Client-to-server packet drops (RSTs from client to server are dropped in transit) indicate the simplest null-routing mechanism: the server's destination IP is null-routed at the censor. The method distinguishes this from server-to-client drops (stateless return-path filtering) and from RST/ICMP injection—cases where the packet is not dropped but a forged termination packet is inserted—which both appear as the 'no-packets-dropped' outcome in the IPID time series.

Approximately 10% of China's IP addresses respond to IPID probes, and 13% of those exhibit globally incrementing IPIDs, meaning roughly 1% of China's total IP address space can serve as passive measurement vantage points with no cooperation from host owners. In contrast, Tor bridge blocking from Chinese clients was observed in 58.91% of server-to-client cases versus 0% for non-China Asia-Pacific clients.

§5, Table 1 evaluation

The GFW blocks Tor primarily via stateless SYN/ACK dropping based on the server's source IP address and port (server-to-client direction, 73.04% of CN,Tor-dir cases). Two specific Tor directory authorities account for 98.8% of client-to-server (null-routed) blocks and 72.7% of error cases, indicating selective deeper blocking of specific IP addresses beyond the common return-path filter.

§5 detection

Over 5 days of measurement, 73.04% of connections from Chinese clients to Tor directory servers were blocked server-to-client (stateless SYN/ACK dropping), 16.73% were blocked client-to-server (null routing), and only 0.63% were unblocked. Of all censored Tor directory server connections measured across all regions, 98% originated from Chinese clients.

§5, Table 1 evaluation

Using TCP IPID side channels combined with SYN backlog state inference, the authors detect intentional packet drops between two arbitrary Internet hosts without controlling either host. The only requirements are a client with a globally incrementing IPID (~1% of IP space) and a server with an open port; an ARMA model handles autocorrelated noise.

§2 evaluation

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

Data center VPSes predominantly experienced TCP connection timeouts and resets—with the highest-blocking VPS censoring 96.8% of tested domains—while residential ISPs were substantially more likely to inject explicit blockpages citing Roskomnadzor's registry, confirming that blocking mechanism varies significantly by network tier even when blocking rates are similar.

2020-ramesh-decentralized §VI-B rst-injectionip-blockingpacket-injectionkeyword-filtering

Pakistan's censorship used layered, evolving mechanisms: DNS redirection by local ISP resolvers appeared in all post-block traces, supplemented by HTTP 3XX redirection to a local provider's error page in Sep 2012 and shifting to RST injection by Aug 2013 (where ≈95% of YouTube HTTP requests received no response, vs. ≈2% pre-block). Porn blocking similarly combined DNS redirection with IP blocking (41% blacklist overlap) in Sep 2012 and RST injection in Aug 2013.

2014-khattak-look §4.2–4.3, Table 4 dns-poisoningrst-injectionip-blockingpacket-injection

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