Among the 44 non-DNS filters, 11 did not reassemble TCP segments and 7 did not reassemble IP fragments before inspection, meaning a censored URL split across segment or fragment boundaries evaded detection. Five filters applied fragment/segment reassembly timeouts of under 2 seconds despite maintaining HTTP request state for more than 8.5 seconds, creating a window where a deliberately fragmented flow with artificial delay avoids inspection entirely.

Of the 55 filters that inspected the HTTP Host header, 26 keyed only on the first Host header in a multi-Host request, 27 keyed only on the last, and only 2 examined both. Placing a benign Host header in the position the filter reads and the blocked URL in the other position bypassed the filter, and this divergence in behavior tracks RFC 7230's requirement to reject multi-Host requests with a 400 error — which none of the tested filters implemented.

§4.1 Mechanism detection

HTTP GET fuzzing via subtle token modifications bypassed large fractions of filters: removing the `\r\n` before the Host header bypassed 36–38 of 44 Host-header filters; embedding the censored URL in the middle of a long hostname string bypassed 33–35 filters; placing the URL in an after-Host field with a non-empty Host bypassed 29–36 filters. Blacklist coverage was also weak: no filter blocked all 100 of the Alexa top adult sites, and some blocked as few as 31.

§4.1 / Appendix A evaluation

Autosonda classified 76 commercial web filters in the NYC metropolitan area into three categories: 21 (27.63%) performed DNS blacklist filtering, 44 (57.89%) matched on the HTTP Host header of GET requests, and 11 (14.47%) performed a DNS lookup of the Host header value and blocked based on the resulting IP. Autosonda found circumvention paths for 100% of filters tested.

§4.1 Results evaluation

All 76 filters inspected only TCP traffic: sending the identical HTTP request over UDP bypassed censorship 100% of the time. Additionally, 17 of the 49 filters that censored requests to EC2 servers only inspected traffic on port 80 and passed through the same requests sent to port 9900 without modification. No filter triggered on URI query strings, so appending query parameters to any censored URL bypassed every tested filter.

§4.1 Mechanism detection

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

HTTP request smuggling (HRS) vectors that exploit CL/TE header parsing divergence between a censor-as-middlebox and a destination web server can circumvent HTTP censorship in China, Iran, and Russia. Of 4,488 test vectors derived from prior HRS research, 2,015 (44.9%) were accepted by at least one web server; CL*/TE vectors achieved a 99.0% web-server acceptance rate while TE/CL* vectors achieved 0%.

2024-niere-http-smuggling §3, §5.1, Table 1 dpikeyword-filteringmiddlebox-interference

India's Airtel HTTP censor fails to reassemble TCP segments: padding any HTTP request to at least 1,449 bytes causes the IP+TCP overhead (52 bytes) to push the total past the Ethernet MTU of 1,500 bytes, forcing segmentation that the censor cannot handle and achieving 100% evasion. Kazakhstan requires the segmentation boundary to fall precisely between the Host header name and value (with two trailing spaces), rather than anywhere in the request.

2022-harrity-get §5.2 dpikeyword-filteringmiddlebox-interference

The GFW performs no stateful TCP stream reassembly, inspecting one packet at a time: splitting the blocked keyword '?falun' across two TCP segments is sufficient to evade detection entirely. Cross-device state is also absent — triggering a block on one border AS (e.g., AS9929) had no effect on traffic transiting a different Chinese border AS.

2006-clayton-ignoring §4.1 dpikeyword-filteringmiddlebox-interference

During the June 2025 shutdown, Iranian authorities blocked international One-Time Password (OTP) SMS delivery, preventing new sign-ins to foreign secure-messaging platforms and VPN services. This forced users toward government-approved domestic platforms that lack security and privacy protections. The blockade of OTPs effectively weaponized account-recovery flows as a secondary shutdown layer, disproportionately affecting users who needed to activate new circumvention tools during the crisis.

2025-iran-shutdown-measurement §Human Rights Implications port-blockingdpikeyword-filtering

Input blocking in Chinese LLM services (DeepSeek, Qwen, Kimi, Doubao) is overwhelmingly consistent: all four services persistently block the exact same queries across all 5 measurement samples in both Simplified and Traditional Chinese. Output blocking is far less consistent, with only 29 out of 349 output-blocked queries blocked across all 5 samples. Baidu-Chat is exceptional: it performs almost no input blocking but instead relies heavily on post-search and output blocking (78.6% of blocks are output-phase).

2026-ablove-characterizing §VI-B keyword-filteringdpi