A single malicious Tor middle router advertising 10 MB/s bandwidth discovered 2,369 distinct bridges in 14 days. The catch probability is determined solely by the aggregated bandwidth M = k·b of malicious middle routers regardless of how that bandwidth is distributed across nodes: three routers at 10 MB/s each achieve strictly greater catch probability than 512 nodes at 50 KB/s each. This means a well-resourced single node is equivalent to or surpasses hundreds of low-bandwidth Sybil nodes.

With 512 PlanetLab nodes each advertising 50 KB/s as malicious Tor middle routers, the theoretical catch probability that at least one bridge circuit traverses a controlled node reaches P(512, 50, 30) ≈ 99% after only 30 circuits. In real-world validation, the 21st circuit created by a bridge client traversed one of the 512 controlled PlanetLab nodes, matching theory. The result generalizes: the 30-circuit exposure threshold applies to any adversary whose nodes' aggregated bandwidth reaches the equivalent of 512 × 50 KB/s = ~25.6 MB/s.

§IV-B, §V-B, Fig. 7, Theorem 4 detection

Tor's bandwidth-weighted path selection creates a structural amplification: 60% of middle routers selected across 430 circuits had bandwidth above 1 MB/s, yet only 10% of all Tor routers exceed 1 MB/s. This skew means that an adversary advertising a single high-bandwidth middle node achieves selection probability far exceeding its proportional count in the network, making high-bandwidth Sybil nodes highly cost-effective for bridge discovery.

§V-B, Fig. 8, Fig. 9 detection

The paper identifies three countermeasure classes against bridge discovery: (i) CAPTCHA on email/HTTPS distribution (limited by automated solving services); (ii) uniform random middle-node selection, which defeats bandwidth-Sybil attacks but degrades Tor throughput by routing through low-bandwidth nodes; (iii) DHT-based P2P architecture where no central server holds all bridge IPs, making systematic enumeration infeasible—though DHT systems introduce Sybil and eclipse-attack vulnerabilities of their own.

§VI defense

Large-scale email and HTTPS enumeration of Tor bridges using 500+ PlanetLab nodes and 2,000 Yahoo accounts discovered 2,365 distinct bridges over approximately one month. The bridge https server rate-limits distribution to 3 bridges per 24-bit IP prefix per day, and the email server to 1 reply per account per day; these controls are circumvented by sourcing requests from hundreds of distinct prefixes. Bridge distribution follows a weighted coupon collector model proportional to bridge bandwidth, not uniform probability.

§III-B, §III-C, §IV-A, §V-A 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

Chinese users treat full proxy/VPN (Shadowsocks, V2Ray/Clash, commercial VPNs) as the '终极大杀器' (ultimate solution) for bypassing GitHub DNS poisoning, implying that lighter-weight DNS-only fixes fail in some network environments where the censor adds firewall-layer blocking beyond DNS.

2026-anon-6-github-dns §3 使用代理或 VPN dns-poisoningip-blocking

Rule-based proxy tools (Clash, Shadowsocks, V2Ray) are documented as the most reliable solution for accessing GitHub from China, with split-tunneling rules routing only GitHub traffic through the proxy while keeping domestic traffic on the direct path. Git command-line tools require explicit proxy configuration (git config --global http.proxy http://127.0.0.1:7890) to route clone/push operations, as they do not inherit system proxy settings automatically.

2026-anon-github-2026-6-dns §方法二 dns-poisoningip-blocking

The GFW blocks GitHub via DNS poisoning across at least four domains — github.com, assets-cdn.github.com, github.global.ssl.fastly.net, and raw.githubusercontent.com — causing connection timeouts and page-load failures for mainland China users. The block is persistent as of February 2026, affecting both browser access and command-line git operations.

2026-anon-github-2026-6-dns Introduction / §方法一 dns-poisoningip-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