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What is a .pac proxy auto config file

A .pac (Proxy Auto Config) file is a file stored on a web server that contains instructions for the browser, to help automate the proxy server setting configuration in a web browser.

The .pac proxy configuration file contains JavaScript code, and must contain one function called FindProxyForURL. This function accepts two parameters, a URL, and a Hostname.

This function returns either “DIRECT” to tell the client to connect to the web server without using a proxy, or “PROXY” followed by the proxy server address, to tell the client to use the specified proxy server. Combinations can be used, separated by a semicolon, to specify fallback options.

The most simple example of a .pac file would be:

function FindProxyForURL(url, host)
	return "PROXY; DIRECT";

This would tell the browser to use the proxy server on, port 3128, and fall back to direct access if the proxy server wasn’t accessible.

For an advanced example of a .PAC file, please check out:
Advanced .PAC proxy auto config file example

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