DSProxy and Cached Exchange mode

Posted on 2010-01-05
Last Modified: 2012-05-08

We are running Exchange 2007 SP1 and AD 2003.

All our Exchange servers are located in HQ. We also have a GC/DC in Branch1 which has about 200 users. All users connect via Outlook 2007 and are set to cached Exchange mode.

From what I understand, there is a process named DSProxy that Exchange uses to refer Outlook to a GC that is in the same site as the Exchange mailbox server. From then on, Outlook uses this GC for address lookups etc.

At the same time, I understand that when Outlook is in cached Exchange mode, it uses the Offline Address Book (OAB) for address book functionality. The OAB is held on a Public Folder.

So, my question is, if Outlook is set to cached mode and uses the OAB, then why does it still need to contact a GC directly? From what I can recall, if I check the connection status of Outlook when in cached mode, I can still see GC's listed for directory access?

Secondly, if there is a need for Outlook to access a GC directly, is it possible to make a change on Exchange so that Outlook uses a local GC, not the one in the same site as Exchange?

Question by:kam_uk
    LVL 31

    Expert Comment

    To force Outlook's GC server:

    Yes, it will refer to it as it refreshes its cache.

    That link should help you, though.

    LVL 58

    Expert Comment


    Outlook has the Offline Address Book, but consider that list is only a cache of the Global Address List. If users need to view another list, or more details about a user which may not be in the OAB, it will need to contact a Global Catalog server.

    The DSProxy service is simply a referral service. As you've indicated, in modern Outlook clients, the requests don't go through the Exchange Server. Instead, the dsproxy simply tells Outlook of a suitable GC as determined by the Exchange Server, and then Outlook connects to that server directly.

    In Exchange 5.5, the Exchange directory was maintained separately from Active Directory. Thus old clients (we're talking older than Outlook 2000) designed specifically for Exchange 5.5 will communicate directly with the Exchange Server, so dsproxy also provides a means for Exchange to proxy their requests to a Global Catalog server and return the response.

    You can force Outlook to connect to a particular GC, but I would advise against it. This is not a good practice - it could cause you terrible troubleshooting issues in the long run. For 200 users at the remote site, I would be considering deploying an Exchange 2007 Mailbox/CAS/Hub Transport at the remote site and moving the mailboxes for users there to that server. This has a number of benefits, but reducing bandwidth in various ways is one of them.

    Also, don't forget Exchange 2007 and Outlook 2007 can use the new Web-Based Distribution method for the OAB, rather than public folders. This has to be enabled on the OAB manually, though.

    LVL 3

    Author Comment

    Thanks guys....I'm still trying to figure out for what other reasons OL would need to contact a GC for though...?

    "Outlook has the Offline Address Book, but consider that list is only a cache of the Global Address List. If users need to view another list, or more details about a user which may not be in the OAB, it will need to contact a Global Catalog server."

    What other lists do you mean, and how do you mean more details about a user which may not be in the OAB?
    LVL 58

    Accepted Solution


    There are a couple of things which aren't cached in the OAB which Outlook is still required to pull from the GAL. You can find these at (look at question 3).

    Essentially, it's the custom properties for any user, the organization hierarchy information for given users and their group membership information.

    So, even with the OAB locally stored, Outlook can look most information it requires up from there, but still needs a connection to a Global Catalog to make any of the above lookups. You'll find that when Outlook is in offline mode (such as when on the road without any connection to the internet) the information mentioned above which isn't cached in the OAB simply won't be available.


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