This document outlines the instances in which the HTTP.sys driver does not cache content. In these situations, Internet Information Services (IIS) 6.0 and later versions must manage caching of content in user mode.
The HTTP.sys response cache caches any request with the appropriate flag in the request header. This cache is disabled on a per-request basis. However, if one or more of the following conditions are true, HTTP.sys does not cache the request response:
- The request is not an anonymous request.
- The request requires authentication. (For example, the request contains an Authorization: header.)
- The website is configured to use a footer.
- Dynamic compression is enabled and is used for the response.
Note Static compression can be used with HTTP.sys caching.
- The static file is a Universal Naming Convention (UNC) file and the DoDirMonitoringForUnc registry key is not enabled.
Note You can use the DoDirMonitoringForUnc registry property (a DWORD value) to switch the static file cache back to a change notification cache. This is set as follows:HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Inetinfo\ParametersThe default value is 0, or not enabled. It can be set to 1 to enable caching of static content based on change notification.
- The request contains a query string.
- The cache is disabled. (That is, the MD_VR_NO_CACHE metabase property equals 1.)
Note More information about the MD_VR_NO_CACHE metabase property is available in the product documentation. To view this documentation, go to the following Microsoft website:For more information about how to access this Help feature, click the following article number to view the article in the Microsoft Knowledge Base:815127 How to access IIS 6.0 Help documentation
- The request has an entity body.
- Certificate mapping is enabled for the URL.
- Custom logging is enabled for the website.
- The request HTTP version is neither 1.1 nor 1.0.
- The request contains a Translate: f header.
- An Expect: header that does not contain exactly "100 continue" is present.
- The request contains either an If-Range: header or a Range: header.
Note HTTP.sys processes only whole responses. HTTP.sys does not try to send ranged responses.
- The response spans multiple SendResponse and SendResponseEntityBody calls.
Note A cacheable response must come down in a single, "vectored" SendResponse* call.
- The total response size is larger than the per-response maximum size. The maximum is controlled by the UriMaxUriBytes registry key, and the default value is 256 KB.
- The response header size is larger than the per-response maximum header size. The default value is 1,024 bytes.
- The cache is already full. The default size is proportional to the physical memory in the computer.
- The response is zero length.
- An ISAPI filter that is not cache-aware is installed.
Note By default, ISAPI filters are not cache-aware. You must set the FilterEnableCache metabase property for the filter to make it cache-aware. All filters in a default installation of IIS are cache-aware. This includes Microsoft FrontPage and Microsoft ASP.NET.
For more information about the FilterEnableCache metabase property, view the product documentation at the following Microsoft website:
- A static file is accessed as a default document. (For example, Default.htm exists in the root directory.) Accessing the specific file by name (http://example.com/default.htm/) causes HTTP.sys to cache the file. Accessing the website by requesting the root folder (http://example.com/) results in a noncached response.
Note If the first page that's listed in the Default Document list is inaccessible, IIS tries to serve the second page in the Default Document list. In this situation, that static page will not be served from the cache.