Iran's HTTP censorship allows the TCP three-way handshake to complete normally before acting on the HTTP GET request: the censor responds with a '403 Forbidden' and simultaneously sends 5 spoofed RST packets to the destination server (3 with in-sequence numbers, 2 with seemingly random offsets). No modifications to TCP/IP or HTTP headers were observed at either endpoint, ruling out a transparent proxy and pointing to inline DPI.

Measurement of Alexa top-500 websites across 18 categories found that over 50% of the internet's most-visited sites were blocked in Iran, with adult content blocked at over 95% and the Art category the third-most censored. DNS hijacking was applied selectively to only three domains (facebook.com, youtube.com, plus.google.com), while HTTP Host filtering accounted for the vast majority of blocks.

§4.1, Table 1 evaluation

Traceroutes from one major Iranian ISP to 3,160 destination IPs across 13 countries consistently showed a single private-address node (10.10._._) as the first observable external hop, preceded by one of only two TCI-owned transit nodes. TTL-based probing confirmed that both HTTP and DNS blocking originated at this same centralized node, suggesting that the processing capacity of this national chokepoint is a key bottleneck in Iran's censorship infrastructure.

§4.5, §6, Figure 5 evaluation

DNS queries for blocked domains were intercepted on-path and never reached the authoritative server; instead, the DNS server received 5 TCP RST packets spoofed from the client's address — despite the original queries being UDP, a likely misconfiguration. Three RST packets carried an identical random sequence number while two had a relative offset of 30 from the first three, the same distinctive 3+2 RST pattern observed in the HTTP blocking mechanism.

§4.3, Figure 4 detection

SSH transfers utilized only 15% of available bandwidth versus 85–89% for HTTP/HTTPS. When SSH was obfuscated by XORing payloads with a constant key (hiding the plaintext handshake), throughput dropped to near-zero during all trials. Applying the same obfuscation to HTTP transfers produced the same near-zero result, supporting the hypothesis that Iran whitelists known-approved protocols rather than blacklisting specific ones, which would preemptively block any unrecognized or randomized transport including Tor's obfsproxy.

§4.4 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

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