Of 2819 public Tor relays in the February 2012 consensus, only 47 (1.6%) were reachable via TCP from within China. After three days, only 1 of those 47 remained reachable. The GFC blocks relays by IP:port tuple rather than by IP to minimize collateral damage to co-hosted services.

A blocked Tor bridge becomes reachable again after approximately 12 hours if Chinese scanners are unable to reach it continuously. In the authors' experiment, one bridge (port 23941) whitelisted to their Chinese VPS via iptables was unblocked within 12 hours despite remaining actively used, while an unrestricted bridge (port 27418) stayed blocked indefinitely.

§4.2 detection

Over 3295 active-probing scans observed across 17 days, 51% (1680) originated from a single IP address (202.108.181.70), while 98% of the remaining 1615 addresses were unique. All scanner IPs belong to three Chinese ASes: AS4837 (65.7%), AS4134 (30.5%), and AS17622 (3.8%). TTL analysis of 85 connections shows the scanner IPs are likely spoofed by the GFC—post-scan ping TTLs differed by +1 from during-scan TTLs.

§4.5 detection

The GFC identifies Tor connections via a unique TLS ClientHello cipher list sent by the Tor client. Once DPI boxes detect this fingerprint on outbound traffic, active scanning is initiated within minutes: scanners connect to the suspected bridge, attempt to build a Tor circuit, and if successful the IP:port tuple is blocked. This two-stage pipeline (fingerprint → confirm → block) allows dynamic bridge blocking without pre-enumeration.

§2, §4.1 detection

Tor DPI fingerprinting by the GFC is applied exclusively to egress traffic (from inside China to the outside world). Simulated Tor connections between domestic Chinese nodes and between external nodes connecting inward to a Chinese VPS attracted zero active scans across multiple experimental runs, indicating the detection infrastructure is positioned on the border for outbound flows only.

§4.4 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

IP and port blocking dropped from 30% of countries historically to only 9% during the study period (six countries), with the decline attributed to difficulty maintaining ephemeral blocklists, CDN collateral damage, and IPv6 expansion. Iran is a significant exception: it has implemented port allowlisting — permitting only ports 80, 443, and 53 — on multiple occasions, blocking all other ports entirely.

2023-master-worldwide §4.3, Table 2 ip-blockingport-blocking

Camoufler's blocking-resistance relies on collateral-damage economics: IM platforms had ~2.5 billion active users as of January 2019 (projected >3 billion by 2022) and are embedded in essential business and commercial operations (airline e-tickets, professional collaboration tools). Blocking all IM to disrupt Camoufler would require the censor to harm its own economy; the threat model requires only that the censor permits at least one IM platform, in which case Camoufler remains operational.

2021-sharma-camoufler §3.1, §7 ip-blockingport-blocking

Protozoa uses the economic and social indispensability of popular WebRTC conferencing services as a censorship deterrent: blocking all WebRTC traffic imposes prohibitive collateral damage on legitimate commerce and communication. This 'parasitism' strategy means the circumvention tool inherits the blocking immunity of the carrier without requiring any protocol mimicry at the network level. Protozoa requires only one reachable WebRTC service to function, and Table 3 confirms at least five services remained unblocked in China during testing.

2020-barradas-poking §3, §7.2 ip-blockingport-blocking

Of 229 Thai Internet users surveyed, 63% (n=144) had attempted to circumvent censorship, and of those, roughly 90% (n=132) reported success using VPNs (32.64%), proxies (32.64%), or Tor (23.61%). Failures were isolated to proxies (n=2), VPNs (n=2), and alternative searches (n=3), indicating that existing circumvention tools were technically adequate but that availability and comprehensibility—not raw capability—were the binding constraints on user success.

2017-gebhart-internet §5.2.1, Table 2 ip-blockingport-blocking

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.

2016-fifield-censors §4 Results ip-blockingport-blocking