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Dynamic Admission Control

Overview

The admission controllers documentation introduces how to use standard, plugin-style admission controllers. However, plugin admission controllers are not flexible enough for all use cases, due to the following:

1.7 introduces two alpha features, Initializers and External Admission Webhooks, that address these limitations. These features allow admission controllers to be developed out-of-tree and configured at runtime.

This page describes how to use Initializers and External Admission Webhooks.

Initializers

What are initializers?

Initializer has two meanings:

Once the controller has performed its assigned task, it removes its name from the list. For example, it may send a PATCH that inserts a container in a pod and also removes its name from metadata.initializers.pending. Initializers may make mutations to objects.

Objects which have a non-empty initializer list are considered uninitialized, and are not visible in the API unless specifically requested by using the query parameter, ?includeUninitialized=true.

When to use initializers?

Initializers are useful for admins to force policies (e.g., the AlwaysPullImages admission controller), or to inject defaults (e.g., the DefaultStorageClass admission controller), etc.

Note: If your use case does not involve mutating objects, consider using external admission webhooks, as they have better performance.

How are initializers triggered?

When an object is POSTed, it is checked against all existing initializerConfiguration objects (explained below). For all that it matches, all spec.initializers[].names are appended to the new object’s metadata.initializers.pending field.

An initializer controller should list and watch for uninitialized objects, by using the query parameter ?includeUninitialized=true. If using client-go, just set listOptions.includeUninitialized to true.

For the observed uninitialized objects, an initializer controller should first check if its name matches metadata.initializers.pending[0]. If so, it should then perform its assigned task and remove its name from the list.

Enable initializers alpha feature

Initializers is an alpha feature, so it is disabled by default. To turn it on, you need to:

Deploy an initializer controller

You should deploy an initializer controller via the deployment API.

Configure initializers on the fly

You can configure what initializers are enabled and what resources are subject to the initializers by creating initializerConfiguration resources.

You should first deploy the initializer controller and make sure that it is working properly before creating the initializerConfiguration. Otherwise, any newly created resources will be stuck in an uninitialized state.

The following is an example initializerConfiguration:

apiVersion: admissionregistration.k8s.io/v1alpha1
kind: InitializerConfiguration
metadata:
  name: example-config
initializers:
  # the name needs to be fully qualified, i.e., containing at least two "."
  - name: podimage.example.com
    rules:
      # apiGroups, apiVersion, resources all support wildcard "*".
      # "*" cannot be mixed with non-wildcard.
      - apiGroups:
          - ""
        apiVersions:
          - v1
        resources:
          - pods

After you create the initializerConfiguration, the system will take a few seconds to honor the new configuration. Then, "podimage.example.com" will be appended to the metadata.initializers.pending field of newly created pods. You should already have a ready “podimage” initializer controller that handles pods whose metadata.initializers.pending[0].name="podimage.example.com". Otherwise the pods will stuck uninitialized.

Make sure that all expansions of the <apiGroup, apiVersions, resources> tuple in a rule are valid. If they are not, separate them in different rules.

External Admission Webhooks

What are external admission webhooks?

External admission webhooks are HTTP callbacks that are intended to receive admission requests and do something with them. What an external admission webhook does is up to you, but there is an interface that it must adhere to so that it responds with whether or not the admission request should be allowed.

Unlike initializers or the plugin-style admission controllers, external admission webhooks are not allowed to mutate the admission request in any way.

Because admission is a high security operation, the external admission webhooks must support TLS.

When to use admission webhooks?

A simple example use case for an external admission webhook is to do semantic validation of Kubernetes resources. Suppose that your infrastructure requires that all Pod resources have a common set of labels, and you do not want any Pod to be persisted to Kubernetes if those needs are not met. You could write your external admission webhook to do this validation and respond accordingly.

How are external admission webhooks triggered?

Whenever a request comes in, the GenericAdmissionWebhook admission plugin will get the list of interested external admission webhooks from externalAdmissionHookConfiguration objects (explained below) and call them in parallel. If all of the external admission webhooks approve the admission request, the admission chain continues. If any of the external admission webhooks deny the admission request, the admission request will be denied, and the reason for doing so will be based on the first external admission webhook denial reason. This means if there is more than one external admission webhook that denied the admission request, only the first will be returned to the user. If there is an error encountered when calling an external admission webhook, that request is ignored and will not be used to approve/deny the admission request.

Note: The admission chain depends solely on the order of the --admission-control option passed to kube-apiserver.

Enable external admission webhooks

External Admission Webhooks is an alpha feature, so it is disabled by default. To turn it on, you need to

Write a webhook admission controller

See caesarxuchao/example-webhook-admission-controller for an example webhook admission controller.

The communication between the webhook admission controller and the apiserver, or more precisely, the GenericAdmissionWebhook admission controller, needs to be TLS secured. You need to generate a CA cert and use it to sign the server cert used by your webhook admission controller. The pem formatted CA cert is supplied to the apiserver via the dynamic registration API externaladmissionhookconfigurations.clientConfig.caBundle.

For each request received by the apiserver, the GenericAdmissionWebhook admission controller sends an admissionReview to the relevant webhook admission controller. The webhook admission controller gathers information like object, oldobject, and userInfo, from admissionReview.spec, sends back a response with the body also being the admissionReview, whose status field is filled with the admission decision.

Deploy the webhook admission controller

See caesarxuchao/example-webhook-admission-controller deployment for an example deployment.

The webhook admission controller should be deployed via the deployment API. You also need to create a service as the front-end of the deployment.

Configure webhook admission controller on the fly

You can configure what webhook admission controllers are enabled and what resources are subject to the admission controller via creating externaladmissionhookconfigurations.

We suggest that you first deploy the webhook admission controller and make sure it is working properly before creating the externaladmissionhookconfigurations. Otherwise, depending whether the webhook is configured as fail open or fail closed, operations will be unconditionally accepted or rejected.

The following is an example externaladmissionhookconfiguration:

apiVersion: admissionregistration.k8s.io/v1alpha1
kind: ExternalAdmissionHookConfiguration
metadata:
  name: example-config
externalAdmissionHooks:
- name: pod-image.k8s.io
  rules:
  - apiGroups:
    - ""
    apiVersions:
    - v1
    operations:
    - CREATE
    resources:
    - pods
  failurePolicy: Ignore
  clientConfig:
    caBundle: <pem encoded ca cert that signs the server cert used by the webhook>
    service:
      name: <name of the front-end service>
      namespace: <namespace of the front-end service>

For a request received by the apiserver, if the request matches any of the rules of an externalAdmissionHook, the GenericAdmissionWebhook admission controller will send an admissionReview request to the externalAdmissionHook to ask for admission decision.

The rule is similar to the rule in initializerConfiguration, with two differences:

Make sure that all expansions of the <apiGroup, apiVersions,resources> tuple in a rule are valid. If they are not, separate them to different rules.

You can also specify the failurePolicy. In 1.7, the system supports Ignore and Fail policies, meaning that upon a communication error with the webhook admission controller, the GenericAdmissionWebhook can admit or reject the operation based on the configured policy.

After you create the initializerConfiguration, the system will take a few seconds to honor the new configuration.

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