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Does Microsoft ISA Server 2004 support compression?

Posted: January 10th, 2005 | Filed under: Uncategorized

One of the most popular questions of 2004 was, without a doubt, “Why does Microsoft ISA Server 2004 break HTTP compression?”

Many users of our httpZip and ZipEnable compression products have been scratching their heads over why implementing ISA Server suddenly makes their compression products stop working. Actually, what is happening is that ISA Server by default removes the Accept-encoding header from all inbound requests. This is the header that the Web server uses to determine whether it is OK to return compressed content to the browser making the request. Without this header, compression products are just doing what they are supposed to do: responding with uncompressed content.

Microsoft has published this informative article that explains how Web publishing rules can be written for ISA Server 2004 to ensure that the Accept-encoding request headers get passed to the web server. If you’re new to ISA, or just confused about why compression worked perfectly on your test server but is mysteriously MIA in your production server environment, check it out:;en-us;838365

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