External MAPI Clients Cannot Connect with RPC

When you are using Server Publishing in Internet Security and Acceleration (ISA) Server 2000 to publish an RPC server (by using either the "Any RPC Server" or "Exchange RPC Server" protocol definition), external Microsoft Windows 2000-based clients can connect to the RPC server behind ISA Server 2000, but clients that are running Microsoft Windows NT 4.0, Microsoft Windows 98, Microsoft Windows 95, or Microsoft Windows Millennium Edition (Me) cannot connect. The error message that is received on the client varies depending on the RPC client program that is being used.

ISA Server 2000 includes an RPC Application Filter feature that you can use to publish RPC servers. This RPC Application Filter functionality checks the RPC Fragmentation flags that are set by the external RPC client that is trying to connect to the published RPC server behind ISA Server.

When Windows 2000 sends an RPC packet that is not fragmented, it indicates this by setting the RPC First Fragment and Last Fragment flag so that it equals 0x3. In Windows NT 4.0, Windows 98, Windows 95, and Windows Me, the First Fragment and Last Fragment flag may be set to 0x0, even for RPC packets that are not fragmented. This cause the RPC Application Filter feature to reject these packets.

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing this specific problem. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language. The English version of this fix should have the following file attributes or later:

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.