- The Practical Proxy PAC file guide

Looking for a simple, practical approach to understanding proxy PAC files?  Need to design, deploy or support a PAC file and don't really know how they work or what's going on?   Want some simple tips and things to watch out for?  Have a question you can't figure out and want to ask the experts?    You're in the right place - welcome to! 

I have spent a number of years designing, deploying and supporting proxies and PAC files for Fortune 100 companies. In those years, I have learned a number of hard lessons and gradually refined my PAC files to become (what I feel to be) well organized and highly efficient. I always had to create my PAC file structures by hand and never had any documents to refer to beyond the basic docs provided by a few vendors.   One of the things that frustrated me most was the lack of good, practical advice online.  There is a lot of basic material out there, but nothing that was written with a practical, engineering-focused mindset.   

This guide is an attempt to wrap up many of the things that I’ve learned to make it easier for others responsable for designing, deploying and maintaining PAC files.  

The content of this site is designed to expand upon the original guide published by Netscape and provide some more advanced examples and some fairly critical warnings. It is assumed that the serious reader has first read through the Netscape document and understands the basic functions listed there before trying to process the content here. There are a lot of functions listed that I don’t use, simply because I haven’t ever had the need. (i.e. time-based calculations), but I do use a lot of them, plus a few other tips and tricks from JavaScript that I’ve picked up over the years.  

JavaScript is, of course, a complex programming language.  It is tremendously flexible and can provide a lot of functionality.   I am, however, NOT a JavaScript developer and won’t ever be.   The target audience of this guide are engineers like myself with little or no formal development background.  If you’re a JavaScript developer you might find some useful tricks and tips here on interaction with proxies but you probably won’t be very interested in the sections that are more focused on the language itself.  

Speaking of things that I'm not...  Web designer.  I admit that this isn't the prettiest site in the world and it could certainly stand a few more bells and whistles, but I'm more focused on function rather than asthetics.  It works, you can navigate around and, hopefully, it has what you need.      I am, of course, always open to suggestions.  Have something that you'd like to see?  Head to the forums and add a post in the Off Topic section.  

September 2011 Update - Site Returns

In 2010, I accidentally let the domain "" expire, and it subsequently snapped up a domain shark.  They offered it back to me at the low, low price of $2,500.  At that point, I threw up my hands and let the site lie.  I had several people contact me, asking for assistance and when the site would be back up, so I 've brought it back online under a new domain.  

 July 31st, 2009 Update - New section on Proxy Load Balancing in a PAC file

I've done some reformatting and editing on the PAC File Tips and Tricks to clean it up and made some edits to improve things while I was at it.    The other day, I found a great article on proxy load balancing and, with permission from the author (Thanks Shaun!) I took some ideas from it and put them in a new section on load balancing between two proxies in a PAC file.    I also created a new sample PAC file with load balancing for a real-world example.   


This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

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The Practical Proxy PAC File Guide