Iran layers SNI-based domain blocking on top of its IP allowlist: even source IPs permitted by the allowlist trigger TCP RSTs when a TLS ClientHello contains a sensitive SNI value. Randomly generated, non-existent domain names pass through without triggering resets, confirming the blocking operates as a domain-name blocklist rather than a generic TLS fingerprinting rule. Blocking behavior was highly consistent, with 97.4% of (source IP, SNI) pairs producing identical outcomes across three test repetitions.

Iran's source-IP allowlist severely restricted outbound access before the May 26, 2026 partial restoration: only ~0.017% (~2,000 of 11,766,454 spoofed Iranian IPs) could reach destinations outside Iran. After restoration, reachable IPs increased nearly fourfold to ~0.07% (~8,000), with most newly reachable addresses originating from Noyan Abr Arvan Co. (ASN 202468), indicating expansion rather than removal of the allowlist.

§4.1.1 evaluation

Before the May 26 restoration, Iran's inbound allowlist granted near-full access (100%) to university ASes (e.g., Shahid Beheshti University AS56765, Iran University of Medical Sciences AS47981) while major residential ISPs had <2% pass rates, demonstrating that the Iranian allowlist enforces differentiated access by institution type rather than a uniform policy. Additionally, roughly half of allowlisted IPs experienced no SNI filtering at all, while the other half faced domain-specific RSTs, suggesting non-uniform application-layer policies even among privileged addresses.

§4.1.2 and §4.2 deployment

Inbound access to Iran remains heavily restricted even after the May 26 restoration: only ~2.8% of tested global /24 blocks (~400,000 of 14,461,944 representative IPs) could reach the Iran vantage point, compared to over 70% for comparable hosts outside Iran — more than 25× lower reachability — demonstrating that Iran enforces allowlist-based destination-IP filtering on inbound traffic as well as outbound.

§4.1.3 evaluation

Unlike the Great Firewall of China, Iran's SNI-triggered blocking produces no residual interference: after a TCP RST caused by a sensitive SNI, subsequent connections from the same source IP using benign or randomly generated SNIs proceed normally without additional disruption. This behavior was observed consistently before and after the May 26, 2026 partial restoration.

§4.2 detection

Iran's SNI filtering is applied inconsistently across allowlisted users: among 737 tested allowlisted IP addresses, approximately half experienced no SNI filtering at all and could reach destinations even using previously-reported blocked domains. For IPs subject to filtering, 49.9% of requests containing hrw.org and 50.2% containing tiktok.com triggered RSTs, while only 0.5% of requests with youtube.com were blocked — suggesting differential access policies tied to specific domains and user classes even within the allowlisted population.

§4.2 detection

As of June 2026, Iranian DPI blocking timescales are: CDN-fronted VLESS+WebSocket domains blocked within 24 hours; SSH dynamic port forwarding server IPs blocked within 8 hours; Reality and WireGuard connections blocked immediately (seconds) after handshake completion. This graduated timeline suggests separate fast-path (handshake-pattern) and slow-path (domain reputation / traffic accumulation) detection pipelines operating in parallel.

2026-pooribitwise-iran-advanced-dpi Issue body — combined observations dpisni-blockingip-blockingrst-injectiondns-poisoning

DNEye detected DoTH (DoT and DoH) blocking across the largest number of ASes in China, with interference against Cloudflare, Quad9, AdGuard, and CleanBrowsing resolvers emerging in early March 2021. Blocking patterns varied per-AS rather than following a centralized GFW DNS-level policy, indicating individual ISP implementation. Saudi Arabia, by contrast, showed coordinated SNI-based blocking of the same DoH resolvers across different ASes, indicating centralized policy.

2022-hoang-measuring §4.2, Table 5 rst-injectionsni-blockingip-blocking

Winter and Lindskog [157] (2012) documented that the GFW used TLS SNI inspection in combination with IP/port filtering and TCP disruption to block Tor, as recorded in the survey's Table 1. This is one of the earliest published accounts of the GFW applying SNI-based blocking specifically to a circumvention protocol, demonstrating that the GFW correlated multiple detection signals rather than relying on any single technique.

2015-aceto-internet Table 1 sni-blockingip-blockingport-blockingrst-injection

Article 19 documents that Iran's National Information Network (NIN / SHOMA) was designed with explicit reference to China's Great Firewall as a model, with institutional mirroring: Iran's Supreme Council of Cyberspace parallels China's Cyberspace Administration of China, and both governments share a "cyber sovereignty" doctrine used to justify domestic content controls and cross-border technology transfer. The report frames Iran's filtering infrastructure as deliberately architected to replicate GFW capabilities, not as an independently developed system.

2026-article19-tightening-the-net §2, §3 dpisni-blockingdns-poisoningip-blockingbgp-hijackthrottling

CensorLess vanilla mode costs $0.27/month for a single proxy processing 6.76 GB of traffic monthly, a 97.1% reduction (34.4×) over SpotProxy's optimal single-NIC configuration ($9.28/month). The private mode, which adds a t4g.micro EC2 VPS for end-to-end encryption via SOCKS, costs $3.41/month — still 63.3% cheaper than SpotProxy's cheapest option. Costs remain below $3.50/day even when scaling to 300 proxies.

2026-kang-censorless-serverless §6.3, Figure 6 ip-blockingsni-blockingdns-poisoning

DPYProxy-DNS's automated probe-and-select mode identified a working DNS circumvention in an average of 13.78 seconds (median 12.90s) in China and 12.32 seconds (median 8.28s) in Iran across 100 runs each; best-case startup was 0.32s (China) and 0.47s (Iran) when the first-tried combination succeeded, while worst-case exceeded 30.72s in China and 58.16s in Iran due to the slow Last Response mode (3s fixed wait per attempt) being selected early in the randomized probe order.

2026-lange-towards §6.3 dns-poisoningsni-blockingip-blocking