Windows 10 does not read a PAC file referenced by a file protocol

Applies to: Windows 10Microsoft Edge

Symptoms


Consider the following scenario:

  • You configured Microsoft Internet Explorer 11 or Microsoft Edge on Windows 10 to use the Use Automatic Configuration script option.
  • You are testing or deploying a proxy auto-configuration (PAC) file. You are storing the file in a local location, such as C:\temp\proxy.pac.
  • You specified the file location in the Use Automatic Configuration script option in Internet Settings by using the file protocol.
  • You created the following registry subkey: 
     

    HKLM\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\

    Value: EnableLegacyAutoProxyFeatures

    Type:  REG_DWORD

    Data:  1

In this scenario, the PAC file is not read by Internet Explorer.

Cause


This issue occurs because Internet Explorer and Edge on Windows 10-based computers use the WinHttp proxy service to retrieve proxy server information. The WinHttp Proxy service does not support using the ftp:// or file:// protocol for a PAC file.

Resolution


To resolve this issue, host the PAC file on a web server, then refer to it by using the http protocol.  

Note The application/x-ns-proxy-autoconfig MIME type has to be specified for the PAC file on the web server in order for the WinHttp proxy service to consume the PAC file.