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Understanding Network Services and Active Directory Domain Controller Placement for Exchange Server 2007 : Exploring DSAccess, DSProxy, and the Categorizer

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4/1/2013 6:22:11 PM

The relationship that Exchange Server 2007 has with Active Directory is complex and often misunderstood. Because the directory is no longer local, special services were written for Exchange to access and process information in AD. Understanding how these systems work is critical for understanding how Exchange interacts with AD.

Understanding DSAccess

DSAccess is one of the most critical services for Exchange Server 2007. DSAccess, via the dsacccess.dll file, is used to discover current Active Directory topology and direct Exchange to various AD components. DSAccess dynamically produces a list of published AD domain controllers and global catalog servers and directs Exchange resources to the appropriate AD resources.

In addition to simple referrals from Exchange to AD, DSAccess intelligently detects global catalog and domain controller failures, and directs Exchange to failover systems dynamically, reducing the potential for downtime caused by a failed global catalog server. DSAccess also caches LDAP queries made from Exchange to AD, speeding up query response time in the process.

DSAccess polls the Active Directory every 15 minutes to identify changes to site structure, domain controller placement, or other structural changes to Active Directory. By making effective use of LDAP searches and global catalog port queries, domain controller and global catalog server suitability is determined. Through this mechanism, a single point of contact for the Active Directory is chosen, which is known as the configuration domain controller.

Determining the DSAccess Roles

DSAccess lists identified domain controllers on the Exchange server properties page and identifies servers belonging to either of two groups, as shown in Figure 1.

  • Domain Controller Servers Being Used by Exchange— Domain controllers that have been identified by DSAccess to be fully operational are shown here.

  • Global Catalog Servers Being Used by Exchange— Global catalog servers are shown here.

Figure 1. Viewing domain controllers and global catalog servers used by Exchange.


A third role, known as the configuration domain controller, was visible on the properties page in Exchange 2003, however, it is not in the same location in Exchange 2007.

  • Configuration domain controller— A single AD domain controller is chosen as the configuration domain controller to reduce the problems associated with replication latency among AD domain controllers. In other words, if multiple domain controllers were chosen to act as the configuration domain controller, changes Exchange makes to the directory could conflict with each other. The configuration domain controller role is transferred to other local domain controllers in a site every 8 hours.

To determine the default configuration domain controller, view the Event Viewer application log and search for Event ID 2150. The results of the dsaccess query are listed here as well, as shown in Figure 2.

Figure 2. Identifying the default configuration domain controller.


In addition, the default configuration domain controller can be changed to one of your choice by performing the following steps:

1.
In the Exchange Management Console, select Server Configuration.

2.
In the action pane on the right side, click Modify Configuration Domain Controller.

3.
You can click Browse to select the appropriate domain, and then place a check in the Configuration Domain Controller check box. Then, you can then click Browse, shown in Figure 3, to manually select the configuration domain controller.

Figure 3. Manually setting the configuration domain controller.


Understanding DSProxy

DSProxy is a component of Exchange that parses Active Directory and creates an address book for down-level Outlook (pre–Outlook 2000 SR2) clients. These clients assume that Exchange uses its own directory, as opposed to directly using the Active Directory by itself, as Outlook 2000 SR2 and greater clients do. The DSProxy service provides these higher-level clients with a referral to an Active Directory global catalog server, which they then use without accessing the Exchange servers directly. The newer Outlook clients do not refresh this information unless a server failure has occurred or the client is restarted.

Note

DSProxy uses Name Service Provider Interface (NSPI) instead of LDAP for address list lookups, because NSPI is a more efficient interface for that type of lookup. Only global catalog servers support NSPI, so they are necessary for all client address list lookups.


Outlining the Role of the Categorizer

The SMTP Categorizer is a component of Exchange that is used to submit mail messages to their proper destination. When a mail message is sent, the Categorizer queries the DSAccess component to locate an Active Directory server list, which is then directly queried for information that can be used to deliver the message.

Although the Categorizer in Exchange gets a list of all global catalog servers from DSAccess, it normally opens only a single LDAP connection to a GC server to send mail, unless a large number of messages are queued for delivery.

Tip

Problems with the Categorizer are often the cause of DNS or AD lookup issues. When troubleshooting mail-flow problems, use message tracking in Exchange Server 2007 to follow the course of a message. If the message stops at the Categorizer, it is often wise to start troubleshooting the issue from a directory access perspective.

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