The endpoint-free methodology fails when bidirectional censorship is absent or when residual censorship is pervasive: experiments in Burundi, Equatorial Guinea, Myanmar, and Kyrgyzstan could not confirm bidirectional censorship, rendering automated triggering-and-measurement inapplicable. Residual censorship causes false positives by making innocuous domains appear blocked following a censored query.

In Brunei, censorship is confined to AS10094, which serves approximately 70% of the country's Internet users. The censor injects RST packets bearing a distinctive fingerprint — the censored query's IP ID field — in response to HTTP requests containing censored Host headers, and censors on all ports without residual censorship. A SYN followed immediately by a PSH+ACK with a censored payload is sufficient to trigger blocking without a completed TCP handshake.

§3 detection

Censoring middleboxes' TCP non-compliance — specifically, their willingness to censor bidirectionally without completing the three-way handshake — enables external vantage points outside a censoring country to trigger and measure censorship without any local endpoint participation. The approach requires only a confirmed censored domain per AS, evidence of bidirectional censorship, and minimal residual censorship.

§2 evaluation

Geneva — originally designed to evolve censorship-evasion packet sequences — was repurposed by inverting its fitness function to discover censorship-triggering packet sequences instead. Training against non-responsive IP addresses allows Geneva to attribute all responses to middleboxes, enabling fully automated discovery of triggering strategies without any endpoint cooperation.

§2 evaluation

Tajikistan routes virtually all national egress and ingress traffic through a single state-run AS (AS51346, Tojiktelecom) under a 2016 national decree, creating a centralized chokepoint. The censor injects RST+ACK packets with a unique 22-byte all-zero payload, censors on all ports, and requires two PSH+ACK packets containing the censored content before injecting — possibly modeling typical multi-resource HTTP browsing behavior.

§3 detection

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

Over 7 months of Hyperquack measurements across 5,555,298 probes targeting 1,632 unique ASes, only a small number of ASes actively interfered with HTTPS connections to FCM endpoints. The majority of blocking incidents occurred in China during September 22–30, 2022, coinciding with the Party's National Congress, when nearly all measurements failed with TCP reset.

2023-xue-use §5.1, Figure 4 sni-blockingrst-injectionmeasurement-platform

Internet-wide IPv4 scanning found 386,187 IP addresses yielding amplification factors ≥ 100× via TCP middlebox reflection, with 82.9% of responses from the top 1 million IPs confirmed as originating from on-path middleboxes rather than endpoints. Nation-state censorship infrastructure dominates: China's GFW alone accounts for approximately 154 million responding IP addresses sharing a 3× RST+ACK (54 bytes each) fingerprint.

2021-bock-weaponizing §5.3, §5.5, Table 4 middlebox-interferencerst-injectionpacket-injectionmeasurement-platform

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.

2020-niaki-iclab §V dns-poisoningpacket-injectionrst-injectionmeasurement-platform

In HTTP tests, more than 50% of filter responses that indicated censorship contained an injected HTML blockpage; the remainder used TCP RST injection or connection timeout. In HTTPS measurements, canonical template matching had a failure rate of only 1.9%, and 95% of Hyperquack measurements completed within 3.5 hours across ~45,000 vantage points.

2020-raman-measuring §III-A, §IV dpirst-injectionsni-blockingmeasurement-platform

A proposed HTTP censorship detection algorithm combining status-code comparison, response-length Z-score, HTML TF-vector cosine similarity, and redirect-hostname matching achieves F1 scores of 0.83 (censored) and 0.77 (uncensored), outperforming OONI (0.80 / 0.70), length-difference methods (0.70 / 0.66), and HTML-similarity methods (0.52 / 0.34) on a manually annotated set of 3,000 responses across six Indian ISPs.

2020-singh-india §4.3, Table 1 measurement-platformkeyword-filteringrst-injectionpacket-injection