CoreDNS

CoreDNS is a DNS server written in Go that delivers its functionality through a chain of plugins. A Corefile declares which plugins handle which zones, and each plugin in the chain either answers a query, mutates the response, or hands the request to the next plugin. The project is a CNCF graduated project and is the default cluster DNS for Kubernetes.

What this codebase contains

This repository (github.com/coredns/coredns) holds:

• A small bootstrap binary (coredns.go, coremain/) that starts the Caddy server framework with the dns server type registered.
• The DNS server type implementation in core/dnsserver/, which parses Corefiles, builds plugin chains, and serves queries over UDP/TCP, DoT, DoH, DoH3, DoQ, and gRPC.
• 60+ in-tree plugins under plugin/ covering everything from caching and forwarding to Kubernetes service discovery, DNSSEC signing, and observability.
• A request abstraction in request/ that gives plugins a uniform view of the client query.
• A large pool of shared packages under plugin/pkg/ that plugins can reuse.
• An end-to-end test suite under test/.

What it does

CoreDNS chains plugins to serve DNS, and each plugin does one job. The shipped plugins let you:

• Serve zone data from files (file, auto, secondary) or from a backend like etcd, Kubernetes, AWS Route53, Azure DNS, Google Cloud DNS, or HashiCorp Nomad.
• Forward queries to upstream resolvers over UDP/TCP/DoT (forward, grpc).
• Cache positive and negative responses (cache).
• Sign zones on the fly (dnssec, sign).
• Rewrite or transform questions and answers (rewrite, template, header, minimal, loadbalance, dns64).
• Enforce access policies (acl, view, tsig).
• Expose Prometheus metrics (prometheus/metrics), structured query logs (log), errors (errors), and dnstap streams (dnstap).
• Listen on additional transports: TLS for DoT (tls), HTTPS for DoH (https), HTTP/3 for DoH3 (https3), QUIC for DoQ (quic), gRPC (grpc_server).

The complete in-tree plugin list lives in plugin.cfg.

How a request flows

graph LR
    Client[DNS client] -->|query| Listener[core/dnsserver Server]
    Listener -->|zone match| Chain[plugin chain]
    Chain -->|ServeDNS| P1[plugin 1]
    P1 -->|next| P2[plugin 2]
    P2 -->|next| Pn[... plugin N]
    Pn -->|response| Listener
    Listener -->|reply| Client

The server matches the query name against configured zones (longest suffix wins), then calls ServeDNS on the head of the plugin chain for that zone. Plugins handle the request or call plugin.NextOrFailure to defer to the next plugin. Order is fixed by plugin.cfg, not by the order plugins appear in the Corefile.

Quick links

What	Where
Build it and run a Corefile	Getting started
Architecture in depth	Architecture
Definitions of project-specific terms	Glossary
Per-plugin docs (READMEs)	plugin/<name>/README.md and man/coredns-<name>.7
External docs	https://coredns.io, https://coredns.io/plugins

Project facts

• Language: Go (>= 1.25.0; tracked in .go-version and go.mod)
• License: Apache 2.0 (see LICENSE)
• Governance: GOVERNANCE.md, CODEOWNERS
• Build entry point: coredns.go → coremain.Run() → caddy.Start(corefile)
• Default Corefile: whoami + log listening on :53

For a quantitative snapshot of the codebase see By the numbers. For its history see Lore.