For each URL request, the filter first looks in a cache of recently allowed addresses, and when that fails, it looks in the ipaddressdb.txt file. This shows an efficient way to authorize connections: a cache allows the filter to quickly allow or disallow access, and because each request comes in through the filter, speed is critical.
The project is a standard appwizard generated ISAPI filter. There are 4 parameters that can be changed to fine tune the filter: the maximum number of cached addresses, the position after which a cached entry will be moved to the front of the list (to make the search time shorter!), the name of the file that contains the IP address list and the name of the html file that indicates to the client that a host with its IP address is not allowed to access this server. All this parameters are #define directives in the IPAddressFilter.h header file.
The filter could be improved in several ways: using a database instead of a file for IP address information (you should consider using stored procedures to search and/or to cache!), load parameters from registry, automatic selection of the number of cached addresses 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:
- 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 ipaddressfilter.dll, usually C:\WinNT\System32\InetSrv\ipaddressfilter.dll. The filter entries are separated by commas. The order is important, if you have other filters with the same priority, the first one listed will receive the requests first.
- Copy the ipaddressfilter.dll file to the directory you specified in the registry.
- Make sure the System account have execute rights on the filter dll file.
Edit the ipaddressdb.txt file so it contains valid IP addresses. The format of the file is:
- Copy the ipaddressdb.txt file to the directory you specified in the IPAddressFilter.h header file for the IP address database.
- Copy the NoAccess.htm file to the directory you specified in the IPAddressFilter.h header file for the page to indicate that the access is denied for this IP address.
- Make sure the System account have read rights on the NoAccess.htm and ipaddressdb.txt files.
- Restart the WWW service.
Last updated: 24 November 1998