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.
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