My goal is to call a service on an AKS cluster (aks1/US) from a pod on a second AKS cluster (aks2/EU).
These clusters will be on different regions and should communicate over a private network.

For the cluster networking I’m using the Azure CNI plugin.

Above you can see a schema of the two possible ending architectures. ExternalName  or ExternalIP  service on the US AKS pointing to a private EU ingress controller IP.

So, after some reading and some video listening, it seemed for me that the best option was to use an externalName service on AKS2 calling a service defined in a custom private DNS zone (ecommerce.private.eu.dev), being these two VNets peered before.

Address space for aks services:
dev-vnet  10.0.0.0/14
=======================================
dev-test1-aks   v1.22.4 - 1 node
dev-test1-vnet  11.0.0.0/16
=======================================
dev-test2-aks   v1.22.4 - 1 node
dev-test2-vnet  11.1.0.0/16

After some trials I can get connectivity between pods networks but I was never able to reach the service network from the other cluster.

  • I don’t have any active firewall
  • I’ve peered all three networks: dev-test1-vnet, dev-test2-vnet, dev-vnet (services CIDR)
  • I’ve create a Private DNS zones private.eu.dev where I’ve put the “ecommerce” A record (10.0.129.155) that should be resolved by the externalName service

dev-test1-aks (EU cluster):

kubectl create deployment eu-ecommerce --image=k8s.gcr.io/echoserver:1.4 --port=8080 --replicas=1

kubectl expose deployment eu-ecommerce --type=ClusterIP --port=8080 --name=eu-ecommerce

kubectl apply -f https://raw.githubusercontent.com/kubernetes/ingress-nginx/controller-v1.1.1/deploy/static/provider/cloud/deploy.yaml

kubectl create ingress eu-ecommerce --class=nginx --rule=eu.ecommerce/*=eu-ecommerce:8080

This is the ingress rule:

❯ kubectl --context=dev-test1-aks get ingress eu-ecommerce-2 -o yaml
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: eu-ecommerce-2
  namespace: default
spec:
  ingressClassName: nginx
  rules:
  - host: lb.private.eu.dev
    http:
      paths:
      - backend:
          service:
            name: eu-ecommerce
            port:
              number: 8080
        path: /ecommerce
        pathType: Prefix
status:
  loadBalancer:
    ingress:
    - ip: 20.xxxxx

This is one of the externalName I’ve tried on dev-test2-aks:

apiVersion: v1
kind: Service
metadata:
  name: eu-services
  namespace: default
spec:
  type: ExternalName
  externalName: ecommerce.private.eu.dev
  ports:
    - port: 8080
      protocol: TCP

These are some of my tests:

# --- Test externalName 
kubectl --context=dev-test2-aks run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- wget -qO- http://eu-services:8080
: '
    wget: cant connect to remote host (10.0.129.155): Connection timed out
'

# --- Test connectivity AKS1 -> eu-ecommerce service
kubectl --context=dev-test1-aks run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- wget -qO- http://eu-ecommerce:8080
kubectl --context=dev-test1-aks run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- wget -qO- http://10.0.129.155:8080
kubectl --context=dev-test1-aks run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- wget -qO- http://eu-ecommerce.default.svc.cluster.local:8080
kubectl --context=dev-test1-aks run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- wget -qO- http://ecommerce.private.eu.dev:8080
# OK client_address=11.0.0.11

# --- Test connectivity AKS2 -> eu-ecommerce POD
kubectl --context=dev-test2-aks run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- wget -qO- http://11.0.0.103:8080
#> OK


# --- Test connectivity - LB private IP
kubectl --context=dev-test1-aks run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- wget --no-cache -qO- http://lb.private.eu.dev/ecommerce
#> OK
kubectl --context=dev-test2-aks run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- wget --no-cache -qO- http://lb.private.eu.dev/ecommerce
#> KO  wget: can't connect to remote host (10.0.11.164): Connection timed out
#>> This is the ClusterIP! -> Think twice!


# --- Traceroute gives no informations
kubectl --context=dev-test2-aks  run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- traceroute -n -m4 ecommerce.private.eu.dev
: '
    *  *  *
    3  *  *  *
    4  *  *  *
'

# --- test2-aks can see the private dns zone and resolve the hostname
kubectl --context=dev-test2-aks run -it --rm --restart=Never busybox --image=gcr.io/google-containers/busybox -- nslookup ecommerce.private.eu.dev
: ' Server:    10.0.0.10
    Address 1: 10.0.0.10 kube-dns.kube-system.svc.cluster.local
    Name:      ecommerce.private.eu.dev
    Address 1: 10.0.129.155
'

I’ve also created inbound and outbound network policies for the AKS networks:

  • on dev-aks (10.0/16) allow all incoming from 11.1/16 and 11.0/16
  • on dev-test2-aks allow any outbound

SOLUTION: Set the LB as an internal LB exposing the external IP to the private subnet

kubectl --context=dev-test1-aks patch service -n ingress-nginx ingress-nginx-controller --patch '{"metadata": {"annotations": {"service.beta.kubernetes.io/azure-load-balancer-internal": "tr
ue"}}}'

This article is also in Medium 🙂

Seen docs: