After approximately four days, Iran placed Cloudflare IP ranges behind a NAT that performed full TCP reassembly with strict checksum and sequence-number validation; any packet with an incorrect checksum or out-of-window sequence number caused immediate connection teardown, eliminating both TTL-spoofing and sequence-desynchronization bypasses.

For non-whitelisted SNIs connecting to allowed Cloudflare IPs, Iran's DPI enforced a strict 6-packet limit per connection, sufficient only to load a webpage but not to sustain any tunnel; connections bearing a whitelisted SNI were unrestricted. Outside DNS providers were blocked within one to two weeks and replaced with a local DNS infrastructure that operated at a maximum of ~100 KB/s with connections dropping at approximately the one-minute mark.

Issue body deployment

Iran implemented a per-connection packet quota tied to SNI whitelisting: connections whose SNI was not on the approved list were classified as 'grey' and hard-limited to exactly 6 packets before DPI terminated the session. Connections presenting a whitelisted SNI were passed without a packet cap.

Issue body (SNI whitelisting paragraph) detection

DNS tunneling operated for 'at most a week or two' before Iran blocked all external DNS resolvers and forced traffic to local DNS providers. The local DNS infrastructure was effectively unusable as a tunneling substrate: maximum throughput was ~100 kb/s and connections were reliably severed around the 1-minute mark.

Issue body (DNS paragraph) evaluation

During the 2026 Iranian internet shutdown, nearly all IP addresses were fully blocked; a small subset of Cloudflare IPs completed TCP three-way handshakes (SYN-ACK returned) but DPI terminated sessions before any data transfer, confirming that the primary enforcement mechanism was payload-level DPI rather than IP-layer blocking alone.

Issue body deployment

During full Iranian blackouts, WebRTC and all UDP/QUIC traffic were completely blocked. DNS-over-QUIC (DoQ) could complete a handshake but the censor severed the connection after a single post-handshake data packet, rendering it unusable for tunneling.

Issue body (WebRTC/UDP paragraph) detection

Two TCP-layer SNI-spoofing techniques briefly circumvented Iran's DPI for approximately four days: (1) sending a fake ClientHello with a TTL too low to reach the server but high enough to pass the censor, and (2) sending a fake ClientHello at a future TCP sequence number that the server would ignore but the stateless DPI would accept. Both were defeated once Iran placed Cloudflare IPs behind a stateful NAT that validated checksums and sequence numbers. Iran's SNI whitelist permitted only specific domains, hcaptcha.com among them.

Issue body (SNI spoofing methods paragraph) defense

The Henan Firewall is stateless in two exploitable ways: (1) it requires the TCP header to be exactly 20 bytes—enabling any TCP option (e.g., TCP Timestamps, which Windows disables by default) to bypass it entirely; (2) it does not perform TCP reassembly, so splitting a TLS ClientHello across two TCP segments such that the SNI extension straddles the boundary bypasses the censor. Both bypasses require only client-side changes and have already been implemented in Xray, GoodbyeDPI, and Shadowrocket. TLS record fragmentation (splitting the ClientHello across multiple TLS records within one TCP segment) also defeats both the Henan Firewall and the GFW, since neither performs TLS reassembly.

2025-wu-regional-censorship §4.3 / §6 sni-blockingdpimiddlebox-interference

The paper identifies a fundamental architectural vulnerability in single-IP circumvention designs: a relay must generate new observable flows (via DNS or TLS SNI) to reach end services after receiving client connections, creating a detectable server-and-client behavioral contrast. A relay accessing user-facing domains (news, social media) scores high on a Relay Suspicion Score (w=0.9) versus infrastructure domains (w=0.1). The paper argues this host-level signal is censorship-invariant and cannot be concealed by link obfuscation.

2026-almutairi-server §2.4, §4 traffic-shapedpisni-blocking

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

The report maps specific Belt and Road Initiative Digital Silk Road projects through which Chinese technology vendors have transferred censorship and surveillance infrastructure to Iran, including fiber backbone investments, data-center co-location agreements, and equipment supply chains. Specific vendors named include Huawei and ZTE as network infrastructure providers, with the report noting that equipment exports include filtering-capable hardware that Iran's ISPs have deployed at network choke points.

2026-article19-tightening-the-net §4, §5 dpisni-blockingthrottling

Testing the Tranco Top 100k domain list from Russian vantage points shows that all 2,644 domains blocked over TLS SNI are also blocked over QUIC SNI, and domains accessible via TLS are accessible via QUIC—indicating the TSPU maintains a unified SNI blocklist across protocols. Additionally, 31 domain patterns including discord.com exhibit 'residual clearance': QUIC connections to those domains are not censored when immediately preceded by non-censored data on the same 4-tuple; 22 domains are only blocked on port 443.

2026-heitmann-russia-s-early-introduction §3, Table 3 sni-blockinghttp3-quic-blockdpi

Reproducing Zohaib et al.'s 10-payload QUIC parser test suite against Russian TSPU devices reveals that the Russian QUIC parser differs meaningfully from the GFW: payload 8 (PING and PADDING frames inserted before the Crypto frame), which circumvents the GFW, does not bypass Russian censorship. Payload 10 (QUICv2) bypasses both censors. Payload 9 (ECH with outer SNI cloudflare-ech.com) is blocked by Russia; payload 3 (version bytes 0x00000002 with invalid MAC) is blocked by both.

2026-heitmann-russia-s-early-introduction Appendix B, Table 2 http3-quic-blockdpisni-blocking