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.

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.

§5.5 evaluation

GFW verification tests confirmed over 90% of OONI-detected DNS anomalies as true blocks: 429/457 domains in Beijing and 422/461 in Shanghai. In total, 527 unique domains were confirmed censored via DNS, HTTP, and HTTPS filters; an additional 718 domains suspected blocked due to IP-address-level blocking of their hosting servers rather than domain-level entries.

§5.6 detection

Only 36.66% of the 139,957 source list URLs (51,313) survived sanitization as live, meaningful pages, with 18,911 URLs removed for lack of content and many more for dead links — underscoring how rapidly manually curated probe lists decay. In Beijing and Shanghai, over 20% of known domains were consistently inaccessible, versus fewer than 4.5% at all other vantage points, and over 68% of known domains remained blocked, suggesting censored topics stay sensitive even as URLs go stale.

§3.1, §5.2 evaluation

Among inaccessible URLs that also triggered OONI anomalies, approximately 58% were generated by the Top2Vec-Trends pipeline (combining Top2Vec topic modeling with Google Trends keyword expansion), while LDA-TFIDF and Top2Vec alone each accounted for only 13–14%. BERTopic-generated pages were least effective at producing censored candidates.

§5.4 evaluation

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

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.

2020-niaki-iclab §III-B measurement-platformip-blocking

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

Validation against the Citizen Lab Block List (CLBL) showed that for 99% of reflectors, more than 56.7% of detected inbound-blocked sites were CLBL-listed (vs. 56.7% CLBL composition of the input dataset); 95% of reflectors showed the same directional bias for outbound filtering, confirming the method detects real censorship rather than measurement noise.

2017-pearce-augur §VI-A, Figure 5 measurement-platformip-blocking