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kubernetes-namespace-reservation

An admission webhook that prevents the creation of specified namespaces

Installation on Kubernetes 1.9+

0. make sure to have at least Kubernetes 1.9, kubectl is working and that jq is installed
1. clone this repo
2. make build-image push-image REPO=<your-docker-username>/namespace-reservation-server
3. adapt the namespace-reservation-server image in artifacts/kube-install/apiserver-list.yaml.template to your chosen Docker REPO.
4. hack/install-kube.sh, compare install-kube.sh

Then test the setup:

5. kubectl create -f artifacts/example/reserve-deads.yaml will reserve the deads namespace, compare reserve-deads.yaml.
6. kubectl create namespace deads should produce "Error from server (Forbidden): "deads" is reserved"

Topology

The webhook is deployed as DaemonSet server in the namespace openshift-namespace-reservation. In a real cluster this is to be restricted to the master nodes. The server pods get a TLS key and cert injected by the secret server-serving-cert, self-signed by a local CA.

In front of the DaemonSet pods is a service named server in the same namespace.

The webhook is an API server itself. An APIService object named v1beta1.admission.online.openshift.io makes the API group v1beta1.admission.online.openshift.io/v1beta1 available within and outside of the cluster via API aggregation of kube-apiserver. The group can be reached at /apis/admission.online.openshift.io/v1beta1/namespacereservations of the kube-apiserver, i.e. via the kubernetes.default.svc service hostname inside the cluster.

There are numerous advantages to registering the webhook server as an aggregated API:

• allows other kubernetes components to talk to the the admission webhook using the kubernetes.default.svc service
• allows other kubernetes components to use their in-cluster credentials to communicate with the webhook
• allows you to test the webhook using kubectl
• allows you to govern access to the webhook using RBAC
• prevents other extension API servers from leaking their service account tokens to the webhook

For more information, see: https://kubernetes.io/blog/2018/01/extensible-admission-is-beta

The admission webhook is registered via a ValidatingWebhookConfiguration object. The webhook URL used for admission requests is https://kubernetes.default.svc/apis/admission.online.openshift.io/v1beta1/namespacereservations, i.e. the kube-apiserver sends admission requests to itself. They are forwarded by the aggregator proxy code to the actual webhook service and finally reach the webhook server.

Trust

• kube-apiserver trusts itself under https://kubernetes.default.svc because the kube-apiserver CA cert is part of the ValidatingWebhookConfiguration object in the caBundle field (KUBE_CA in artifacts/kube-install/apiserver-list.yaml.template)
• kube-apiserver trusts https://server.openshift-namespace-reservation.svc because the local CA cert is part of the APIService object as field caBundle (SERVICE_SERVING_CERT_CA in artifacts/kube-install/apiserver-list.yaml.template), therefore trusted by kube-apiserver and the webhook server answers with its server cert which is signed by that local CA.
• the webhook server trusts the admission requests from the kube-apiserver because
  • it finds the extension-apiserver-authenticationa ConfigMap in the kube-system namespace
  • which includes a CA cert of the kube-apiserver that the request's client cert is signed with.