In this blog, I describe the Gatekeeper integration with Red Hat Advanced Cluster Management for Kubernetes. I focus on the value that Red Hat Advance Cluster Management provides by integrating Gatekeeper. Gatekeeper is an open source, general-purpose policy engine that enables unified, context-aware policy enforcement and can evaluate the compliance of K8s resources to policies. It leverages Open Policy Agent (OPA) as the policy engine, which uses Rego as the policy language.

Continue reading to learn about the following aspects and components to integrate Gatekeeper with Red Hat Advanced Cluster Management:

  • Create the Red Hat Advanced Cluster Management gatekeeper operator policy to install and monitor the Gatekeeper Operator.
  • Create a policy to configure which namespaces that you want to be handled by Gatekeeper, and configure cluster-wide configuration options.
  • Create policies that distribute Gatekeeper rules onto your clusters.
  • Learn how to monitor the Gatekeeper-related policies in Red Hat Advanced Cluster Management.

Integration: High-level overview

View the following diagram for a visual of the integration:


Notice the policies that are handled by the Kubernetes configuration policy controller. In theory, you can put all Gatekeeper-related configuration into a single policy; the options are discussed later in this blog.

Support for Gatekeeper Operator in the context of Red Hat Advanced Cluster Management

Let's use and test the Gatekeeper Operator from the upstream project. This Operator can be used without Red Hat Advanced Cluster Management, however Red Hat Advanced Cluster Management supports optimal integration and verification of the usecases.

Note: Red Hat Advanced Cluster Management 2.2 fully supports Gatekeeper Operator for OpenShift 4.6+ Clusters. Review our documentation for more information.

Install Gatekeeper as Operator using policies

Use the gatekeeper operator policy to install the community version of Gatekeeper. Be sure to leverage the Red Hat Advanced Cluster Management features like PlacementRules to distribute rules among the clusters, where you want the policies to be applied.

Configure namespaces to be excluded

Once you have installed the Operator, apply the policy-gatekeeper-config-exclude-namespaces.yaml to configure the namespaces where you do not want Gatekeeper to be handled. Check out the Exempting Namespaces from Gatekeeper README for more information.

List the namespaces that you want to exclude. View the following YAML example where several OpenShift and Open Cluster Management namespaces are excluded:

kind: Config
name: config
namespace: openshift-gatekeeper-system
- excludedNamespaces:
- hive
- kube-system
- kube-public
- openshift-kube-apiserver
- openshift-monitoring
- open-cluster-management-agent
- open-cluster-management
- open-cluster-management-agent-addon
- openshift-sdn
- openshift-machine-config-operator
- openshift-machine-api
- openshift-ingress-operator
- openshift-ingress
- sdn-controller
- openshift-cluster-csi-drivers
- openshift-kube-controller-manager-operator
- openshift-kube-controller-manager
- '*'

Write and distribute gatekeeper-policies with Red Hat Advanced Cluster Management

In this section, I demonstrate and describe how to write a policy for Gatekeeper.

As a first step, create a ConstraintTemplate and a Constraint. The purpose of the constraint template is to define both the Rego code that you use to enforce the policy, and the schema that the constraint can be applied to. View the following examples:

  • A constraint template that includes rego parameter:

    kind: ConstraintTemplate
    name: k8srequiredlabels
    rego: |
    package k8sminreplicacount
    violation[{"msg": msg, "details": {"missing_replicas": missing}}] {
    provided :=
    required := input.parameters.min
    missing := required - provided
    missing > 0
    msg := sprintf("you must provide %v more replicas", [missing])
  • A concrete constraint which defines that every deployment in the deploymenttest namespace must have at least 5 replicas:

    kind: K8sMinReplicaCount
    name: deployment-must-have-min-replicas
    - apiGroups:
    - apps
    - Deployment
    - apiGroups:
    - autoscaling
    - Scale
    - deploymenttest
    min: 5
  • An audit template to check for violations that are in existing resources:

    name: policy-gatekeeper-audit
    remediationAction: inform # will be overridden by remediationAction in parent policy
    severity: low
    - complianceType: musthave
    kind: K8sMinReplicaCount
    name: must-have-minreplica
    totalViolations: 0
  • An admission template to check for violation events (event_type) generated by unallowed actions against the previously defined constraint:

    name: policy-gatekeeper-admission
    remediationAction: inform # will be overridden by remediationAction in parent policy
    severity: low
    - complianceType: mustnothave
    apiVersion: v1
    kind: Event
    namespace: openshift-gatekeeper-system
    constraint_action: deny
    constraint_kind: K8sMinReplicaCount
    constraint_name: must-have-minreplica
    event_type: violation

