Kubernetes DNS Concepts

Internal

• Kubernetes Concepts
• Kubernetes Networking Concepts

Overview

Each Kubernetes cluster runs its own DNS service. The DNS service is exposed as a kube-dns Kubernetes ClusterIP service, running in the "kube-system" namespace. The service is backed by two coredns highly-available pods, also deployed in the "kube-system" namespace.

The DNS Service

Each Kubernetes cluster runs an internal DNS service. The DNS service is exposed as a regular Kubernetes ClusterIP service, deployed in the "kube-system" namespace, which in turn exposes the Kubernetes internal DNS server:

NAME       TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)                  AGE
kube-dns   ClusterIP   10.96.0.10   <none>        53/UDP,53/TCP,9153/TCP   49d

The static ClusterIP address allocated to the DNS service (10.96.0.10) is hardcoded into every pod in the cluster, allowing the pods to resolve service names.

Every time a new Kubernetes service is deployed, the Kubernetes cluster automatically updates the internal database with service name to ClusterIP address mappings, making the ClusterIP address of the new service dynamically available to all other pods in the cluster. StatefulSets and the individual pods managed by a StatefulSet are also registered with the DNS service.

DNS Operations

• Kubernetes DNS Operations

The DNS Service

The DNS service is built on CoreDNS.

Testing name resolution:

kubectl run -it --rm --restart=Never --image=infoblox/dnstools:latest dnstools

Also see:

Services and DNS

DNS Implementation Details

CoreDNS

NAME                         READY   STATUS    RESTARTS   AGE
coredns-5644d7b6d9-kcxt6     1/1     Running   0          8h
coredns-5644d7b6d9-mztf9     1/1     Running   0          8h

kube-dns

Note that kube-dns seems to be both the name of the Kubernetes DNS service and the name of a backing provider.

SkyDNS

---

Explain default.svc.cluster.local, svc.cluster.local, cluster.local.

TODO: https://medium.com/kubernetes-tutorials/kubernetes-dns-for-services-and-pods-664804211501

Name Resolution inside a Pod

Each pod gets an /etc/resolv.conf with a name server hardcoded to the IP address of the DNS service kube-dns:

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

The local DNS library is thus configured to use by default the name server behind the Kubernetes DNS service.