• OPA is not Kubenretes specific
• general purpose policy engine
• An admission controller is a piece of code that intercepts requests to the Kubernetes API server prior to persistence of the object, but after the request is authenticated and authorized.
• Leveraging Rego language
• works with JSON/YAML
• in Kubernetes it uses Admission Controllers
• does not know anything about concepts like pods or deployments
• OPA Gatekeeper (with Custom Resource Definitions)

How it works

• create ConstraintTemplate

Install Gatekeeper OPA to your cluster

Thanks to Dynamic Admission Controller there is no need to configure kube-api server.

There are two kinds of webhooks:

• validating admission webhook
• mutating admission webhook

kubectl create -f https://raw.githubusercontent.com/killer-sh/cks-course-environment/master/course-content/opa/gatekeeper.yaml

# Check what was created in gatekeeper-system namespace
 k get pod,svc -n gatekeeper-system
NAME                                                 READY   STATUS    RESTARTS   AGE
pod/gatekeeper-audit-65f658df68-rfg4j                1/1     Running   0          62s
pod/gatekeeper-controller-manager-5fb6c9ff69-vqqxj   1/1     Running   0          62s

NAME                                 TYPE        CLUSTER-IP       EXTERNAL-IP   PORT(S)   AGE
service/gatekeeper-webhook-service   ClusterIP   10.103.249.167   <none>        443/TCP   62s

Create your first template

ConstraintTemplate: allwaysdeny_template

cat <<'EOF' > alwaysdeny_template.yaml
---
apiVersion: templates.gatekeeper.sh/v1beta1
kind: ConstraintTemplate
metadata:
  name: k8salwaysdeny
spec:
  crd:
    spec:
      names:
        kind: K8sAlwaysDeny
      validation:
        # Schema for the `parameters` field
        openAPIV3Schema:
          properties:
            message:
              type: string
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8salwaysdeny
        violation[{"msg": msg}] {
          1 > 0
          msg := input.parameters.message
        }        
EOF

A new Custom Resource Definition called K8sAlwaysDeny will be created

root@scw-k8s-cks:~# k get crd
NAME                                                 CREATED AT
configs.config.gatekeeper.sh                         2022-01-24T14:44:30Z
constraintpodstatuses.status.gatekeeper.sh           2022-01-24T14:44:30Z
constrainttemplatepodstatuses.status.gatekeeper.sh   2022-01-24T14:44:30Z
constrainttemplates.templates.gatekeeper.sh          2022-01-24T14:44:30Z

# Run kubectl create -f ...
root@scw-k8s-cks:~# k create -f  alwaysdeny_template.yaml
constrainttemplate.templates.gatekeeper.sh/k8salwaysdeny created

# Check a new Custom Resource Definition k8salwaysdeny.constraints.gatekeeper.sh
root@scw-k8s-cks:~# k get crd
NAME                                                 CREATED AT
configs.config.gatekeeper.sh                         2022-01-24T14:44:30Z
constraintpodstatuses.status.gatekeeper.sh           2022-01-24T14:44:30Z
constrainttemplatepodstatuses.status.gatekeeper.sh   2022-01-24T14:44:30Z
constrainttemplates.templates.gatekeeper.sh          2022-01-24T14:44:30Z
k8salwaysdeny.constraints.gatekeeper.sh              2022-01-24T15:05:08Z

Now create that k8salwaysdeny.constraints.gatekeeper.sh

cat <<'EOF' > all_pod_always_deny.yaml

apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sAlwaysDeny
metadata:
  name: pod-always-deny
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]
  parameters:
    message: "ACCESS DENIED!"
EOF

Apply this file …

# Kubectl it ...
root@scw-k8s-cks:~# kubectl  create -f all_pod_always_deny.yaml
k8salwaysdeny.constraints.gatekeeper.sh/pod-always-deny created

Now, we should not be able to create any pod since 1>0 in Rego language

root@scw-k8s-cks:~# k run podx --image=nginx
Error from server ([pod-always-deny] ACCESS DENIED!): admission webhook "validation.gatekeeper.sh" denied the request: [pod-always-deny] ACCESS DENIED!

