Add the DNS entry to DNSCore in kubernetes

 Edit configmap with name coredns in namespace 

kubectl edit cm coredns   -n kube-system

apiVersion: v1
kind: ConfigMap
metadata:
name: coredns
namespace: kube-system
data:
Corefile: |
.:53 {
errors
log
health
rewrite name foo.example.com foo.default.svc.cluster.local
kubernetes cluster.local 10.0.0.0/24
file /etc/coredns/cluster.db cluster.local
proxy . /etc/resolv.conf
cache 30
}
cluster.db: |

cluster.local.               IN      SOA     ns.dns.cluster.local. hostmaster.cluster.local. 2015082541 7200 3600 1209600 3600

something.cluster.local.     IN      A       10.0.0.1

otherthing.cluster.local.    IN      CNAME   google.com.

Add the highlight  content above( The example.db is included dns entries the you want to add).

and we also need to edit the volumes section of the Pod template spec(DNSCore deployment):

      volumes:

and we also need to edit the volumes section of the Pod template spec:

volumes:
- name: config-volume
configMap:
name: coredns
items:
- key: Corefile
path: Corefile
- key: cluster.db
path: cluster.db

Ref: https://coredns.io/2017/06/08/how-queries-are-processed-in-coredns/