Proxy Auto-config - The PAC File

The PAC File

The Proxy auto-config file format was originally designed by Netscape in 1996 for the Netscape Navigator 2.0 and is a text file that defines at least one JavaScript function, FindProxyForURL(url, host), with two arguments: url is the URL of the object and host is the host-name derived from that URL. By convention, the PAC file is normally named proxy.pac. The WPAD standard uses wpad.dat.

To use it, a PAC file is published to a HTTP server, and client user agents are instructed to use it, either by entering the URL in the proxy connection settings of the browser or through the use of the WPAD protocol.

Even though most clients will process the script regardless of the MIME type returned in the HTTP request, for the sake of completeness and to maximize compatibility, the HTTP server should be configured to declare the MIME type of this file to be either application/x-ns-proxy-autoconfig or application/x-javascript-config.

There is little evidence to favor the use of one MIME type over the other. It would be, however, reasonable to assume that application/x-ns-proxy-autoconfig will be supported in more clients than application/x-javascript-config as it was defined in the original Netscape specification, the latter type coming into use more recently.

A very simple example of a PAC file is:

function FindProxyForURL(url, host) { return "PROXY proxy.example.com:8080; DIRECT"; }

This function instructs the browser to retrieve all pages through the proxy on port 8080 of the server proxy.example.com. Should this proxy fail to respond, the browser contacts the Web-site directly, without using a proxy. The latter may fail if firewalls, or other intermediary network devices, reject requests from sources other than the proxy; a common configuration in corporate networks.

A more complicated example demonstrates some available JavaScript functions to be used in the FindProxyForURL function:

function FindProxyForURL(url, host) { // our local URLs from the domains below example.com don't need a proxy: if (shExpMatch(host, "*.example.com")) { return "DIRECT"; } // URLs within this network are accessed through // port 8080 on fastproxy.example.com: if (isInNet(host, "10.0.0.0", "255.255.248.0")) { return "PROXY fastproxy.example.com:8080"; } // All other requests go through port 8080 of proxy.example.com. // should that fail to respond, go directly to the WWW: return "PROXY proxy.example.com:8080; DIRECT"; }