---
approvers:
- bprashanth
- davidopp
- derekwaynecarr
- erictune
- janetkuo
- thockin
title: Using Admission Controllers
---

* TOC
{:toc}

## What are they?

An admission control plug-in is a piece of code that intercepts requests to the Kubernetes
API server prior to persistence of the object, but after the request is authenticated
and authorized.  The plug-in code is in the API server process
and must be compiled into the binary in order to be used at this time.

Each admission control plug-in is run in sequence before a request is accepted into the cluster.  If
any of the plug-ins in the sequence reject the request, the entire request is rejected immediately
and an error is returned to the end-user.

Admission control plug-ins may mutate the incoming object in some cases to apply system configured
defaults.  In addition, admission control plug-ins may mutate related resources as part of request
processing to do things like increment quota usage.

## Why do I need them?

Many advanced features in Kubernetes require an admission control plug-in to be enabled in order
to properly support the feature.  As a result, a Kubernetes API server that is not properly
configured with the right set of admission control plug-ins is an incomplete server and will not
support all the features you expect.

## How do I turn on an admission control plug-in?

The Kubernetes API server supports a flag, `admission-control` that takes a comma-delimited,
ordered list of admission control choices to invoke prior to modifying objects in the cluster.

## What does each plug-in do?

### AlwaysAdmit

Use this plugin by itself to pass-through all requests.

### AlwaysPullImages

This plug-in modifies every new Pod to force the image pull policy to Always. This is useful in a
multitenant cluster so that users can be assured that their private images can only be used by those
who have the credentials to pull them. Without this plug-in, once an image has been pulled to a
node, any pod from any user can use it simply by knowing the image's name (assuming the Pod is
scheduled onto the right node), without any authorization check against the image. When this plug-in
is enabled, images are always pulled prior to starting containers, which means valid credentials are
required.

### AlwaysDeny

Rejects all requests.  Used for testing.

### DefaultStorageClass

This plug-in observes creation of `PersistentVolumeClaim` objects that do not request any specific storage class
and automatically adds a default storage class to them.
This way, users that do not request any special storage class do not need to care about them at all and they
will get the default one.

This plug-in does not do anything when no default storage class is configured. When more than one storage
class is marked as default, it rejects any creation of `PersistentVolumeClaim` with an error and administrator
must revisit `StorageClass` objects and mark only one as default.
This plugin ignores any `PersistentVolumeClaim` updates; it acts only on creation.

See [persistent volume](/docs/concepts/storage/persistent-volumes/) documentation about persistent volume claims and
storage classes and how to mark a storage class as default.

### DefaultTolerationSeconds

This plug-in sets the default forgiveness toleration for pods to tolerate
the taints `notready:NoExecute` and `unreachable:NoExecute` for 5 minutes,
if the pods don't already have toleration for taints
`node.kubernetes.io/not-ready:NoExecute` or
`node.alpha.kubernetes.io/unreachable:NoExecute`.

### DenyExecOnPrivileged (deprecated)

This plug-in will intercept all requests to exec a command in a pod if that pod has a privileged container.

If your cluster supports privileged containers, and you want to restrict the ability of end-users to exec
commands in those containers, we strongly encourage enabling this plug-in.

This functionality has been merged into [DenyEscalatingExec](#denyescalatingexec).

### DenyEscalatingExec

This plug-in will deny exec and attach commands to pods that run with escalated privileges that
allow host access.  This includes pods that run as privileged, have access to the host IPC namespace, and
have access to the host PID namespace.

If your cluster supports containers that run with escalated privileges, and you want to
restrict the ability of end-users to exec commands in those containers, we strongly encourage
enabling this plug-in.

### GenericAdmissionWebhook (alpha)

This plug-in is related to the [Dynamic Admission Control](/docs/admin/extensible-admission-controllers)
introduced in v1.7.
The plug-in calls the webhooks configured via `ExternalAdmissionHookConfiguration`,
and only admits the operation if all the webhooks admit it.
Currently, the plug-in always fails open.
In other words, it ignores the failed calls to a webhook.

### ImagePolicyWebhook

The ImagePolicyWebhook plug-in allows a backend webhook to make admission decisions. You enable this plug-in by setting the admission-control option as follows:

```shell
--admission-control=ImagePolicyWebhook
```

#### Configuration File Format
ImagePolicyWebhook uses the admission config file `--admission-control-config-file` to set configuration options for the behavior of the backend. This file may be json or yaml and has the following format:

```javascript
{
  "imagePolicy": {
     "kubeConfigFile": "path/to/kubeconfig/for/backend",
     "allowTTL": 50,           // time in s to cache approval
     "denyTTL": 50,            // time in s to cache denial
     "retryBackoff": 500,      // time in ms to wait between retries
     "defaultAllow": true      // determines behavior if the webhook backend fails
  }
}
```

The config file must reference a [kubeconfig](/docs/concepts/cluster-administration/authenticate-across-clusters-kubeconfig/) formatted file which sets up the connection to the backend. It is required that the backend communicate over TLS.

