Iran's censorship infrastructure began dropping all fragmented TLS ClientHello packets around late July 2026, rendering the tlshello fragmentation bypass technique non-functional. The blocking took effect approximately July 27, 2026, based on user-reported timelines in the net4people/bbs community report.

Iran's fragmentation-detection capability targets the fragmentation mechanism itself, not the SNI field content: testers report blocking occurs even when the fragmented ClientHello carries a clean, non-blocked SNI. This indicates the censor is classifying flows by structural packet properties (fragment boundary patterns) rather than by inspected SNI value.

Issue body (Jul 31 2026) detection

Community testing in Iran following the fragmentation bypass failure confirms that REALITY and Shadowsocks remain viable circumvention mechanisms as of late July 2026. Neither protocol relies on ClientHello fragmentation for evasion, which accounts for their continued functionality under the updated Iranian filtering regime.

Abstract / issue comments evaluation

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.

2026-tuleo1-internet-analysis-shutdown Issue body sni-blockingdpimiddlebox-interference

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.

2026-tuleo1-internet-analysis-shutdown Issue body (SNI spoofing methods paragraph) dpisni-blockingmiddlebox-interference

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

TSG's Context-Based Detector (CBD) evaluates circumvention tool signatures across JA3/JA4 TLS fingerprints, SSL certificate issuer fields, UDP payload length, first-packet byte patterns, server IP ranges, and FQDN matches, often combining multiple conditions with logical AND. Of 29 VPN and circumvention tool signatures logged in the VPN Signature File Update Log (March–November 2024), 21 carry risk level 5 (the maximum), indicating aggressive prioritization of blocking major tools including ExpressVPN, NordVPN, Tor Browser, and Psiphon.

2026-ablove-technical-analysis-geedge §4.1, §4.1.2, Table 3, Table 4 dpitls-fingerprinttraffic-shapeip-blockingsni-blocking

TLS ClientHello length-field permutation tests show that GFW II's parsing behavior matches an older tagged version of TSG's SSL module, while Kazakhstan and Pakistan (DROP mode) exhibit parsing behaviors consistent with TSG QUIC plugin deployments. QUIC version fingerprinting further confirms Kazakhstan runs an older version of TSG's QUIC parsing code, and Myanmar's supported QUIC versions exactly match TSG's, spanning at least five countries confirmed or plausibly running TSG components.

2026-ablove-technical-analysis-geedge §6.4, §6.5, Table 8, Table 9 dpitls-fingerprinthttp3-quic-blocksni-blocking

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