In approximately 100,000 DNS queries over 9 days from within a censored network, injected packets were reliably distinguishable: legitimate IP TTLs were stable at either 44 or 42, while injected TTL values ranged across [0–255], and injected packets arrived well before legitimate replies because the injector co-resided within the same ISP while the recursive resolver was in another country. With a TTL threshold of ±1 and an RTT threshold of 0.5× expected RTT, the Hold-On prototype achieved 0% false positive rate and 0% false negative rate.

Without DNSSEC, Hold-On can be defeated by a sophisticated censor that crafts injected packets with TTL and timing matching the expected legitimate reply, injecting just before its predicted arrival. When combined with DNSSEC, Hold-On is robust even against this attack because the censor cannot forge a valid DNSSEC signature; injection can still cause a denial-of-service by forcing a 'Bogus' result, but Hold-On prevents that by waiting for the legitimate validating reply.

§VI defense

Over 11,700,000 DNS requests across 6 days at ICSI's border network and 15,200,000 DNS transactions in a 1.5-hour trace at UC Berkeley's border, secondary differing DNS replies were essentially absent in normal traffic, yielding effectively 0 false positives. Only two benign authority servers produced anomalous dual replies at Berkeley—one for the BBC returning two addresses within the same /24, one for businessinsider.com returning a SERVFAIL—neither of which would disrupt a Hold-On resolver.

§IV.A evaluation

A prototype Hold-On DNS proxy introduced no perceptible additional latency for either cached or uncached DNS queries in live testing; query-time measurements for both sets of names overlapped entirely with baseline (Hold-On disabled) measurements. The Hold-On timer (set to 5 seconds initial, 10s second try, 15s third try) is only reached under anomalous conditions; under normal operation the resolver returns as soon as the legitimate reply validates.

§V.B evaluation

On-path censors commonly operate on traffic mirrors rather than inline (in-path), making their systems failure-tolerant and easier to deploy. This architectural choice means on-path injectors cannot suppress the legitimate DNS reply—both the forged and authentic replies reach the resolver—creating a detectable anomaly. The same structural weakness applies to TCP RST injection and other on-path packet injection attacks.

§I, §II.A detection

Wallbleed was a buffer over-read in the GFW's DNS injection subsystem that caused middleboxes to append up to 125 bytes of their own process memory to forged DNS responses. The bug persisted for at least two years (confirmed from October 2021); the GFW issued an incorrect partial patch in November 2023 (Wallbleed v2 remained exploitable) and fully patched it in March 2024. Over 5.1 billion Wallbleed responses were collected during continuous measurement, and an IPv4-wide scan found 242 million IP addresses across 381 autonomous systems receiving Wallbleed-injected responses — including some traffic whose source and destination were both outside China, due to routing through China's network border.

2025-fan-wallbleed §1–§3, §7 dns-poisoningpacket-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

Over 2.5 months (Nov 2024–Jan 15, 2025), IRBlock scanned all 11M Iranian IPv4 addresses daily, finding 6.8M IPs subject to DNS poisoning and HTTP blockpage injection, and 5.4M IPs subject to UDP-based traffic disruption. Testing over 700M FQDNs (500M apex domains) revealed 6M banned FQDNs from 3.3M censored apex domains. Of 537 active ASes in Iran, 485 (90.3%) exhibited blocking for at least 25% of assigned IPs. DNS and HTTP censorship overlapped at >99% of censored IPs; UDP blocking was a strict subset of DNS-censored IPs, affecting ~5M addresses.

2025-tai-irblock §5.1, §1 dns-poisoningpacket-injectionhttp3-quic-block

The GFI operates three distinct DNS/HTTP injectors with different fake IP addresses (10.10.34.34, 10.10.34.35, 10.10.34.36) and partially overlapping blocklists—mirroring the GFW's triplet-censor architecture. Injector 10.10.34.35 exhibits TTL reflection (injected response TTL = probe TTL − hop count), identical to the GFW. No IP exclusively receives injections from 10.10.34.34 (a smaller, selective component); the two primary injectors 10.10.34.35 and 10.10.34.36 handle the majority of censorship. Different injectors maintain distinct domain blocklists, meaning which domains a user sees as censored depends on routing through their AS.

2025-tai-irblock §5.4, Table 2 dns-poisoningpacket-injectiondpi

The GFW DNS injector vulnerability enabled reflective amplification attacks with a baseline factor of 4.04× (46-byte payload → 186-byte response). Combined with routing loops — approximately 1,000 destination IP addresses in China were found to loop packets across the GFW more than 30 times, with 159 persisting after two days and a maximum of 119 loop iterations per query — the effective amplification factor reached 481.17×, sufficient to generate 100 Gbps of attack traffic from just over 200 Mbps of source traffic.

2024-sakamoto-bleeding §5.2 Reflective Amplification Attack dns-poisoningpacket-injection

The GFW's DNS packet injector (Injector 3, identified by TTL mirroring and zero IP ID) contained an out-of-bounds read vulnerability: due to missing label-length and null-terminator validation, malformed DNS requests caused the injector to copy adjacent stack memory into forged responses. Over three days in October 2023, researchers collected over 1 TB of data containing over 13 billion leaks, ~87.43% with non-duplicate content, including live Internet traffic transiting China's backbone and stack frames of the GFW's packet-handling processes.

2024-sakamoto-bleeding §3 Vulnerability dns-poisoningpacket-injectiondpi