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How to make an ISAPI redirection filter

, 10 Sep 2003
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How to build a simple ISAPI Filter
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Introduction

Redirecting web traffic is a fairly common task, we can redirect from html, from ASP (or other scripting engines) and from ISAPI. This tutorial will demonstrate how to reduce server workload by redirecting from ISAPI using MFC using a “redirection filter”. This example will redirect a requested page, while retaining the URL the user sees in their browser

Html Redirect:

The code to redirect a page looks something like the following…
<META http-equiv="refresh" content="5; URL=http://somewhere.com/page.asp">

ASP Redirect

The ASP code to redirect looks like…
response.redirect “chrismaunderfanclub.com”

The first relies on you having each page set up to redirect, the second is done via ASP which means it requires use of the scripting engine (server hit).

ISAPI Redirection Filter

We can reduce the server load by using ISAPI filters in C++. This will have the same effect – the downside is that you need to know how to use Visual Studio to do it. The upside is that you don’t use ASP. This is filter will change the URL of the file IIS thinks it wants to return, without changing the URL the user sees in their browser

The Project

Open Visual Studio (I use version 6.0) Make a new project and select “ISAPI Extension Wizard”.

New ISAPI Project

Give your project a name. I’m using “redirector”, click "OK". You should now be presented with the following.

Tick “Generate Filter Object” and un-tick “Generate a Server Extension Object”. In case I’ll also link statically.

Click Next...

Because we only want to redirect the actual page and not the page the user thinks it is, we’ll tick “Post-processing of the request headers” and select the notification priority we want the filter to have, in this case we’ll select “high” to make sure it gets attention before any other filters. Click “Finish” and Visual Studio will now prepare the project for you.

The Code

We will now add our code to redirect all requests from .cfm to .asp The function of interest is in redirector.cpp and is called OnPreprocHeaders and it looks something like the following;

DWORD CRedirectorFilter::OnPreprocHeaders(
                             CHttpFilterContext* pCtxt, 
                             PHTTP_FILTER_PREPROC_HEADERS pHeaderInfo
                         )
{
    return SF_STATUS_REQ_NEXT_NOTIFICATION;
}

This function is called when the server has preprocessed the client headers. In this example we want to modify the URL that IIS thinks its wants to return. We’ll try something simple - replacing all Coldfusion (.cfm) extensions with .asp, Why? Perhaps we’re using ASP instead of ColdFusion and don’t want to change the links on the pages themselves.

We need to find out which page has been requested, and then replace .cfm with .asp if necessary. To do this we need to use the HTTP_FILTER_PREPROC_HEADERS pointer, pHeaderInfo, and the filter context pointer pCtxt.

We want to see the URL requested, to do this we can ask IIS what the URL was by way of the GetHeader function

DWORD CRedirectorFilter::OnPreprocHeaders(
                             CHttpFilterContext* pCtxt, 
                             PHTTP_FILTER_PREPROC_HEADERS pHeaderInfo
                         )
{
    char buffer[256]
    DWORD buffSize = sizeof(buffer);
    BOOL  bHeader=pHeaderInfo->GetHeader(pCtxt->m_pFC, "url",
                                             buffer, &buffSize); 
    ...
}
Once we have the URL we can decide whether to redirect it. I’m going to cheat a little here and use CString to deal with the string code, you can use whatever string code you like.
DWORD CRedirectorFilter::OnPreprocHeaders(
                             CHttpFilterContext* pCtxt,
                             PHTTP_FILTER_PREPROC_HEADERS pHeaderInfo
                         )
{
    char buffer[256];
    DWORD buffSize = sizeof(buffer);
    BOOL bHeader = pHeaderInfo->GetHeader(pCtxt->m_pFC, "url",
                                              buffer, &buffSize); 
    CString urlString(buffer);
    urlString.MakeLower(); // for this exercise 
    if (urlString.Find(".cfm") != -1) //we want to redirect this file
    {
        urlString.Replace(".cfm",".asp");
        char *newUrlString= urlString.GetBuffer(urlString.GetLength());
        pHeaderInfo->SetHeader(pCtxt->m_pFC, "url", newUrlString);
        return SF_STATUS_REQ_HANDLED_NOTIFICATION;
    }
    //we want to leave this alone and let IIS handle it
    return SF_STATUS_REQ_NEXT_NOTIFICATION;
}

Now, build the file and with any luck there will be no errors!

Adding the Filter to IIS

Next you will want to add the filter to IIS, or Personal Web Service if you are using it. I’m now going to describe how to add the filter to Personal Web Service.

