Kubernetes Admission Control

In Kubernetes, Admission Controllers enforce semantic validation of objects during create, update, and delete operations. With OPA you can enforce custom policies on Kubernetes objects without recompiling or reconfiguring the Kubernetes API server.

Goals

This tutorial shows how to enforce custom policies on Kubernetes objects using OPA. In this tutorial, you will define admission control rules that prevent users from creating Kubernetes Ingress objects that violate the following organization policy:

  • Ingress hostnames must be whitelisted on the Namespace containing the Ingress.
  • Two ingresses in different namespaces must not have the same hostname.

Prerequisites

This tutorial requires Kubernetes 1.9 or later. To run the tutorial locally, we recommend using minikube in version v0.28+ with Kubernetes 1.10 (which is the default).

Steps

To implement admission control rules that validate Kubernetes resources during create, update, and delete operations, you must enable the ValidatingAdmissionWebhook when the Kubernetes API server is started. the admission controller is included in the recommended set of admission controllers to enable

Start minikube:

minikube start

Make sure that the minikube ingress addon is enabled:

minikube addons enable ingress

2. Create a new Namespace to deploy OPA into

When OPA is deployed on top of Kubernetes, policies are automatically loaded out of ConfigMaps in the opa namespace.

kubectl create namespace opa

Configure kubectl to use this namespace:

kubectl config set-context opa-tutorial --user minikube --cluster minikube --namespace opa
kubectl config use-context opa-tutorial

3. Deploy OPA on top of Kubernetes

Communication between Kubernetes and OPA must be secured using TLS. To configure TLS, use openssl to create a certificate authority (CA) and certificate/key pair for OPA:

openssl genrsa -out ca.key 2048
openssl req -x509 -new -nodes -key ca.key -days 100000 -out ca.crt -subj "/CN=admission_ca"

Generate the TLS key and certificate for OPA:

cat >server.conf <<EOF
[req]
req_extensions = v3_req
distinguished_name = req_distinguished_name
[req_distinguished_name]
[ v3_req ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
extendedKeyUsage = clientAuth, serverAuth
EOF
openssl genrsa -out server.key 2048
openssl req -new -key server.key -out server.csr -subj "/CN=opa.opa.svc" -config server.conf
openssl x509 -req -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt -days 100000 -extensions v3_req -extfile server.conf

Note: the Common Name value you give to openssl MUST match the name of the OPA service created below.

Create a Secret to store the TLS credentials for OPA:

kubectl create secret tls opa-server --cert=server.crt --key=server.key

Next, use the file below to deploy OPA as an admission controller.

admission-controller.yaml:

# Grant OPA/kube-mgmt read-only access to resources. This let's kube-mgmt
# replicate resources into OPA so they can be used in policies.
kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: opa-viewer
roleRef:
  kind: ClusterRole
  name: view
  apiGroup: rbac.authorization.k8s.io
subjects:
- kind: Group
  name: system:serviceaccounts:opa
  apiGroup: rbac.authorization.k8s.io
---
# Define role for OPA/kube-mgmt to update configmaps with policy status.
kind: Role
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  namespace: opa
  name: configmap-modifier
rules:
- apiGroups: [""]
  resources: ["configmaps"]
  verbs: ["update", "patch"]
---
# Grant OPA/kube-mgmt role defined above.
kind: RoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  namespace: opa
  name: opa-configmap-modifier
roleRef:
  kind: Role
  name: configmap-modifier
  apiGroup: rbac.authorization.k8s.io
subjects:
- kind: Group
  name: system:serviceaccounts:opa
  apiGroup: rbac.authorization.k8s.io
---
kind: Service
apiVersion: v1
metadata:
  name: opa
  namespace: opa
spec:
  selector:
    app: opa
  ports:
  - name: https
    protocol: TCP
    port: 443
    targetPort: 443
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  labels:
    app: opa
  namespace: opa
  name: opa
spec:
  replicas: 1
  selector:
    matchLabels:
      app: opa
  template:
    metadata:
      labels:
        app: opa
      name: opa
    spec:
      containers:
        # WARNING: OPA is NOT running with an authorization policy configured. This
        # means that clients can read and write policies in OPA. If you are
        # deploying OPA in an insecure environment, be sure to configure
        # authentication and authorization on the daemon. See the Security page for
        # details: https://www.openpolicyagent.org/docs/security.html.
        - name: opa
          image: openpolicyagent/opa:0.10.2
          args:
            - "run"
            - "--server"
            - "--tls-cert-file=/certs/tls.crt"
            - "--tls-private-key-file=/certs/tls.key"
            - "--addr=0.0.0.0:443"
            - "--addr=http://127.0.0.1:8181"
          volumeMounts:
            - readOnly: true
              mountPath: /certs
              name: opa-server
        - name: kube-mgmt
          image: openpolicyagent/kube-mgmt:0.6
          args:
            - "--replicate-cluster=v1/namespaces"
            - "--replicate=extensions/v1beta1/ingresses"
      volumes:
        - name: opa-server
          secret:
            secretName: opa-server
---
kind: ConfigMap
apiVersion: v1
metadata:
  name: opa-default-system-main
  namespace: opa
data:
  main: |
    package system

    import data.kubernetes.admission

    main = {
      "apiVersion": "admission.k8s.io/v1beta1",
      "kind": "AdmissionReview",
      "response": response,
    }

    default response = {"allowed": true}

    response = {
        "allowed": false,
        "status": {
            "reason": reason,
        },
    } {
        reason = concat(", ", admission.deny)
        reason != ""
    }
kubectl apply -f admission-controller.yaml

When OPA starts, the kube-mgmt container will load Kubernetes Namespace and Ingress objects into OPA. You can configure the sidecar to load any kind of Kubernetes object into OPA. The sidecar establishes watches on the Kubernetes API server so that OPA has access to an eventually consistent cache of Kubernetes objects.

