DNS resolver
The server that looks DNS names up on behalf of clients, querying authoritative servers from the root down and caching the answers.
What it means
Applications do not usually query authoritative servers themselves. They ask a recursive resolver, run by an ISP, a company or a public service, which follows referrals from the root to the authoritative nameservers and caches each answer for its TTL.
Mail servers rely on their resolver for every MX, SPF, DKIM, DMARC and blocklist lookup. Two practical effects: what you see from your laptop may differ from what a receiver's resolver has cached, and a slow or failing resolver produces SPF and DMARC temperrors on the receiving side.
DNSSEC validation, when enabled, also happens at the resolver.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 9499: DNS Terminology, section 6rfc-editor.org
- RFC 1034: Domain Names: Concepts and Facilities, section 5rfc-editor.org
All 432 terms in the glossaryThe wiki: blocklists and bounce codes