So, go to Administrative tools (in Windows 2000) and select Internet Services Manger, something like the following will now appear.

Right click on “Default Web Site” (or whatever web server you want to add it to) and select “properties”, the following windows should then appear.

From the tabs, click on “ISAPI Filters”

Now, assuming there are no filters already present, the box in the middle should be clear (as this example shows) Click the “Add” button…

Give your filter a “name”, and use the browse button to locate the redirection filter .dll file.

Click “OK” and the filter will now appear, if you want your filter to have higher priority than others that may appear in the list, use the buttons on the left hand side to organize the correct order.

Click “OK”. The tabbed dialog will disappear.

Next we want to stop and start the web service. Right click on the “Default Web Site”, select “Stop” then right click again and select “Start”.

Alternatively you can click the black square on the rebar to stop the service, and click the “Start item” (black triangle) to restart it.

Testing it

Make a file test.asp and place it in your wwwroot directory of inetpub. Open your web browser and open location http://localhost/test.cfm. If all goes well then the contents of test.asp will be displayed in the browser.

That’s it, you’re done!

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

bryce
Software Developer (Senior)
Australia Australia
No Biography provided

Comments and Discussions

 
GeneralRe: Debugging ISAPI Filter? [modified] Pinmemberbryce7-Aug-07 19:22 
GeneralRe: Debugging ISAPI Filter? PinmemberLang Deng7-Aug-07 21:39 
GeneralDetect HTTP Agent - and redirect PinmemberQuantumDJ18-Jun-07 0:01 
GeneralRe: Detect HTTP Agent - and redirect Pinmemberbryce18-Jun-07 0:24 
QuestionVSTS (VS 2005) PinmemberOfflinesurfer11-Dec-06 2:37 
AnswerRe: VSTS (VS 2005) PinmemberOfflinesurfer11-Dec-06 11:49 
GeneralRe: VSTS (VS 2005) Pinmemberbryce14-Dec-06 21:55 
GeneralRe: VSTS (VS 2005) PinmemberOfflinesurfer15-Dec-06 4:58 
Hi Bryce
 
Thanks for the reply...
 
I came across this:
http://support.microsoft.com/kb/910382[^]
 
and this
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/iissdk/html/b6fcdaac-14cc-48b2-a186-09e1c24f3443.asp[^] there is a link for ISAPI filters at the bottom of the page.
 
and then this, which is interesting:
http://blogs.msdn.com/david.wang/archive/2006/03/25/MFC-ISAPI-Template-and-ISAPI-Development.aspx[^]
 
I have just downloaded the Windows® Server 2003 SP1 Platform SDK Web Install for a play...
 
Cheers !
Roll eyes | :rolleyes:
GeneralRe: VSTS (VS 2005) Pinmemberbryce4-Jan-07 19:52 
GeneralRe: VSTS (VS 2005) PinmemberOfflinesurfer9-Jan-07 3:50 
GeneralRe: VSTS (VS 2005) PinmemberYUserID3-Apr-07 5:23 
QuestionAny filter to add a banner or footer to all static and dynamic pages? Pinmembermarcelo_imai23-Nov-06 5:08 
AnswerRe: Any filter to add a banner or footer to all static and dynamic pages? Pinmemberbryce23-Nov-06 11:54 
GeneralHi Bryce,how to Get unicode url and redirect Pinmembernetvc24-Jun-06 2:07 
GeneralISAPI Filter is not active PinmemberVijay Chandra Sekhar Parepalli4-Apr-06 10:17 
QuestionI want to show the new URL to the browser. PinsussJohn T. Kelly8-Oct-05 13:59 
Generalhelp me! How can I control speeds. Pinmember10244225-Aug-05 20:46 
GeneralIs there some ISAPI Book PinmemberVasudevan Deepak Kumar19-Jul-05 3:03 
GeneralRedirecting across app pools PinsussAnonymous28-Mar-05 15:50 
Generalfull url required Pinmemberjagatnibas28-Nov-04 22:24 
GeneralRe: full url required PinsussAnonymous24-Mar-05 7:22 
GeneralRe: full url required Pinmemberfbalicki29-Jun-06 5:54 
GeneralReading the IIS metabase Pinmemberjfprince14-Oct-04 1:24 
GeneralThe browser sees the re write in URL as if it is a redirect PinsussAnonymous1-Oct-03 5:22 
GeneralRe: The browser sees the re write in URL as if it is a redirect PinsussAnonymous2-Oct-03 12:13 

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