tracebox identified a transparent HTTP proxy or IDS within a National Research Network (SUNET) that intercepted port-80 SYN probes but not port-21 SYN probes, producing shorter observed path lengths to port 80. It also found proxy misconfigurations causing forwarding loops for non-HTTP traffic, where ICMP replies alternated between two routers indefinitely.

In 80% of measured paths (72 PlanetLab VPs × 5,000 Alexa targets), at least one intermediate router returns the full IP packet in ICMP time-exceeded replies (RFC1812-compliant), enabling per-hop detection of packet modifications. The majority of these full-ICMP routers reside in the network core rather than the access segment.

§3.2 evaluation

Middleboxes that randomize TCP sequence numbers do not update the sequence numbers inside TCP SACK blocks; tracebox found two PlanetLab VPs with stateful seq-number randomizers that cycled approximately every 20 seconds. When SACK blocks reference sequence numbers outside the current window, the Linux TCP stack waits for a full RTO instead of fast-retransmitting, producing up to 50% throughput degradation in controlled measurements.

§3.3 evaluation

Of 72 PlanetLab vantage points, 7 (~10%) automatically stripped or replaced TCP options (Multipath TCP, MD5, and Window Scale) with NOPs at the very first hop, and 2 VPs always altered TCP sequence numbers. These modifications occurred without any corresponding update to dependent fields, corrupting the TCP stream for higher-layer protocols.

§3.1 evaluation

tracebox can estimate middlebox location with an error of ≤4 hops in 61% of cases; errors above 13 hops (the length of ~60% of paths) are each below 1% individually. Of MSS-modifying middleboxes detected, 52% were located in the network core and only 2.7% close to the source vantage point.

§3.4 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