GFC keyword filtering is distributed across the backbone, not confined to border routers: only 29.6% of filtering occurred at the first hop into China's address space, 11.8% occurred beyond the third hop (with as many as 13 hops past the border in one case), and 28.3% of the 296 probed Chinese hosts were reachable via paths with no filtering at all.

ChinaNET (CHINANET-*) performed 324/389 = 83.3% of all filtering observed across 296 probed hosts over a two-week period, and 99.1% of all filtering that occurred at the first hop past the Chinese border, despite constituting only 77% of first-hop routers encountered.

§3.3.1 evaluation

GFC keyword filtering exhibits strong diurnal patterns in which filtering effectiveness drops markedly during busy network periods, sometimes letting more than one fourth of packets containing known filtered keywords pass through unimpeded; the blocking timeout after a keyword RST was measured at 90 seconds for the tested route.

§3.2 detection

Latent semantic analysis applied to the Chinese-language Wikipedia (942,033 terms across 94,863 documents, k=600 rank reduction) discovered 122 previously unknown GFC-filtered keywords starting from only 12 seed concepts; each list of 2,500 candidate terms required 1.2–6.7 hours to probe, with an average of 3.5 hours.

§4.3 evaluation

When the GFC keyword blacklist is known, multiple server-side-only evasion techniques become viable requiring no client modification: IP packet fragmentation to split keywords across MTU boundaries, HTML comment injection mid-keyword (e.g., 'Fa<!- Comment ->lun Gong'), alternative URL percent-encodings (e.g., 'F%61lun Gong'), and spam-style character substitution ('F@1un G0-ng'); the GFC implementation was observed not to check control characters in URL requests.

§5 defense

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