Automatic client proxy configuration

In the mid '90s, Netscape developed Javascript to address client-side processing (do NOT get Javascript confused with Java, SUN's powerful cross-platform language).

This (at the time) added a huge amount of functionality to web pages, in that some of the page could be 'dynamic'. The browser could understand this JavaScript code and it had the responsibility for processing the code. On the face of it, it looked like an answer to 'static' boring web pages, but of course, came with a whole host of inherent issues. Nonetheless, JavaScript is part of web programming today and I don't think it's going to go away in a hurry.

We can use this functionality in configuring our browser. If visiting 100 client machines fills you with dread, then this may be one way of solving the problem. Of course, you will need to visit your 100 clients at least once, but if anything changes in the future, no need to revisit them. There are of course other ways of skinning this cat (like editing the registry on Windows machines using an automated logon script), but we're certainly not going to delve into that here.

Here I am referring to a proxy.pac script, which is a very simple script, understood by most (if not all) modern browser that will automatically configure the browser for the Internet via your proxy. Of course it does not need to be called proxy.pac, but I use that name here, so let's go with the flow.

In order to ge this right, you will need to:

  1. Have a web server set up on your network which will contain the proxy.pac script

  2. Configure your browser (as in the GUI configuration above) to use Automatic Proxy configuration URL. In this box, you will need to enter the url, "" where platapus is the name of the web server.

I've included my simple proxy.pac script below, so take a look at it, and then visit for a list of functions that can be included in your own proxy.pac script.

My proxy.pac script:

function FindProxyForURL(url, host) {
        if ( isInNet(host, "", "") ||
                dnsDomainIs(host,"") ||
                dnsDomainIs(host,"") ) {
                return "DIRECT";
        else {
                return "PROXY";

This script will set the browser to go directly to sites in the 192.168.1.x network or in the domains or domains, bypassing the need for the proxy. If however, the client machine tries to go to anything else, this script will have configured the browser to use the proxy Bingo, your 100 (or more) browsers are configured. If at a later stage you split your network, putting in a secondary proxy, all that needs to change is the proxy.pac file. No need to ever revisit the individual client workstations.