This is mostly by way of a "note to self". I've recently started working at a customer where connecting my computer to their network is not just allowed, but necessary. Once connected, if I want to use the Internet, I have to go through their filtering proxy - presumably to keep the badness of the Internet from their systems (and yes, they do pay a lot of attention to ensuring the machine is virus-free). Previously, when I worked there for a day or two, setting up the proxy was a minor irritation, but as I'm going to be there rather longer, the idea of reconfiguring my networking twice a day started to look pretty unattractive. My first attempt at solving this had been to have a couple of scripts that set up the proxy by making the relevant registry settings, but unfortunately, Windows doesn't pick these up immediately. Yeah - sure - if I could remember to run the scripts before shutting down it might work, but I'm not that obsessive.Or I could get Windows to pick up the settings by opening the various screens... Internet Options... Connections.... LAN Settings... oh wait... there had to be a better way.

It turns out that there's something called a Proxy Auto-configuration file. If you select "Automatically detect settings", then Windows will try and locate one of these on the network using the Web Proxy Auto-discovery Protocol, however the customer in question doesn't do this. My needs were simple enough, though, so I checked the next box down: the one that says "Use automatic configuration script". All that remained was to create the script.

It turns out that you write such things in JavaScript, and it's simply a matter of writing a function which is named in the PAC standard, and using other functions that are made available. Here's what I ended up with (although I'll probably add refinements):

function FindProxyForURL(url, host) {
	var customerProxy = "PROXY 10.62.40.42:1234";

	if (atCustomer()){
		if(dnsDomainIs(host, ".internal.customer.com") || dnsDomainIs(host, "localhost")|| dnsDomainIs(host,".local")){
			return "DIRECT";
		}
		else return customerProxy;
	} else {
		return "DIRECT";
	}
	
}

function atCustomer(){
	return isResolvable("server.not.on.external.dns");
	// or maybe
	// return isInNet(myIpAddress(), "10.62.0.0", "255.255.0.0"); 
}

Nothing fancy, but it works. I suspect I'll find a few edge cases where I maybe have to enhance the script or even configure things by hand, but for now I have the satisfaction of knowing I can just turn up, plug in, and start work.