My team us currently trying to install Brotli compression on a VPS: Windows Server 2012 R2 with IIS8.5 using the 64bit module that can be downloaded here: https://www.iispeed.com/pagespeed/products/iisbrotli

But whatever we try, when I fill out the https domain name under: https://tools.keycdn.com/brotli-test I get a message

Negative! www.zorgbeurs.nl does not support Brotli compression.

In IIS these two modules are active for that site:

What we've tried so far:

Added this to the applicationHost.Config file:

    <httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files">
        <scheme name="br" dll="C:\inetpub\iisbrotli64.dll" />
        <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" />
            <add mimeType="text/*" enabled="true" />
            <add mimeType="message/*" enabled="true" />
            <add mimeType="application/x-javascript" enabled="true" />
            <add mimeType="application/javascript" enabled="true" />
            <add mimeType="*/*" enabled="false" />
            <add mimeType="text/*" enabled="true" />
            <add mimeType="message/*" enabled="true" />
          <add mimeType="application/x-javascript" enabled="true" />
            <add mimeType="application/javascript" enabled="true" />
            <add mimeType="application/atom+xml" enabled="true" />
            <add mimeType="application/xaml+xml" enabled="true" />
            <add mimeType="*/*" enabled="false" />

I tried with out without the gzip line <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" />

I've stopped and started IIS.

I wanted to check if caching was the issue so I cleared files in folders:
"C:\Windows\Temp", "C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root" and "%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files" and then restarted the webserver.

In the web.config of the site I tried with and without this line in the <system.webServer> section:

<urlCompression doStaticCompression="true" doDynamicCompression="true" />

Still I see this in Chrome dev console for the 200 request to zorgbeurs.nl:

Response Headers

Content-Type:text/html; charset=utf-8
Date:Fri, 30 Dec 2016 19:01:48 GMT
Set-Cookie:showcookiebar=false; path=/

It's as if the configuration is completely ignored.

What else can I try?

  • Does your request header include br in accept-encoding header? – Mahmoud Moravej Mar 14 '17 at 6:02
  • Thanks. Response Header contains: Content-Encoding:gzip, Request Headers contains Accept-Encoding:gzip, deflate, sdch, br...what else can I do? – Flo Mar 19 '17 at 20:08
  • You should remove gzip config otherwise iis prioritize gzip over br. Did u check it?if yes whst's the response content-encoding? – Mahmoud Moravej Mar 19 '17 at 21:27

It appears the Brotli module you referenced requires a paid license, so I haven't tried it, but I encountered a similar issue with my own open source Brotli plugin for IIS.

Current browsers advertise Brotli support after gzip and deflate in the Accept-Encoding header. Typical headers will look like: Accept-Encoding: gzip, deflate, br.

The HTTP RFC gives no specific guidance on how to choose from many Accept-Encoding values with the same priority, so it would be acceptable to return br content to those clients. However, IIS will choose the first one (left to right) that matches one of its configured compression schemes. This means it won't choose br if either gzip or deflate compression is also enabled.

The obvious solution is to disable gzip and deflate on your server so that br is the only match. However, because roughly 20-25% of Internet users (as of early 2018) are still using older web browsers that don't support Brotli, you probably want to keep gzip enabled on your server to support compression for those clients, at least for a while longer.

If you wish to leave both (or all three) schemes enabled, you must, therefore, take some action to force IIS to choose br when acceptable. To accomplish this, you can modify the Accept-Encoding header value on requests as they enter your IIS pipeline. The IIS URL Rewrite Module makes it easy.

The Accept-Encoding header is represented by the HTTP_ACCEPT_ENCODING Server Variable in the IIS pipeline, and you can modify it before it reaches the Compression Module(s). Here is a sample configuration:

        <add name="HTTP_ACCEPT_ENCODING" />
        <rule name="Prioritize Brotli">
            <match url=".*" />
                <add input="{HTTP_ACCEPT_ENCODING}" pattern="\bbr(?!;q=0)\b" />
                <set name="HTTP_ACCEPT_ENCODING" value="br" />

The rule above simply looks for the string br (surrounded by word boundaries and not immediately followed by ;q=0) in the Accept-Encoding header and re-writes it to be just plain br, giving IIS only one choice.

Note that the default URL Rewrite configuration does not allow modification of the HTTP_ACCEPT_ENCODING variable. The allowedServerVariables element overrides that restriction and must be configured in applicationHost.config. The rewrite rule can then be defined at any level in the config hierarchy, although it probably makes sense to make it global.


Brotli compression is now officially supported on IIS:




According to the documentation all versions of IIS from 8.5 up no longer allow preference of compression schemes to be configured at the server level. Since browsers must "ask" the server which compression schemes it supports, and all browsers have Brotli behind gzip, no one will be able to use Brotli on IIS 8.5 onwards. :-(


https://github.com/saucecontrol/BrotliIIS might help you with the problem

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