TCP segmentation — splitting DNS-over-TCP messages into 20-byte fragments — successfully circumvented DNS censorship for 40 of 41 tested resolvers in China. In Iran, TCP segmentation is inconsistently effective: it succeeds in some scan runs and fails entirely in others, suggesting the Iranian censor can reassemble TCP fragments when processing capacity permits.

All major browsers (Firefox, Chromium) issue an unencrypted DNS-over-UDP query to resolve their configured DoH resolver's domain before initiating any encrypted DNS session. In Iran, nearly all tested DoH resolver domains are directly censored at the DNS layer (returning block-page IPs), which renders browser-native encrypted DNS ineffective regardless of whether the underlying encrypted protocol would otherwise succeed. Additionally, browsers always include the SNI extension in TLS handshakes with DNS resolvers even though no tested resolver requires it.

§7 detection

DPYProxy-DNS's automated probe-and-select mode identified a working DNS circumvention in an average of 13.78 seconds (median 12.90s) in China and 12.32 seconds (median 8.28s) in Iran across 100 runs each; best-case startup was 0.32s (China) and 0.47s (Iran) when the first-tried combination succeeded, while worst-case exceeded 30.72s in China and 58.16s in Iran due to the slow Last Response mode (3s fixed wait per attempt) being selected early in the randomized probe order.

§6.3 evaluation

The GFW operates as an on-path censor that injects forged DNS responses faster than the real resolver but cannot suppress the legitimate response from also arriving. Waiting approximately 3 seconds and accepting the last-received UDP response circumvented GFW DNS injection for 40 of 41 tested public resolvers in China; the single exception (Cloudflare 1.1.1.1) was IP-blocked via packet dropping rather than injection racing.

§6.2.1 defense

Iran's DNS censorship is largely ineffective against encrypted DNS: DoQ is not censored at all (with or without SNI present), DoH3 works for all tested Cloudflare and NextDNS resolvers, and most DoT/DoH resolvers work when the SNI extension is omitted. Iran's censorship of unencrypted DNS is in-path (queries never reach the real resolver), which means the GFW-style 'last response' technique fails entirely in Iran because the client's original query is dropped before reaching its destination.

§6.2.2 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

TCP segmentation (splitting a DNS message into 20-byte TCP fragments) successfully circumvented DNS censorship in China for nearly all resolvers that support TCP. In Iran, TCP segmentation was only partially effective due to the censor's ability to reassemble TCP fragments when system load permits—some runs succeeded completely, others failed entirely across all resolvers. The "Last Response" mode (wait 3 seconds for the final UDP reply) was highly effective against China's on-path GFW injector for all resolvers except the fully IP-blocked Cloudflare 1.1.1.1 resolver.

2026-lange-towards §4.1, §6.2.1, §6.2.2 dns-poisoningdpi

Of 6,915,266 domains extracted from the 572 GiB Geedge Networks Leak (GNL), 298,955 censored domains (93.7% of all GNL-censored domains) appear in neither Tranco top-1M nor CitizenLab test lists. Measurements across China (Guangzhou/Nanjing), Myanmar, Pakistan, and Algeria confirmed censorship via DNS injection and SNI-based TLS connection termination. The GNL covers 25–62% of Tranco-censored domains across countries, showing substantial but incomplete overlap. This vendor-side ground truth reveals a censorship surface roughly two orders of magnitude larger than curated academic test lists.

2026-sheffey-geedge §4.1, Table 2 dpisni-blockingdns-poisoningmeasurement-platform

TTL-based path analysis showed that all censorship actions (DNS poisoning, HTTP injection, TLS resets) in the June 2025 shutdown occurred at the same network hop across all tested ISPs, indicating a single centralized national border gateway—likely TCI AS gateways—rather than per-ISP enforcement. Global BGP announcements were kept intact throughout, making the shutdown invisible to routing monitors while domestic connectivity collapsed.

2025-aryapour-stealth-blackout §4.5 dpidns-poisoningrst-injection

Over 90% of tested censored domains returned private IP addresses in the 10.10.34.0/24 range (chiefly 10.10.34.34) via injected DNS replies during the June 2025 shutdown, with poisoned response TTLs often very low—consistent with inline DPI injection rather than a recursive DNS lookup. A small set of domains including Google and state-approved services were whitelisted and resolved correctly.

2025-aryapour-stealth-blackout §4.1 dns-poisoningdpi