Splitting the TLS ClientHello so that the first TCP segment is ≤4 bytes (less than the 5-byte TLS record header) defeats the GFW's ESNI detection with near 100% reliability. Geneva expressed this as `[TCP:flags:PA]-fragment{tcp:4:True}-|` (client-side) or a server-side window-size reduction to 4 bytes that forces the client to segment. This suggests the GFW's ESNI classifier cannot reassemble TCP segments across all protocol contexts.

Geneva discovered 6 client-side and 4 server-side TCP-layer evasion strategies against GFW ESNI blocking within 48 hours of training, all achieving near 100% reliability. Effective strategies include desynchronization attacks (triple SYN with corrupt sequence number, FIN+SYN flag confusion, TCB turnaround via pre-handshake SYN+ACK) and TCB teardown via corrupted-checksum RST injection. All strategies operate at the TCP layer and require no changes to the application sending ESNI.

Evasion strategies / Summary on Circumvention Strategies defense

The GFW's ESNI detector is keyed specifically to extension value `0xffce` (ESNI draft-01). Replacing `0xffce` with ECH draft values `0xff02`, `0xff03`, or `0xff04` produced no blocking as of August 2020. This indicates the GFW deployed a detector matching on a specific extension ID rather than detecting encrypted SNI generically.

The GFW censors ESNI, but not omit-SNI / New extension values are not blocked detection

The GFW blocks ESNI by dropping client-to-server packets whenever a TLS ClientHello containing the `0xffce` encrypted_server_name extension is sent over a completed TCP handshake. Unlike GFW censorship of SNI and HTTP (which uses RST injection to both endpoints), ESNI censorship uses unidirectional packet dropping with no injected packets. The blocking applies on all ports from 1 to 65535.

Details About the Blocking detection

After a GFW ESNI block is triggered, residual censorship persists for 120–180 seconds (varying by vantage point), blocking all traffic on the same (srcIP, dstIP, dstPort) 3-tuple. Additional ESNI handshakes sent during the residual window do not reset the timer, and it takes at least 1 second for the GFW to enable blocking rules after the triggering packet.

Residual Censorship detection

During the June 2025 Iran shutdown, circumvention tool performance diverged sharply by transport design. Psiphon's multi-protocol architecture sustained 1.5 million concurrent users—roughly one-third of its normal Iranian base. Lantern's "proxyless" protocol (domain-fronting via CDN, ~40% of Lantern's Iranian traffic) showed moderate success. Tor usage collapsed during the blackout but bridge connections surged and rebounded quickly after lifting. BeePass (serving 500k+ daily users at shutdown onset) used live A/B testing of port/obfuscation-prefix combinations to probe the censors' blocking parameters in real time. The Ceno Browser's P2P network grew from 600 active peers on June 13 to ~8,000 by July 11, indicating that decentralized fallback paths stayed up even during peak blocking.

2025-iran-shutdown-measurement §Tool Performance dpithrottling

The June 2025 Iran shutdown—carried out during the Iran-Israel war beginning ~June 19—did not use BGP route withdrawals as in 2019. Instead, authorities applied service-level restrictions at the national border: DNS poisoning of foreign destinations, protocol whitelisting permitting only pre-approved domestic services, and DPI to block circumvention-tool traffic. Iran's international traffic fell roughly 90% while the country's BGP routes remained advertised, making the shutdown invisible to BGP-based monitoring systems. OONI measurement volume, which totalled 121,333 in June 2025, collapsed to under 200 submissions on June 19-20.

2025-iran-shutdown-measurement Executive Summary / §2 dpidns-poisoningport-blockingip-blocking

During the June 2025 shutdown, Iranian authorities blocked international One-Time Password (OTP) SMS delivery, preventing new sign-ins to foreign secure-messaging platforms and VPN services. This forced users toward government-approved domestic platforms that lack security and privacy protections. The blockade of OTPs effectively weaponized account-recovery flows as a secondary shutdown layer, disproportionately affecting users who needed to activate new circumvention tools during the crisis.

2025-iran-shutdown-measurement §Human Rights Implications port-blockingdpikeyword-filtering

Input blocking in Chinese LLM services (DeepSeek, Qwen, Kimi, Doubao) is overwhelmingly consistent: all four services persistently block the exact same queries across all 5 measurement samples in both Simplified and Traditional Chinese. Output blocking is far less consistent, with only 29 out of 349 output-blocked queries blocked across all 5 samples. Baidu-Chat is exceptional: it performs almost no input blocking but instead relies heavily on post-search and output blocking (78.6% of blocks are output-phase).

2026-ablove-characterizing §VI-B keyword-filteringdpi

A three-stage detection pipeline exploiting the "dual-role" behavioral fingerprint of single-IP circumvention relays achieved 23.2% recall (96/414 ground-truth relays) with a 0.18% false-positive rate against 97,651 benign TLS servers, for an overall accuracy of 99.5%. The ground-truth set covered OpenVPN, WireGuard, and SOCKS relays identified in a 17 TB single-day backbone trace (WIDE Project, April 9, 2025).

2026-almutairi-server §3.4, Table 1 traffic-shapedpiflow-correlation

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