Kubernetes Policy Engines: A Deep Dive Comparison
An in-depth guide comparing OPA Gatekeeper, Kyverno, and Polaris for Kubernetes security, compliance, and governance.
📋 Prerequisites
- A running Kubernetes cluster (e.g., Minikube, kind, or a cloud provider's)
kubectlconfigured with cluster-admin access- Helm package manager installed
- Strong understanding of Kubernetes resources (Pods, Deployments, etc.) and YAML manifests
- Familiarity with the concept of Admission Controllers
The Role of Kubernetes Policy Engines
Out of the box, Kubernetes is incredibly flexible, but this flexibility can lead to security vulnerabilities and misconfigurations. A policy engine acts as a gatekeeper for your cluster. It integrates with the Kubernetes API server via Dynamic Admission Controllers to intercept resource requests (like kubectl apply) and enforce rules before objects are persisted in the cluster's database.
OPA Gatekeeper
The powerhouse for complex, custom logic. It uses Open Policy Agent (OPA) and its query language, Rego, offering maximum flexibility. Best for environments with strict, nuanced compliance needs.
Kyverno
The Kubernetes-native choice. Policies are defined directly in YAML as Kubernetes resources, making it easy to learn for anyone familiar with K8s. Best for teams that want fast adoption and powerful mutation capabilities.
Polaris
The best-practices auditor. Polaris focuses on scanning for a predefined set of configuration issues related to security, efficiency, and reliability. Best for CI/CD scanning and getting a quick compliance score.
Feature Comparison: Gatekeeper vs. Kyverno vs. Polaris
Choosing the right tool depends on your team's needs and skills. Here’s a breakdown of their key features.
| Feature | OPA Gatekeeper | Kyverno | Polaris |
|---|---|---|---|
| Policy Language | Rego (Powerful, general-purpose) | YAML (Kubernetes-native resources) | YAML Configuration (Pre-defined checks) |
| Ease of Use | Steeper learning curve (requires Rego) | Very easy (familiar YAML syntax) | Easiest (primarily configuration) |
| Validation | ✅ Yes (Core feature) | ✅ Yes (Core feature) | ✅ Yes (Core feature, audit-focused) |
| Mutation | ✅ Yes (Via `gator` CLI in beta) | ✅ Yes (Mature and powerful) | ❌ No |
| Resource Generation | ❌ No | ✅ Yes (Can create resources like NetworkPolicies) | ❌ No |
| Primary Use Case | Complex, custom validation policies at scale. | All-purpose validation, mutation, and generation. | Auditing for best practices and CI/CD scanning. |
Deep Dive: OPA Gatekeeper
Gatekeeper decouples policy logic from its definition. You create a ConstraintTemplate with Rego code, then apply it with a Constraint resource.
1. Installation (Helm)
{`helm repo add gatekeeper https://open-policy-agent.github.io/gatekeeper/charts
helm install gatekeeper/gatekeeper --name-template gatekeeper --namespace gatekeeper-system --create-namespace`}2. Policy Example: Disallow 'latest' Image Tag
First, define the logic in a ConstraintTemplate. Then, apply it with a Constraint.
{`# 1. The Template (The "How")
apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
name: k8sdisallowlatesttag
spec:
crd:
spec:
names:
kind: K8sDisallowLatestTag
targets:
- target: admission.k8s.gatekeeper.sh
rego: |
package k8sdisallowlatesttag
violation[{"msg": msg}] {
container := input.review.object.spec.containers[_]
endswith(container.image, ":latest")
msg := sprintf("Image '%v' uses the 'latest' tag.", [container.image])
}
---
# 2. The Constraint (The "What" and "Where")
apiVersion: constraints.gatekeeper.sh/v1beta1
kind: K8sDisallowLatestTag
metadata:
name: disallow-latest-tag
spec:
match:
kinds:
- apiGroups: [""]
kinds: ["Pod"]
- apiGroups: ["apps"]
kinds: ["Deployment", "StatefulSet", "DaemonSet"]`}Deep Dive: Kyverno
With Kyverno, the policy logic and definition are combined into a single, easy-to-read YAML manifest, typically a ClusterPolicy.
1. Installation (Helm)
{`helm repo add kyverno https://kyverno.github.io/kyverno/
helm install kyverno kyverno/kyverno -n kyverno --create-namespace`}2. Policy Example: Disallow 'latest' Image Tag
The same "disallow latest tag" policy is a single resource in Kyverno, making it much simpler to manage for common use cases.
{`apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
name: disallow-latest-tag
spec:
validationFailureAction: Enforce
background: true
rules:
- name: validate-image-tag
match:
any:
- resources:
kinds:
- Pod
- Deployment
- StatefulSet
- DaemonSet
validate:
message: "Using the 'latest' tag on images is not allowed."
pattern:
spec:
containers:
# The 'image' field of each container must NOT end with ':latest'
- image: "!*:latest"`}Deep Dive: Polaris
Polaris is less of an admission controller (though it can be) and more of a best-practices auditor. It shines in CI/CD and as a dashboard to give your cluster a "grade."
1. Installation (Helm)
{`helm repo add fairwinds-stable https://charts.fairwinds.com/stable
helm install polaris fairwinds-stable/polaris -n polaris --create-namespace`}2. Auditing and Configuration
The simplest way to use Polaris is via its dashboard or CLI. Checks are configured in the Helm values.yaml file.
{`# Access the dashboard
kubectl port-forward --namespace polaris svc/polaris-dashboard 8080:80
# In your values.yaml, configure checks from 'warning' to 'error'
config:
images:
tagNotSpecified: error # Was 'warning'
security:
runAsRootAllowed: error
networking:
hostPortSet: warning`}How to Choose the Right Tool
💡 Key Takeaways
- Start with Polaris: Run a Polaris audit first. It provides an immediate cluster score and a list of actionable items based on best practices, giving you a great starting point.
- Use Kyverno for Simplicity and Speed: If your team lives in Kubernetes YAML, Kyverno offers the lowest barrier to entry and is incredibly powerful for validation, mutation, and resource generation.
- Use Gatekeeper for Ultimate Flexibility: If you have complex, custom logic that requires a true query language (Rego) or needs to reference external data, OPA Gatekeeper is the industry standard.
- Don't Forget Mutation: Policy isn't just about blocking things. Use Kyverno to automatically add security contexts, labels, or sidecars to enforce standards without manual developer effort.
- Integrate into CI/CD: The most effective policy enforcement happens "left" of the cluster. Use the CLI versions of these tools (
kyverno-cli,gator,polaris) to scan manifests inside your pull requests.