ISA Server 2004 returns reverse-proxy custom error pages, even in forward-proxy situations

Consider the following scenario. You have a computer that is running Microsoft Internet Security and Acceleration (ISA) Server 2004, Enterprise Edition. On this computer, you create different versions of custom error pages to be returned in forward-proxy situations and in reverse-proxy situations. However, only reverse-proxy error messages are returned to the clients, even in forward-proxy situations. This problem occurs only if the client computers are configured as SecureNAT clients.

Service pack information

To resolve this problem, obtain the latest service pack for ISA Server 2004. For more information about how to obtain the latest service pack for ISA Server 2004, click the following article number to view the article in the Microsoft Knowledge Base:

To work around this problem, configure the client computers as Web-proxy clients instead of as SecureNAT clients.

When internal clients in forward-proxy scenarios receive reverse-proxy custom error pages, the error messages may be misleading. These error messages may not contain sufficient information for troubleshooting.

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