China's censoring device is stateful: it inspects only the first HTTP GET request after a TCP handshake and ignores subsequent requests or those without a preceding handshake. After blocking a request, it records the (src IP, dst IP, port, protocol) tuple and denies all further communication between that machine pair for approximately 12 hours, even for traffic that would not independently trigger censorship.

China's censoring devices send four spoofed RST packets per filtered connection with varying sequence and ACK numbers and TTL values corresponding to roughly the hop count to the Chinese border; the IP ID field increments sequentially per TTL group, strongly implying a small cluster of out-of-band machines co-located at each border router. Because the device is out-of-band, the actual server response still arrives at the client but is preempted by the injected RSTs.

§4.2 detection

Across 11 countries, censorship execution falls into at least six distinct categories: DNS redirect to localhost (Malaysia, Russia, Turkey), DNS redirect with warning page (South Korea), connection timeout with no notification (Bangladesh, India), spoofed TCP RST injection (China), spoofed HTTP 403 with warning page (Bahrain, Iran), HTTP 302 redirect (South Korea, Thailand), and spoofed HTTP 200 iframe response (Saudi Arabia). Four countries censor at DNS and eight at routers, with South Korea employing both layers simultaneously.

Table 3 evaluation

Thailand uses an out-of-band device to inject spoofed HTTP 302 redirect responses, so the destination server still receives and responds to the original request — unlike inline censors in Bangladesh and India where the request is dropped before reaching the server. Saudi Arabia similarly uses an out-of-band device to inject a spoofed HTTP 200 response containing an iframe warning page loaded from a separate IP address, allowing the warning page content to be updated without modifying the censoring module.

§4.2, §5 detection

South Korea operates DNS-based and router-based censorship simultaneously; sites blocked at the DNS resolver are a strict subset of those blocked at the router, verified by switching to an external DNS resolver and observing continued blocking at the router layer. Alternate DNS resolvers alone are therefore insufficient to circumvent South Korean censorship, in contrast to Malaysia, Russia, and Turkey where DNS-only bypass is adequate.

§4.1, §4.2 evaluation

TLS connections to blocked services (instagram.com, telegram.org) were terminated by TCP RST immediately after the client's ClientHello, before any certificate exchange, confirming SNI-based DPI that reads the plaintext SNI extension and aborts the handshake. HTTP filtering additionally matched Host headers and URL keywords case-sensitively, with injected HTTP 403 pages or TCP RST responses, and case-change evasions were sometimes effective.

2025-aryapour-stealth-blackout §4.2, §4.3 sni-blockingdpirst-injectionkeyword-filtering

Iran's HTTP censor exhibits several parsing inconsistencies exploitable for evasion: (1) it is case-sensitive and ignores lowercase method variant "gET"; (2) it does not censor the Host header for HTTP version strings "HTTP", "1.1", and "example" (suggests a version regex of HTTP/.*); (3) when the Host header is absent, the path is not censored for versions "HTTP" and "HTTP/1"; (4) the body is never analyzed regardless of version. All HTTP and DNS censorship occurs at the same last-hop border node, suggesting centralized architecture.

2025-lange-i-ra-nconsistencies §3.2, Table 1 dpikeyword-filteringrst-injection

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

China's GFW exhibited unusually inconsistent HTTP censorship behavior: 13 of the evaluated HRS test vectors circumvented the GFW in some executions but not others, with per-vector success rates between 10% and 35% across 100 executions per domain. The authors attribute this to two distinct parts of GFW infrastructure employing different HTTP censorship mechanisms, a departure from the GFW's typical consistency.

2024-niere-http-smuggling §5.2 (China paragraph), §5 intro dpirst-injectionkeyword-filtering

Extending Geneva's genetic algorithm to the application layer automatically discovered 77 unique HTTP evasion strategies and 9 DNS evasion strategies against censors in China, India, and Kazakhstan — all requiring only unprivileged usermode modifications with no TCP/IP header access. Against India's Airtel censor, 56 of the 77 strategies succeeded; 29 worked against Kazakhstan; 22 evaded China's keyword-based HTTP censorship and 27 evaded its Host-header censorship.

2022-harrity-get §5.1, §6 dpikeyword-filteringrst-injectiondns-poisoning

The GFW only inspects two locations within an HTTP request for censored keywords: the path component of the request line and the Host header, in UTF-8 and GB 18030 encodings (with %-decoding applied). Cookie headers, custom headers (e.g., X-Tension), and POST body fields are not monitored. Even in monitored positions, only approximately 75% of requests containing censored keywords actually trigger a TCP RST disconnection.

2021-rambert-chinese §4.4 keyword-filteringdpirst-injection