IIS: How to Disable Caching of Specific MIME Types (247389)

MORE INFORMATION

MIME stands for "Multipurpose Internet Mail Extensions," which refers to a way of configuring browsers or mail clients to view files that are in multiple formats based on a "MIME type." "MIME mapping" is a process by which Internet Information Server (IIS) "maps" files by their extensions to a particular MIME type. For example, a file with the extension .htm has a MIME type of "text/html", whereas a file with the extension .gif has a MIME Type of "image/gif".

When a request is made by a client for a particular file, IIS uses the MIME map to determine the correct MIME type to tell a client it will be receiving. IIS contains a large list of default MIME types to use and will return a MIME type of "application/octet-stream" for any file extension that is not explicitly mapped.

Cache Disabling Techniques

• The easiest way to disable caching of a given MIME type is to create folders based on the types to disable and then set custom headers for those folders. For example, the following steps will disable caching of files in an images folder:
  
  
  1. Create a folder under your Wwwroot folder named Images.
  2. Open the Internet Services Manager (ISM).
  3. Locate the Images folder under your default Web site.
  4. Right-click the Images folder and select Properties.
  5. Select the HTTP Headers tab.
  6. Click Add in the Custom HTTP Headers section.
  7. Add a header named Cache-Control with a value of no-cache.
  8. Click ok to apply the changes.
  When creating content, any files stored in this folder will no longer be cached. This is especially useful when working with images that are frequently updated.
  
• The following steps describe an unsupported method of tricking IIS using Windows Scripting Host (WSH) and Active Directory Service Interfaces (ADSI) into returning a "no-cache" header for a specific MIME type, in this case .gif files, for an entire Web server by modifying the IIS metabase.
  
  
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  1. 
     
     
  2. Save the following WSH code in your %SystemDrive%\InetPub\AdminScripts folder as MimeChange.vbs:

     Option Explicit
     
     Dim boolFound
     Dim intCount
     Dim intMimeMap
     Dim objMimeMap
     Dim varMimeMap
     Dim varMimeExt
     Dim varMimeTyp
     
     ' specify the extension and MIME type to work with
     varMimeExt = ".gif"
     ' NOTE - this entry stores an extra HTTP header with the MIME information
     ' this "tricks" IIS into sending the extra cache control header
     varMimeTyp = "image/gif" & vbCrLf & "Cache-Control: no-cache"
     
     ' create the ADSI object & current MIME map at that path
     Set objMimeMap = GetObject("IIS://localhost/w3svc")
     varMimeMap = objMimeMap.GetEx("MimeMap")
     
     ' get the MIME map count & set search status to false
     intCount = UBound(varMimeMap) + 1
     boolFound = False
     
     ' loop through MIME map searching for existing extension information
     For intMimeMap = 0 to intCount - 1
       If varMimeMap(intMimeMap).Extension = varMimeExt Then
         boolFound = True
         Exit For
       End If
     Next
     
     ' if no extension information is found, create the new mapping
     If boolFound = False Then
       intMimeMap = intCount
       ReDim Preserve varMimeMap(intMimeMap)
       Set varMimeMap(intMimeMap) = CreateObject("MimeMap")
       varMimeMap(intMimeMap).Extension = varMimeExt
     End If
     
     ' store the new information in the MIME map
     varMimeMap(intMimeMap).MimeType = varMimeTyp
     objMimeMap.PutEx 2,"MimeMap",varMimeMap
     objMimeMap.SetInfo
     							

  3. Execute the following command to modify the metabase: CSCRIPT %SystemDrive%\InetPub\AdminScripts\MimeChange.vbs
     
     Future requests for .gif files will now returned with the desired "no-cache" header.