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Resolve internal (on-premise) DNS server in AWS EKS via CoreDNS

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Prerequisites

• CoreDNS running on your EKS cluster (default since EKS v1.12). If CoreDNS isn’t installed, follow the AWS CoreDNS documentation to install it.
• kubectl access to the EKS cluster running your Domino deployment. Update your local kubeconfig or connect directly in AWS before proceeding.

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Update the CoreDNS configmap

1. Open the CoreDNS configmap for editing:
   kubectl -n kube-system edit configmap coredns
   
2. Locate the Corefile section. It looks similar to this:
   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
     }
   
3. Add a new forwarding block after the closing } of the existing stanza. Replace google.com with your on-premise domain and 8.8.8.8 with the IP address of your DNS server (for example, the load balancer IP for your VPN tunnel). Keep the period (.) between forward and the IP address. The updated Corefile section should look like this:
   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
     }
     google.com:53 {
      forward . 8.8.8.8
     }
   
4. Save the file. The editor displays a confirmation message when the configmap saves successfully.
5. Restart the CoreDNS pods to apply the change immediately:
   kubectl get pods -n kube-system -oname |grep coredns |xargs kubectl delete -n kube-system
   
6. Verify that the CoreDNS pods return to a Running status:
   kubectl get pods --namespace=kube-system -l k8s-app=kube-dns
   
   All listed pods should show Running in the STATUS column before you proceed.

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Configure the Domino environment

1. In Domino, navigate to Environments and select the environment you want to update.
2. Click Edit Definition.
3. Scroll down and expand the Advanced section.
4. Enter the following script in the Pre Setup Script field, replacing google.com with your on-premise domain:
   cat /etc/resolv.conf >> /tmp/resolv.conf.tmp
   sed -i '2s/$/ google.com/' /tmp/resolv.conf.tmp
   cat /tmp/resolv.conf.tmp > /etc/resolv.conf
   
5. Save and build the environment.

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Verify DNS resolution

1. Start a new Jupyter workspace and open a terminal.
2. Run an nslookup for a hostname in your domain. Using the Google example:
   nslookup maps
   
3. Confirm that the output resolves the short hostname to a fully qualified domain name (FQDN). Expected output:
   ubuntu@run-5ea9d22e84e7430006080fdb-7bfkr:/mnt$ nslookup maps
   Server:     10.100.0.10
   Address:    10.100.0.10#53
   
   Non-authoritative answer:
   Name:  maps.google.com
   Address: 172.217.14.206
   Name:  maps.google.com
   Address: 2607:f8b0:400a:808::200e
   
   Successful output shows the short name resolving to the FQDN (for example, maps → maps.google.com), confirming that on-premise DNS resolution is working