Both GFW and GC injected packets share a distinctive implementation side-channel: the IP TTL field progressively increments on successive packets injected into the same connection, paired with an incrementing TCP window size. Using this compound fingerprint, the authors identified GC activity in 8 months of Lawrence Berkeley National Laboratory enterprise border traces with only a single false-positive source IP, and used per-hop TTL probing to localize both the GFW and GC to the same network link on China Telecom (hop 12–13, 144.232.12.211→202.97.33.37) and China Unicom (hop 17–18, 219.158.101.61→219.158.101.49).

Analysis of GreatFire.org's server logs (16.6M requests, 13K unique source IPs, March 18–19 2015) showed 67% of DDoS attack traffic originated from Taiwan and Hong Kong, while mainland China accounted for only 18 requests — confirming the GC weaponizes foreign browsers by intercepting traffic at China's network border, not domestic ones. The dominant attack vector (38% of requests) was pos.baidu.com (Baidu's ad network), meaning any user globally visiting a non-Baidu site that loads Baidu ad scripts became an unwitting DDoS participant without visiting any Chinese site directly.

§5, §7.1 policy

The Great Cannon (GC) operates as a distinct in-path system — not an extension of the GFW — capable of both injecting and suppressing traffic, enabling full man-in-the-middle capability against targeted IP addresses. Unlike the on-path GFW, the GC only examines the first data packet of each connection (avoiding TCP bytestream reassembly), targets specific destination IP addresses rather than all border traffic, and maintains a per-source-IP flow cache of approximately 16,000 entries to ignore already-processed connections.

§3, §3.1 detection

The GC acted probabilistically, responding to only approximately 1.75% of eligible requests (526 out of 30,000 from three measurement IP addresses) and completely ignoring one of four measurement source IPs. Flow-cache exhaustion tests confirmed the probabilistic decision is made per-flow at cache insertion time: once the ~16,000-entry cache was filled, injections resumed on previously-ignored source ports, ruling out connection-tuple hashing as the selection mechanism.

§3.1 evaluation

TLS/HTTPS provides complete protection against GC-style content injection: the GC can only replace unencrypted HTTP responses and cannot inject into TLS-encrypted streams. GitHub's universal TLS enforcement prevented the GC from selectively targeting GreatFire.org's repositories despite sustained attack — China had previously attempted to block GitHub entirely but reversed the block within two days due to domestic programmer backlash, leaving TLS as the effective barrier.

§6, §8, §9 defense

An internet-wide scan of 500k IP addresses from an in-country VPS vantage point found TCP establishment-interception injections on 43,479 addresses (8.7% of scanned), with over 70% concentrated in two Akamai ASes (AS16625 and AS20940). The injection pattern — triggered by the first packet sent to these addresses — is consistent with targeted blocking of domain-fronting proxies hosted on Akamai CDN.

2025-alaraj-iran-refraction §4.1.2 packet-injectionrst-injectionip-blocking

Iran's censorship of refraction-networking proxies (Conjure via Psiphon) is not monolithic: different ISPs independently deploy different techniques and timelines. Over 800 million logged Conjure connections from July 2023–February 2025 across 10+ Iranian ASes show TCI (AS58224, ~33% of traffic) uses packet injection, while MCCI/Hamrah-e Avval (AS197207, ~22%) applies IP-based blocking, and some ASes (Parsonline AS16322, Shatel AS31549) show no proxy blocking at all.

2025-alaraj-iran-refraction §4 rst-injectionip-blockingpacket-injectiondpi

Iran's DNS censor now injects two distinct block-page IPs: 10.10.34.36 (≈87% of 47,633 censored domains) and 10.10.34.34 (≈13%). Both originate from the same network node at Iran's border. Prior research (Aryan et al. 2013) described only 10.10.34.34. The IP injected correlates strongly with the HTTP censorship method applied: domains with 10.10.34.34 in DNS receive TCP RST via HTTP (86.8% of RST cases), while domains with 10.10.34.36 in DNS receive HTTP block pages (84.6% of block-page cases).

2025-lange-i-ra-nconsistencies §3.1, Table 2 dns-poisoningrst-injectionpacket-injection

The GFI's HTTP and HTTPS filters are now stateful (requiring initial SYN packet with matching sequence numbers) and have been activated on all TCP ports—not only standard ports 80 and 443 as reported by prior studies. This is a significant departure from previous work that found stateless HTTP/HTTPS blocking limited to standard ports. The HTTP filter injects a 403 Forbidden blockpage (not RST packets as used by the GFW), while HTTPS injects a single RST+ACK packet. The GFI also exhibits TCP non-compliance (not requiring a full three-way handshake to trigger filtering), enabling outside-in measurement without in-country servers.

2025-tai-irblock §2.1, §3.2 dpirst-injectionpacket-injectionmiddlebox-interference

In Brunei, censorship is confined to AS10094, which serves approximately 70% of the country's Internet users. The censor injects RST packets bearing a distinctive fingerprint — the censored query's IP ID field — in response to HTTP requests containing censored Host headers, and censors on all ports without residual censorship. A SYN followed immediately by a PSH+ACK with a censored payload is sufficient to trigger blocking without a completed TCP handshake.

2023-nourin-detecting §3 rst-injectionpacket-injectionmiddlebox-interference

Tajikistan routes virtually all national egress and ingress traffic through a single state-run AS (AS51346, Tojiktelecom) under a 2016 national decree, creating a centralized chokepoint. The censor injects RST+ACK packets with a unique 22-byte all-zero payload, censors on all ports, and requires two PSH+ACK packets containing the censored content before injecting — possibly modeling typical multi-resource HTTP browsing behavior.

2023-nourin-detecting §3 rst-injectionpacket-injectionmiddlebox-interference