# Customizing CoreDNS forwarders on Kubernetes

In some cases you will need to use your internal DNS servers for name resolution.

To achieve this you will need to modify the [CoreDNS](https://coredns.io/) configuration of your Kubernetes cluster and add your DNS servers as forwarders.

CoreDNS is a DNS server that is modular and pluggable, and each plugin adds new functionality to CoreDNS. This can be configured by maintaining a [Corefile](https://coredns.io/2017/07/23/corefile-explained/) , which is the CoreDNS configuration file. As a cluster administrator, you can modify the [ConfigMap](https://kubernetes.io/docs/concepts/configuration/configmap/) for the CoreDNS Corefile to change how DNS service discovery behaves for that cluster.

In Kubernetes, CoreDNS is installed with the following default Corefile configuration:

```plaintext
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
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
        cache 30
        loop
        reload
        loadbalance
    }
```

To explicitly force all non-cluster DNS lookups to go through your specific nameservers, at 172.16.0.1 and 172.16.0.2 for example, point the forward to the ip adresses of your servers instead of /etc/resolv.conf

To do so, edit the coredns ConfigMap

```plaintext
kubectl -n kube-system edit cm coredns
```

and replace **forward . /etc/resolv.conf** by **forward . 172.16.0.1 172.16.0.2**

```plaintext
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
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 . 172.16.0.1 172.16.0.2
        cache 30
        loop
        reload
        loadbalance
    }
```

Don't forget to keep the dot (.) in the forward line.

You can now force your CoreDNS deployment do redeploy the coredns pods with a scale 0 to 1 operation.

```plaintext
kubectl -n kube-system scale deployment coredns --replicas=0
kubectl -n kube-system scale deployment coredns --replicas=1
```

With this configuration the pods running on the cluster can now perform name resolution to your internal DNS servers.

Feel free to comment this article if you have questions.

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References https://kubernetes.io/docs/tasks/administer-cluster/dns-custom-nameservers/