Censorship Detector Extension

A browser extension to detect website censorship methods as you surf.

Demo

demo.mp4

The story

In 2021, the Lebanese government blocked local exchange rate websites. Some reports said that the internet service providers were using DNS filtering (i.e. blocking the websites by tampering the DNS answers). However, upon testing my own network, I found something peculiar: even when I changed my local DNS resolver, the websites were still blocked. Ofcourse, one can imagine how that would happen over HTTP—a censor would filter based on the host header. Yet, the websites were also blocked over HTTPS! Upon further inspection, it turned out to be SNI (Server Name Indication) filtering; in other words, my ISP was filtering the requested hostname in the TLS handshake.

I wondered what other censorship methods Lebanese ISPs were using. I got in touch with friends on other networks, but there was a problem: running traditional censorship detection tools was either too technical or yielded inaccurate results. Hence this extension!

The extension can currently detect DNS, HTTP, HTTPS, and IP filtering. It is built with a modular architecture, allowing for other checks to be added in the future. Do note that it is highly experimental and relies on testing the limits of some browser extension APIs, so use it critically.

But why a browser extension?

There are multiple benefits to detecting censorship methods from a browser extension:

• Browser extensions are easy to install
• We can continuously detect censorship, since the extension sees all browser traffic
• The extension doesn't rely on a fixed set of URLs, allowing for ubiquitous censorship detection
• It is technically cool

Installation

Apologies for having to build the extension and create a release! The extension is still in an experimental phase.

1. Build the browser extension:

$ npm run build

2. Create the releases:

$ npm run release

The releases will be located at /releases/${BROWSER}/. You can distribute those freely.

3. To load into Firefox, browse to about:debugging#/runtime/this-firefox and load the Firefox release zip file via Load Temporary Add-on.... To load into a Chromium browser, unzip the Chrome file, browse to chrome://extensions, and Load unpacked the unzipped directory.