6.6 KiB
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 CLUSTER_IP EXTERNAL_IP PORT(S) SELECTOR AGE
dns-backend 10.0.2.3 <none> 8000/TCP name=dns-backend 1d
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 CLUSTER_IP EXTERNAL_IP PORT(S) SELECTOR AGE
dns-backend 10.0.2.4 <none> 8000/TCP name=dns-backend 1d
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
NAME READY STATUS RESTARTS AGE
dns-frontend 0/1 ExitCode:0 0 1m
Wait until the pod succeeds, then we can see the output from the client pod:
$ kubectl logs 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 logs 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.svc.cluster.local:8000, or shorthand version http://dns-backend:8000
tl; dr;
For those of you who are impatient, here is the summary of the commands we ran in this tutorial. Remember to set first $CLUSTER_NAME and $USER_NAME to the values found in ~/.kube/config.
# create dev and prod namespaces
kubectl create -f examples/cluster-dns/namespace-dev.yaml
kubectl create -f examples/cluster-dns/namespace-prod.yaml
# create two contexts
kubectl config set-context dev --namespace=development --cluster=${CLUSTER_NAME} --user=${USER_NAME}
kubectl config set-context prod --namespace=production --cluster=${CLUSTER_NAME} --user=${USER_NAME}
# create two backend replication controllers
kubectl config use-context dev
kubectl create -f examples/cluster-dns/dns-backend-rc.yaml
kubectl config use-context prod
kubectl create -f examples/cluster-dns/dns-backend-rc.yaml
# create backend services
kubectl config use-context dev
kubectl create -f examples/cluster-dns/dns-backend-service.yaml
kubectl config use-context prod
kubectl create -f examples/cluster-dns/dns-backend-service.yaml
# create a pod in each namespace and get its output
kubectl config use-context dev
kubectl create -f examples/cluster-dns/dns-frontend-pod.yaml
kubectl logs dns-frontend
kubectl config use-context prod
kubectl create -f examples/cluster-dns/dns-frontend-pod.yaml
kubectl logs dns-frontend