Automatic proxy configuration support provides a form of transparency, in that clients do not have to configure a browser to point to a specific proxy or SOCKS server, but to a proxy automatic configuration (PAC) file. This lets the system administrator modify the configuration with little impact to the clients, who update their automatic configuration files and are automatically directed to the new configuration. Server administrators can use this to reroute requests when servers are down, to balance workload, to send requests for specific URLs to specific proxies, or other reasons specific to their installation. Note that new PAC files are only reloaded when a browser is restarted.
Automatic proxy configuration is a browser function that enables more dynamic server selection. The PAC file is a JavaScript file that consists of functions that get called by the client browser before each URL retrieval. The functions return values indicating whether a proxy server, SOCKS server, or a direct connection is used to service the request. The file can also redirect the request if the initial connection to be used is down. When a client's browser is set to autoproxy, it calls the JavaScript PAC file each time a URL is requested by the user.
The Proxy Auto-Configuration configuration page allows you to create a PAC file that contains some basic functions.