A Proxy Automatic Configuration (PAC) file is that acts as a URL/site routing control file for a web browser. Simply put: The PAC controls how the browser retrieves an object– directly, bypassing the proxy or via a proxy. The PAC can also return different proxy servers based on conditions found in the file.
PAC file are most commonly used in corporate environments with complex web environments. Chances are, if your company has a proxy it has a PAC file. PAC files can also be used in the home to block specific unwanted web content - Banner pages, malware, etc.
PAC files are written in the JavaScript programming language. Many common JavaScript functions can be used when developing a PAC file.
PAC files are the heart and soul of a large corporate network. They control the browser. A well-written and carefully-planned PAC file can be simple and efficient, yet offer tremendous flexibility. An organically-grown PAC file, made of a number of thrown-together statements might work, but can result in less effient processing and significantly more network bandwidth for distribution.