DNSResolvers

DNS resolvers are network servers that provide domain name resolution on behalf of clients. When a client application queries a domain, it usually asks a resolver rather than contacting authoritative servers directly. The resolver then performs the necessary lookups by querying root servers, then top-level domain servers, and finally authoritative servers for the domain. Depending on configuration, a resolver may be recursive (fully resolving the name) or iterative (returning the next server to ask). Most consumer and enterprise resolvers are recursive.

Caching and TTL: The resolver stores responses in a cache for the duration of the time-to-live and can respond quickly to repeated queries without hitting upstream servers. It may also cache negative responses and serve them until the negative TTL expires.

Deployment and use: Resolvers are commonly operated by internet service providers, organizations, and by public services such as Google Public DNS, Cloudflare, and Quad9. They may implement privacy features, rate limiting, and DNSSEC validation to verify authenticity of responses. Dozens of resolvers offer configurable privacy and performance options.

Protocols and security: DNS queries typically use UDP on port 53, with TCP used for larger responses or zone transfers. Encrypted variants include DNS over TLS (DoT) and DNS over HTTPS (DoH), which help prevent on-path observers from seeing queries. DNSSEC provides cryptographic validation of responses, reducing certain types of spoofing.

Impact and challenges: The choice of resolver affects latency, privacy, and reliability. Public and private resolvers can log queries, influencing user privacy; operators must balance performance with security and transparency.