Packets carrying an unsolicited TCP MD5 option header (RFC 2385) are silently ignored by modern Linux servers (kernel ≥ 2.6) that have not negotiated MD5 authentication, yet are accepted and processed by the GFW as normal packets that update its TCB. Crucially, none of the observed middleboxes dropped packets with MD5 options, making the MD5 header the most universally applicable insertion packet type — usable with any TCP flag (SYN, RST, or data) and immune to middlebox filtering.

Tested across 11 vantage points in 9 Chinese cities against 77 Alexa-ranked websites (50 trials each, April–May 2017), most prior TCB evasion strategies are largely broken: TCB creation with SYN achieves only 6.9% success (88.9% Failure 2), TCB teardown with FIN achieves only 11.1% success (87.9% Failure 2), while in-order data overlapping with TTL-based insertion still achieves 90.6% success and only 3.7% Failure 2. Without any evasion strategy the baseline success rate is 2.8%.

§3, Table 1 evaluation

The GFW evolved to create a TCB not only on SYN packets but also on SYN/ACK packets, and enters a 're-synchronization state' upon seeing multiple SYN packets, multiple SYN/ACK packets, or a SYN/ACK with an incorrect acknowledgment number. Once in this state, it re-synchronizes its TCB using the next client-to-server data packet or server SYN/ACK, invalidating prior TCB-creation evasion strategies that assumed the GFW used only the first SYN sequence number.

§4 detection

INTANG, a measurement-driven tool that caches the best-performing TCP evasion strategy per server IP, achieves an average success rate of 98.3% (range 93.7%–100%) from vantage points inside China. Four combined new strategies — Improved TCB Teardown, Improved In-order Data Overlapping, TCB Creation + Resync/Desync, and TCB Teardown + TCB Reversal — each independently achieve average success rates of 94.5%–96.2% inside China and 84.6%–92.7% outside China, with Failure 2 rates below 1.1%.

§7, Table 4 defense

Client-side middleboxes at every tested vantage point interfere with IP-layer evasion tactics: Aliyun (6/11 nodes) discards all IP fragments, while the Tianjin China Unicom node drops packets with wrong TCP checksums or no TCP flag. IP-layer discrepancies that survive routers (e.g., IP total-length > actual length) are still dropped by some middleboxes, making IP-layer manipulations unreliable across Chinese ISPs. TCP-layer manipulations are significantly more consistent across paths.

§3, Table 2 evaluation

Russian TSPU devices directly block ECH by dropping ClientHello messages that contain both an ECH extension and the outer SNI hostname "cloudflare-ech.com" — the static outer SNI Cloudflare advertises in all its ECH configurations. Blocking affects both TLS and QUIC. ECH connections to servers with Cloudflare ECH support but outside Cloudflare's official IP ranges are NOT blocked. TCP segmentation alone or TLS record fragmentation alone did NOT bypass TSPU ECH blocking, but combining both techniques did circumvent it. TSPU has also added TCP reassembly capabilities that defeat previously effective fragmentation-only bypasses.

2025-niere-encrypted §5, Figure 5 dpirst-injectionmiddlebox-interference

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.

2025-tai-irblock §2.1, §3.2 dpirst-injectionpacket-injectionmiddlebox-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

Longitudinal GFWeb data spanning 20 months shows the GFW actively patched previously-published evasion findings during the measurement period: overblocking bugs reported in academic papers were fixed, and fragmented-packet reassembly failures that researchers used to bypass blocking were corrected. This demonstrates that the GFW operator monitors published research and iterates on the system in response to disclosed vulnerabilities.

2024-hoang-gfweb Abstract, §5.4, §6 dpimiddlebox-interference

The SAT Móvil app (Mexico's official tax service, 1M+ downloads) consistently fetches its 'Chat' page over cleartext HTTP, exposing citizen ID numbers (CURP, RFC), passwords, and tax documents to any in-path attacker. None of the four major Latin American telco apps (MiTelcel, MiTigo, MiClaro, MiMovistar) implement HSTS on SMS-delivered external links, making all of them uniformly vulnerable to SSL strip downgrade attacks.

2024-kujath-analyzing §7.2, Table 3 dpimiddlebox-interference

China's Great Firewall showed anomalous inconsistency: 13 test vectors produced mixed outcomes—TCP RST injection on some executions and a clean server response on others—with circumvention rates between 10% and 35% across 100 executions per vector. The authors attribute this to heterogeneous GFW infrastructure components applying different HTTP parsing logic, a departure from the GFW's usual consistency.

2024-m-ller-turning §5.2 dpirst-injectionkeyword-filteringmiddlebox-interference