---
reviewers:
- smarterclayton
- lavalamp
- whitlockjc
- caesarxuchao
- deads2k
- liggitt
- mbohlool
title: Dynamic Admission Control
content_template: templates/concept
weight: 40
---

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The [admission controllers documentation](/docs/reference/access-authn-authz/admission-controllers/)
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:

* They need to be compiled into kube-apiserver.
* They are only configurable when the apiserver starts up.

*Admission Webhooks* (beta in 1.9) addresses these limitations. It allows
admission controllers to be developed out-of-tree and configured at runtime.

This page describes how to use Admission Webhooks.

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### What are admission webhooks?

Admission webhooks are HTTP callbacks that receive admission requests and do
something with them. You can define two types of admission webhooks,
[validating admission Webhook](/docs/reference/access-authn-authz/admission-controllers/#validatingadmissionwebhook)
and
[mutating admission webhook](/docs/reference/access-authn-authz/admission-controllers/#mutatingadmissionwebhook).
With validating admission Webhooks, you may reject requests to enforce custom
admission policies. With mutating admission Webhooks, you may change requests to
enforce custom defaults.

### Experimenting with admission webhooks

Admission webhooks are essentially part of the cluster control-plane. You should
write and deploy them with great caution. Please read the [user
guides](/docs/reference/access-authn-authz/extensible-admission-controllers/#write-an-admission-webhook-server) for
instructions if you intend to write/deploy production-grade admission webhooks.
In the following, we describe how to quickly experiment with admission webhooks.

### Prerequisites

* Ensure that the Kubernetes cluster is at least as new as v1.9.

* Ensure that MutatingAdmissionWebhook and ValidatingAdmissionWebhook
  admission controllers are enabled.
  [Here](/docs/reference/access-authn-authz/admission-controllers/#is-there-a-recommended-set-of-admission-controllers-to-use)
  is a recommended set of admission controllers to enable in general.

* Ensure that the admissionregistration.k8s.io/v1beta1 API is enabled.

### Write an admission webhook server

Please refer to the implementation of the [admission webhook
server](https://github.com/kubernetes/kubernetes/blob/v1.13.0/test/images/webhook/main.go)
that is validated in a Kubernetes e2e test. The webhook handles the
`admissionReview` requests sent by the apiservers, and sends back its decision
wrapped in `admissionResponse`.

the `admissionReview` request can have different versions (e.g. v1beta1 or `v1` in a future version).
The webhook can define what version they accept using `admissionReviewVersions` field. API server
will try to use first version in the list which it supports. If none of the versions specified
in this list supported by API server, validation will fail for this object. If the webhook
configuration has already been persisted, calls to the webhook will fail and be
subject to the failure policy.

The example admission webhook server leaves the `ClientAuth` field
[empty](https://github.com/kubernetes/kubernetes/blob/v1.13.0/test/images/webhook/config.go#L47-L48),
which defaults to `NoClientCert`. This means that the webhook server does not
authenticate the identity of the clients, supposedly apiservers. If you need
mutual TLS or other ways to authenticate the clients, see
how to [authenticate apiservers](#authenticate-apiservers).

### Deploy the admission webhook service

The webhook server in the e2e test is deployed in the Kubernetes cluster, via
the [deployment API](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#deployment-v1beta1-apps).
The test also creates a [service](/docs/reference/generated/kubernetes-api/{{< param "version" >}}/#service-v1-core)
as the front-end of the webhook server. See
[code](https://github.com/kubernetes/kubernetes/blob/v1.13.0/test/e2e/apimachinery/webhook.go#L227).

You may also deploy your webhooks outside of the cluster. You will need to update
your [webhook client configurations](https://github.com/kubernetes/kubernetes/blob/v1.13.0/staging/src/k8s.io/api/admissionregistration/v1beta1/types.go#L247) accordingly.

