Kubernetes Admission Controllers for API Server Validation: Enforcing Policies and Best Practices

Kubernetes Admission Controllers for API Server Validation

In Kubernetes, Admission Controllers are a powerful feature that help control and validate requests to the Kubernetes API server before they are persisted in the cluster. These controllers can be used to enforce policies, apply default configurations, or modify requests based on specific conditions. Admission Controllers are executed after the request is authenticated and authorized but before it is written to the Kubernetes API server's data store (etcd).

In this guide, we will explore what Admission Controllers are, how they work, and how you can use them to enforce validation, defaults, and other policies in your Kubernetes cluster.

---

What Are Admission Controllers?

Admission Controllers are plugins that intercept and validate or mutate API requests to the Kubernetes API server. They are essentially the gatekeepers of your cluster, ensuring that only valid and appropriate resources are created or modified.

There are two types of Admission Controllers:

1. Mutating Admission Controllers: These controllers have the ability to modify the incoming request before it is stored in etcd. For example, a mutating controller can automatically add default values to fields that are missing in a resource specification.

2. Validating Admission Controllers: These controllers can only validate incoming requests. They cannot modify the request but can reject it if it does not meet specific validation criteria.

Admission Controllers are executed in the order in which they are configured, allowing for complex workflows. For example, you might first apply mutating controllers to modify the request and then apply validating controllers to ensure it meets policy requirements.

---

How Admission Controllers Work

When a request is made to the Kubernetes API server (for example, to create or update a resource like a Pod or a Deployment), the request passes through several stages:

1. Authentication: The request is checked to ensure the requester is who they say they are.
2. Authorization: The request is checked to determine if the requester has the necessary permissions to perform the operation.
3. Admission Control: This is where the Admission Controllers come into play. Before the request is persisted in etcd, Admission Controllers validate the request and may mutate it.
4. Persistence: If the request passes all checks and validation, the resource is saved to etcd and becomes part of the cluster's state.

---

Types of Admission Controllers

There are many built-in Admission Controllers in Kubernetes, and they can be grouped into two categories: mutating and validating.

Mutating Admission Controllers

• DefaultTolerationSeconds: Adds default toleration for Pods that do not specify one.
• LimitRanger: Adds default resource requests and limits for containers in Pods that do not specify them.
• NamespaceLifecycle: Ensures that resources are not created or modified in a namespace that is being deleted.
• PodNodeSelector: Enforces Pod scheduling constraints by modifying the Pod spec to add node selectors.
• ResourceQuota: Enforces resource usage limits by automatically adding resource quotas to the request.

Validating Admission Controllers

• PodSecurityPolicy: Validates whether Pods meet the required security policies (e.g., privilege escalation, volume types, and security contexts).
• DenyEscalatingExec: Prevents users from opening a shell or executing commands inside containers with escalated privileges.
• AlwaysPullImages: Ensures that Kubernetes always pulls the latest image from the registry, preventing the use of locally cached images.
• K8sPodSecurity: Enforces Pod security standards for workloads running in a cluster.
• ValidatingAdmissionWebhook: Allows you to define custom validation logic for your Kubernetes resources.

---

Enabling and Configuring Admission Controllers

Admission Controllers are configured through the --enable-admission-plugins flag when starting the Kubernetes API server. By default, Kubernetes enables a set of standard controllers, but you can configure additional controllers depending on your use case.

Example: Enable Admission Controllers in Kubernetes API Server

You can enable specific Admission Controllers when launching the API server by passing the --enable-admission-plugins flag. The controllers are specified as a comma-separated list.

kube-apiserver --enable-admission-plugins=MutatingAdmissionWebhook,ValidatingAdmissionWebhook,NamespaceLifecycle,LimitRanger

If you're using a managed Kubernetes service (e.g., GKE, EKS, or AKS), the ability to configure admission controllers might be limited, and you'll need to rely on the default set of controllers provided by the platform.

---

Admission Webhooks

Kubernetes also supports Admission Webhooks that allow you to define custom validation or mutation logic. Webhooks can either mutate the object (MutatingAdmissionWebhook) or validate it (ValidatingAdmissionWebhook).

To configure an admission webhook, you need to:

1. Create a Webhook Server: The server must be able to handle HTTP requests and must be secured using HTTPS.
2. Configure the API Server: Once the webhook server is ready, configure the Kubernetes API server to call it by specifying it in the admission webhook configuration.

Example: Mutating Admission Webhook

Here is an example of how to configure a MutatingAdmissionWebhook that automatically adds labels to Pods during creation.

1. Create a webhook server that accepts POST requests and adds a label to the incoming Pod spec.

2. Create a webhook configuration to register the webhook with the API server.
   

apiVersion: admissionregistration.k8s.io/v1
kind: MutatingWebhookConfiguration
metadata:
  name: mutating-webhook
webhooks:
  - name: add-labels.k8s.io
    clientConfig:
      service:
        name: my-service
        namespace: my-namespace
        path: "/mutate"
      caBundle: <CA-BUNDLE>
    rules:
      - operations: ["CREATE"]
        apiGroups: [""]
        apiVersions: ["v1"]
        resources: ["pods"]
    admissionReviewVersions: ["v1"]
    sideEffects: None

1. The webhook server will then be triggered whenever a CREATE request is made for a Pod. The server can mutate the Pod spec, such as adding labels, before it is persisted.

---

Common Use Cases for Admission Controllers

1. Enforcing Security Policies: Use PodSecurityPolicy or custom webhooks to enforce security policies, such as preventing privileged containers, ensuring proper security contexts, or enforcing namespaces for resources.

2. Resource Management: Use LimitRanger to apply default resource requests and limits for containers that do not specify them, helping to avoid resource contention.

3. Enforcing Naming Conventions: Custom mutating admission webhooks can be used to enforce naming conventions for resources, such as appending a prefix to all Pod names or labels.

4. Validating Resource Creation: Use ValidatingAdmissionWebhook to validate requests before they are persisted. For example, you could prevent Pods from being created unless they contain a specific label or annotation.

5. Ensuring Default Configuration: Use MutatingAdmissionWebhook to apply default configurations to resources that do not explicitly define them, such as adding tolerations, labels, or node selectors to Pods.

---

Best Practices for Admission Controllers

1. Use Default Controllers: Enable the default set of admission controllers provided by Kubernetes to ensure basic resource management, security, and validation.

2. Use Admission Webhooks for Custom Logic: If you need to enforce custom policies or validation that is not provided by default admission controllers, consider creating admission webhooks.

3. Avoid Overuse of Webhooks: While webhooks offer great flexibility, they can introduce latency in API server requests. Only use them when necessary and ensure they are optimized for performance.

4. Test Admission Controllers: Before deploying admission controllers in a production environment, thoroughly test them in a staging environment to ensure that they do not interfere with legitimate workloads.

5. Use Webhook Failures Carefully: Admission controllers that interact with external webhooks should be carefully configured to ensure that failures in the webhook do not cause widespread issues in your cluster.

---

Conclusion

Admission Controllers are a key feature in Kubernetes that help enforce policies, validate resources, and apply defaults in a flexible and powerful way. Whether you use the built-in controllers or custom admission webhooks, understanding and leveraging admission controllers is crucial for maintaining a secure and well-governed Kubernetes environment. By using these controllers, you can prevent misconfigurations, enforce security standards, and ensure that resources meet your organization’s requirements.

---