This service is the Exchange 2000 component that provides an address book service to Outlook clients. Implemented as a DLL file called DSPROXY.DLL, it provides two essential functions:

• To emulate a MAPI (Messaging Application Programming Interface) address book service and proxy requests to an Active Directory server.

• To provide a referral mechanism, so the (appropriate) Outlook clients can directly contact Active Directory servers.

Early versions of Outlook assume that the directory service was contained within Exchange server. This is no longer true since Exchange 2000, but is still essential in providing compatibility to these versions. In reality, this proxy service actually forwards requests to, and receives responses from, the global catalogue servers on behalf of the client.

Later versions of Outlook (2000 SR2 and later) and 2002, were designed with the assumption that Exchange 2000 does not have its own directory service. Consequently, DSProxy can continue to either handle requests from these clients to and from the directory or alternatively refer the client to communicate directly with the global catalogue.

These communication processes need to be fully understood when designing operation of Exchange and Outlook across firewall devices, as it will directly impact the configuration requirements of your firewall policies and port assignments.

It's also a potentially important consideration in the scalability of larger systems, as you may benefit from reducing the proxy load on the server and instruct Outlook (where possible) to communicate direct with the catalogue services. Again, this is an extensive subject and is covered in detail within various Microsoft knowledge base articles.

Another useful point is that MAPI clients perform name resolution using NSPI (Name Service Provider Interface), as opposed to LDAP (Lightweight Directory Access Protocol), and recognising that DSProxy is emulating NSPI. This can be particularly beneficial when troubleshooting name resolution problems, by setting Exchange system logging of this process to high.

Scenario assumptions
In this particular scenario, we are going to make the following assumptions. Firstly, that the Exchange 2000 server has been configured as non-referral, forcing the Outlook clients to communicate via the DSProxy service, emulating NSPI, and passing the requests to and from the global catalogue servers.

Secondly, that the Exchange server has been configured with static ports and will only communicate across assigned ports. Consideration needs to be applied when choosing a port number (see Microsoft knowledge base), but in this example we will use the following:

• NSPI - Port 49153
• Information Store (IS) Interface - Port 49152
• RPC Locator Service - 135

The IS and RPC services form essential communication channels between Exchange and Outlook, but again, are not covered in any detail for the purpose of this article.