### Configure admission webhooks on the fly

You can dynamically configure what resources are subject to what admission
webhooks via
[ValidatingWebhookConfiguration](https://github.com/kubernetes/kubernetes/blob/v1.13.0/staging/src/k8s.io/api/admissionregistration/v1beta1/types.go#L84)
or
[MutatingWebhookConfiguration](https://github.com/kubernetes/kubernetes/blob/v1.13.0/staging/src/k8s.io/api/admissionregistration/v1beta1/types.go#L114).

The following is an example `validatingWebhookConfiguration`, a mutating webhook
configuration is similar.

```yaml
apiVersion: admissionregistration.k8s.io/v1beta1
kind: ValidatingWebhookConfiguration
metadata:
  name: <name of this configuration object>
webhooks:
- name: <webhook name, e.g., pod-policy.example.io>
  rules:
  - apiGroups:
    - ""
    apiVersions:
    - v1
    operations:
    - CREATE
    resources:
    - pods
    scope: "Namespaced"
  clientConfig:
    service:
      namespace: <namespace of the front-end service>
      name: <name of the front-end service>
    caBundle: <pem encoded ca cert that signs the server cert used by the webhook>
  admissionReviewVersions:
  - v1beta1
  timeoutSeconds: 1
```

The scope field specifies if only cluster-scoped resources ("Cluster") or namespace-scoped
resources ("Namespaced") will match this rule. "*" means that there are no scope restrictions.

{{< note >}}
When using `clientConfig.service`, the server cert must be valid for
`<svc_name>.<svc_namespace>.svc`.
{{< /note >}}

{{< note >}}
Default timeout for a webhook call is 30 seconds but starting in kubernetes 1.14 you
can set the timeout and it is encouraged to use a very small timeout for webhooks.
If the webhook call times out, the request is handled according to the webhook's 
failure policy.
{{< /note >}}

When an apiserver receives a request that matches one of the `rules`, the
apiserver sends an `admissionReview` request to webhook as specified in the
`clientConfig`.

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

### Authenticate apiservers

If your admission webhooks require authentication, you can configure the
apiservers to use basic auth, bearer token, or a cert to authenticate itself to
the webhooks. There are three steps to complete the configuration.

* When starting the apiserver, specify the location of the admission control
  configuration file via the `--admission-control-config-file` flag.

* In the admission control configuration file, specify where the
  MutatingAdmissionWebhook controller and ValidatingAdmissionWebhook controller
  should read the credentials. The credentials are stored in kubeConfig files
  (yes, the same schema that's used by kubectl), so the field name is
  `kubeConfigFile`. Here is an example admission control configuration file:

```yaml
apiVersion: apiserver.k8s.io/v1alpha1
kind: AdmissionConfiguration
plugins:
- name: ValidatingAdmissionWebhook
  configuration:
    apiVersion: apiserver.config.k8s.io/v1alpha1
    kind: WebhookAdmission
    kubeConfigFile: <path-to-kubeconfig-file>
- name: MutatingAdmissionWebhook
  configuration:
    apiVersion: apiserver.config.k8s.io/v1alpha1
    kind: WebhookAdmission
    kubeConfigFile: <path-to-kubeconfig-file>
```

The schema of `admissionConfiguration` is defined
[here](https://github.com/kubernetes/kubernetes/blob/v1.13.0/staging/src/k8s.io/apiserver/pkg/apis/apiserver/v1alpha1/types.go#L27).

* In the kubeConfig file, provide the credentials:

```yaml
apiVersion: v1
kind: Config
users:
# DNS name of webhook service, i.e., <service name>.<namespace>.svc, or the URL
# of the webhook server.
- name: 'webhook1.ns1.svc'
  user:
    client-certificate-data: <pem encoded certificate>
    client-key-data: <pem encoded key>
# The `name` supports using * to wildmatch prefixing segments.
- name: '*.webhook-company.org'
  user:
    password: <password>
    username: <name>
# '*' is the default match.
- name: '*'
  user:
    token: <token>
```

Of course you need to set up the webhook server to handle these authentications.
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