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.

Only 8% of keywords censored by Chinese chat clients (WeChat, Sina Weibo — ~63,200 total terms) are also censored by GFW packet inspection, demonstrating independently maintained blocklists. The GFW's packet-inspection chat-derived blocklist contains up to 1,221 distinct censored keywords for outbound traffic; just 68 keyword components account for all censored terms from Beijing, with 「六四」(June Fourth) alone responsible for more than half.

§4.2, §4.2.2, §4.3 evaluation

After a censored connection, 50–75% of subsequent connections from the same client IP to the same server IP and port are blocked for 90 seconds even without censored keywords ("penalty box"). The penalty box is strictly scoped to the (client IP, server IP, server port) triple — other ports at the same server IP or other server IPs are unaffected. The GFW monitors HTTP keyword traffic on every TCP port, not just port 80.

§4.4, §4.5 detection

The GFW enforces SNI-based blocking on every TCP port (not just 443), triggering TCP RST injection and a penalty box for known-censored hostnames (e.g., facebook.com, zh.wikipedia.org) in the TLS ClientHello. The SNI blocklist is separate from the HTTP keyword blocklist — keyword-derived subdomains in the SNI did not trigger censorship. No evidence was found for indiscriminate HTTPS decryption or certificate substitution.

§4.6 detection

The GFW maintains two HTTP keyword sublists: 15 terms censored unconditionally, and approximately 60–63 additional terms censored only when the English word "search" also appears in the request URL. No other English word among the 10,000 most common, no Chinese search synonym (搜索, 查找, 关键词), and no common URL parameter abbreviation ("q", "kw", "s") replicates this expanded-censorship trigger.

§4.1, §4.2.1 detection

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

Table 1 of the survey documents that by 2013–2014 censors were deploying simultaneous blocking across BGP, DNS, IP/port filtering, TCP disruption, TLS, and application-layer keyword filtering. No single detection tool in the survey covers all six layers; the most comprehensive, OONI (2012), covers DNS, IP/port, TCP, TLS, keyword, and HTTP but notes only partial BGP coverage.

2015-aceto-internet Table 1 bgp-hijackdns-poisoningport-blockingip-blockingrst-injectiontls-fingerprintkeyword-filteringdpi