Bridges that carry clients are highly stable: their median lifetime is 116 days (~4 months) and 84% never change IP address, with 90% having at most one IP change. This means current censor policies that remove bridge IP blocks every 25 hours are far more conservative than necessary — an adversary could sustain blocks for months without significant collateral damage.

77% of public bridges offer only vanilla Tor, which is trivially detectable via TLS certificate pattern matching. An additional 15% offer Pluggable Transports with conflicting security properties (e.g., obfs4 + obfs3 + obfs2 co-deployed on the same bridge), allowing a censor to confirm and block the bridge via the weakest PT and thereby disable all stronger PTs on the same IP — including active-probing-resistant transports like obfs4 and ScrambleSuit.

§V-C, Table I defense

Default bridges — whose IP addresses are hardcoded in the Tor Browser Bundle — carry 91.4% of all bridge clients globally in April 2016, and 86.1% in Iran and 69.2% in Syria. Because these addresses are trivially obtainable from the Tor Browser Bundle configuration files, a censor can block the vast majority of bridge users in a country at any time.

§V-E, Table II evaluation

Four OR ports (443, 8443, 444, 9001) account for 82% of all active public bridge fingerprints as of April 2016, down from 95% in March 2013 but still concentrated. Scanning just three of these ports (443, 8443, 9001) is sufficient to deanonymize 71% of all active public bridges. Additionally, CollecTor's published per-bridge usage statistics allow a censor to rank bridges by client count per country and identify the highest-impact OR ports to scan next.

§V-D, Figure 6, Table IV detection

Tor's vanilla TLS certificate presents a distinctive pattern (SubjectCN=www.[random].com; IssuerCN=www.[random].net using base32 random strings), which never changes across certificate rotations every 2 hours. Using this pattern against Censys and Shodan scan data without running any active scans, the authors discovered 694 private bridges and 645 private proxies, and deanonymized the IP address of 35% of public bridges with clients (23% of all active public bridges) in April 2016.

§II-B, §VI-A, Table V 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