Next, generate the manifest that will be used to register OPA as an admission controller:

cat > webhook-configuration.yaml <<EOF
kind: ValidatingWebhookConfiguration
apiVersion: admissionregistration.k8s.io/v1beta1
metadata:
  name: opa-validating-webhook
webhooks:
  - name: validating-webhook.openpolicyagent.org
    rules:
      - operations: ["CREATE", "UPDATE"]
        apiGroups: ["*"]
        apiVersions: ["*"]
        resources: ["*"]
    clientConfig:
      caBundle: $(cat ca.crt | base64 | tr -d '\n')
      service:
        namespace: opa
        name: opa
EOF

The generated configuration file inclues a base64 encoded representation of the CA certificate so that TLS connections can be established between the Kubernetes API server and OPA.

Finally, register OPA as and admission controller:

kubectl apply -f webhook-configuration.yaml

You can follow the OPA logs to see the webhook requests being issued by the Kubernetes API server:

kubectl logs -l app=opa -c opa

4. Define a policy and load it into OPA via Kubernetes

To test admission control, create a policy that restricts the hostnames that an ingress can use (ingress-whitelist.rego):

package kubernetes.admission

import data.kubernetes.namespaces

deny[msg] {
    input.request.kind.kind = "Ingress"
    input.request.operation = "CREATE"
    host = input.request.object.spec.rules[_].host
    not fqdn_matches_any(host, valid_ingress_hosts)
    msg = sprintf("invalid ingress host %q", [host])
}

valid_ingress_hosts = {host |
    whitelist = namespaces[input.request.namespace].metadata.annotations["ingress-whitelist"]
    hosts = split(whitelist, ",")
    host = hosts[_]
}

fqdn_matches_any(str, patterns) {
    fqdn_matches(str, patterns[_])
}

fqdn_matches(str, pattern) {
    pattern_parts = split(pattern, ".")
    pattern_parts[0] = "*"
    str_parts = split(str, ".")
    n_pattern_parts = count(pattern_parts)
    n_str_parts = count(str_parts)
    suffix = trim(pattern, "*.")
    endswith(str, suffix)
}

fqdn_matches(str, pattern) {
    not contains(pattern, "*")
    str = pattern
}

Store the policy in Kubernetes as a ConfigMap. By default kube-mgmt will try to load policies out of configmaps in the opa namespace OR configmaps in other namespaces labelled openpolicyagent.org/policy=rego.

kubectl create configmap ingress-whitelist --from-file=ingress-whitelist.rego

The OPA sidecar will notice the ConfigMap and automatically load the policy into OPA.

5. Exercise the policy

Create two new namespaces to test the Ingress policy.

qa-namespace.yaml:

apiVersion: v1
kind: Namespace
metadata:
  annotations:
    ingress-whitelist: "*.qa.acmecorp.com,*.internal.acmecorp.com"
  name: qa

production-namespace.yaml:

apiVersion: v1
kind: Namespace
metadata:
  annotations:
    ingress-whitelist: "*.acmecorp.com"
  name: production
kubectl create -f qa-namespace.yaml
kubectl create -f production-namespace.yaml

Next, define two Ingress objects. One of the Ingress objects will be permitted and the other will be rejected.

ingress-ok.yaml:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: ingress-ok
spec:
  rules:
  - host: signin.acmecorp.com
    http:
      paths:
      - backend:
          serviceName: nginx
          servicePort: 80

ingress-bad.yaml:

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: ingress-bad
spec:
  rules:
  - host: acmecorp.com
    http:
      paths:
      - backend:
          serviceName: nginx
          servicePort: 80

Finally, try to create both Ingress objects:

kubectl create -f ingress-ok.yaml -n production
kubectl create -f ingress-bad.yaml -n qa

The second Ingress is rejected because it's hostname does not match the whitelist in the qa namespace.

6. Modify the policy and exercise the changes

OPA allows you to modify policies on-the-fly without recompiling any of the services that offload policy decisions to it.

To enforce the second half of the policy from the start of this tutorial you can load another policy into OPA that rejects Ingress objects in different namespaces from sharing the same hostname.

ingress-conflicts.rego:

package kubernetes.admission

import data.kubernetes.ingresses

deny[msg] {
    input.request.kind.kind = "Ingress"
    input.request.operation = "CREATE"
    host = input.request.object.spec.rules[_].host
    ingress = ingresses[other_ns][other_ingress]
    other_ns != input.request.namespace
    ingress.spec.rules[_].host = host
    msg = sprintf("invalid ingress host %q (conflicts with %v/%v)", [host, other_ns, other_ingress])
}
kubectl create configmap ingress-conflicts --from-file=ingress-conflicts.rego

The OPA sidecar annotates ConfigMaps containing policies to indicate if they were installed successfully. Verify the ConfigMap was installed successfully.

kubectl get configmap ingress-conflicts -o yaml

Test that you cannot create an Ingress in another namespace with the same hostname as the one created earlier.

staging-namespace.yaml:

apiVersion: v1
kind: Namespace
metadata:
  annotations:
    ingress-whitelist: "*.acmecorp.com"
  name: staging
kubectl create -f staging-namespace.yaml
kubectl create -f ingress-ok.yaml -n staging

Wrap Up

Congratulations for finishing the tutorial!

This tutorial showed how you can leverage OPA to enforce admission control decisions in Kubernetes clusters without modifying or recompiling any Kubernetes components. Furthermore, once Kubernetes is configured to use OPA as an External Admission Controller, policies can be modified on-the-fly to satisfy changing operational requirements.

For more information about deploying OPA on top of Kubernetes, see Deployments - Kubernetes.

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