Controlling the Internet Information Server
Through ActiveX ISAPI Filters

NOTE

Refer back to Chapter 5, "ActiveX Documents," for background on OLE DocObjects; Chapter 6, "ActiveX Controls," to review those controls; and Chapter 7, "ActiveX Scripting," for information on ActiveX scripting.

Other types of business uses for ISAPI filters are data encryption or compression for transmitting data in a custom format that only specific client browsers can interpret. For example, say you ran a stock-options business and had a set of clients that paid for high-speed access to sensitive market research information. You could allow access through a client-server architecture by passing the data back to the clients in a browsable or hyperlinked format.

An ISAPI filter can be used to transform the data before sending it to the client's browser. The client's browser would have to decrypt or decompress the data for display on the client side. In this way, your data is secured by encrypting the data before tranfer; by compressing the data, it can be transferred at higher speeds over the network. This is illustrated in Figure 12.4.

Figure 12.4. Data encryption and compression on the fly.

You can create an ISAPI filter for a customized tracing mechanism of HTTP requests that pass through your HTTP server. You could then use the information this filter reports to make decisions affecting capabilities and security of your business intranet and its connection the Internet. This alone is a good reason for creating an ISAPI filter—to offer a custom security scheme beyond what your current HTTP server can offer.

For instance, you may have certain files that only upper-level management should have access to, such as employee salaries or employee profiles. These files could be made accessible through the Web exclusively to upper-level management. You could write an ISAPI filter that would perform an authentication check before allowing access to the files.

When these files are accessed, your ISAPI filter would get an Authentication event. At this time, the filter would check the IDs against a database of the current upper-level management (since it changes so frequently). If the requester's ID is valid, then allow access; if not, he or she would get a "gentle" reminder in HTML format that this is (now) forbidden territory.

Your business can also use an ISAPI filter to modify certain documents for presentations. A filter could be used to change a file's data before the HTTP server passes it to the client's browser for display, or one could be written to convert any document references into HTML URL references. For example, assume a text file (FileStore\filelist.txt) is requested containing the following two lines:

Third Quarter 1996 Business Plan

c:\INetSrv\WWWRoot\FileStor\BusinessPlan3rdQ96.doc

A filter could be written to process the text file into an HTML file when it receives an URL Map event for the FileStore directory. For the sake of illustration, say the filter takes the second line and makes a hyperlink from it with the textual description on the first line being the URL's highlighted text, as follows:

<A HREF="FileStor\BusinessPlan3rdQ96.doc.htm">Third Quarter 1996 Business Plan</a><p>

This could certainly produce a nice business presentation, especially if you had the CVTDOC example, mentioned in the introduction, which takes *.doc.htm URL Maps and converts the *.doc file into an *.htm file (HTML). You could then click and get a Word-like display of the *.doc file in HTML. Figure 12.5 shows two screen shots illustrating the difference in appearance that converting the text file to HTML can make.

Figure 12.5. Simple text file versus converting to HTML.

Event Notification Paradigm

The IIS HTTP server calls ISAPI filters for different types of HTTP-related requests. These are refered to as events and the process can be thought of as an event notification paradigm—the event notification paradigm for ActiveX ISAPI filters. The events sent to filter DLLs are specified by each individual DLL during initialization of IIS. When an IIS service is started, the DLLs listed in a special place in the registry get loaded. As each filter is loaded, an initialization routine (GetFilterVersion()) is called for each filter to return information; part of this information is which types of events it wants to be notified of. When IIS generates such events, it send them to the filter DLLs that had expressed an interest in them. Your ISAPI filter DLL is listed in the registry in the following key

"HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\W3Svc\Parameters\"

under the value

"Filter DLLs"

The DLLs are separated by commas. The order in which filters are notified is based on priority: high, medium, or low. If there are two or more filters listed with the same priority, the order of notification is based on where the DLLs sit, left to right, as they are listed in this registry value. Once the DLL is initialized, you get notified—or called—for the events specified to IIS during Server startup.

Notification flags

  CString URLPath;
  int index=0; // index of your old directory in the path
  
  // copy into a CString for simple search
  URLPath = pMapInfo->pszPhysicalPath;
  
  // Find the start of the "\\OldDir\\"
  stringindex = URLPath.Find ( "\\OldDir\\" );
  
  // If the directory name was found; replace it
  if (-1 != index)
  {
    // Get the left half of the directory path
    CString newDir = URLPath.Left( index );
    
    // Add the new directory
    newDir += "\\NewDir\\";
    
    // Add the right half of the directory path
    newDir += URLPath.Mid ( index + strlen ( "\\OldDir\\" ) );
    
    // copy the new directory path back into the input path
    strcpy ( pMapInfo->pszPhysicalPath, (LPCTSTR)newDir );
  }

// Set the flags we are interested inpVer->dwFlags |= SF_NOTIFY_ORDER_LOW| SF_NOTIFY_SECURE_PORT| SF_NOTIFY_NONSECURE_PORT| SF_NOTIFY_URL_MAP;

<a href="http://yourservername/OldDir/FileStore/filelist.txt">The Biz</a>

c:\INetSrv\WWWRoot\OldDir\FileStore\filelist.txt

c:\INetSrv\WWWRoot\NewDir\FileStore\filelist.txt

http://yourservername/FileStore/UC/FileList.htm