Efficient ad blocking via proxy autoconfig

proxy.pac.js

/* the network your computer is on, and the subnet mask of that network */
var local_network = '192.168.1.0';
var local_netmask = '255.255.255.0';

/* the host and port of your regular proxy, if any; otherwise an empty string */
var upstreamProxy  = '';

/* space-separated list of hosts or domains that should always be connected to directly without using a proxy */
var directZones  = 'plasmasturm.org delicious.com paulgraham.com yahoo.com';

/* space-separated list of hosts or domains that should never connected to */
var blockedZones = 'googlesyndication.com google-analytics.com snap.com ixnp.com adbrite.com doubleclick.com doubleclick.net hitbox.com 2o7.net';

/*-------------------------------------------------------------------*/

function TrimString(string) { return string.replace(/^ +| +$/,'') }
function ListFromString(list_string) { return TrimString(list_string).split(/ +/) }

local_network = TrimString(local_network);
local_netmask = TrimString(local_netmask);
upstreamProxy = TrimString(upstreamProxy);
directZones   = ListFromString(directZones);
blockedZones  = ListFromString(blockedZones);

function TryMatch(hosts, matcher) {
	for (var i = 0; i < hosts.length; ++i)
		if (matcher(hosts[i])) return true;
	return false;
}

function FindProxyForURL(url, host) {
	if (isInNet(host, '127.0.0.0', '255.0.0.0'))
		return 'DIRECT';

	var hostCheck    = function(domain) { return host == domain };
	var dnsZoneCheck = function(domain) { return dnsDomainIs(host, '.' + domain) };

	if (isPlainHostName(host) || TryMatch(directZones, dnsZoneCheck))
		return 'DIRECT';

	if (TryMatch(blockedZones, hostCheck) || TryMatch(blockedZones, dnsZoneCheck))
		return 'PROXY 127.0.0.1:1';

	if (isInNet(host, local_network, local_netmask))
		return 'DIRECT';

	if (upstreamProxy)
		return 'PROXY ' + upstreamProxy;

	return 'DIRECT';
}