IIS 7 returns 304 instead of 200

asp.netiis-7windows-server-2008

I have a strange issue with IIS 7.
Sometimes it seems to return a 304 instead of a 200.

Here is a sample request captured with Fiddler:
(Note that the file requested is not located in my browsers cache yet.)

GET https://[mysite]/Content/js/jquery.form.js HTTP/1.1
Accept: */*
Referer: https://[mysite]/Welcome/News
Accept-Language: sv-SE
User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; InfoPath.2; OfficeLiveConnector.1.4; OfficeLivePatch.1.3; .NET4.0C; .NET4.0E)
Accept-Encoding: gzip, deflate
Host: [mysite]
Connection: Keep-Alive
Cache-Control: no-cache
Cookie: ...

Note that there is no If-Modified-Since or If-None-Match in the request.
But still the response is:

HTTP/1.1 304 Not Modified
Cache-Control: public
Expires: Tue, 02 Mar 2010 06:26:08 GMT
Last-Modified: Mon, 22 Feb 2010 21:58:44 GMT
ETag: "1CAB40A337D4200"
Server: Microsoft-IIS/7.5
X-Powered-By: ASP.NET
Date: Mon, 01 Mar 2010 17:06:34 GMT

Does anyone have a clue of what could be wrong here?

I'm running IIS 7 on Windows Web Server 2008 R2.

EDIT:

I've found a workaround, enable caching and then disable it on an extension level did the trick for me.

<configuration>
  <system.webServer>
    <caching enabled="true" enableKernelCache="true">
      <profiles>
        <add extension=".png" policy="DisableCache" kernelCachePolicy="DisableCache" />
        <add extension=".gif" policy="DisableCache" kernelCachePolicy="DisableCache" />
        <add extension=".js" policy="DisableCache" kernelCachePolicy="DisableCache" />
        <add extension=".css" policy="DisableCache" kernelCachePolicy="DisableCache" />
      </profiles>
    </caching>
    <staticContent>
      <clientCache cacheControlMode="NoControl" />
    </staticContent>
  </system.webServer>
</configuration>

Best Answer

According to section 14.9 of the HTTP1.1 spec, the no-cache directive for the Cache-Control header is only imposable by the origin server, which means IIS is ignoring the header in your request.

The cache-control directives can be broken down into these general categories:

  - Restrictions on what are cacheable; these may only be imposed

by the origin server.

Section 14.9.1 defines public, private, and no-cache as the directives restricting what is cacheable, which can only be imposed by the server.

If you don't want your .js file to be cached you'll either need to set the no-cache directive in the app (ie- the ASP.NET code) or you'll need to change the Cache-Control header in the request to use the no-store directive instead of no-cache.

EDIT:
Based on your comment - yes I assumed you did not want the file cached. The 304, then, might be coming as a result of the file being in one of IIS's internal caches. Have a look at these:

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