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Posted 6 Dec 1999


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ISAPI Authentication Filter

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6 Dec 1999CPOL
This article demonstrates the use of the MFC ISAPI classes to write filters.

In this article, the Microsoft SDK ISAPI authentication filter sample was rewritten using MFC ISAPI classes. The filter's functionality remains exactly the same, the purpose of the article is to demonstrate the use of the MFC ISAPI classes to write filters. Note that this authentication filter is not the most trivial sample, it provides pretty serious functionality (Microsoft refers to it as "A Filter for Advanced Authentication"). The filter functionality is described below using an exception of Microsoft documentation.

"AuthFilt demonstrates how to write an authentication filter based on an external datasource. Authentication is the process of accepting or denying a request from a client, so AuthFilt will be notified each time an authentication request comes in. This sample uses a file (userdb.txt) to keep track of authorized users, but you might modify this sample to access a database which holds user information.

For each authentication request, AuthFilt first looks in a cache of recently authenticated users, and when that fails, AuthFilt looks in the userdb.txt file. This shows an efficient way to authorize connections: a cache allows the filter to quickly authenticate users, and because each request comes in through the filter, speed is critical."

The project is a standard appwizard generated ISAPI filter. The global functions of the AuthFilt Microsoft sample were encapsulated in the filter class. There are 3 parameters that can be changed to fine tune the filter: the maximum number of cached users, the position after which a cached entry will be moved to the front of the list (to make the search time shorter!) and the name of the file that contains the username/password pairs and the appropriate NT account the username/password should be mapped to. All these parameters are #define directives in the authflit.h header file.

The filter could be improved in several ways: using a database instead of a file for authentication information (you should consider using stored procedures to search and/or to cache!), load parameters from registry, automatic selection of the number of cached users and the list reorder parameter, etc.

The full source code is provided, you will have to compile it in order to get a working filter. Once you have compiled the project, you will need to take the following steps to install:

  1. Run REGEDT32.EXE and modify the server's registry as follows. Select the Filter DLLs key in HKEY_LOCAL_MACHINE\CurrentControlSet\Services\W3SVC\Parameters. Add a local path to authfilt.dll, usually C:\WinNT\System32\InetSrv\authfilt.dll. The filter entries are separated by commas. The order is important, if you have other authentication filter with the same priority, the first one listed will receive the authentication request.
  2. Copy the authfilt.dll file to the directory you specified in the registry.
  3. Make sure the System account has execute rights on the filter DLL file.
  4. Edit the userdb.txt file so it contains valid users and passwords. The format of the file is:
    User1:Password1, NTUser1:NTPassword1
    User2:Password2, NTUser2:NTPassword2
    User3:Password3, NTUser3:NTPassword3
  5. Copy the userdb.txt file to the directory you specified in the authfilt.h header file for the user database.
  6. Make sure the System account has read rights on the userdb.txt file.
  7. Restart the WWW service.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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Comments and Discussions

QuestionISAPI Development Question Pin
FranklinIssac11-Jun-08 8:13
MemberFranklinIssac11-Jun-08 8:13 
QuestionISAPI for Windows 2003 Server IIS Service not work !? Pin
jackie hsu18-Mar-08 18:04
Memberjackie hsu18-Mar-08 18:04 
AnswerRe: ISAPI for Windows 2003 Server IIS Service not work !? Pin
shanhe24-Aug-09 4:23
Membershanhe24-Aug-09 4:23 
GeneralISAPI authentication filter problem Pin
imazing1039-Dec-07 19:35
Memberimazing1039-Dec-07 19:35 
GeneralISAPI on ISA Server Pin
LeoNicolas15-Mar-05 6:01
MemberLeoNicolas15-Mar-05 6:01 
GeneralISAPI filter for Windows Shared Services Pin
Sarma Pisapati25-Sep-04 2:10
MemberSarma Pisapati25-Sep-04 2:10 
QuestionGetting client IP address in ISAPI filters? Pin
gtyhf8-Jul-04 20:32
Membergtyhf8-Jul-04 20:32 
GeneralGetting Session ID without using cookies. Pin
gtyhf8-Jul-04 20:26
Membergtyhf8-Jul-04 20:26 
GeneralRe: Getting Session ID without using cookies. Pin
Gumba14-Sep-04 20:28
MemberGumba14-Sep-04 20:28 
GeneralCannot execute Pin
Anonymous7-Apr-04 22:36
MemberAnonymous7-Apr-04 22:36 
QuestionHow to use this?? Pin
alan9325-Aug-03 12:25
Memberalan9325-Aug-03 12:25 
GeneralProject compiled in VC++.NET is not working Pin
Udhaya Moorthi5-Aug-03 20:16
MemberUdhaya Moorthi5-Aug-03 20:16 
GeneralRe: Project compiled in VC++.NET is not working Pin
slimtim25-Nov-03 13:15
Memberslimtim25-Nov-03 13:15 
Questionuser/password not being passed?? Pin
hanzhanquan2002atyahoocomcn20-Mar-03 16:03
Memberhanzhanquan2002atyahoocomcn20-Mar-03 16:03 
QuestionHow can I activate this filter!! Pin
sayhappy2-Dec-02 21:02
Membersayhappy2-Dec-02 21:02 
AnswerRe: How can I activate this filter!! Pin
hanzhanquan2002atyahoocomcn20-Mar-03 16:11
Memberhanzhanquan2002atyahoocomcn20-Mar-03 16:11 
GeneralConcurrency issues with the IIS Log Pin
16-Oct-02 11:53
suss16-Oct-02 11:53 
GeneralGetting IIS 5 SessionID vai ISAPI Filter Pin
19-May-02 22:00
suss19-May-02 22:00 
GeneralRe: Getting IIS 5 SessionID vai ISAPI Filter Pin
taliesins13-Sep-02 21:04
Membertaliesins13-Sep-02 21:04 
QuestionWinNT account from ISAPI????? Pin
7-May-02 15:43
suss7-May-02 15:43 
QuestionISAPI filter sends HTTP client call? Pin
thorsten25-Apr-02 21:20
Memberthorsten25-Apr-02 21:20 
GeneralUser not being passed Pin
19-Mar-02 16:06
suss19-Mar-02 16:06 
GeneralRe: User not being passed Pin
hanzhanquan2002atyahoocomcn20-Mar-03 15:35
Memberhanzhanquan2002atyahoocomcn20-Mar-03 15:35 
GeneralRe: User not being passed Pin
Muhammad Asim Sajjad25-Sep-07 22:29
MemberMuhammad Asim Sajjad25-Sep-07 22:29 
GeneralInstall of ISAPI Filter Pin
6-Feb-02 11:58
suss6-Feb-02 11:58 

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