Private Docker Registry in Kubernetes

Kubernetes offers an optional private Docker registry addon, which you can turn on when you bring up a cluster or install later. This gives you a place to store truly private Docker images for your cluster.

How it works

The private registry runs as a Pod in your cluster. It does not currently support SSL or authentication, which triggers Docker's "insecure registry" logic. To work around this, we run a proxy on each node in the cluster, exposing a port onto the node (via a hostPort), which Docker accepts as "secure", since it is accessed by localhost.

Turning it on

Some cluster installs (e.g. GCE) support this as a cluster-birth flag. The ENABLE_CLUSTER_REGISTRY variable in cluster/gce/config-default.sh governs whether the registry is run or not. To set this flag, you can specify KUBE_ENABLE_CLUSTER_REGISTRY=true when running kube-up.sh. If your cluster does not include this flag, the following steps should work. Note that some of this is cloud-provider specific, so you may have to customize it a bit.

Make some storage

The primary job of the registry is to store data. To do that we have to decide where to store it. For cloud environments that have networked storage, we can use Kubernetes's PersistentVolume abstraction. The following template is expanded by salt in the GCE cluster turnup, but can easily be adapted to other situations:

kind: PersistentVolume
apiVersion: v1
metadata:
  name: kube-system-kube-registry-pv
spec:
{% if pillar.get('cluster_registry_disk_type', '') == 'gce' %}
  capacity:
    storage: {{ pillar['cluster_registry_disk_size'] }}
  accessModes:
    - ReadWriteOnce
  gcePersistentDisk:
    pdName: "{{ pillar['cluster_registry_disk_name'] }}"
    fsType: "ext4"
{% endif %}

If, for example, you wanted to use NFS you would just need to change the gcePersistentDisk block to nfs. See here for more details on volumes.

Note that in any case, the storage (in the case the GCE PersistentDisk) must be created independently - this is not something Kubernetes manages for you (yet).

I don't want or don't have persistent storage

If you are running in a place that doesn't have networked storage, or if you just want to kick the tires on this without committing to it, you can easily adapt the ReplicationController specification below to use a simple emptyDir volume instead of a persistentVolumeClaim.

Claim the storage

Now that the Kubernetes cluster knows that some storage exists, you can put a claim on that storage. As with the PersistentVolume above, you can start with the salt template:

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: kube-registry-pvc
  namespace: kube-system
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: {{ pillar['cluster_registry_disk_size'] }}

This tells Kubernetes that you want to use storage, and the PersistentVolume you created before will be bound to this claim (unless you have other PersistentVolumes in which case those might get bound instead). This claim gives you the right to use this storage until you release the claim.

Run the registry

Now we can run a Docker registry:

apiVersion: v1
kind: ReplicationController
metadata:
  name: kube-registry-v0
  namespace: kube-system
  labels:
    k8s-app: registry
    version: v0
spec:
  replicas: 1
  selector:
    k8s-app: registry
    version: v0
  template:
    metadata:
      labels:
        k8s-app: registry
        version: v0
    spec:
      containers:
      - name: registry
        image: registry:2
        resources:
          limits:
            cpu: 100m
            memory: 100Mi
        env:
        - name: REGISTRY_HTTP_ADDR
          value: :5000
        - name: REGISTRY_STORAGE_FILESYSTEM_ROOTDIRECTORY
          value: /var/lib/registry
        volumeMounts:
        - name: image-store
          mountPath: /var/lib/registry
        ports:
        - containerPort: 5000
          name: registry
          protocol: TCP
      volumes:
      - name: image-store
        persistentVolumeClaim:
          claimName: kube-registry-pvc

Note: that if you have set multiple replicas, make sure your CSI driver has support for the ReadWriteMany accessMode.

Expose the registry in the cluster

Now that we have a registry Pod running, we can expose it as a Service:

apiVersion: v1
kind: Service
metadata:
  name: kube-registry
  namespace: kube-system
  labels:
    k8s-app: registry
    kubernetes.io/name: "KubeRegistry"
spec:
  selector:
    k8s-app: registry
  ports:
  - name: registry
    port: 5000
    protocol: TCP

Expose the registry on each node

Now that we have a running Service, we need to expose it onto each Kubernetes Node so that Docker will see it as localhost. We can load a Pod on every node by creating following daemonset.

apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: kube-registry-proxy
  namespace: kube-system
  labels:
    k8s-app: kube-registry-proxy
    version: v0.4
spec:
  template:
    metadata:
      labels:
        k8s-app: kube-registry-proxy
        kubernetes.io/name: "kube-registry-proxy"
        version: v0.4
    spec:
      containers:
      - name: kube-registry-proxy
        image: gcr.io/google_containers/kube-registry-proxy:0.4
        resources:
          limits:
            cpu: 100m
            memory: 50Mi
        env:
        - name: REGISTRY_HOST
          value: kube-registry.kube-system.svc.cluster.local
        - name: REGISTRY_PORT
          value: "5000"
        ports:
        - name: registry
          containerPort: 80
          hostPort: 5000

When modifying replication-controller, service and daemon-set definitions, take care to ensure unique identifiers for the rc-svc couple and the daemon-set. Failing to do so will have register the localhost proxy daemon-sets to the upstream service. As a result they will then try to proxy themselves, which will, for obvious reasons, not work.

This ensures that port 5000 on each node is directed to the registry Service. You should be able to verify that it is running by hitting port 5000 with a web browser and getting a 404 error:

$ curl localhost:5000
404 page not found

Using the registry

To use an image hosted by this registry, simply say this in your Pod's spec.containers[].image field:

    image: localhost:5000/user/container

Before you can use the registry, you have to be able to get images into it, though. If you are building an image on your Kubernetes Node, you can spell out localhost:5000 when you build and push. More likely, though, you are building locally and want to push to your cluster.

You can use kubectl to set up a port-forward from your local node to a running Pod:

$ POD=$(kubectl get pods --namespace kube-system -l k8s-app=registry \
            -o template --template '{{range .items}}{{.metadata.name}} {{.status.phase}}{{"\n"}}{{end}}' \
            | grep Running | head -1 | cut -f1 -d' ')

$ kubectl port-forward --namespace kube-system $POD 5000:5000 &

Now you can build and push images on your local computer as localhost:5000/yourname/container and those images will be available inside your kubernetes cluster with the same name.

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