Описание тега pac

A proxy auto-config (PAC) file defines how web browsers and other user agents can automatically choose the appropriate proxy server (access method) for fetching a given URL.

A PAC file contains a JavaScript function FindProxyForURL(url, host). This function returns a string with one or more access method specifications that cause the user agent to use a particular proxy server, or to connect directly.

This is an example of a very simple PAC file:

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

this PAC file 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 tries to contacts the Web-site directly, without using a proxy.

A more complicated PAC file can be like this:

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";
}