DNS State Exhaustion: Water Torture, NXNS Amplification, TsuNAME & DoT/DoH Socket Starvation
UDP DNS is stateful in practice: recursive resolvers hold per-query client tables, delegation chain depth counters, DNSSEC validation state, and TCP connection pools for DoT/DoH. Each layer is exhaustible at different cost ratios. Water torture generates maximum valid NXDOMAIN workload at minimum bandwidth. NXNS amplification (CVE-2020-8616) forces resolver delegation storms via NS referrals. TsuNAME creates infinite cyclic delegation. NRDelegation combines both. Full BIND9/Unbound configuration hardening and rate-limiting defense included.
