Proxy.Pac files can be tricky. The way to identify the client is:
myIPAddress()
But this fails occassionally, reporting the LocalHost address instead, especially in Linux.
Additionally, on Windows 7 multiple enabled interfaces can lead to the wrong one reporting back.
There is no fix but DISABLING the inappropriate interfaces and re-enabling them can work.
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harlekwinblog
Crib Sheet 
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