proxy.register()

You can exchange messages between the PAC file and your extension's background page (or any other privileged pages, like popup pages) using runtime.sendMessage() and runtime.onMessage.

To send a message to the PAC file, you must set the toProxyScript option:

// background.js

// Log any messages from the proxy.
browser.runtime.onMessage.addListener((message, sender) => {
  if (sender.url === browser.extension.getURL(proxyScriptURL)) {
    console.log(message);
  }
});

let messageToProxy = {
  enabled: true,
  foo: "A string",
  bar: 1234
};

browser.runtime.sendMessage(messageToProxy, {toProxyScript: true});

// pac.js

browser.runtime.onMessage.addListener((message) => {
  if (message.enabled) {
    browser.runtime.sendMessage("I'm enabled!");
  }
});

The basic PAC file syntax is described in the PAC documentation, but the implementation used by the proxy API differs from standard PAC design in several ways, which are described in this section.

The standard FindProxyForURL() returns a string. In Firefox 55 and 56, the PAC file used with the proxy API also returns a string. In Firefox 55 only, you must pass an argument to the "DIRECT" return value, even though it doesn't need an argument.

From Firefox 57 onwards, FindProxyForURL() may still return a string, but may alternatively (and preferably) return an array of proxy.ProxyInfo objects.

If the array contains more than one object, then all ProxyInfo objects after the first one represent failovers: if the proxy at position N in the array is not reachable when its ProxyInfo.failoverTimeout expires, then the browser will try the proxy at position N+1.

For example:

const proxySpecification = [
  {
    type: "socks",
    host: "foo.com",
    port: 1080,
    proxyDNS: true,
    failoverTimeout: 5
  },
  {
    type: "socks",
    host: "bar.com",
    port: 1060,
  }
];

The first proxy in the array will be tried first. If it does not respond in failoverTimeout seconds, the next will be tried, until the end of the array is reached.

The global helper functions usually available for PAC files (isPlainHostName(), dnsDomainIs(), and so on) are not available.

Code running in the PAC file does not get access to:

• any DOM functions (for example, window or any of its properties)
• any WebExtension APIs except runtime.sendMessage() and runtime.onMessage
• the Console API - to log messages from a PAC, send a message to the background script:

//  pac.js

// send the log message to the background script
browser.runtime.sendMessage(`Proxy-blocker: blocked ${url}`);

// background-script.js

function handleMessage(message, sender) {
  // only handle messages from the proxy script
  if (sender.url !== browser.extension.getURL(proxyScriptURL)) {
    return;
  }
  console.log(message);
}

browser.runtime.onMessage.addListener(handleMessage);

url

String. URL pointing to the PAC file to load. PAC files must be bundled with the extension, and url must be relative to the extension's manifest.json file.

A Promise that will be fulfilled with no arguments when the PAC file has been registered, or rejected if there was an error.