This tutorial shows how to deploy OPA as an admission controller from scratch. It covers the OPA-kubernetes version that uses kube-mgmt. The OPA Gatekeeper version has its own docs. For the purpose of the tutorial we will deploy two policies that ensure:

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

💡 Kubernetes does not guarantee consistency across resources. If two ingresses are created in parallel, there is no guarantee that OPA (or any other admission controller) will observe the creation of one ingress before the other. This means that it’s not possible to enforce these policies during admission control 100% of the time. There will be a small window of time (usually on the order of milliseconds) when the eventually consistent cache inside of OPA (or any other admission controller) is out-of-date. To catch these violations we recommend you periodically audit the state of the cluster against your policies. Offline auditing is one of the features provided by the OPA Gatekeeper project.

Prerequisites

This tutorial requires Kubernetes 1.20 or later. To run the tutorial locally ensure you start a cluster with Kubernetes version 1.20+, we recommend using minikube or KIND.

Steps

1. Enable recommended Kubernetes Admission Controllers

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 ValidatingAdmissionWebhook 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

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. Create TLS credentials for OPA

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
prompt = no
[req_distinguished_name]
CN = opa.opa.svc
[ v3_req ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
extendedKeyUsage = clientAuth, serverAuth
subjectAltName = @alt_names
[alt_names]
DNS.1 = opa.opa.svc
EOF

openssl genrsa -out server.key 2048
openssl req -new -key server.key -out server.csr -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 and Subject Alternative Name 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 --namespace opa

4. Define OPA policy

Let’s define a couple of policies to test admission control. First create a new folder to store our policies:

mkdir policies && cd policies

Policy 1: Restrict Hostnames

Create a policy that restricts the hostnames that an ingress can use. Only hostnames matching the specified regular expressions will be allowed.

ingress-allowlist.rego:

package kubernetes.admission

import data.kubernetes.namespaces

operations = {"CREATE", "UPDATE"}

deny[msg] {
	input.request.kind.kind == "Ingress"
	operations[input.request.operation]
	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 |
	allowlist := namespaces[input.request.namespace].metadata.annotations["ingress-allowlist"]
	hosts := split(allowlist, ",")
	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
}

Policy 2: Prohibit Hostname Conflicts

Now let’s define another policy to test admission control. The following policy prevents Ingress objects in different namespaces from sharing the same hostname.

ingress-conflicts.rego:

package kubernetes.admission

import data.kubernetes.ingresses

deny[msg] {
    some other_ns, other_ingress
    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])
}

Combine Policies

Let’s define a main policy that imports the Restrict Hostnames and Prohibit Hostname Conflicts policies and provides an overall policy decision.

main.rego:

package system

import data.kubernetes.admission

main = {
  "apiVersion": "admission.k8s.io/v1",
  "kind": "AdmissionReview",
  "response": response,
}

default uid = ""

uid = input.request.uid

response = {
    "allowed": false,
    "uid": uid,
    "status": {
        "message": reason,
    },
} {
    reason = concat(", ", admission.deny)
    reason != ""
}
else = {"allowed": true, "uid": uid}

⚠️ When OPA receives a request, it executes a query against the document defined data.system.main by default.

5. Build and Publish OPA Bundle

Build an OPA bundle containing policies defined in the previous step. In our setup, OPA will download policies from the bundle service and the kube-mgmt container will load Kubernetes resources into OPA. Since we load policy and data into OPA from multiple sources, we need to scope the bundle to a subset of OPA’s policy and data cache by defining a manifest. More information about this can be found here. Run the following commands in the policies folder created in the previous step.

cat > .manifest <<EOF
{
    "roots": ["kubernetes/admission", "system"]
}
EOF

opa build -b .

