The CleanFeed first stage populates its IP blocklist by automatically resolving hostnames from the IWF database via DNS. Content providers can serve false DNS results pointing to high-traffic third-party IP addresses (e.g., Google cache servers at 66.102.9.104), causing the first stage to redirect legitimate traffic through the proxy. Automated IP-update processes cannot reliably distinguish a genuine IP migration from a spoofed DNS result, and this can cause legitimate sites to be blocked collaterally.

A hybrid two-stage blocking system (IP-redirect first stage, URL-proxy second stage) can be exploited as an oracle to enumerate blocked IP addresses by sending TCP packets with a TTL sufficient to reach the first-stage redirector but insufficient to reach the destination. Non-redirected IPs return ICMP TTL-expired from an intermediate router, while redirected IPs return a SYN/ACK from the web proxy impersonating the destination. A live scan of a /24 subnet confirmed 17 redirected IP addresses, yielding 91 associated hostnames across 9 of those IPs.

§5.2 defense

Using a simple dialup connection, the CleanFeed oracle scan enumerated IP addresses at up to 98 addresses/second. At this rate, the ~8.3 million Russian IP addresses (the IWF reported 25% of known illegal sites were hosted in Russia) could be scanned in under 24 hours, and the full routable IPv4 space (32% of 2^32 addresses) in approximately 160 days. A suitable filtered dialup account was available for free, with phone costs under £15.

§5.2 evaluation

The hybrid two-stage design's architectural vulnerability is that circumventing either stage independently defeats the system: end-users can tunnel via Tor or JAP to bypass both stages entirely, while content providers can serve different content to IWF crawlers versus real users, exploiting the fact that only 33% of IWF hotline reports were substantiated as potentially illegal. The system's precision is entirely contingent on content-provider cooperation, which cannot be assumed.

§4, §6 defense

Brightview's countermeasure requiring a minimum probe TTL of 24 (to prevent low-TTL scans from stopping at the proxy) was bypassed by sending probes with TTL=128 and examining the TTL of returned SYN/ACK packets. The UK web proxy consistently returned TTL=49 (64−15 hops), while Russian destination servers returned TTL=45–49 or TTL=113–238 depending on initial OS TTL settings. The two populations were cleanly distinguishable, defeating the fix with no change to scan logic beyond raising the probe TTL.

Postscript defense

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

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

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.

2026-niere-dpyproxy-dns §6.3 dns-poisoningsni-blockingip-blocking