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.

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.

§4.1.2 detection

Two Iranian ASes apply a protocol allowlist that drops traffic not matching known application-layer protocol patterns (after ~6 packets), independently of the destination IP. Experiments with fresh /26 phantom subnets showed that prefixing Conjure connections with a plain HTTP GET payload evaded this blocking for four weeks, while TLS Client Hello-prefixed and SSH-prefixed connections were blocked within 72 hours (TLS) or 72 hours after port rotation (SSH). HTTP GET on port 80 was the only tested prefix that survived the full experiment window.

§4.4, §5 detection

MCCI (AS197207) blocks proxy IPs proportionally to observed connection volume: the more connections a phantom IP receives, the faster it gets blocked. A controlled experiment with a fresh /27 IPv4 subnet divided into 7 /30 sub-ranges with increasing weights confirmed that higher-weighted subnets were blocked first, demonstrating that the censor infers proxy IP reputation from traffic rate rather than from a static blocklist.

§4.2.3 detection

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.

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From September 5–20, 2023, the GFW blocked 1.1.1.1:443 via TCP RST injection; starting October 1, 2023, the mechanism shifted to HTTP packet injection on port 80, while port 443 behavior became inconsistent across ASes — from one AS45090 vantage point, HTTPS connections to 1.1.1.1 still succeeded while other observers confirmed RST injection.

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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.

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OONI data shows anomaly rates in Russia's top five ASes (including Rostelecom AS12389, Vimpelcom AS8402) rose from roughly 7–11% in January and early February 2022 to 12–21% in mid-March 2022, with social-media and news domains such as Facebook, Twitter, Instagram, and BBC going from available to near-completely blocked after the invasion.

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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.

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Data center VPSes predominantly experienced TCP connection timeouts and resets—with the highest-blocking VPS censoring 96.8% of tested domains—while residential ISPs were substantially more likely to inject explicit blockpages citing Roskomnadzor's registry, confirming that blocking mechanism varies significantly by network tier even when blocking rates are similar.

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