Require labels on namespaces and pods

To achieve such a goal we need to create ConstraintTemplate first with some appropriate Rego code.

cat <<'EOF' > k8srequiredlabels_template.yaml
apiVersion: templates.gatekeeper.sh/v1beta1
kind: ConstraintTemplate
metadata:
  name: k8srequiredlabels
spec:
  crd:
    spec:
      names:
        kind: K8sRequiredLabels
      validation:
        # Schema for the `parameters` field
        openAPIV3Schema:
          properties:
            labels:
              type: array
              items: string
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8srequiredlabels
        violation[{"msg": msg, "details": {"missing_labels": missing}}] {
          provided := {label | input.review.object.metadata.labels[label]}
          required := {label | label := input.parameters.labels[_]}
          missing := required - provided
          count(missing) > 0
          msg := sprintf("you must provide labels: %v", [missing])
        }
EOF

# kubectl create this file
root@scw-k8s-cks:~# k create -f k8srequiredlabels_template.yaml
constrainttemplate.templates.gatekeeper.sh/k8srequiredlabels created

Same with all namespaces. We need to change only K8sRequiredLabels Custom Resource Definition.

cat <<'EOF' > k8srequiredlabels_on_namespaces.yaml
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sRequiredLabels
metadata:
  name: ns-must-have-cks
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Namespace"]
  parameters:
    labels: ["cks"]
EOF

Then create K8sRequiredLabels Custom Resource Definition to ensure POD have some labels.

cat <<'EOF' > all_pod_must_have_cks.yaml
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sRequiredLabels
metadata:
  name: pod-must-have-cks
spec:
  match:
    kinds:
      - apiGroups: [""]
        kinds: ["Pod"]
  parameters:
    labels: ["cks"]
EOF

Make sure to apply both files and by now we should have labels cks enforced on both PODS and namespaces.

root@scw-k8s-cks:~# k create -f all_pod_must_have_cks.yaml
k8srequiredlabels.constraints.gatekeeper.sh/pod-must-have-cks created
root@scw-k8s-cks:~# k create -f k8srequiredlabels_on_namespaces.yaml
k8srequiredlabels.constraints.gatekeeper.sh/ns-must-have-cks created

Let’s try to create some namespace

root@scw-k8s-cks:~# k create ns new-namespace
Error from server ([ns-must-have-cks] you must provide labels: {"cks"}): admission webhook "validation.gatekeeper.sh" denied the request: [ns-must-have-cks] you must provide labels: {"cks"}

Restrict number of replicaCount

cat <<'EOF' > k8sminreplicacount_template.yaml
apiVersion: templates.gatekeeper.sh/v1beta1
kind: ConstraintTemplate
metadata:
  name: k8sminreplicacount
spec:
  crd:
    spec:
      names:
        kind: K8sMinReplicaCount
      validation:
        # Schema for the `parameters` field
        openAPIV3Schema:
          properties:
            min:
              type: integer
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8sminreplicacount
        violation[{"msg": msg, "details": {"missing_replicas": missing}}] {
          provided := input.review.object.spec.replicas
          required := input.parameters.min
          missing := required - provided
          missing > 0
          msg := sprintf("you must provide %v more replicas", [missing])
        }        
EOF

Then object create K8sMinReplicaCount

cat <<'EOF' > all_deployment_must_have_min_replicacount.yaml
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sMinReplicaCount
metadata:
  name: deployment-must-have-min-replicas
spec:
  match:
    kinds:
      - apiGroups: ["apps"]
        kinds: ["Deployment"]
  parameters:
    min: 2
EOF

Create new Custom Resource Definitions

# kubectl create ...
kubectl create -f k8sminreplicacount_template.yaml
kubectl create -f all_deployment_must_have_min_replicacount.yaml

Test replicaCount restriction

root@scw-k8s-cks:~# k create  deployment test  --image=nginx  -oyaml --dry-run=client > deployment.yaml
root@scw-k8s-cks:~# vim deployment.yaml
root@scw-k8s-cks:~# k create -f  deployment.yaml
Error from server ([deployment-must-have-min-replicas] you must provide 1 more replicas): error when creating "deployment.yaml": admission webhook "validation.gatekeeper.sh" denied the request: [deployment-must-have-min-replicas] you must provide 1 more replicas

Rego playground

https://play.openpolicyagent.org/

Old way - creating a configMap with a special label and in a special namespace

 kubectl  get cm unique-host -o yaml
apiVersion: v1
data:
  unique-host.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])
    }    
kind: ConfigMap
metadata:
  annotations:
    openpolicyagent.org/policy-status: '{"status":"ok"}'
  creationTimestamp: "2021-05-01T12:37:05Z"
  labels:
    openpolicyagent.org/policy: rego
  name: unique-host
  namespace: opa