Question

We’re running into the nasty sporadic IE6 bug where gzip compression enabled on js and css files makes things go bad (see Can i gzip-compress all my html content(pages) for example).

Therefore, what seems to be the best way to deal with this would be to use the URL Rewrite Module in IIS7/7.5 to check for requests from < IE6 and serve them uncompressed as per http://sebduggan.com/posts/ie6-gzip-bug-solved-using-isapi-rewrite.

1. I want to use the IIS7 Url Rewrite Module
2. Only the IIS7 Url Rewrite Module 2.0 RC supports rewriting headers

But the following results in a 500 error for the affected resources:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
    <rewrite>
        <rules>
            <rule name="IE56 Do not gzip js and css" stopProcessing="true">
                <match url="\.(css|js)" />
                <conditions>
                    <add input="{HTTP_USER_AGENT}" pattern="MSIE\ [56]" />
                </conditions>
                <action type="None" />
                <serverVariables>
                    <set name="Accept-Encoding" value=".*" /> <!-- This is the problem line -->
                </serverVariables>
            </rule>
        </rules>
    </rewrite>
</system.webServer>

What to put in the Server Variable for Accept-Encoding? I’ve verified that this is the problem line (as everything else has been isolated and operates as required). I’ve tried everything I can think of and I’m beginning to think that there just isn’t support for setting the Accept-Encoding header.

I’ve tried:

<set name="HTTP_ACCEPT_ENCODING" value=" " />
<set name="HTTP_ACCEPT_ENCODING" value=".*" />
<set name="HTTP_ACCEPT_ENCODING" value="0" />

Specifically, it results in a “HTTP/1.1 500 URL Rewrite Module Error.”

Answer 1

Well, it turns out that for security reasons you need to explicitly allow whatever server variables you wish to modify in the applicationHost.config (see http://learn.iis.net/page.aspx/665/url-rewrite-module-20-configuration-reference#Allowed_Server_Variables_List).

Therefore, the following does the trick in the Web.config:

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
    <rewrite>
        <rules>
            <rule name="IE56 Do not gzip js and css" stopProcessing="false">
                <match url="\.(css|js)" />
                <conditions>
                    <add input="{HTTP_USER_AGENT}" pattern="MSIE\ [56]" />
                </conditions>
                <action type="None" />
                <serverVariables>
                    <set name="HTTP_ACCEPT_ENCODING" value="0" />
                </serverVariables>
            </rule>
        </rules>
    </rewrite>
</system.webServer>

As long as the applicationHost.config has:

<location path="www.site.com">
    <system.webServer>
        <rewrite>
            <allowedServerVariables>
                <add name="HTTP_ACCEPT_ENCODING" />
            </allowedServerVariables>
        </rewrite>
    </system.webServer>
</location>

See http://www.andornot.com/about/developerblog/2009/11/ie6-gzip-bug-solved-using-iis7s-url.aspx for a blog post detailing everything.

EDIT: Added official documentation link.

EDIT: Added link to blog post summarizing.