As a UK resident, I have had to resort to workarounds in order to access certain US-based sites, due to geo-blocks caused by GDPR “compliance”.

On my home network I run a Squid proxy that can redirect all traffic through a separate USA-based HTTP proxy.

I’d rather only redirect certain traffic through the proxy (only sites that actually return HTTP 451 Unavailable For Legal Reasons). To achieve this I use PAC (Proxy Auto Configuration) with a curated list of sites that I want to access;

// wpad.dat

function FindProxyForURL(url, host) {

  if (isPlainHostName(host) || shExpMatch(host, "*.local"))
    return "DIRECT";

  var usaSites = [
    "nydailynews.com",
    "fayobserver.com",
    "wthr.com",
    "simpsonsworld.com",
    "bowmanextra.com",
    "baltimoresun.com",
    "providencejournal.com",
    "wdrb.com",
    "journaltimes.com"
  ]

  for (i = 0; i < usaSites.length; i++) {
    if (dnsDomainIs(host, usaSites[i])) return "PROXY nas.dipple.local:3128"
  }

  return "DIRECT";

}

I host this file in GitHub, and a scheduled task clones the latest version from my private repository and hosts it on my local network as wpad.dipple.local/wpad.dat.

I have messed this file up a few times in the past while making changes, as I didn’t have a way of easily validating the file for incorrect syntax.

The pac-resolver package in NPM is perfect for automating PAC file validation - it will parse a PAC file, then tell you which value the PAC file will return for a specific host.

Using this module, I built the following mocha test suite;

const assert = require('assert');

var fs = require('fs');
var pac = require('pac-resolver');

const testPAC = async (pacFile, endpoint) => {
  const FindProxyForURL = pac(fs.readFileSync(pacFile))
  return await FindProxyForURL(endpoint)
}

describe('Test PAC file wpad.dat', function() {
  it('Default to DIRECT', async function() {
    assert.strictEqual(await testPAC("wpad.dat","http://www.google.com"), "DIRECT");
  });

  it('Switch proxy for certain USA-based sites', async function() {
    assert.strictEqual(await testPAC("wpad.dat","http://nydailynews.com"), "PROXY nas.dipple.local:3128");
  });

});

These tests can be executed using the following command;

./node_modules/mocha/bin/mocha test

I then integrated these tests into a Github action. When I open a Pull Request, I can see if the changes will break the file in the main branch.