Configuring the Active Directory Lightweight Directory Service (Part 1)

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The Lightweight Directory Service is useful for situations in which applications need access to a directory service, but you do not want to risk compromising your Active Directory database. In this article, you will be introduced to the Lightweight Directory Services, its uses, and capabilities.

When Microsoft introduced the Active Directory with Windows 2000, it didn’t take long before people began to realize that the Active Directory was really little more than a centralized database, and that the Active Directory could be used for purposes for which it was never intended.

For a while, it seemed as though almost every software vendor was designing their wares to be Active Directory integrated. Many such applications stored their configuration information in the Active Directory, and some even whet so far as to actually treat the Active Directory as an alternative to a SQL database and store actual application data in the Active Directory database.

Today, most of the third party software publishers seem to take less invasive approach to the way that they interface with the Active Directory. Many applications read Active Directory data, but not nearly as many applications seem to store data within the Active Directory as did a few years back. Although I can only speculate on the reasons for this, I suspect that it has something to do with the fact that the Active Directory has become a critical component of the network infrastructure, and many administrators are reluctant to perform unnecessary schema extensions (which are almost always necessary to support applications that store data within the Active Directory).

Even though software publishers may not use the Active Directory to quite the extent that they once did, I think that it is safe to say that the Active Directory can be very useful for supporting various applications. To show you what I mean, consider the fact that Microsoft still designs many of their server applications with a high degree of Active Directory integration. Exchange Server 2007 and Exchange Server 2010 for example, are designed in such a way that all of the server configuration information is stored in the Active Directory, rather than being stored locally on the server. The advantage to doing so is that it makes it possible to regenerate a failed server on the fly.

Suppose for instance that you had a catastrophic hard disk failure on an Exchange 2010 server that was hosting the Hub Transport Server Role. Because of the way that Exchange stores its configuration information in the Active Directory, you wouldn’t even have to restore a backup in order to fix the problem. Instead, you would start out by resetting the Computer account for the failed server within the Active Directory. You would then install Windows and any applicable service packs onto a new server. Next, you would assign that server the same computer name as your failed server had used, and join the new server to the Active Directory. Because you reset the Active Directory computer account, the new server is able to use it.

From there, fixing the problem is as simple as running Exchange Server’s Setup program with a special switch. Setup installs the necessary binaries, and then configures the server according to the configuration information found in the Active Directory. The new server can be up and running in less than an hour, and without ever restoring a backup.

My point is that the Active Directory can be very useful for application support, but that many software publishers are reluctant to use it to the extent that Microsoft does, because of the stigma that’s attached to making Active Directory schema extensions.

Another reason why you don’t see more software publishers storing a lot of data in the Active Directory has to do with Active Directory replication. Generally speaking, any data that is stored in the Active Directory must be replicated to all of the domain controllers in the domain (possibly even all of the domain controllers in the forest). As such, if an application were to store a large volume of data in the Active Directory, that data could impact the speed of the normal replication process – especially if that data changes frequently.

In spite of these challenges, there is a way to reap the benefits of Active Directory integration, without impacting your Active Directory database in the process. Windows Server 2008 and Windows Server 2008 R2 include a service called the Active Directory Lightweight Directory Service, or AD LDS.  A similar service also exists in Windows Server 2003, but goes by the name Active Directory Application Mode (or ADAM).

In case you are not familiar with AD LDS, it provides you with an environment that is very similar to, but completely separate from, the Active Directory. AD LDS is a standalone service that has no dependency on the Active Directory Directory Service. In fact, it is common to deploy AD LDS in environments in which no Active Directory domains exist.

A perfect example of such a situation is Microsoft Exchange Server. Earlier I said that Exchange Server 2007 and 2010 are both designed to store all of their configuration information in the Active Directory database. There is one big exception to this however.

Exchange Server defines a series of roles that dictate how an Exchange Server is configured, and what tasks the server performs. All but one of the server roles are designed to store the server configuration in the Active Directory.

The server role that does not use the Active Directory is known as the Edge Transport Server Role. The Edge Transport Server is designed to reside at the network perimeter and keep the other Exchange Servers from being directly exposed to the Internet.

Because the Edge Transport Server is exposed to various Internet based threats, making it a member of an Active Directory domain could be a potential security risk. If someone were able to compromise the edge transport server, they may be able to use it to gain information about the Active Directory.

To keep this from happening, the Edge Transport Server cannot be a domain member, and it cannot host any other Exchange Server roles. Even so, the Edge Transport Server does require access to a minimal amount of Active Directory information so that it can do its job. Rather than provide the server with direct access to the Active Directory, Microsoft has designed the Edge Transport Server role to use AD LDS.

One of the backend Exchange Servers reads the required information from the Active Directory, and sends the information to the AD LDS partition on the Edge Transport Server. That way, the Edge Transport Server has access to the information that it needs, without being able to access the Active Directory. Incidentally, the Edge Transport Server also stores its own configuration information in the AD LDS partition, just as other Exchange Server roles store configuration information in the Active Directory.


Now that I have talked about what the AD LDS is and what it is used for, I want to turn my attention to using this service in your own organization. In Part 2, I will begin discussing the hardware and the software requirements for using AD LDS.

If you would like to read the other parts in this article series please go to:

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