What the Autoconfiguration File Does

The autoconfiguration file is written in JavaScript, a compact, object-based scripting language for developing client and server Internet applications. The browser interprets the JavaScript file.

When the browser is first loaded, it downloads the autoconfiguration file. The file can be kept anywhere that the browser can get to it by using a URL. For example, the file can be kept on a web server. The file could even be kept on a network file system, provided the browser can get to it using a file:// URL.

The proxy configuration file is written in JavaScript. The JavaScript file defines a single function (called FindProxyForURL) that determines which proxy server, if any, the browser should use for each URL. The browser sends the JavaScript function two parameters: the host name of the system from which the browser is running and the URL it is trying to obtain. The JavaScript function returns a value to the browser that tells it how to proceed.

By using an autoconfiguration file, you can specify different proxies (or no proxy at all) for various types of URLs, various servers, or even various times of the day. In other words, you can have multiple specialized proxies so that, for example, one serves the .com domain, another the .edu domain, and yet another serves everything else. This method enables you to divide the load and use your proxies’ disks more effieciently because only a single copy of any file is stored in the cache instead of multiple proxies all storing the same documents.

Autoconfiguration files also support proxy failover, so if a proxy server is unavailable, the browser will transparently switch to another proxy server.