Day 31/40

Understanding CoreDNS In Kubernetes

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@piyushsachdeva
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In this section, we're looking at coredns concept in kubernetes.

workloads in kubernetes cluster can communicate with each other with coredns.

root@localhost:~# kubectl get pod -n=kube-system
NAME                                               READY   STATUS    RESTARTS   AGE
coredns-7db6d8ff4d-7dwv7                           1/1     Running   0          9d
coredns-7db6d8ff4d-tmb52                           1/1     Running   0          9d
...
root@localhost:~# kubectl get svc -n=kube-system
NAME       TYPE        CLUSTER-IP   EXTERNAL-IP   PORT(S)                  AGE
kube-dns   ClusterIP   10.96.0.10   <none>        53/UDP,53/TCP,9153/TCP   9d

root@localhost:~# kubectl get pod
NAME          READY   STATUS    RESTARTS   AGE
task-pv-pod   1/1     Running   0          3d21h
root@localhost:~# kubectl exec task-pv-pod -- cat /etc/resolv.conf
search default.svc.cluster.local svc.cluster.local cluster.local
nameserver 10.96.0.10
options ndots:5

As we can see, the dns is set in resolv.conf file inside the pod, so everything across the cluster can be resolved with coredns service.

root@localhost:~# kubectl exec task-pv-pod -- cat /etc/hosts
# Kubernetes-managed hosts file.
127.0.0.1       localhost
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
fe00::0 ip6-mcastprefix
fe00::1 ip6-allnodes
fe00::2 ip6-allrouters
192.168.2.202   task-pv-pod
root@localhost:~# kubectl exec task-pv-pod -- hostname -i
192.168.2.202

Let's check something on one of the coredns pods:

root@localhost:~# kubectl describe pod coredns-7db6d8ff4d-7dwv7 -n=kube-system
Name:                 coredns-7db6d8ff4d-7dwv7
Namespace:            kube-system
Priority:             2000000000
Priority Class Name:  system-cluster-critical
Service Account:      coredns
...
    Image:         registry.k8s.io/coredns/coredns:v1.11.1
...
    Mounts:
      /etc/coredns from config-volume (ro)
      /var/run/secrets/kubernetes.io/serviceaccount from kube-api-access-t6mss (ro)
...
Volumes:
  config-volume:
    Type:      ConfigMap (a volume populated by a ConfigMap)
    Name:      coredns
    Optional:  false
...

There's a ConfigMap for coredns which is mounted as volume to the pod.

root@localhost:~# kubectl get cm -n=kube-system
NAME                                                   DATA   AGE
coredns                                                1      9d
...

root@localhost:~# kubectl describe cm coredns -n=kube-system
Name:         coredns
Namespace:    kube-system
Labels:       <none>
Annotations:  <none>

Data
====
Corefile:
----
.:53 {
    errors
    health {
       lameduck 5s
    }
    ready
    kubernetes cluster.local in-addr.arpa ip6.arpa {
       pods insecure
       fallthrough in-addr.arpa ip6.arpa
       ttl 30
    }
    prometheus :9153
    forward . /etc/resolv.conf {
       max_concurrent 1000
    }
    cache 30
    loop
    reload
    loadbalance
}


BinaryData
====

Events:  <none>

The official documentation is here and for debugging is here and sample ConfigMap yaml file:

apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        log
        errors
        health
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          upstream
          fallthrough in-addr.arpa ip6.arpa
        }
        prometheus :9153
        forward . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }

If you have no error and your coredns doesn't come up, we can look at the CNI at first because it can be the first dependency for our coredns service.

40 Days Of Kubernetes (31/40)

Day 31/40

Understanding CoreDNS In Kubernetes