IIS and Static content?

I understand your situation. Sometime its confusing how IIS handles a file. Its also different for IIS 6 vs IIS 7 and different for Classic App Pools and Integrated mode app pools. My experience is mostly with Integrated App Pools on IIS 7.5, so thats the environment I can comment on most accurately.

First Question

But how does IIS knows what is actually a static content and what is
not?

Is it just images , css , js and not ASPX , ashx…?

Where can I see in the IIS what is already considered to be static and
what not ?

You can inspect the list of file handlers in IIS by navigating to your website and then click ‘Handler Mappings’. By default these are inherited from the .Net base web.config which is in a different location depending on your .Net framework version.

• C:\Windows\Microsoft.NET\Framework\v2.0.50727\CONFIG\web.config
• C:\Windows\Microsoft.NET\Framework\v4.0.30319\Config\web.config

If a file being requested isn’t already explicitly mapped to another handler it falls to a catch all handler (*) as the last option (System.Web.DefaultHttpHandler) which determines if it is a static file or a directory browsing request. So Static files are simply files not bound to another handler already. For example you’ll see that *.aspx is already mapped to System.Web.UI.PageHandlerFactory prior to this default handler. So its going to be processed by that handler and not be considered a static file. If you removed that mapping you could technically serve *.aspx as a static file if you really wanted to (just for proof of how it works).

But you can also explicitly list a file type as a static file by adding an entry in your web.config’s httpHandlers section mapping the file extensions to System.Web.StaticFileHandler in IIS. For example:

<configuration>
  <system.webServer>
    <handlers>
      <add name="StaticHandler" verb="*" path="*.zip" type="System.Web.StaticFileHandler" preCondition="integratedMode" />
    </handlers>
  </system.webServer>
</configuration>

This example is using the <system.webServer> config section, so its for an App Pool running in Integrated Mode.

Second Question

What about the scenario where a page has been declared with <%@
OutputCache header(without location) . does the images,css,js src
files inside of it , are also being output cached with the same
properties?

No. Because the page is being server as a separate request (maybe even by a separate handler) it can have totally different cache headers/hints. The host page and the resources it may use are not related from a caching perspective.

In fact you may even want to have a shorter cache period for *.html and a longer cache period for *.jpg or *.png? Something to consider.

Third Question

As a best prcatice , I should set one year into the future as the
maximum expiration time.I should use that as the default for all
static content on the site

Hmm… I might not go as far as one year. How about one month? I would set a global policy like this:

<configuration>
  <system.webServer>
    <staticContent>
      <!-- Set expire headers to 30 days for static content-->
      <clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="30.00:00:00" />
    </staticContent>
  </system.webServer>
</configuration>

This is the same as the sample you showed above, but is not inside a <location> element, instead it is right in the root <configuration> element so it is the default policy. Again this is for an App Pool running in Integrated Mode. Sometimes you also need to turn on:

<configuration>
    <system.webServer>
        <modules runAllManagedModulesForAllRequests="true">
            <!-- stuff -->
        </modules>
    </system.webServer>
<system.webServer>

This just makes sure that static files are processed through the managed static file handler which respects the above configuration elements.

Edit to Address Comments

The documentation for the configuration dialog you posted above is located here: Configure the HTTP Expires Response Header (IIS 7)

Apparently these settings are saved in C:\Windows\System32\inetsrv\config\applicationHost.config

I do not have IIS7 and personally develop on IIS 7.5 now. So please post a comment if you can verify this location is accurate!