rewrites

rewriting SubjectAccessReviews example

Note to try this out with minikube, make sure you enable RBAC correctly as explained here.

RBAC differentiates in two types, that need to be authorized, resources and non-resources. A resource request authorization, could for example be, that a requesting entity needs to be authorized to perform the get action on a particular Kubernetes Deployment.

Take the following example. We want to deploy a prometheus-example-app, and protect it with the kube-rbac-proxy. In this example we require a requesting entity to be allowed to call the metrics subresource on a Kubernetes Namespace, the name of which is passed by the HTTP URL query parameter namespace. This is configured in the file passed to the kube-rbac-proxy with the --config-file flag. Additionally the --upstream flag has to be set to configure the application that should be proxied to on successful authentication as well as authorization.

The kube-rbac-proxy itself also requires RBAC access, in order to perform TokenReviews as well as SubjectAccessReviews. These are the APIs available from the Kubernetes API to authenticate and then validate the authorization of an entity.

$ kubectl create -f deployment.yaml

The content of this manifest is:

apiVersion: v1
kind: ServiceAccount
metadata:
  name: kube-rbac-proxy
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: kube-rbac-proxy
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: kube-rbac-proxy
subjects:
- kind: ServiceAccount
  name: kube-rbac-proxy
  namespace: default
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: kube-rbac-proxy
rules:
- apiGroups: ["authentication.k8s.io"]
  resources:
  - tokenreviews
  verbs: ["create"]
- apiGroups: ["authorization.k8s.io"]
  resources:
  - subjectaccessreviews
  verbs: ["create"]
---
apiVersion: v1
kind: Service
metadata:
  labels:
    app: kube-rbac-proxy
  name: kube-rbac-proxy
spec:
  ports:
  - name: https
    port: 8443
    targetPort: https
  selector:
    app: kube-rbac-proxy
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: kube-rbac-proxy
data:
  config-file.yaml: |+
    authorization:
      rewrites:
        byQueryParameter:
          name: "namespace"
      resourceAttributes:
        apiVersion: v1
        resource: namespace
        subresource: metrics
        namespace: "{{ .Value }}"
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: kube-rbac-proxy
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: kube-rbac-proxy
    spec:
      securityContext:
        runAsUser: 65532
      serviceAccountName: kube-rbac-proxy
      containers:
      - name: kube-rbac-proxy
        image: quay.io/brancz/kube-rbac-proxy:v0.9.0
        args:
        - "--secure-listen-address=0.0.0.0:8443"
        - "--upstream=http://127.0.0.1:8081/"
        - "--config-file=/etc/kube-rbac-proxy/config-file.yaml"
        - "--logtostderr=true"
        - "--v=10"
        ports:
        - containerPort: 8443
          name: https
        volumeMounts:
        - name: config
          mountPath: /etc/kube-rbac-proxy
        securityContext:
          allowPrivilegeEscalation: false
      - name: prometheus-example-app
        image: quay.io/brancz/prometheus-example-app:v0.1.0
        args:
        - "--bind=127.0.0.1:8081"
      volumes:
      - name: config
        configMap:
          name: kube-rbac-proxy

Once the prometheus-example-app is up and running, we can test it. In order to test it, we deploy a Job, that performs a curl against the above deployment. Because it has the correct RBAC roles, the request will succeed.

The Dockerfile of this container can be found here.

$ kubectl create -f client-rbac.yaml

The content of this manifest is:

apiVersion: rbac.authorization.k8s.io/v1
kind: RoleBinding
metadata:
  name: namespace-metrics
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: Role
  name: namespace-metrics
subjects:
- kind: ServiceAccount
  name: default
  namespace: default
---
apiVersion: rbac.authorization.k8s.io/v1
kind: Role
metadata:
  name: namespace-metrics
rules:
- apiGroups: [""]
  resources:
  - namespace/metrics
  verbs: ["get"]