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Set up CoreDNS as DNS provider for Cluster Federation

Note: Federation V1, the current Kubernetes federation API which reuses the Kubernetes API resources ‘as is’, is currently considered alpha for many of its features. There is no clear path to evolve the API to GA; however, there is a Federation V2 effort in progress to implement a dedicated federation API apart from the Kubernetes API. The details are available at sig-multicluster community page.

This page shows how to configure and deploy CoreDNS to be used as the DNS provider for Cluster Federation.

Objectives
Before you begin
Deploying CoreDNS and etcd charts
Deploying Federation with CoreDNS as DNS provider
Setup CoreDNS server in nameserver resolv.conf chain

Objectives

Configure and deploy CoreDNS server
Bring up federation with CoreDNS as dns provider
Setup CoreDNS server in nameserver lookup chain

Before you begin

You need to have a running Kubernetes cluster (which is referenced as host cluster). Please see one of the getting started guides for installation instructions for your platform.
Support for LoadBalancer services in member clusters of federation is mandatory to enable CoreDNS for service discovery across federated clusters.

Deploying CoreDNS and etcd charts

CoreDNS can be deployed in various configurations. Explained below is a reference and can be tweaked to suit the needs of the platform and the cluster federation.

To deploy CoreDNS, we shall make use of helm charts. CoreDNS will be deployed with etcd as the backend and should be pre-installed. etcd can also be deployed using helm charts. Shown below are the instructions to deploy etcd.

helm install --namespace my-namespace --name etcd-operator stable/etcd-operator
helm upgrade --namespace my-namespace --set cluster.enabled=true etcd-operator stable/etcd-operator

Note: etcd default deployment configurations can be overridden, suiting the host cluster.

After deployment succeeds, etcd can be accessed with the http://etcd-cluster.my-namespace:2379 endpoint within the host cluster.

The CoreDNS default configuration should be customized to suit the federation. Shown below is the Values.yaml, which overrides the default configuration parameters on the CoreDNS chart.

isClusterService: false
serviceType: "LoadBalancer"
plugins:
  kubernetes:
    enabled: false
  etcd:
    enabled: true
    zones:
    - "example.com."
    endpoint: "http://etcd-cluster.my-namespace:2379"

The above configuration file needs some explanation:

isClusterService specifies whether CoreDNS should be deployed as a cluster-service, which is the default. You need to set it to false, so that CoreDNS is deployed as a Kubernetes application service.
serviceType specifies the type of Kubernetes service to be created for CoreDNS. You need to choose either “LoadBalancer” or “NodePort” to make the CoreDNS service accessible outside the Kubernetes cluster.
Disable plugins.kubernetes, which is enabled by default by setting plugins.kubernetes.enabled to false.
Enable plugins.etcd by setting plugins.etcd.enabled to true.
Configure the DNS zone (federation domain) for which CoreDNS is authoritative by setting plugins.etcd.zones as shown above.
Configure the etcd endpoint which was deployed earlier by setting plugins.etcd.endpoint

Now deploy CoreDNS by running

helm install --namespace my-namespace --name coredns -f Values.yaml stable/coredns

Verify that both etcd and CoreDNS pods are running as expected.

Deploying Federation with CoreDNS as DNS provider

The Federation control plane can be deployed using kubefed init. CoreDNS can be chosen as the DNS provider by specifying two additional parameters.

--dns-provider=coredns
--dns-provider-config=coredns-provider.conf

coredns-provider.conf has below format:

[Global]
etcd-endpoints = http://etcd-cluster.my-namespace:2379
zones = example.com.
coredns-endpoints = <coredns-server-ip>:<port>

etcd-endpoints is the endpoint to access etcd.
zones is the federation domain for which CoreDNS is authoritative and is same as –dns-zone-name flag of kubefed init.
coredns-endpoints is the endpoint to access CoreDNS server. This is an optional parameter introduced from v1.7 onwards.

Note: plugins.etcd.zones in the CoreDNS configuration and the --dns-zone-name flag to kubefed init should match.

Setup CoreDNS server in nameserver resolv.conf chain

Note: The following section applies only to versions prior to v1.7 and will be automatically taken care of if the coredns-endpoints parameter is configured in coredns-provider.conf as described in section above.

Once the federation control plane is deployed and federated clusters are joined to the federation, you need to add the CoreDNS server to the pod’s nameserver resolv.conf chain in all the federated clusters as this self hosted CoreDNS server is not discoverable publicly. This can be achieved by adding the below line to dnsmasq container’s arg in kube-dns deployment.

--server=/example.com./<CoreDNS endpoint>

Replace example.com above with federation domain.

Now the federated cluster is ready for cross-cluster service discovery!

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