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k3s/examples/cluster-dns/README.md

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Kubernetes DNS example

This is a toy example demonstrating how to use kubernetes DNS.

Step Zero: Prerequisites

This example assumes that you have forked the repository and turned up a Kubernetes cluster. Make sure DNS is enabled in your setup, see DNS doc.

$ cd kubernetes
$ hack/dev-build-and-up.sh

Step One: Create two namespaces

We'll see how cluster DNS works across multiple namespaces, first we need to create two namespaces:

$ kubectl create -f examples/cluster-dns/namespace-dev.yaml
$ kubectl create -f examples/cluster-dns/namespace-prod.yaml

Now list all namespaces:

$ kubectl get namespaces
NAME          LABELS             STATUS
default       <none>             Active
development   name=development   Active
production    name=production    Active

For kubectl client to work with each namespace, we define two contexts:

$ kubectl config set-context dev --namespace=development --cluster=${CLUSTER_NAME} --user=${USER_NAME}
$ kubectl config set-context prod --namespace=production --cluster=${CLUSTER_NAME} --user=${USER_NAME}

You can view your cluster name and user name in kubernetes config at ~/.kube/config.

Step Two: Create backend replication controller in each namespace

Use the file examples/cluster-dns/dns-backend-rc.yaml to create a backend server replication controller in each namespace.

$ kubectl config use-context dev
$ kubectl create -f examples/cluster-dns/dns-backend-rc.yaml

Once that's up you can list the pod in the cluster:

$ kubectl get rc
CONTROLLER    CONTAINER(S)   IMAGE(S)              SELECTOR           REPLICAS
dns-backend   dns-backend    ddysher/dns-backend   name=dns-backend   1

Now repeat the above commands to create a replication controller in prod namespace:

$ kubectl config use-context prod
$ kubectl create -f examples/cluster-dns/dns-backend-rc.yaml
$ kubectl get rc
CONTROLLER    CONTAINER(S)   IMAGE(S)              SELECTOR           REPLICAS
dns-backend   dns-backend    ddysher/dns-backend   name=dns-backend   1

Step Three: Create backend service

Use the file examples/cluster-dns/dns-backend-service.yaml to create a service for the backend server.

$ kubectl config use-context dev
$ kubectl create -f examples/cluster-dns/dns-backend-service.yaml

Once that's up you can list the service in the cluster:

$ kubectl get service dns-backend
NAME          LABELS    SELECTOR           IP(S)          PORT(S)
dns-backend   <none>    name=dns-backend   10.0.236.129   8000/TCP

Again, repeat the same process for prod namespace:

$ kubectl config use-context prod
$ kubectl create -f examples/cluster-dns/dns-backend-service.yaml
$ kubectl get service dns-backend
NAME          LABELS    SELECTOR           IP(S)         PORT(S)
dns-backend   <none>    name=dns-backend   10.0.35.246   8000/TCP

Step Four: Create client pod in one namespace

Use the file examples/cluster-dns/dns-frontend-pod.yaml to create a client pod in dev namespace. The client pod will make a connection to backend and exit. Specifically, it tries to connect to address http://dns-backend.development.cluster.local:8000.

$ kubectl config use-context dev
$ kubectl create -f examples/cluster-dns/dns-frontend-pod.yaml

Once that's up you can list the pod in the cluster:

$ kubectl get pods dns-frontend
POD            IP           CONTAINER(S)   IMAGE(S)               HOST                                    LABELS              STATUS    CREATED     MESSAGE
dns-frontend   10.244.2.9                                         kubernetes-minion-sswf/104.154.55.211   name=dns-frontend   Running   3 seconds
                            dns-frontend   ddysher/dns-frontend                                                               Running   2 seconds

Wait until the pod succeeds, then we can see the output from the client pod:

$ kubectl log dns-frontend
2015-05-07T20:13:54.147664936Z 10.0.236.129
2015-05-07T20:13:54.147721290Z Send request to: http://dns-backend.development.cluster.local:8000
2015-05-07T20:13:54.147733438Z <Response [200]>
2015-05-07T20:13:54.147738295Z Hello World!

Please refer to the source code about the log. First line prints out the ip address associated with the service in dev namespace; remaining lines print out our request and server response.

If we switch to prod namespace with the same pod config, we'll see the same result, i.e. dns will resolve across namespace.

$ kubectl config use-context prod
$ kubectl create -f examples/cluster-dns/dns-frontend-pod.yaml
$ kubectl log dns-frontend
2015-05-07T20:13:54.147664936Z 10.0.236.129
2015-05-07T20:13:54.147721290Z Send request to: http://dns-backend.development.cluster.local:8000
2015-05-07T20:13:54.147733438Z <Response [200]>
2015-05-07T20:13:54.147738295Z Hello World!

Note about default namespace

If you prefer not using namespace, then all your services can be addressed using default namespace, e.g. http://dns-backend.default.cluster.local:8000, or shorthand version http://dns-backend:8000

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