Put the elements together into a single policy

As mentioned before, you can put all Gatekeeper-related configurations into a single policy, however this is not best practice. A single policy is recommended to define which previously mentiond elements you want to implement. After all of the resources have been defined, include them in a Red Hat Advanced Cluster Management configuration policy. You can either define one ConfigurationPolicy and add the four defined resources in the objectDefinition parameter section, or create one ConfigurationPolicy for each of the four resources of the policy. There are advantages with creating a configuration policy for each resource. For example, it is more clear which resources of the policy are non-compliant when checking from the Red Hat Advanced Cluster Management console.

Policies, configuration policies, and object-templates

Let’s discuss the use case further to see how the policy works:

First let’s try to generate a deployment that violates the defined constraints. Download the deploymenttest_noprobe_missingreplicas.yaml before you run the following command:

oc apply -f deploymenttest_noprobe_missingreplicas.yaml 

Notice how the gatekeeper admission webhook is defined to deny the request because of the following three rules that have been applied:

  • Liveness probe must be present
  • Readiness probe must be present
  • At least five replicas must be present (as previously mentioned)

View the following logs generated from Gatekeeper:

[denied by deployment-must-have-min-replicas] you must provide 2 more replicas): error when creating "deploymenttest_noprobe_missingreplicas.yaml": admission webhook "" denied the request: [denied by containerlivenessprobenotset] Deployment/gatekeepertest: container 'nginx' has no livenessProbe. See:
[denied by containerlivenessprobenotset] Deployment/gatekeepertest: container 'nginx' has no livenessProbe. See:
[denied by containerreadinessprobenotset] Deployment/gatekeepertest: container 'nginx' has no readinessProbe. See:
[denied by containerreadinessprobenotset] Deployment/gatekeepertest: container 'nginx' has no readinessProbe. See:

After that, let’s create a deployment which passes the validation, and then let’s scale the deployment to 3 replicas:

oc scale deployment my-application-web --replicas=3 -n deploymenttest

Run the following command to scale the deployment to 3:

oc patch deployment -p '{"spec": {"replicas": 3}}' 

Note, the current setup gets denied and generates a violation event. Continue reading for an explanation of why scaling replicas is possible, and how to configure Gatekeeper to also detect and forbid the scaling operation.

Now let’s log in to the Red Hat Advanced Cluster Management console to view the status of the defined policy. In the following image, you can see that two of the four resource templates are not compliant:


Further explanation:

The first constraint is non-compliant against the audit template, because the constraint has more than one violation (totalViolations > 1). Once you scale the deployment back to 5, the constraint becomes compliant again.

Click the View history link for more details about the violation.


The second constraint is considered not compliant because there are events in the openshift-gatekeeper-namespace, which was generated after attempting to generate the Deployment object.

You can either delete this event manually or wait until the event is automatically purged after the default value of 2 hours (event-ttl).

oc delete event -n openshift-gatekeeper-system my-application-web.165ba7be7e8eebf7

After the violation event is removed from the configuration policy, it becomes compliant again.

Modifications to make scale operations work

As previously demonstrated, it is possible to scale the deployments to 3. The policy detected the violation, but why was it possible to perform the scale operation at all?

In this example, policy modifications need to be applied to the Gatekeeper Operator and its related policies to include Scale, which can be described and configured as a subresource.

It is necessary to add the following contents to the gatekeeper policy:

  - apiGroups:
- autoscaling
- Scale

Further, we need to modify the validating-webhook-configuration object by running the following command:

oc edit ValidatingWebhookConfiguration gatekeeper-validating-webhook-configuration


After you complete the modifications, scaling the replicas to be less than 5 is not allowed anymore. This generates the described event in the openshift-gatekeeper namespace, which is recognized by the admission template.

Recap of gatekeeper-related policies in the policy-collection repository

Recall the gatekeepr-related policies that were discussed in this blog, that can also be found in the policy-collection repository:

Enabling mutation

I also want to introduce two policies with the new Gatekeeper feature, Mutating-Webhooks (alpha). The following policies are available in the community folder of the policy-collection repository:


We are continuously adding more examples to our policy-collection repository and happy for any contribution from the open source community. Learn more about contributing by reviewing the Contributing policies document guidelines.


OpenShift 4, Red Hat Advanced Cluster Management, Gatekeeper

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