Create a Secure and Compliant Kubernetes Admission Controller Using Java

Creating a Kubernetes Admission Controller in Java

Kubernetes is one of the most popular and widely used container orchestration platforms. It simplifies the deployment, management, and scaling of applications. But with this power comes the responsibility to ensure that all objects created in the Kubernetes cluster meet the organization's standards and guidelines.

Admission controllers are an important part of the Kubernetes security story. They are pieces of code that enable checks and validations to be performed on resources before they are accepted and committed to the cluster. For example, you might use an admission controller to enforce limits on the size of persistent volumes, or to ensure that only authorized users are allowed to create resources.

In this tutorial, we'll show you how to create your own custom admission controller for Kubernetes using the Java programming language. We will outline the steps involved in setting up the Admission controller and implementing a few basic checks.

Prerequisites

The following prerequisites are required in order to complete this tutorial:

• A Kubernetes cluster running version 1.15 or higher
• Java 8 or higher
• Maven 3.5.0 or higher

Step 1: Configure the Admission Controller

The first step is to configure the admission controller. This can be done by adding an annotation to the Kubernetes manifest file. The annotation should contain the following:

``` admissionregistration.k8s.io/v1beta1: kind: DefaultAdmissionConfig rules: - apiGroups: ["*"] apiVersions: ["*"] operations: ["CREATE", "UPDATE", "DELETE"] resources: ["*/*"] admissionReviewVersions: ["v1beta1"] ```

This defines the admission controller and its parameters. The operations parameter specifies which operations it applies to (CREATE, UPDATE, DELETE); the resources parameter defines which requests it applies to; and the admissionReviewVersions parameter defines which version of the admission review API should be used.

Step 2: Create the Admission Controller

Once the configuration is in place, the next step is to create the actual admission controller. This can be done by creating a new Java class in your project. The class should extend the AbstractAdmissionController class, which is part of the Kubernetes client library. For example:

```java public class MyAdmissionController extends AbstractAdmissionController { // Implementation goes here } ```

Once the class is created, it needs to be registered. This can be done by calling the registerAdmissionController() method in the main() method of your application.

Step 3: Implement the Admission Controller

Now that the admission controller is configured and registered, you can begin implementing the logic. This is done in the handle() method of the controller. This method takes a AdmissionRequest object, containing details about the request such as its resource type, name, operation, and API version. You can implement your custom logic to check whether the request meets your requirements.

For example, you might want to make sure that all pods created in the cluster have a valid label. You could do this by checking the request's object to see if it contains a valid label. If it does not, you can reject the request and return an appropriate error message.

Step 4: Deploy the Admission Controller

Once the admission controller has been implemented, it needs to be deployed. This is done by creating a Kubernetes manifest file containing the necessary configuration and deploying it to the cluster. The manifest should contain the following:

```yaml apiVersion: admissionregistration.k8s.io/v1beta1 kind: ValidatingWebhookConfiguration metadata: name: my-admission-controller webhooks: - name: my-admission-controller rules: - operations: ["CREATE", "UPDATE", "DELETE"] apiGroups: ["*"] apiVersions: ["*"] resources: ["*/*"] clientConfig: caBundle: ```

In this configuration, the webhook is named my-admission-controller, and it is configured to apply to all resources in all API groups and versions. The clientConfig section contains the public certificate of the application serving the admission controller. This certificate must be encoded in base64 before being added to the manifest.

Conclusion

In this tutorial, we demonstrated how to create a custom Kubernetes admission controller in Java. We outlined the steps involved in configuring and registering the admission controller, as well as implementing the logic. Finally, we showed how to deploy the admission controller to the cluster. By implementing admission controllers, organizations can ensure that all resources created in the cluster meet their standards and guidelines.