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

Short description

Use CoreDNS to configure a conditional forwarder for DNS queries that are sent to the domains that a customized DNS server resolves. For more information, see Customizing DNS service on the Kubernetes website.

Important: Before you make configuration changes to the CoreDNS Amazon EKS add-on, you must determine the settings that Amazon EKS manages. You can find this information in the Amazon EKS add-on configuration. When you modify a field that Amazon EKS manages, Amazon EKS can't manage the add-on. Amazon EKS might overwrite your changes when an add-on is updated.

Resolution

The following resolution applies to CoreDNS self-managed and Amazon EKS add-on configurations.

Configure a self-managed add-on

Run the following command to modify the CoreDNS ConfigMap and add the custom DNS conditional forwarder configuration:

$ kubectl -n kube-system edit configmap coredns

The output looks similar to the following:

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 and custom-dns-server with your custom DNS server IP address.

Configure an Amazon EKS add-on

To make the changes to the Amazon EKS managed CoreDNS add-on, complete the following steps:

1. Open the Amazon EKS console.

2. In the navigation pane, choose Clusters.

3. Choose the name of your cluster.

4. Choose the Add-ons tab.

5. Select the CoreDNS add-on, and then choose Edit.

6. On the edit page, choose the Optional configuration settings section. In the Configuration values section, add the custom DNS conditional forwarder configuration:

   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 and custom-dns-server with your custom DNS server IP address.

Verify the domain name resolution

To verify that domain name resolution works, run the following commands:

$ 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.