Error message when a Web browser tries to access external Web sites from behind an ISA Server 2004 computer that has Service Pack 2 installed

Consider the following scenario. You have Web browser clients that are located behind a Microsoft Internet Security and Acceleration (ISA) Server 2004 computer. This ISA Server 2004 computer has Service Pack 2 (SP2) installed. One of these Web browsers tries to access external Web sites. In this scenario, you experience the following symptoms:

• A Web browser that is behind an ISA Server 2004 SP2 Enterprise Edition computer receives the following error message:
  Error Code 400: Bad Request
• A Web browser that is behind an ISA Server 2004 SP2 Standard Edition computer receives an error message that is similar to the following:
  Error Code 11001: Host not found
  Additionally, the URL in the Web browser's address box may change to the following:
  http://%20example.comom

This problem occurs because ISA Server adds space between some headers and their values. For example, the following header information may be sent by an ISA Server computer to the Web server:When this behavior occurs in the host header, some Web servers incorrectly interpret this as a host name that starts with a space. Therefore, the Web server may redirect the browser to a URL that contains a space. For example, the Web server may redirect the browser to a URL that has the following format: Then, the Web browser tries unsuccessfully to access the malformed URL.

To resolve this problem, obtain the Update Rollup for ISA Server Services. For more information about the Update Rollup for ISA Server Services, click the following article number to view the article in the Microsoft Knowledge Base:

ISA server adds space between some headers and their values when certain Web filters are enabled. To work around this problem, disable the following Web filters:

• Compression filter
• Caching compressed content filter

Note You may also have to disable some third-party Web filters to resolve this problem.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

For more information about the description of the standard terminology that is used to describe Microsoft software updates, click the following article number to view the article in the Microsoft Knowledge Base: