Some obfs4 bridges exhibited a roughly 24-hour periodic semi-blocking pattern from China, where bridges cycled between reachable and blocked states with a ~24-hour period. This diurnal pattern differed between the two China probe sites and between bridges, and one blocking failure coincided with a documented nationwide GFW outage that also briefly restored access to Google services.

All bridges in a given Tor Browser release batch were blocked simultaneously within a 20-minute window, and every blocking event occurred during China Standard Time business hours (between 10:40 and 17:00 CST). The combination of unpredictable multi-day delay followed by abrupt simultaneous batch blocking suggests a semi-manual process: human analysts discover bridges after an irregular delay, then an automated system applies blocks.

§4 Results, Table 3 evaluation

China's firewall never blocked a bridge before its public Tor Browser release, despite bridges being discoverable earlier via bug-tracker tickets and source code commits. The four bridges distributed only in Orbot (not Tor Browser) remained unblocked throughout the experiment, indicating the GFW monitors end-user software releases rather than upstream repositories or alternative distribution channels.

§4 Results detection

The Great Firewall of China blocked newly published obfs4 Tor Browser default bridges after delays of 7, 2, 18, 11, and 36 days following the first public software release, and up to 57 days after bridges were first discoverable via bug-tracker ticket filing. Iran showed no blocking of the same default bridges across the entire five-month measurement period.

§4 Results, Table 2 evaluation

GFW blocking was keyed on both IP address and port number, not IP address alone. Bridges with port 22 (SSH) open had that port remain reachable even as other ports on the same IP were blocked, confirming per-(IP, port) tuple granularity in the GFW blocklist.

§4 Results 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

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