CacheBrowser bypasses GFW DNS poisoning by directly fetching CDN content from known edge server IPs, using a low-bandwidth out-of-band bootstrapper to seed its edge-server database. The SWEET email-based bootstrapper achieves median 5.4-second resolution latency with 95% of queries answered within 10 seconds across 100 runs—acceptable because CDN provider migrations occur only every few months.

Of GFW-blocked websites in the Alexa top 1000, 82% are already hosted on CDN infrastructure; for news websites specifically, the figure rises to 85%. This was measured by scraping GreatFire.org blocked-site data and verifying CDN hosting for each domain.

§5.2 evaluation

Akamai's China-based edge servers self-censor, returning HTTP 403 for GFW-forbidden content, while Akamai's mapping system (located outside China) returns valid edge server IPs to Chinese users even for forbidden domains, and non-Chinese Akamai edge servers serve all content freely. This partial self-censorship structure is driven by the requirement to operate CDN infrastructure inside China.

§3.2.1 detection

CacheBrowser achieves significantly lower download latency than Tor when fetching CDN-hosted content from China, because content is retrieved directly from CDN edge servers without traversing third-party proxies. Fetching from non-default alternative CDN edge servers increases latency relative to the CDN-mapped optimum, but the overhead is not prohibitive for real-world browsing; geographically proximate alternative servers minimize the penalty.

§5.3 evaluation

The GFW universally uses DNS poisoning rather than IP blocking to censor CDN-hosted content. Across all tested CDN providers (Akamai, CloudFlare, CloudFront, EdgeCast, Fastly, SoftLayer), no CDN edge server IPs were IP-filtered, because a single provider like Akamai hosts content on 170,000 shared edge servers—blocking any IP would collaterally block hundreds of thousands of unrelated publishers.

§3.2, §3.3 detection

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

Article 19 documents that Iran combines technical filtering with formal coercion of major foreign platforms (including Telegram, Instagram, and WhatsApp) to comply with content removal orders under threat of full blocking. The report notes that Iran's 2022 Women Life Freedom protests accelerated platform blocking when foreign operators refused compliance, demonstrating that the censorship system operates in two modes: coerce-and-allow for compliant platforms, block for non-compliant ones. Domain fronting via these platforms is therefore subject to sudden revocation if political conditions change.

2026-article19-tightening-the-net §6 ip-blockingdns-poisoningthrottling

Both Firefox and Chromium leak cleartext DNS before establishing encrypted DNS connections: they first send an unencrypted UDP DNS query to resolve the DoH server's domain (e.g., doh.opendns.com). An in-path censor can intercept and poison this initial query, making encrypted DNS in browsers completely ineffective without additional circumvention of the resolver-lookup step. Additionally, Chromium always includes the SNI extension in the encrypted DNS TLS handshake (e.g., "doh.opendns.com"), leaking the resolver identity even after the initial lookup. No resolver requires SNI to be present for certificate validation when the resolver's IP certificate is configured.

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DNS censorship of encrypted protocols is inconsistent in both China and Iran. In China, Yandex resolvers are censored only when the SNI extension is present; omitting SNI bypasses censorship for these resolvers. In Iran, DoH requires SNI omission for Quad9, Google, Adguard, CleanBrowsing, and NextDNS resolvers, but works with SNI for Yandex and Cisco resolvers. These inconsistencies suggest resolvers have been accidentally missed by censors, highlighting the value of automated tools that trial all resolver-mode combinations rather than hard-coding a single strategy. The support evaluation found 47 resolvers supporting DoH, 16 supporting DoH3, and only 8 supporting DoQ out of ~65 tested.

2026-lange-towards §6.2, Table 2 dns-poisoningsni-blockingip-blocking

DPYProxy-DNS tested 8 circumvention modes against DNS censorship from vantage points in Iran (AS201295, Mashhad) and China (AS4837, China Unicom). In Iran, DoQ was entirely uncensored even with the SNI extension present; DoH3 worked for all Cloudflare and NextDNS resolvers. Iran's censor operates in-path (not on-path like the GFW), making the "Last Response" mode (wait 3s for the last UDP reply) ineffective in Iran but highly effective in China. Auto-mode averaged 12.32s (median 8.28s) in Iran and 13.78s (median 12.90s) in China to discover a working combination.

2026-lange-towards §6.2, §6.3 dns-poisoningsni-blockingip-blocking