China started blocking OONI


July 7, 2023 - ongoing
published by Maria Xynou, Elizaveta Yachmeneva on October 31, 2023

On 7th July 2023, China started blocking access to our website ( and censorship measurement app (OONI Probe).

The following graph aggregates OONI measurement coverage from the testing of across networks in China between 1st June 2023 to 25th October 2023.


Chart: OONI Probe testing of on 17 ASNs in China between 1st June 2023 to 25th October 2023 (source: OONI MAT).

As is evident, all measurements failed on 7th July 2023, and most measurements collected thereafter either failed or presented anomalies. While measurements are generally annotated as “failed” when the OONI Probe experiment fails to perform as expected (for example, as the result of a bug), such measurements can also be symptomatic of censorship. Meanwhile, measurements are annotated as “anomalies” when they present signs of internet censorship.

It’s worth noting that OONI measurements collected from the testing of in China were previously (mostly) successful, showing that was accessible on tested networks in China before 7th July 2023. Through a more detailed data analysis, we conclude that is blocked by means of both DNS injection and TLS interference. DNS blocking consists of injecting responses containing invalid IP addresses. TLS blocking consists of interfering with the TLS handshake and resetting the TCP connection.

China also appears to be attempting to prevent OONI Probe users from submitting measurements. For starters, this is suggested by the large and persistent drop in OONI measurement coverage from China, suggesting that most OONI Probe users in China may not have been able to submit measurements for publication.


Chart: OONI Probe Web Connectivity measurements collected from China between 1st June 2023 to 28th July 2023 (source: OONI MAT).

To block OONI Probe, ISPs in China would have to block our backend services, preventing OONI Probe users from submitting measurements. We therefore analyzed OONI data (collected through the use of Snowflake as a proxy) pertaining to the testing of our API and test helpers. As part of our analysis, we found that our API and test helpers are blocked by means of DNS tampering, TLS interference, and IP blocking.

In response to the block, we shared circumvention advice. While access to remains blocked in China, the overall OONI measurement coverage from China has increased in recent months nonetheless.

Learn more through our research report.