We're looking for a person or people to help audit MDN to find places we could speed up. Is this you or someone you know? Check out the RFP: https://mzl.la/2IHcMiE

Use the proxy API to register an extended Proxy Auto-Configuration (PAC) file, which implements a policy for proxying web requests. This implementation deviates from standard PAC design in several ways because the de-facto specification for PAC files hasn't changed since its initial implementation circa 1995. There is no standards body maintaining the specification.

Note that Google Chrome provides an extension API also called "proxy" which is functionally similar to this API, in that extensions can use it to implement a proxying policy. However, the design of the Chrome API is completely different to this API. With Chrome's API an extension can register a PAC file, but can also define explicit proxying rules. Since this is also possible using the extended PAC files, this API only supports the PAC file approach. Because this API is incompatible with the Chrome proxy API, this API is only available through the browser namespace.

To use this API you need to have the "proxy" permission.

Communicating with PAC files

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)) {

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!");

PAC file specification

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.

FindProxyForURL() return value

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 objects. Each object has the following properties:

String. This must be one of: "http"|"https|"socks4"|"socks"|"direct". "socks" refers to the SOCKS5 protocol.
String. Hostname for the proxy to use.
String. Port for the proxy.
username Optional
String. Username for the proxy. This is usable with "socks". For HTTP proxy authorizations, use webRequest.onAuthRequired.
password Optional
String. Password for the proxy. This is usable with "socks". For HTTP proxy authorizations, use webRequest.onAuthRequired.
proxyDNS Optional
Boolean. If true, the proxy server is used to resolve certain DNS queries (only usable with "socks4" and "socks"). Defaults to false.
failoverTimeout Optional
Integer. Number of seconds before timing out and trying the next proxy in the array. Defaults to 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.

PAC file environment

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:

//  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)) {



Registers the given proxy script.
Unregisters the proxy script.


Fired when the system encounters an error running the proxy script.

Browser compatibility

ChromeEdgeFirefoxFirefox for AndroidOpera
onProxyError No No5555 No
register No No


55 *

55 No
unregister No No5656 No

Example extensions


Microsoft Edge compatibility data is supplied by Microsoft Corporation and is included here under the Creative Commons Attribution 3.0 United States License.

Document Tags and Contributors

 Contributors to this page: ericjung, vorwrath, wbamberg, yfdyh000, andrewtruongmoz, mwein
 Last updated by: ericjung,