IPC
   handles, an alternate event handler would have to be provided instead
   of using Winsock's select() function.

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6.  Security Considerations

   This document describes a mechanism for accessing an LDAP server that
   is co-resident with the client machine.  As such, it is inherently
   immune to security issues associated with using LDAP across a
   network.  The mechanism also provides a means for a client to
   authenticate itself to the server without exposing any sensitive
   passwords.  The security of this authentication is equal to the
   security of the host machine.

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7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2717]  Petke, R. and I. King, "Registration Procedures for URL
              Scheme Names", BCP 35, RFC 2717, November 1999.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4511]  Sermersheim, J., "Lightweight Directory Access Protocol
              (LDAP): The Protocol", RFC 4511, June 2006.

   [RFC4513]  Harrison, R., "Lightweight Directory Access Protocol
              (LDAP): Authentication Methods and Security Mechanisms",
              RFC 4513, June 2006.

   [RFC4516]  Smith, M. and T. Howes, "Lightweight Directory Access
              Protocol (LDAP): Uniform Resource Locator", RFC 4516,
              June 2006.

7.2.  Informative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

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Appendix A.  IANA Considerations

   This document satisfies the requirements of [RFC2717] for
   registration of a new URL scheme.

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Author's Address

   Howard Chu
   Symas Corp.
   18740 Oxnard Street, Suite 313A
   Tarzana, California  91356

   Phone: +1 818 757-7087

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