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An Introduction to LDAP

by Luke A. Kanies
08/16/2001

A year or two ago, it seemed like everyone had heard about LDAP, and quite a few people were talking about it, but no one was really doing anything with it.

That seems to finally be changing, which is especially good for administrators and developers. LDAP can play a vital role in networks of all sizes, but like most new technology, it suffers from the Catch-22 of no one using it because it's not supported and developers not supporting it because no one is using it.

To understand why and how LDAP is going to be such an important tool in the life of a network administrator, it is necessary to understand what problems LDAP was developed to solve and how it will do so. This means it is also necessary to understand LDAP itself, both as a technology and as a tool.


Because of the difficulty in truly separating the job of system administrator from the job of network administrator, and because there is often so much cross-over, especially in smaller environments, I will generally refer to people of either category as network administrators. I choose "network administrator" as the generic term because a quality system or network administrator is concerned with the entire network of devices or systems, not individual nodes. The term "network administrator" places more stress on viewing the network as a whole, making it a more appropriate term.

In this introductory article, I hope to introduce LDAP and the concept of online directories, and explain why you might want them and what you can do with them. In later articles, I'll provide a more in-depth technical explanation of how to use LDAP, along with some example applications.

What is LDAP?

LDAP is the latest iteration in a rather lengthy development process beginning with the X.500 directory specification and its corresponding Directory Access Protocol (DAP) in the late 1980s and early 1990s. (For a more complete history of LDAP, and more information in general, see "Understanding and Deploying LDAP Directory Services", by T. Howes, M. Smith, and G. Good.)

DAP was a consistently difficult protocol to work with and implement, so easier protocols were developed with most of its functionality but significantly less complexity. Eventually, these versions were passed on to the IETF and OSI-DS and got merged into the Lightweight Directory Access Protocol, or LDAP, specification, first published as RFC 1487 in 1993. LDAP gained some widespread use in version 2, specified in RFC 1777.

LDAP is a protocol definition for accessing specialized databases called directories. It is similar to SQL in that it is a language for interacting with databases without specifying a particular database. In fact, the back-end for LDAP directories is nearly always a more general RDBMS system, such as LDBM or Oracle.

Using LDAP to interact with a database does place constraints on that database, because of the assumptions the protocol makes and the specialized needs of a directory versus a standard relational database. But these constraints are necessary to be able to gain all of the desired features of a directory.

What is a directory, and why does my network need one?

Use of the word "directory" in this context may confuse people into thinking that most networks currently don't make use of directories, or that LDAP will be the only directory on the network. In actuality, directories are already a mainstay of life, especially in the computer world. Most people are familiar with talking about a phonebook or a map of the mall as a directory, but for some reason insist upon using the term "database" in computing, even when directory would be more specific and correct. As an example, I call Unix's system of storing user information the "passwd database," but that database easily qualifies as a directory.

At its most basic definition, a directory is any database specialized more for reading than for writing: The phonebook only comes out once a year, the mall directory is only changed when stores change, and the passwd database is only updated when user information is changed -- but all of that information is read frequently. The definition really does not get more specific than that because there are so many different information stores which can qualify as directories, although generally speaking a directory is much more likely to be searched than browsed. The listing most often referred to as a directory in the technical industry, a file system directory, also fits this definition, because the directory is read whenever a listed file is accessed in any way, but is only written when files are created or destroyed. Also, the directory is far more likely to be read when searching for a specific file rather than just browsing the listing.

Almost anyone involved in the development or maintenance of networked applications or services is already working with at least one directory: a system of maintaining user information. Nearly all services require some sort of authentication services, thus mandating that those same services maintain a user directory. Because of this, the most common form of online directory is for user information. Directories are useful for a larger variety of information than that, though. For instance, Unix systems maintain directories for services, groups, and many other data types, the Domain Name System (DNS) is a very specialized global directory for host-name-to-address correlation, and web directories used for navigating sets of web sites are springing up everywhere.

If I'm already using directories, why switch to LDAP?

We have already seen that almost any network uses a variety of directories, often for specific services. In database-speak, that means the directory data is not normalized, which means many pieces of data are stored in more than one place, and thus must be changed in more than one place when changes are necessary.

This is a problem for many reasons. The most obvious is that every time any information in any of these directories gets changed, all of the other directories must be hunted through to make that same change. This is not only difficult, it is often completely unmanageable -- witness the ease with which passwords for the same user in different services go out of sync.

Beyond that, every time two services implement their own versions of the same style of directory, there is significant redundant effort. Not only did the developers for each service have to develop their own directory, but now the managers of each service have to separately maintain the directories -- a single user of both services will almost certainly have a different user experience with each service, and it is nearly impossible to centrally manage these multiple directories.

Security is an even worse problem. Probably every developer and administrator is familiar with the headaches associated with user security. Are the passwords secure? Is the transport secure? Has the user really proved his/her identity? Did I accidentally leave a loophole to gain higher access? Will the user directory always be available? When multiple services implement separate directories for the same information, each of them must completely cover the security issues. Basic statistics virtually guarantee more security holes than a unified directory, and this also means that the services are likely to have differing and even conflicting security policies.

What you really want in a network is unification of directories, and this is exactly what LDAP was designed for. With this unification, you get data normalization, central management, consistent user experience, consistent management and security policies, fewer security holes, and less wasted development time.

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