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Introduction to LDAP (Lightweight Directory Access Protocol)

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1.4.2 A new standard is born

In 1991, after a few false starts with other potential standards, LDAP was brought forth as a lightweight means for accessing the DAP-enabled directories of the X.500 world. The first set of standards, LDAPv2, were eventually defined and accepted by the Internet Engineering Task Force (IETF), an important standards body responsible for many important Internet standards, as RFCs 1777 and 1778.

These standards provided basic authentication, search, and compare operations, as well as additional operations for changing the directory. From the start, LDAP made X.500 more accessible, as intended. Figure 1.8 shows an X.500 server being accessed by an LDAP gateway service that is forwarding requests from an LDAP client.

Figure 1.8
The X.500 client goes away, replaced by an LDAP client talking to an LDAP server. Here, the LDAP server acts as a gateway between LDAP-aware clients and DAP-aware X.500 DSAs.

Almost as important as the protocol itself was the release of a standard API and the production of a client development kit. For the first time, it was possible to access these servers programmatically without wandering knee-deep into an arcane protocol.

1.4.3 LDAP goes solo

As time went by, some people began to wonder what made X.500 so special in the first place. The University of Michigan, which had developed the reference implementation of LDAP, released a stand-alone server called Slapd that would allow the LDAP server to present data from a local data store rather than simply act as a gateway to an X.500 server.

Slapd was followed by a second service called Slurpd, which read the changes from one server and replicated those changes via the LDAP protocol to other Slapd servers. Figure 1.9 shows a typical stand-alone LDAP environment.

Click here for a larger image.

Figure 1.9
An LDAP client talks to a Slapd server. X.500 is no longer involved.

At this point, Netscape hired most of the original developers from the University of Michigan Slapd server to develop the Netscape Directory Server. Netscape, which was riding high with an incredible share of the Internet browser market, decided that networks would require directories and that LDAP, not X.500, should be the standard. Nearly 40 other companies announced support at that time, bringing LDAP the focus and support it needed to become the de facto standard for directory services.

1.4.4 LDAPv3

LDAP may have gained acceptance as a stand-alone service, but it was far from complete. Due primarily to its reliance on X.500 servers to provide the server-to-server communications, access control, and other functionality, LDAP was still only a skeleton of a full directory service by the mid-1990s.

Many interested parties pushed forward with the development of the next generation of the LDAP standards. In December 1996, the new version was published as RFCs 2251 to 2256. These new specifications covered items including the protocol itself, mandatory and optional schema, and LDAP URLs. A set of standard authentication mechanisms and a standard for session encryption were added to the list of core specifications in 2000. Figure 1.10 shows the core specifications that make up the LDAP standard.

Figure 1.10
The IETF has been the primary standards body for most of the existing LDAPv3 specifications. This figure shows a list of published RFCs that are considered the core LDAP standards.


LDAPv3 was considered a great leap forward in several key areas, but it takes more than a protocol to make a directory service successful. It is now up to several standards bodies and industry consortia to enhance the LDAP core specifications and build a framework that allows directories from different vendors to interoperate, or at least share some of the most crucial information in a standard way, and play a more pivotal role in e-business. Figure 1.11 shows some of the many standards bodies and industry consortia that shape directory standards and define best practices in deployment and management.

Figure 1.11
Many industry consortia and standards bodies are involved with LDAP and related standards, but most have a narrow focus.

1.5.1 Replication and access control

Version 3 of the LDAP protocol was greatly improved from version 2, but lacked two important items: replication and access control. The IETF has created workgroups to deliver these missing pieces and others, as shown in figure 1.12.

Figure 1.12
IETF workgroups are trying to fill in the gaps left after the initial publication of LDAPv3.

Lack of a standard replication process has since become an interoperability nightmare as each LDAP server vendor implemented its own proprietary solution. Many products use simple LDAP protocol operations to distribute data as shown in figure 1.13. However, even those solutions using the LDAP protocol sometimes require proprietary controls or attributes.

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Figure 1.13
Supplier-to-consumer replication exists in some products using the LDAP protocol. Unfortunately, most need to use proprietary attributes or controls to get around current limitations in the specifications.

Many parties recognized that replication was critical to obtaining scalability, redundancy, and other important benefits. To resolve this issue, the Lightweight Directory Update Protocol (LDUP) working group was created within the IETF. At the time of this writing, the group has completed draft documents detailing requirements, a model for meeting those requirements, conflict resolution processes, and a protocol specification. The use of replication is discussed further in chapter 6.

In addition to the supplier-consumer model of replication available in most existing directory servers, LDUP was chartered with allowing for multiple directory masters for the same information, which is shown in figure 1.14. It also documents a process for resolving conflicts that may arise when different and potentially conflicting changes are made independently to the same entry on each master. In addition, LDUP defines a protocol that can be used for both supplier-initiated and consumer-initiated replication.

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Figure 1.14
Multimaster replication will allow changes to the same directory tree in multiple directories.

Security was further along in some respects. The Simple Authentication and Security Layer (SASL), originally developed for the Internet Mail Access Protocol (IMAP), was added as a core LDAP standard early on as a way to negotiate an appropriate type of client and/or server authentication and even session encryption.

Developing a standard for access control has proven to be much more time consuming and has produced fewer results. As shown in figure 1.15, such a standard will allow a server to determine if an authenticated entity should be able to read or update a particular entry or an entire portion of the directory.

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Figure 1.15
LDAP access control standards will include a mechanism for determining in advance whether an operation will be permitted.

The task of creating such a standard fell into the hands of the LDAP extensions (LDAPEXT) workgroup within the IETF. This workgroup was formed to handle any extensions needed to the LDAPv3 standards outside of replication. As this book is being written, most activities of the LDAPEXT workgroup have been moved to individual submissions and will likely become an informational RFC rather than a full standard. Some aspects of access control may be pursued as part of the interoperability requirements for replication.

To understand why access control might be bundled with the replication workgroup, think about the fact that any replication of information outside a vendor's products will render that data insecure—other vendors will not know the access control rules of the source data. Any practical solution for replication is dependent on a standard for access control. We will look at access control further in chapter 13 when we discuss directory security in more detail.

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This article was originally published on April 25, 2003

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