Scenario

From time to time we come across cases where customers complain that the proxy exception list does not work for certain URLs and (Winhttp) clients still try to connect to the destination server using the proxy instead of going directly.

Affected applications vary, we have seen issues with outlook 2007, SCCM, but the list might not be limited to those two – any Winhttp application might show the same symptoms.

Upon investigating these issues we found that the problem is a case-sensitivity issue – applications might provide an uppercase (or mixed-case) name as URL, whereas the ISA generated pac file works with lowercase exception names (no matter how you entered them in the UI) and case sensitive comparison.

The behavior of TMG is by default the same as that of ISA – however, we now have an option to influence whether we perform case sensitive comparison.

ISA behavior for generating the pac file

Luckily, a new property ConvertUrlToLowerCase was added to the IFPCClientAutoScript3 interface (see http://msdn.microsoft.com/en-us/library/ff824480(VS.85).aspx) to allow lower-casing the URLs passed to the routing script before performing comparison, thus making the routing script case-insensitive.
This behavior is OFF by default. You can however enable this with the following script, provided here as an example.

'
' set wpad script to lowercase its input url – for Internal network
'
set fpc = CreateObject("FPC.ROOT")
set net_internal = fpc.GetContainingArray().NetworkConfiguration.Networks("Internal")
set wpad = net_internal.ClientConfig.Browser.AutoScript
wpad.ConvertUrlToLowerCase = -1
wpad.save

http://blogs.technet.com/b/isablog/archive/2010/11/11/case-sensitivity-of-isa-tmg-generated-proxy-auto-configuration-pac-files.aspx