README.md

Setup DNS for two clusters

This example shows how to simply configure three k8s clusters to know each other. Can be skipped if clusters setupped with external DNS.

Run

Expose dns service for first cluster

kubectl --kubeconfig=$KUBECONFIG1 expose service kube-dns -n kube-system --port=53 --target-port=53 --protocol=TCP --name=exposed-kube-dns --type=LoadBalancer

Wait for assigning IP address (note: you should see IP address in logs. If you dont see repeat this):

kubectl --kubeconfig=$KUBECONFIG1 get services exposed-kube-dns -n kube-system -o go-template='{{index (index (index (index .status "loadBalancer") "ingress") 0) "ip"}}'
ip1=$(kubectl --kubeconfig=$KUBECONFIG1 get services exposed-kube-dns -n kube-system -o go-template='{{index (index (index (index .status "loadBalancer") "ingress") 0) "ip"}}')
if [[ $ip1 == *"no value"* ]]; then 
    ip1=$(kubectl --kubeconfig=$KUBECONFIG1 get services exposed-kube-dns -n kube-system -o go-template='{{index (index (index (index .status "loadBalancer") "ingress") 0) "hostname"}}')
    ip1=$(dig +short $ip1 | head -1)
fi
echo Selected externalIP: $ip1 for cluster1

Expose dns service for the second cluster:

kubectl --kubeconfig=$KUBECONFIG2 expose service kube-dns -n kube-system --port=53 --target-port=53 --protocol=TCP --name=exposed-kube-dns --type=LoadBalancer

Wait for assigning IP address (note: you should see IP address in logs. If you dont see repeat this):

kubectl --kubeconfig=$KUBECONFIG2 get services exposed-kube-dns -n kube-system -o go-template='{{index (index (index (index .status "loadBalancer") "ingress") 0) "ip"}}'
ip2=$(kubectl --kubeconfig=$KUBECONFIG2 get services exposed-kube-dns -n kube-system -o go-template='{{index (index (index (index .status "loadBalancer") "ingress") 0) "ip"}}')
if [[ $ip2 == *"no value"* ]]; then 
    ip2=$(kubectl --kubeconfig=$KUBECONFIG2 get services exposed-kube-dns -n kube-system -o go-template='{{index (index (index (index .status "loadBalancer") "ingress") 0) "hostname"}}')
    ip2=$(dig +short $ip2 | head -1)
fi
echo Selected externalIP: $ip2 for cluster2

Add DNS forwarding from cluster1 to cluster2:

cat > configmap.yaml <<EOF
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
        }
        k8s_external my.cluster1
        prometheus :9153
        forward . /etc/resolv.conf {
            max_concurrent 1000
        }
        loop
        reload 5s
    }
    my.cluster2:53 {
      forward . ${ip2}:53 {
        force_tcp
      }
    }
EOF
kubectl --kubeconfig=$KUBECONFIG1 apply -f configmap.yaml
cat > custom-configmap.yaml <<EOF
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns-custom
  namespace: kube-system
data:
  server.override: |
    k8s_external my.cluster2
  proxy1.server: |
    my.cluster2:53 {
      forward . ${ip2}:53 {
        force_tcp
      }
    }
EOF

kubectl --kubeconfig=$KUBECONFIG1 apply -f custom-configmap.yaml

Add DNS forwarding from cluster2 to cluster1:

cat > configmap.yaml <<EOF
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
        }
        k8s_external my.cluster2
        prometheus :9153
        forward . /etc/resolv.conf {
            max_concurrent 1000
        }
        loop
        reload 5s
    }
    my.cluster1:53 {
      forward . ${ip1}:53 {
        force_tcp
      }
    }
EOF
kubectl --kubeconfig=$KUBECONFIG2 apply -f configmap.yaml
cat > custom-configmap.yaml <<EOF
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns-custom
  namespace: kube-system
data:
  server.override: |
    k8s_external my.cluster1
  proxy1.server: |
    my.cluster1:53 {
      forward . ${ip1}:53 {
        force_tcp
      }
    }
EOF
kubectl --kubeconfig=$KUBECONFIG2 apply -f custom-configmap.yaml

Cleanup

kubectl --kubeconfig=$KUBECONFIG1 delete service -n kube-system exposed-kube-dns
kubectl --kubeconfig=$KUBECONFIG2 delete service -n kube-system exposed-kube-dns