We will now serve the OPA bundle using Nginx.

docker run --rm --name bundle-server -d -p 8888:80 -v ${PWD}:/usr/share/nginx/html:ro nginx:latest

6. Deploy OPA as an Admission Controller

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 lets 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: 8443
---
apiVersion: apps/v1
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:0.39.0-rootless
          args:
            - "run"
            - "--server"
            - "--tls-cert-file=/certs/tls.crt"
            - "--tls-private-key-file=/certs/tls.key"
            - "--addr=0.0.0.0:8443"
            - "--addr=http://127.0.0.1:8181"
            - "--set=services.default.url=http://host.minikube.internal:8888"
            - "--set=bundles.default.resource=bundle.tar.gz"
            - "--log-format=json-pretty"
            - "--set=status.console=true"
            - "--set=decision_logs.console=true"
          volumeMounts:
            - readOnly: true
              mountPath: /certs
              name: opa-server
          readinessProbe:
            httpGet:
              path: /health?plugins&bundle
              scheme: HTTPS
              port: 8443
            initialDelaySeconds: 3
            periodSeconds: 5
          livenessProbe:
            httpGet:
              path: /health
              scheme: HTTPS
              port: 8443
            initialDelaySeconds: 3
            periodSeconds: 5
        - name: kube-mgmt
          image: openpolicyagent/kube-mgmt:2.0.1
          args:
            - "--replicate-cluster=v1/namespaces"
            - "--replicate=networking.k8s.io/v1/ingresses"
      volumes:
        - name: opa-server
          secret:
            secretName: opa-server

⚠️ If using kind to run a local Kubernetes cluster, the bundle service URL should be http://host.docker.internal:8888.

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. This webhook will ignore any namespace with the label openpolicyagent.org/webhook=ignore.

cat > webhook-configuration.yaml <<EOF
kind: ValidatingWebhookConfiguration
apiVersion: admissionregistration.k8s.io/v1
metadata:
  name: opa-validating-webhook
webhooks:
  - name: validating-webhook.openpolicyagent.org
    namespaceSelector:
      matchExpressions:
      - key: openpolicyagent.org/webhook
        operator: NotIn
        values:
        - ignore
    rules:
      - operations: ["CREATE", "UPDATE"]
        apiGroups: ["*"]
        apiVersions: ["*"]
        resources: ["*"]
    clientConfig:
      caBundle: $(cat ca.crt | base64 | tr -d '\n')
      service:
        namespace: opa
        name: opa
    admissionReviewVersions: ["v1"]
    sideEffects: None
EOF

The generated configuration file includes a base64 encoded representation of the CA certificate created in Step 3 so that TLS connections can be established between the Kubernetes API server and OPA.

Next label kube-system and the opa namespace so that OPA does not control the resources in those namespaces.

kubectl label ns kube-system openpolicyagent.org/webhook=ignore
kubectl label ns opa openpolicyagent.org/webhook=ignore

Finally, register OPA as an 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:

# ctrl-c to exit
kubectl logs -l app=opa -c opa -f

7. Exercise Restrict Hostnames policy

Now let’s exercise the Restrict Hostnames policy by creating two new namespaces.

qa-namespace.yaml:

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

production-namespace.yaml:

apiVersion: v1
kind: Namespace
metadata:
  annotations:
    ingress-allowlist: "*.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: networking.k8s.io/v1
kind: Ingress
metadata:
  name: ingress-ok
spec:
  rules:
  - host: signin.acmecorp.com
    http:
      paths:
      - pathType: ImplementationSpecific
        path: /
        backend:
          service:
            name: nginx
            port:
              number: 80

ingress-bad.yaml:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: ingress-bad
spec:
  rules:
  - host: acmecorp.com
    http:
      paths:
      - pathType: ImplementationSpecific
        path: /
        backend:
          service:
            name: nginx
            port:
              number: 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 its hostname does not match the allowlist in the qa namespace.

It will report an error as follows:

Error from server: error when creating "ingress-bad.yaml": admission webhook "validating-webhook.openpolicyagent.org" 
denied the request: invalid ingress host "acmecorp.com"

8. Exercise Prohibit Hostname Conflicts policy

Test the Prohibit Hostname Conflicts policy by verifying 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-allowlist: "*.acmecorp.com"
  name: staging

kubectl create -f staging-namespace.yaml

kubectl create -f ingress-ok.yaml -n staging

The above command will report an error as follows:

Error from server (BadRequest): error when creating "ingress-ok.yaml": admission webhook 
"validate.nginx.ingress.kubernetes.io" denied the request: host "signin.acmecorp.com" and 
path "/" is already defined in ingress production/ingress-ok

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, with OPA’s Bundle feature policies can be periodically downloaded from remote servers to satisfy changing operational requirements.

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