How to write a custom ISAPI filter to replace http header

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Background:

UrlScan is usually used to replace/remove http “server” header. But if the server gets Exchange installed, UrlScan could cause various issues and a lot of configuration and testing need to be done to make Exchange functional properly.

We can write a simple ISAPI filter to replace the server header instead of using UrlScan.

For general introduction of ISAPI filter, refer to https://msdn.microsoft.com/en-us/library/ms524610(v=vs.90).aspx

 

Solution:

  1. Use Visual Studio to create a C++ empty project and create the following two files.

sample_isapi_filter.DEF

LIBRARY sample_isapi_filter

 

;DESCRIPTION 'filter to remove server header'

 

EXPORTS

    HttpFilterProc

GetFilterVersion

 

sample_isapi_filter.C

#include <windows.h>

#include <stdio.h>

#include <stdlib.h>

#include <httpfilt.h>

 

DWORD DoSendResponse(HTTP_FILTER_CONTEXT * pfc,HTTP_FILTER_SEND_RESPONSE * pResponse);

 

BOOL WINAPI __stdcall GetFilterVersion(HTTP_FILTER_VERSION *pVer)

{

   pVer->dwFlags = ( SF_NOTIFY_ORDER_LOW | SF_NOTIFY_SEND_RESPONSE );

   pVer->dwFilterVersion = HTTP_FILTER_REVISION;

   return TRUE;

}

 

DWORD WINAPI __stdcall HttpFilterProc(HTTP_FILTER_CONTEXT *pfc, DWORD NotificationType, VOID *pvData)

{

   switch (NotificationType)

   {

   case SF_NOTIFY_SEND_RESPONSE :

         return DoSendResponse(pfc, (HTTP_FILTER_SEND_RESPONSE *) pvData);

   default :

         break;

   }

   return SF_STATUS_REQ_NEXT_NOTIFICATION;

}

 

DWORD DoSendResponse(HTTP_FILTER_CONTEXT * pfc,HTTP_FILTER_SEND_RESPONSE * pResponse)

{

   BOOL fServer = TRUE;

   DWORD dwServerError;

   fServer = pResponse->SetHeader(pfc, "Server:", "Apache http 2.4.2");

   if ( !fServer )

   {

         dwServerError = GetLastError();

         pfc->pFilterContext = (LPVOID)(DWORD64)pResponse->HttpStatus;

   }

   return SF_STATUS_REQ_NEXT_NOTIFICATION;

}

 

  1. In Build-Configuration Manager choose your target platform (win32 or x64) and build into a dynamic link library (sample_isapi_filter.dll).
  2. Put this file on the web server and add the filter from IIS Manager – Web Site Properties

 

 

  1. Verify the response header by fiddler. The header is successfully replaced by the filter.

HTTP/1.1 200 OK

Content-Length: 1433

Content-Type: text/html

Content-Location: https://xxxx/iisstart.htm

Last-Modified: Fri, 21 Feb 2003 10:48:30 GMT

Accept-Ranges: bytes

ETag: "05367c596d9c21:a54"

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Server: Apache http 2.4.2

Date: Mon, 02 Jul 2012 06:36:52 GMT

Connection: close

 

 Regards,

Aaron from APGC DSI Team