How do I configure a conditional forwarder with CoreDNS in my Amazon EKS cluster?

Last updated: 2021-09-13

I want to configure a conditional forwarder with CoreDNS in my Amazon Elastic Kubernetes Service (Amazon EKS) cluster.

Short description

You can use CoreDNS to configure conditional forwarding for DNS queries sent to the domains resolved by a customized DNS server. For more information, see Customizing DNS Service on the Kubernetes website.

Important: Apply the modifications in the following resolution to self-managed CoreDNS only. To make configuration changes to the CoreDNS Amazon EKS add-on, determine the settings that Amazon EKS manages by checking the Amazon EKS add-on configuration. Check the add-on configuration before making any changes, because modifying a field managed by Amazon EKS prevents Amazon EKS from managing the add-on. This could result in your changes being overwritten when an add-on is updated.

Resolution

1.    Modify the CoreDNS ConfigMap and add the conditional forwarder configuration:

$ kubectl -n kube-system edit configmap coredns

Output:

apiVersion: v1
kind: ConfigMap
metadata:
  annotations:
  labels:
    eks.amazonaws.com/component: coredns
    k8s-app: kube-dns
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        health
        kubernetes cluster.local in-addr.arpa ip6.arpa {
          pods insecure
          fallthrough in-addr.arpa ip6.arpa
        }
        prometheus :9153
        forward . /etc/resolv.conf
        cache 30
        loop
        reload
        loadbalance
    }
    domain-name:53 {
        errors
        cache 30
        forward . custom-dns-server
        reload
    }

Note: Replace domain-name with your domain name. Replace custom-dns-server with your custom DNS server IP address.

2.    Verify that domain name resolution works:

$ kubectl run busybox --restart=Never --image=busybox:1.28 -- sleep 3600
$ kubectl exec busybox -- nslookup domain-name

Note: Replace domain-name with your domain name.

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