Unbound is a production-grade, validating, recursive caching DNS resolver developed by NLnet Labs. It operates as a daemon with a modular architecture, supporting DNS over UDP, TCP, TLS (DoT), HTTPS (DoH), and QUIC (DoQ). The core resolution logic is implemented in the iterator module, which performs recursive queries with QNAME minimisation, delegation point chasing, and DNSSEC validation. The validator module handles signature verification, NSEC/NSEC3 denial-of-existence proofs, and trust anchor management. Caching is provided by LRU hash tables and slab hashes, with optional external backends like Redis. The system includes a rich plugin system (Python, dynamic libraries, EDNS subnet, ipset, response policy zones) and a comprehensive test suite with replay-based integration tests and unit tests.