How OONI Measures Internet Censorship

Imagine you're trying to send postcards to friends around the world, but some postcards never arrive. Was the postal service blocking them on purpose? Did the mail carrier simply lose them? Or is the postal system temporarily overwhelmed? Without a systematic way to test which postcards get through and which don't, you'd only know about the ones you never heard back from — and you'd never know the true pattern.

Internet censorship works similarly. Governments, internet service providers, and other intermediaries block access to websites, throttle traffic, or interfere with communication. But how do we know where and how this happens? Who measures it? And how do we distinguish between deliberate blocking and a legitimate outage?

The Open Observatory of Network Interference (OONI) is a global project that does exactly this kind of systematic testing. Run by volunteers and maintained by the Tor Project, OONI collects evidence of network interference — what the internet community calls "censorship" — and makes the data publicly available to researchers, journalists, and civil society organizations.

What OONI Actually Does

OONI operates a network of volunteer "probes" — small software programs that run on computers, servers, or routers around the world. These probes perform automated tests to see whether websites, apps, and online services are accessible from that location. When a volunteer installs the OONI Probe app (available for Android, iOS, and Linux), their device automatically tests connections to a curated list of websites and services. The probe records what happens: Did the request succeed? Was it redirected somewhere else? Did the server respond with an error?

The results are sent back to OONI's servers, where they are stored and eventually published in a searchable database called OONI Explorer. A researcher in Berlin can see whether a particular website was accessible in Egypt on a specific date. A journalist investigating a government crackdown can find evidence of what was blocked when. This transparency is the heart of OONI's work.

What makes OONI different from, say, simply asking people "Is this website blocked where you live?" is that it generates objective, verifiable evidence. A human can be mistaken or misremembered. An automated test produces data that others can examine, replicate, and audit.

How OONI Tells Blocking from Technical Problems

This is where the work becomes genuinely difficult. The internet fails for many reasons. A server might be down. A network connection might be interrupted. A request might time out because of legitimate latency. A website might be unreachable because its domain name server (the service that translates domain names like google.com into the numerical IP addresses that computers use to find each other) is misconfigured. How do you know if a government blocked a website or if it just happened to be offline that moment?

OONI uses a combination of techniques to make an educated judgment. When a probe tries to access a website and the request fails, OONI performs additional diagnostic tests. It might check whether the server itself is online by contacting it from a different network. It might test whether DNS resolution is being tampered with — that is, whether the system that translates website names into addresses is returning false information. It tests whether the connection is being reset or the response redirected to a different IP address.

If a website is consistently unreachable from many probes in a specific country, but accessible everywhere else in the world, that's strong evidence of blocking. If a website is unreachable from everywhere simultaneously, it's more likely to be a legitimate outage. If access is blocked for some users but not others within the same country, OONI can identify whether the blocking is happening at the ISP level or through a different mechanism.

But OONI itself is honest about the limits here. Some forms of censorship are deliberately subtle. "Deep packet inspection" — a technique where a network operator examines the contents of data packets to make decisions about what to block — can be designed to look like random connection failures. A government could intentionally drop connections in an unpredictable pattern to make it hard to distinguish blocking from a technical problem. OONI documents these ambiguities rather than guessing.

The Role of Test Lists

OONI doesn't test every website on the internet. Instead, it maintains curated lists of websites relevant to different regions. These "test lists" include websites about human rights, LGBTQ+ issues, news outlets, social media platforms, and government sites. Different countries have different lists that reflect the kinds of content that are typically targeted for censorship in those regions.

Who decides what goes on these lists? This is important. Test lists are crowdsourced; civil society organizations, journalists, and researchers in each country can suggest websites that are likely to be blocked or are culturally relevant to test. This helps make the testing locally meaningful rather than driven by outside assumptions.

But there's a tradeoff: If a website isn't on a test list, OONI won't test it, so blocking of that site won't be documented. OONI measures what it looks for, not everything that happens on the internet.

Who Uses OONI Data

OONI's public database is used by human rights organizations documenting censorship for advocacy, journalists investigating control of information during elections or protests, and academic researchers studying censorship patterns globally. During major political events — elections, demonstrations, internet shutdowns — OONI data often provides some of the first objective evidence of what was blocked and when.

The data isn't perfect, but it's transparent. Anyone can download it, examine it, and verify the findings themselves.

Why This Matters

Internet censorship is often invisible. When access to a website is blocked, most users simply see an error message and move on. Without systematic measurement, censorship remains anecdotal — a complaint here, a rumor there. OONI transforms individual experiences into documented patterns that inform policy debates, legal cases, and human rights investigations.

Understanding how OONI works means understanding both what we can know about censorship and what we can't — a crucial distinction in a world where information is both powerful and contested.

Next, you might explore how different countries implement censorship technically (DNS blocking, IP-based filtering, traffic analysis), how the Tor network relates to censorship measurement, or how internet shutdowns differ from selective site blocking.