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The Internet Explorer and Microsoft Internet Explorer Administration Kit documentation specifies that you can use the following file formats for your Wpad.dat file when you are implementing automatic discovery.
From the Internet Explorer 5 Resource Kit: (pages 261-264):
DHCP implementation: In the Data Type list, click the String data type. For the default value of the string, type the URL that points to your configuration file. This file can be a .pac, .jvs, .js, or .ins configuration file.
In the DNS database file, enter a host record named wpad that points to the IP address of the Web server that contains the .pac, .jvs, .js, or .ins automatic configuration file.
DNS implementation: Enter a CNAME alias named wpad that points to the name (the resolved name, not the IP address) of the server that contains the .pac, .jvs, .js, or .ins automatic configuration file.
However, renaming an .ins file that was generated with IEAK or manually created with Notepad to Wpad.dat does not work when implementing Autodiscovery with DNS.
None of the settings get applied to the browser. The Wpad.dat file does get queried and cached to the temporary Internet cache (assuming your DNS or DHCP servers are properly configured), but it does not get processed.
In a DNS Implementation of Autodiscovery, the browser only interprets a renamed proxy automatic configuration file or .pac, .jvs, or .js file. In a DHCP Implemtation of Autodiscovery a .ins can be used, as the browser is being told where the file is by the DHCP Server, as oppossed to a DNS implemation where the browser is looking for a "hardcoded" URL of http://wpad.domain.com/wpad.dat".
To work around this problem, set up a DNS Implementation of Automatic Discovery as specified in the documentation (the Internet Explorer 5 Resource Kit). When it is set up correctly, specify that the Wpad.dat file be a redirector to your .ins file. To do this in Microsoft Internet Information Server (IIS):
Start Internet Service Manager in Microsoft Management Console (MMC).
Locate the folder in which your Wpad.dat file is located (usually the root of your Web server).
Right-click the Wpad.dat file, click Properties, and then on the File tab click A redirection to a URL. Point to the .ins file (anywhere on the server).
When you perform automatic configuration this way, standard automatic configuration rules still apply. If you want the restrictions to come from .cab files, you must either sign the .cab files or change the security on the client.
NOTE: Implementing automatic discovery by using DHCP is not possible with Windows NT Server 4.0. This is only possible in Windows 2000.
For additional information about setting up automatic discovery in Windows 2000, click the article number below to view the article in the Microsoft Knowledge Base:
252898 How to Enable Proxy Autodiscovery in Windows 2000