DNS & CoreDNS

How Kubernetes DNS works, the FQDN format, CoreDNS configuration, and how to debug DNS failures.

intermediate45 minRead → Lab → Quiz → Practice

🧠 Brain Warm-Up

🧠 Brain Warm-Up: When a pod in namespace "frontend" tries to connect to "backend-svc", how does it know which IP to use? There is no /etc/hosts entry. Think about how DNS is injected into pods before reading.

How Kubernetes DNS Works

Every pod gets /etc/resolv.conf injected with:

nameserver 10.96.0.10          ← CoreDNS ClusterIP
search default.svc.cluster.local svc.cluster.local cluster.local
options ndots:5

CoreDNS is a pod running in kube-system that handles DNS for the cluster.

FQDN Format

<service>.<namespace>.svc.cluster.local
    ↑          ↑           ↑
  svc name  namespace   cluster domain

Examples:

my-api.default.svc.cluster.local — full form
my-api.default — works due to search domains
my-api — works within the same namespace

Search Domains Enable Short Names

From a pod in namespace frontend:

backend-svc resolves (search appends frontend.svc.cluster.local)
backend-svc.backend resolves cross-namespace (search appends svc.cluster.local)
backend-svc.backend.svc.cluster.local — always works

Pod DNS Policy

Policy	Behavior
`ClusterFirst` (default)	Cluster DNS first, then upstream
`Default`	Node's /etc/resolv.conf (no cluster DNS)
`None`	Custom — must set dnsConfig
`ClusterFirstWithHostNet`	For hostNetwork pods

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