GFW failures are both persistent and intermittent: four client/server pairs showed all 22 hourly measurements over a full day returning No-packets-dropped (entirely unblocked), while many others showed only sporadic failures. Temporal analysis showed failures cluster in bursts of hours, with probability of a second failure decaying sharply beyond ~5 hours after the first.

The GFW blocks Tor primarily by dropping SYN/ACK segments entering China from blacklisted IP/port pairs, not by dropping SYN segments leaving China. Of 142,802 CN→Tor-Relay measurements, 81.52% were Server-to-client-dropped versus only 0.55% Client-to-server-dropped. Blocking Tor directory authorities also showed substantial Client-to-server drops (19.61%), suggesting authorities may be treated differently.

§4.1, Table 1 detection

GFW filtering failures — cases where blocked Tor traffic passed through — showed no conspicuous geographic patterns across China. The maximum observed Pearson correlation coefficient between neighboring clients' failure counts was 0.26 (near-zero), and failure cases were geographically distributed in proportion to Internet penetration, not clustered by province or ISP region.

§4.2, Figure 6, Figure 8 evaluation

Routing is the dominant structural factor in GFW failures. CERNET (the Chinese Educational and Research Network) accounted for 503 of 135 destination IPs' failures — by far the most of any network — and packets transiting CERNET→CERNET links reached Tor destinations at an r=0.9896 ratio, near 1.0. Within CHINANET and CNC Group backbones, the Tor-to-non-Tor traversal ratio dropped to 0.403 and 0.272 respectively (Table 4), indicating heavy intra-ISP filtering.

§5.2, §5.3, Table 3, Table 4 evaluation

The hybrid idle scan technique converts approximately 1% of the total IPv4 address space into passive measurement vantage points without requiring control of either the censored client or the destination server, enabling full bipartite connectivity measurements across 161 geographically stratified Chinese clients and 176 servers over 27 days. After data pruning for quality, 36% of raw measurements were usable; ARMA modeling was sufficient (over Hidden Markov Models) because only level-shift detection was needed.

§2.3, §3.2.1, §4 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

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

Brussee measures a systematic pattern of Chinese government websites actively blocking access from outside China (the "reverse Great Firewall"), publishing a CSV dataset of affected domains (available at zenodo.org/records/18172145). The paper frames this outbound geo-blocking as a cybersecurity-motivated practice — Chinese authorities classify foreign access to domestic government infrastructure as an attack surface — distinct from the inbound information control goal of the GFW.

2026-brussee-reverse-great-firewall §3, §5 ip-blockingasn-blackholing

Brussee develops a conceptual framework distinguishing two logics of government geo-blocking: (1) information control (blocking inbound foreign content from domestic users) and (2) data sovereignty / attack-surface reduction (blocking outbound access by foreign actors to domestic systems). Chinese government site blocking of external IPs is motivated primarily by the second logic, creating an asymmetric internet topology where CN citizens cannot reach the outside world, and outside actors cannot probe CN government infrastructure.

2026-brussee-reverse-great-firewall §2, §4 ip-blockingasn-blackholing

ShieldShare demonstrates that an Android application can route all hotspot-client traffic through a VPN tunnel without root access by using a SOCKS5/HTTP/HTTPS proxy layer between the hotspot and the VPN, with per-client traffic accounting and quota management. The system works because Android's native hotspot does not forward VPN routing tables to connected clients; ShieldShare interposes a proxy that handles this. Released as open-source.

2026-edorh-shieldshare §Abstract ip-blocking