Tuesday, April 19, 2016

How-to have IE11 pick up local PAC file

In our company we utilize a cloud-hosted Proxy Automatic Configuration script (PAC file). On occasion it is convenient to switch to a local version, e.g. to investigate or troubleshoot. When we still were at IE9 this was easy, but since we’ve upgraded the company browser to IE11 it becomes more difficult. To promote interoperability across network stacks, Microsoft deprecated the option for a local PAC file. You can still configure it in Internet Settings, but IE 11 will simple ignore a local proxy auto-config file.
However, Microsoft also understands it’s backwards compatible responsibility. And therefore build in support to overrule the deprecation of local PAC file. The approach consists of 2 steps:
  • Step 1: [source: Understanding Web Proxy Configuration] Via registry value 'HKLM\SOFTWARE\Policies\Microsoft\Windows\CurrentVersion\Internet Settings\EnableLegacyAutoProxyFeatures' specify that it is ok to utilize the legacy Internet Settings. Note that in our company the regular users are not enabled to set this, as they are not administrator on their workstation. But the occasional usage of local proxy file is then also not for these regular end-users.
  • Step 2: [source: local proxy which is set in the proxy auto config of IE11 is not processed correctly by IE11] Create the local PAC file as "c:/windows/system32/drivers/etc/proxy", and specify in IE as setting for automatic configuratio script = 'file://c:/windows/system32/drivers/etc/proxy'

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