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Problem DCOM-ing from 7 to XP

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  • Problem DCOM-ing from 7 to XP

    Our app uses DCOM to connect to our propriety server. I'm now setting up my first Windows 7 workstation, and have encountered a problem connecting to a dev server running on an XP machine. Here are the details:

    1. Connection (CoCreateInstanceEx) returns a "The RPC server is unavailable" error.
    2. Event Viewer has a message saying "DCOM was unable to communicate with the computer n.n.n.n using any of the configured protocols".
    3. I'm logged into both machines with the same (domain) user.
    4. From both machines I can access the other machine's C$.
    5. No firewall in either machine.
    6. The app is run as an administrator.
    7. On the 7 machine, the app can connect to a server running on a Win2003 machine with no problem, meaning there's no problem with the app's installation.
    8. An app running on a different XP machine can connect to the XP server with no problem, meaning there's no problem with the server's installation.
    9. The call never seems to get to the server, as the server service is not being automatically started once a call is done from the 7 machine.

    I've had my share of DCOM connection problems over the years, but this one beats me. Any ideas what might be the problem?

  • #2
    Re: Problem DCOM-ing from 7 to XP

    Any outbound firewall rules in place?
    Caesar's cipher - 3




    • #3
      Re: Problem DCOM-ing from 7 to XP

      I know this seems like a firewall problem as there was just no communication between the two machines (AFAIK), but both machines have the firewall turned off. Also, the said app on the 7 machine was able to communicate with two other servers (Win2003) without a problem, and the XP server was used by an app running on an XP client.


      • #4
        Re: Problem DCOM-ing from 7 to XP

        Sounds like an RPC issue that I've seen before. On the Windows 7 machine open up a custom MMC and add the WMI Control snap-in. Connect to the Windows XP machine and see if you get any errors such as RPC Server Unavailable when you right click on the machine name and select Properties.

        I've seen this type of issue before on an SBS 2003 Server where my old friend the Scalable Networking Pack had caused it. If you do get the RPC Server Unavailable error from WMI it might be worth checking if you have the advanced features (E.g. Receive Side Scaling, TCP Checksum Offload etc) enabled. I haven't heard of it causing issues on XP before, but it depends on the NIC driver I suppose. Broadcom are particularly bad for releasing NIC Drivers that are too clever for older OSs like XP to work with reliably.
        BSc, MCSA: Server 2008, MCSE, MCSA: Messaging, MCTS
        Cruachan's Blog


        • #5
          Re: Problem DCOM-ing from 7 to XP

          Connceting WMI to the XP machine has indeed failed, with the error you've mentioned. I hadn't have the advanced network features on, but I've updated the network adapter (Intel 82567LM-3) driver and tried again with the advanced features both enabled and disabled, to no avail.

          BTW, I have two XPs, one with SP2 and one with SP3, and I wasn't able to connect to neither. However, I was able to connect to someone else's XP SP2. All three XPs are running on the same hardware (3 year old Lenovo ThinkCentres). Windows 7 is running on a new Dell Optiplex 760.