By comparing echo-server (bidirectional) versus discard-server (inbound-only) results across 11 censoring countries, Quack finds that only 4 countries (China, Egypt, Jordan, Turkey) also block inbound traffic; the remaining 7 apply DPI exclusively to outbound data. Direction-sensitive blocking is a confirmed capability of deployed middleboxes.

Across all tested countries, circumvention and anonymization tools are the most consistently blocked category: www.hotspotshield.com is blocked in 5 of 13 detected censoring countries, and three Tor Project properties (bridges.torproject.org, www.torproject.org, ooni.torproject.org) each appear in the top-10 most broadly blocked domains. Collateral damage is also documented — Iran blocks psiphonhealthyliving.com as a substring match for the psiphon.ca circumvention domain.

§6.2, Figure 12 evaluation

Quack's echo-server technique achieves vantage-point coverage of 4,458 autonomous systems across 184 countries — nearly an order of magnitude more than OONI's 678 ASes in 113 countries — while processing over 500 domain-server pairs per second from a single measurement machine. The public IPv4 space contains over 50,000 active echo servers daily, with 47,276 stable over 24 hours.

§5.4, Figure 7 evaluation

Iran's number of blocked domains increases from 25 (HTTP keyword blocking) to 374 (TLS SNI-based blocking) — a 15× increase — with the newly blocked domains shifting composition to predominantly News, Human Rights, and Anonymization tools. This demonstrates that Iran maintains a distinct, more aggressive SNI blocklist for HTTPS traffic that is largely invisible to HTTP-only measurement.

§6.4 detection

Stateful DPI disruption in censoring countries disengages within approximately 100 seconds in 99.9% of observed cases, with roughly 50% of servers recovering within 60 seconds. A 2-minute empirically determined delay is sufficient to distinguish stateful per-connection blocking from persistent blocking when retrying with innocuous payloads against the same server.

§6.1, Figure 3 detection

Iran's censorship of refraction-networking proxies (Conjure via Psiphon) is not monolithic: different ISPs independently deploy different techniques and timelines. Over 800 million logged Conjure connections from July 2023–February 2025 across 10+ Iranian ASes show TCI (AS58224, ~33% of traffic) uses packet injection, while MCCI/Hamrah-e Avval (AS197207, ~22%) applies IP-based blocking, and some ASes (Parsonline AS16322, Shatel AS31549) show no proxy blocking at all.

2025-alaraj-iran-refraction §4 rst-injectionip-blockingpacket-injectiondpi

The GFI's HTTP and HTTPS filters are now stateful (requiring initial SYN packet with matching sequence numbers) and have been activated on all TCP ports—not only standard ports 80 and 443 as reported by prior studies. This is a significant departure from previous work that found stateless HTTP/HTTPS blocking limited to standard ports. The HTTP filter injects a 403 Forbidden blockpage (not RST packets as used by the GFW), while HTTPS injects a single RST+ACK packet. The GFI also exhibits TCP non-compliance (not requiring a full three-way handshake to trigger filtering), enabling outside-in measurement without in-country servers.

2025-tai-irblock §2.1, §3.2 dpirst-injectionpacket-injectionmiddlebox-interference

Manual analysis of 700+ unique packet groupings from possibly tampered connections yielded 19 high-confidence tampering signatures — up from 6 in prior work — covering 86.9% of all possibly tampered connections. Post-SYN signatures account for 43.2% of possibly tampered connections (99.5% matching a known signature), post-ACK for 16.1% (98.7%), and post-first-data-packet (PSH+ACK) for 5.3% (97.9%), with 19 signatures described as flag-sequence patterns of the form ⟨X→Y⟩ in Table 1.

2023-raman-global §4.1, Table 1 rst-injectionpacket-injectiondpi

The Great Firewall of China does not inject blockpages — it resets connections via TCP RST injection — making it invisible to blockpage-based detection systems. In contrast, the Iran firewall accounted for 97.1% of disruptions observed in Iranian vantage points, and the Bahrain and Saudi Arabia firewalls caused 71.2% and 80.2% of disruptions respectively, all using application-layer blockpage injection.

2020-raman-measuring §V-A-2 rst-injectiondpipacket-injection

Censors in Russia, Iran, and India implement all three measured censorship techniques simultaneously: block pages, RST injection, and TTL anomalies. Iran and Cyprus censoring ASes censor content across many URL categories (including General News, Internet Services, Pornography, Gambling), while most other censoring ASes restrict only a few category types.

2017-cho-churn §4, Table 2 rst-injectionpacket-injectiondpi

Yemen's national ISP (YemenNet) uses explicit blockpages for social and Internet-tools content while applying stealthy techniques — TCP RST injection and unrequited HTTP GETs — specifically for political and conflict content that is constitutionally protected. Censorship also ceases intermittently when the ISP exhausts filtering product licenses.

2015-gill-characterizing §4.2, Fig. 6 rst-injectionpacket-injectiondpi