Of 19,493,925 TCP packet injection events ICLab detected, only 0.7% (143,225) could be definitively attributed to censorship after multi-heuristic filtering; a further 58% (15,589,882) were RST-or-ICMP-unreachable events classified only as 'probable censorship' because ordinary network failure could not be excluded. Block pages appeared in just 3.4% of definitively-censored injections, meaning the vast majority of censor-side TCP disruption is covert. DNS manipulation detection achieved a false positive rate of approximately 10⁻⁴ using a threshold of θ=11 autonomous systems, cross-checked against block page observations.

ICLab's semi-automated block page discovery — combining HTML tag-frequency vector clustering with locality-sensitive hashing (LSH) of page text — identified 48 previously unknown block page signatures from 13 countries: 15 via structural clustering across 5 countries and 33 via textual similarity clustering across 8 countries. The system seeds from 308 manually verified regular expressions and uses a URL-to-country ratio sort (largest ratio discovered: 286) to prioritize candidates for manual review, eliminating reliance on brittle hand-maintained regex lists alone.

§IV-C detection

Between January 2017 and September 2018, ICLab conducted 53,906,532 measurements of 45,565 URLs across 62 countries and 234 ASes, detecting blocking of 3,602 unique URLs in 60 countries via DNS manipulation, TCP packet injection, and block page delivery. Iran blocked 20–30% of Alexa top-500 URLs — more than any other monitored country — while Saudi Arabia consistently blocked roughly 10%. The global trend in detected censorship shows a steady decrease, which the authors attribute to rising adoption of TLS and circumvention tools.

§V evaluation

ICLab's longitudinal monitoring detected censorship shifts coinciding with political events weeks before press coverage: Turkey's filtering rate rose from roughly 3% to 5% in late April 2017 — with blocked content shifting from pornography to news and political sites — ahead of a June 2017 constitutional referendum. India's censorship dropped from roughly 2% to 0.8% following a net neutrality announcement in late 2017, then partially recovered to roughly 1.5% after mid-2018 regulations clarified that illegal-content filtering would continue. Within the same country, different blocking techniques were applied to different content categories simultaneously (e.g., Turkey used DNS manipulation for illegal/streaming URLs but block pages for pornography and news).

§V-C, Table III evaluation

ICLab's commercial VPN vantage points reside in data-center ('content') ASes for 41% of monitored networks, which may experience less aggressive censorship than residential ISPs, making VPN-based measurements a systematic lower bound on blocking rates faced by ordinary users. In countries where both VPN and volunteer-operated device (VOD) vantages coexist, identical block pages were observed from both AS types, indicating similar overt blocking policies, but covert IP-based or RST-injection blocking may still differ by AS class.

§III-B evaluation

Iran's DNS censor now injects two distinct block-page IPs: 10.10.34.36 (≈87% of 47,633 censored domains) and 10.10.34.34 (≈13%). Both originate from the same network node at Iran's border. Prior research (Aryan et al. 2013) described only 10.10.34.34. The IP injected correlates strongly with the HTTP censorship method applied: domains with 10.10.34.34 in DNS receive TCP RST via HTTP (86.8% of RST cases), while domains with 10.10.34.36 in DNS receive HTTP block pages (84.6% of block-page cases).

2025-lange-i-ra-nconsistencies §3.1, Table 2 dns-poisoningrst-injectionpacket-injection

Over one month, 54K measurements from 1.7K ASes in 164 countries detected I2P blocking in exactly five countries: China (DNS poisoning of homepage and 3 of 10 reseed servers), Iran (TCP RST injection with HTTP 403 on mirror site), Oman and Qatar (SNI-based blocking of HTTPS homepage plus TCP injection with block-page redirect on HTTP mirror), and Kuwait (TCP injection on mirror site at AS47589 only). All other tested countries left I2P fully reachable.

2019-hoang-measuring §5, Table 2 dns-poisoningsni-blockingrst-injectionpacket-injection

Pakistan's censorship used layered, evolving mechanisms: DNS redirection by local ISP resolvers appeared in all post-block traces, supplemented by HTTP 3XX redirection to a local provider's error page in Sep 2012 and shifting to RST injection by Aug 2013 (where ≈95% of YouTube HTTP requests received no response, vs. ≈2% pre-block). Porn blocking similarly combined DNS redirection with IP blocking (41% blacklist overlap) in Sep 2012 and RST injection in Aug 2013.

2014-khattak-look §4.2–4.3, Table 4 dns-poisoningrst-injectionip-blockingpacket-injection

On-path censors commonly operate on traffic mirrors rather than inline (in-path), making their systems failure-tolerant and easier to deploy. This architectural choice means on-path injectors cannot suppress the legitimate DNS reply—both the forged and authentic replies reach the resolver—creating a detectable anomaly. The same structural weakness applies to TCP RST injection and other on-path packet injection attacks.

2012-duan-hold-on §I, §II.A dns-poisoningpacket-injectionrst-injection

Across 11 countries, censorship execution falls into at least six distinct categories: DNS redirect to localhost (Malaysia, Russia, Turkey), DNS redirect with warning page (South Korea), connection timeout with no notification (Bangladesh, India), spoofed TCP RST injection (China), spoofed HTTP 403 with warning page (Bahrain, Iran), HTTP 302 redirect (South Korea, Thailand), and spoofed HTTP 200 iframe response (Saudi Arabia). Four countries censor at DNS and eight at routers, with South Korea employing both layers simultaneously.

2012-verkamp-inferring Table 3 dns-poisoningrst-injectionpacket-injectiondpi

An internet-wide scan of 500k IP addresses from an in-country VPS vantage point found TCP establishment-interception injections on 43,479 addresses (8.7% of scanned), with over 70% concentrated in two Akamai ASes (AS16625 and AS20940). The injection pattern — triggered by the first packet sent to these addresses — is consistent with targeted blocking of domain-fronting proxies hosted on Akamai CDN.

2025-alaraj-iran-refraction §4.1.2 packet-injectionrst-injectionip-blocking