การค้นหาบริการและ DNS ใน Kubernetes

Container IPs are ephemeral — pods restart and get new addresses constantly. Hardcoding IPs is impossible.

Service discovery lets workloads find each other by stable names instead of changing IPs.

A Kubernetes Service provides a stable virtual IP (ClusterIP) and DNS name that load-balances across a set of pods selected by labels.

Every cluster runs a DNS server (CoreDNS). It automatically creates records for each Service so workloads can resolve them by name.

Services follow a predictable FQDN pattern:

service.namespace.svc.cluster.local

Within the same namespace, just orders works. Across namespaces use orders.payments.

Each pod's /etc/resolv.conf points at the cluster DNS and includes search domains, so short names expand to full Service FQDNs automatically.

A ClusterIP is virtual — kube-proxy programs iptables or IPVS rules so traffic to it is distributed across the healthy backing pods. Clients never see individual pod IPs.

Set clusterIP: None for a headless Service. DNS then returns the individual pod IPs directly, which is ideal for stateful sets and client-side load balancing.

Behind each Service, Kubernetes maintains EndpointSlices listing the ready pod IPs. As pods come and go, these update so DNS and routing stay current.

You can debug discovery by running nslookup from a pod to verify a Service name resolves to its ClusterIP.

An ExternalName Service maps a cluster DNS name to an outside hostname via a CNAME, letting you reference external systems with cluster-internal names.

When name resolution fails, check:

• Label selector mismatch (no endpoints)
• Wrong namespace in the FQDN
• CoreDNS pods unhealthy
• NetworkPolicy blocking DNS (port 53)

Test your service discovery knowledge.

You now understand container service discovery:

• Services give stable ClusterIPs and DNS names
• CoreDNS resolves service.namespace.svc.cluster.local
• EndpointSlices track ready pods
• Headless and ExternalName Services for special cases
• Debug with nslookup from inside pods

This builds on your Docker network modes and Kubernetes networking basics.