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.

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

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.

§IV-B, §V-A detection

The automated probe list generation system discovered 45.79 potentially blocked domains per 1,000 domains crawled, compared to 4.11 for FilteredWeb — over 10× higher efficacy. It uncovered 1,490 potentially blocked domains in crawls of just 71,960 URLs, versus 1,255 blocked domains found by Hounsel et al. in crawls of 1,000,000 URLs, with 1,473 of the 1,490 domains not overlapping with prior work.

2024-tang-automatic §5.5 measurement-platformdns-poisoningip-blocking

VPS-based vantage points in Singapore and India detected censorship patterns similar to 'free' locations, failing to observe blocking known to be enforced by local ISPs following government directives. This occurred because ISP-level censorship is implemented per-carrier rather than centrally, and the VPS provider's ISP did not enforce those blocks — confirmed by re-testing from a residential IP that did observe the expected blocks.

2024-tang-automatic §6.2 measurement-platformip-blocking

The authors' blockpage-based methodology cannot detect transit censorship implemented via TCP RST injection or packet drops, because distinguishing these from transient network errors requires identifying their location on the routing path. As a result, the 8-country, 6-AS finding is explicitly characterized as a lower bound on the true extent of Russian transit censorship.

2023-ortwein-towards §4.1 Gathering Blockpages / §6 Conclusion rst-injectionip-blockingmeasurement-platform

29 of 80 VPN providers — including paid services — configure clients to resolve DNS through third-party public resolvers (Google Public DNS, Cloudflare, OpenDNS, Quad9) rather than provider-operated infrastructure. Three self-hosted solutions (Algo, Streisand, Outline) hardcode public DNS with no easy override, causing connection failures in regions where those services are blocked.

2022-ramesh-vpnalyzer §VI (Fig. 5) ip-blockingmeasurement-platform

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.

2017-matic-dissecting §V-D, Figure 6, Table IV ip-blockingmeasurement-platform

Aggregate measurements across nearly 180 countries over 17 days found that 60% of reflectors experienced some degree of connectivity disruption; the bias of detected blocks toward Citizen Lab Block List sites held for both inbound and outbound filtering, and temporal variability corroborated documented censorship events around political timelines.

2017-pearce-augur §VI-B measurement-platformip-blocking