Sigh. The team at Port80 Software discovered this little nugget around Microsoft’s ISA Server. I quote it here in its entirety because the permalink is returning a 404.

UPDATE (January 10, 2005, 21:29 EST): The article is available here.

Does Microsoft ISA Server 2004 support compression?

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.

Although they have not done a good job of getting the word out,
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