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A micro-framework for building Kubernetes Admission Controllers.

• Provides an extensible AdmissionHandler type that accepts a custom admission function (or AdmitFunc), making it easy for you to add new validating or mutating webhook endpoints.
• Makes it easy to set up:
• Includes an example AdmitFunc for denying the creation of public Ingress and Services in GKE.
• Provides example Deployment, Service and ValidatingWebhookConfiguration definitions for you to build off of.

Note: Looking to extend admission-control for your own uses? It's first-and-foremost exposed as a library - simply import it as github.com/elithrar/admission-control and reference the example server at cmd/admissiond.

You'll need:

• Access to a Kubernetes cluster (GKE, minikube, AKS, etc) with support for admission webhooks (v1.9+)
• cfssl as part of the process of generating a TLS key-pair, and some familiarity with creating TLS (SSL) certificates (CSRs, PEM-encoded certificates, keys).
• Experience writing Go - for implementing your own AdmitFuncs (refer to the example DenyPublicServices AdmitFunc included).
• Experience building OCI (Docker) containers via docker build or similar.

⚠ Note: Admission webhooks must support HTTPS (TLS) connections; k8s does not allow webhooks to be reached over plain-text HTTP. If running in-cluster, the Service fronting the controller must be reachable via TCP port 443. External webhooks only need to satisfy the HTTPS requirement, but can be reached on any valid TCP port.

Having your k8s cluster create a TLS certificate for you will dramatically simplify the configuration, as self-signed certificates require you to provide a .webhooks.clientConfig.caBundle value for verification.

The key steps include:

1. Generate a TLS keypair for the admission controller by issuing a CertificateSigningRequest against the Kubernetes cluster, and obtain the CA certificate from the k8s cluster.
2. Creating a Deployment and a Service that makes the admission controller available to the cluster.
3. Creating a ValidatingWebhookConfiguration that points matching k8s API requests to a route on your admission controller. i.e. you may want to configure different validation policies between Services and Pods.

As noted above, we need to make our webhook endpoint available over HTTPS (TLS), which requires generating a CA cert (required as the caBundle value), key and certificate. You can can choose to have your k8s cluster sign & provide a cert for you, or otherwise provide your own self-signed cert & CA cert.

We're going to have our cluster issue a certificate for us, which simplifies the process:

1. Create a k8s CertificateSigningRequest for the hostname(s) you will deploy the Service as. There is an example CSR in demo-certs/csr.yaml for the admission-control-service.default.svc hostname. This hostname must match the .webhooks.name[].clientConfig.service.name described in your ValidatingWebhookConfiguration.

2. Approve and then fetch the certificate from the k8s API server.

3. Create a Secret that contains the TLS key-pair - the key you created alongside the CSR in step 1, and the certificate you fetched via kubectl get csr <name> ... - e.g. kubectl create secret tls <name> --cert=cert.crt --key=key.key.

4. Retrieve the k8s cluster CA cert - this will be the .webhooks.clientConfig.caBundle value in our ValidatingWebhookConfiguration: `

kubectl config view --raw --minify --flatten -o jsonpath='{.clusters[].cluster.certificate-authority-data}'

Specifically, you'll want to make sure your manifest looks like this:

apiVersion: admissionregistration.k8s.io/v1beta1
kind: ValidatingWebhookConfiguration
metadata:
  name: deny-public-services
webhooks:
  - name: deny-public-services.questionable.services
    # <snip, for brevity>
    clientConfig:
      service:
        # This is the hostname our certificate needs in its Subject Alternative
        # Name array - name.namespace.svc
        name: admission-control-service
        namespace: default
        path: "/admission-control/deny-public-services"
      # This will be the CA cert from your k8s cluster, or the CA cert you
      # generated if you took the DIY approach.
      caBundle: "<your-base64-encoded-PEM-certificate-here>"

With the TLS certificates in hand, you can now move on to deploying the controller.

With the TLS certificates generated & the associated Secret created, we can update our Deployment, Service and ValidatingWebhookConfiguration in-kind. Refer to the samples/ directory if you need a reference config.

1. Create a Deployment and make sure the -host flag passed to the admissiond container matches the hostname (ServerName) you used in the CSR.
2. Update the .spec.containers[].volumes.secret.secretName to refer to the Secret you created in step 3.
3. Create a Service that exposes the Deployment in step no. 4 to the cluster. Remember: the name of the Service should match one of the names in step 1.
4. Create a ValidatingWebhookConfiguration that matches the objects (kinds, versions) and actions (create, update, delete), and configure the .webhooks.clientConfig.service map to point to the Service you created.

Note: A set of example manifests - both admissiond-deployment.yml and deny-public-admissions-config.yml- are available in the samples/ directory.

To deploy the built-in server to your cluster with its existing validation endpoints, you'll need to build the container image and push it to an image registry that your k8s cluster can access.

If you're using Google Container Registry, you can push images to the same project as your GKE cluster:

docker build -t yourco/admissiond .
docker tag yourco/admissiond gcr.io/$PROJECTNAME/admissiond
docker push gcr.io/$PROJECTNAME/admissiond

Make sure to update/copy samples/admission-control-service.yaml with the new container image URL before deploying it:

# An example Deployment we'll try to expose
kubectl apply -f samples/hello-app.yaml
# Install the Admission Controller into the cluster
kubectl apply -f samples/admission-control-service.yaml
# Add our ValidatingWebhookConfiguration
kubectl apply -f samples/deny-public-webhook-config.yaml

Let's now attempt to deploy a kind: Service of type: LoadBalancer without the internal-only annotations:

kubectl apply -f samples/public-service.yaml

You should see the following output:

Error from server (hello-service does not have the cloud.google.com/load-balancer-type: Internal annotation.): error when creating "samples/public-service.yaml": admission webhook "deny-public-services.questionable.services" denied the request: Services of type: LoadBalancer without an internal annotation are not allowed on this cluster

Perfect! 🎉

The core type of the library is the AdmitFunc - a function that takes a k8s AdmissionReview object and returns an (*AdmissionResponse, error) tuple.

You can then pass pass the AdmissionHandler to your favorite HTTP router, and define a path that the function is avaiable on:

	r := mux.NewRouter().StrictSlash(true)
	admissions := r.PathPrefix("/admission-control").Subrouter()
	admissions.Handle("/deny-public-services", &admissioncontrol.AdmissionHandler{
		AdmitFunc:  admissioncontrol.DenyPublicServices,
		Logger:     logger,
    }).Methods(http.MethodPost)

The example server admissiond provides a step-by-step of how to configure & serve your admission controller endpoints.

If you run into problems setting up the admission-controller, make sure that:

• Your certificates are valid / key-pairs match
• You've inspected the Pod logs - e.g. via kubectl logs -f -l app=admission-control - all HTTP handler errors are logged to the configured logger.
• Your ValidatingWebhookConfiguration is matching the right API versions, namespaces & objects vs. what you have configured as an AdmitFunc endpoint in the admission-control server.

If you're stuck, open an issue with the output of:

kubectl version
# replace the label if you've authored your own Deployment manifest
kubectl logs -f -l app=admission-control

... and any relevant error messages from attempting to kubectl apply -f <manifest> that match your ValidatingWebhookConfiguration.

This project is open to contributions!

As a courtesy: please open an issue with a brief proposal of your idea first (and the use-cases surrounding it!) before diving into implementation.

Apache 2.0 licensed. Copyright Google, LLC (2019). See the LICENSE file for details.