National-level filtering is not homogeneous: the administrative burden of maintaining up-to-date filtering rules at national scale leads states to delegate implementation to regional authorities or individual ISPs, producing measurable filtering differences between geographic regions and providers within the same country.

Censorship mapping tools that detect filtering by probing blocked content create concentrated access patterns that are qualitatively different from normal user behavior, potentially exposing volunteers to scrutiny even in countries where individual access to filtered content would not ordinarily trigger enforcement action. The paper identifies this as a fundamental ethical tension intrinsic to any filtering measurement methodology.

§5 policy

Censors responding to encryption-based circumvention have two escalation options: block all encrypted connections outright, or identify the underlying protocol via traffic signatures that persist even inside encrypted tunnels. The paper frames these as the two dominant censor responses to DPI being defeated by encryption.

§3 detection

Open DNS resolvers, widely available across the internet as public services, make DNS poisoning trivially detectable globally: a researcher can connect to a resolver in a target country and compare responses against a trusted reference resolver, without requiring volunteer proxies or in-country infrastructure.

§4.2 evaluation

At the time of writing, the Tor network had no publicly announced exit nodes located on the Chinese mainland, making direct Tor-based measurement of GFW filtering unavailable. The paper generalizes this: heavily filtered countries show systematically low availability of relay services, precisely where measurement need is highest.

§4.2 evaluation

The June 2025 Iran shutdown—carried out during the Iran-Israel war beginning ~June 19—did not use BGP route withdrawals as in 2019. Instead, authorities applied service-level restrictions at the national border: DNS poisoning of foreign destinations, protocol whitelisting permitting only pre-approved domestic services, and DPI to block circumvention-tool traffic. Iran's international traffic fell roughly 90% while the country's BGP routes remained advertised, making the shutdown invisible to BGP-based monitoring systems. OONI measurement volume, which totalled 121,333 in June 2025, collapsed to under 200 submissions on June 19-20.

2025-iran-shutdown-measurement Executive Summary / §2 dpidns-poisoningport-blockingip-blocking

Article 19 documents that Iran's National Information Network (NIN / SHOMA) was designed with explicit reference to China's Great Firewall as a model, with institutional mirroring: Iran's Supreme Council of Cyberspace parallels China's Cyberspace Administration of China, and both governments share a "cyber sovereignty" doctrine used to justify domestic content controls and cross-border technology transfer. The report frames Iran's filtering infrastructure as deliberately architected to replicate GFW capabilities, not as an independently developed system.

2026-article19-tightening-the-net §2, §3 dpisni-blockingdns-poisoningip-blockingbgp-hijackthrottling

GFWeb tested 1.02 billion domains against the GFW over 20 months and discovered 943,000 pay-level domains blocked by HTTP filters and 55,000 by HTTPS filters — the largest GFW blocklist dataset ever published. The HTTP-to-HTTPS ratio (17:1) confirms that the GFW's HTTPS keyword-based and SNI-based blocking covers far fewer domains than its HTTP host-header blocking, likely because HTTPS blocks carry higher collateral-damage risk.

2024-hoang-gfweb Abstract, §5.1 dpidns-poisoningip-blockingkeyword-filteringsni-blocking

GFW verification tests confirmed over 90% of OONI-detected DNS anomalies as true blocks: 429/457 domains in Beijing and 422/461 in Shanghai. In total, 527 unique domains were confirmed censored via DNS, HTTP, and HTTPS filters; an additional 718 domains suspected blocked due to IP-address-level blocking of their hosting servers rather than domain-level entries.

2024-tang-automatic §5.6 dns-poisoningip-blockingdpi

The largest measurement study of Turkmenistan censorship to date tested 15.5 million domains and found more than 122,000 domains censored using separate blocklists for DNS, HTTP, and HTTPS. Reverse-engineering the blocking rules revealed approximately 6,000 over-blocking rules that cause incidental filtering of more than 5.4 million additional domains — a 44x collateral damage ratio relative to intentionally blocked domains.

2023-nourin-measuring §Results dns-poisoningdpikeyword-filteringip-blocking

OONI data shows anomaly rates in Russia's top five ASes (including Rostelecom AS12389, Vimpelcom AS8402) rose from roughly 7–11% in January and early February 2022 to 12–21% in mid-March 2022, with social-media and news domains such as Facebook, Twitter, Instagram, and BBC going from available to near-completely blocked after the invasion.

2023-ramesh-network §4.1 dpirst-injectionip-blockingdns-poisoning