The kubeconfig file's cluster field must point to the remote service, and the user field must contain the returned authorizer.

```yaml
# clusters refers to the remote service.
clusters:
- name: name-of-remote-imagepolicy-service
  cluster:
    certificate-authority: /path/to/ca.pem    # CA for verifying the remote service.
    server: https://images.example.com/policy # URL of remote service to query. Must use 'https'.

# users refers to the API server's webhook configuration.
users:
- name: name-of-api-server
  user:
    client-certificate: /path/to/cert.pem # cert for the webhook plugin to use
    client-key: /path/to/key.pem          # key matching the cert
```
For additional HTTP configuration, refer to the [kubeconfig](/docs/concepts/cluster-administration/authenticate-across-clusters-kubeconfig/) documentation.

#### Request Payloads

When faced with an admission decision, the API Server POSTs a JSON serialized api.imagepolicy.v1alpha1.ImageReview object describing the action. This object contains fields describing the containers being admitted, as well as any pod annotations that match `*.image-policy.k8s.io/*`.

Note that webhook API objects are subject to the same versioning compatibility rules as other Kubernetes API objects. Implementers should be aware of looser compatibility promises for alpha objects and check the "apiVersion" field of the request to ensure correct deserialization. Additionally, the API Server must enable the imagepolicy.k8s.io/v1alpha1 API extensions group (`--runtime-config=imagepolicy.k8s.io/v1alpha1=true`).

An example request body:

```
{  
  "apiVersion":"imagepolicy.k8s.io/v1alpha1",
  "kind":"ImageReview",
  "spec":{  
    "containers":[  
      {  
        "image":"myrepo/myimage:v1"
      },
      {  
        "image":"myrepo/myimage@sha256:beb6bd6a68f114c1dc2ea4b28db81bdf91de202a9014972bec5e4d9171d90ed"
      }
    ],
    "annotations":[  
      "mycluster.image-policy.k8s.io/ticket-1234": "break-glass"
    ],
    "namespace":"mynamespace"
  }
}
```

The remote service is expected to fill the ImageReviewStatus field of the request and respond to either allow or disallow access. The response body's "spec" field is ignored and may be omitted. A permissive response would return:

```
{
  "apiVersion": "imagepolicy.k8s.io/v1alpha1",
  "kind": "ImageReview",
  "status": {
    "allowed": true
  }
}
```

To disallow access, the service would return:

```
{
  "apiVersion": "imagepolicy.k8s.io/v1alpha1",
  "kind": "ImageReview",
  "status": {
    "allowed": false,
    "reason": "image currently blacklisted"
  }
}
```

For further documentation refer to the `imagepolicy.v1alpha1` API objects and `plugin/pkg/admission/imagepolicy/admission.go`.

#### Extending with Annotations

All annotations on a Pod that match `*.image-policy.k8s.io/*` are sent to the webhook. Sending annotations allows users who are aware of the image policy backend to send extra information to it, and for different backends implementations to accept different information.

Examples of information you might put here are:

 * request to "break glass" to override a policy, in case of emergency.
 * a ticket number from a ticket system that documents the break-glass request
 * provide a hint to the policy server as to the imageID of the image being provided, to save it a lookup

In any case, the annotations are provided by the user and are not validated by Kubernetes in any way. In the future, if an annotation is determined to be widely useful, it may be promoted to a named field of ImageReviewSpec.

### Initializers (alpha)

This plug-in is introduced in v1.7.
The plug-in determines the initializers of a resource based on the existing
`InitializerConfiguration`s. It sets the pending initializers by modifying the
metadata of the resource to be created.
For more information, please check [Dynamic Admission Control](/docs/admin/extensible-admission-controllers).

### InitialResources (experimental)

This plug-in observes pod creation requests. If a container omits compute resource requests and limits,
then the plug-in auto-populates a compute resource request based on historical usage of containers running the same image.
If there is not enough data to make a decision the Request is left unchanged.
When the plug-in sets a compute resource request, it does this by *annotating* the
the pod spec rather than mutating the `container.resources` fields.
The annotations added contain the information on what compute resources were auto-populated.

See the [InitialResouces proposal](https://git.k8s.io/community/contributors/design-proposals/autoscaling/initial-resources.md) for more details.

### LimitPodHardAntiAffinity

This plug-in denies any pod that defines `AntiAffinity` topology key other than
`kubernetes.io/hostname` in `requiredDuringSchedulingRequiredDuringExecution`.

### LimitRanger

This plug-in will observe the incoming request and ensure that it does not violate any of the constraints
enumerated in the `LimitRange` object in a `Namespace`.  If you are using `LimitRange` objects in
your Kubernetes deployment, you MUST use this plug-in to enforce those constraints. LimitRanger can also
be used to apply default resource requests to Pods that don't specify any; currently, the default LimitRanger
applies a 0.1 CPU requirement to all Pods in the `default` namespace.

See the [limitRange design doc](https://git.k8s.io/community/contributors/design-proposals/resource-management/admission_control_limit_range.md) and the [example of Limit Range](/docs/tasks/configure-pod-container/limit-range/) for more details.

### NamespaceAutoProvision

This plug-in examines all incoming requests on namespaced resources and checks
if the referenced namespace does exist.
It creates a namespace if it cannot be found.
This plug-in is useful in deployments that do not want to restrict creation of
a namespace prior to its usage.

### NamespaceExists

This plug-in checks all requests on namespaced resources other than `Namespace` itself.
If the namespace referenced from a request doesn't exist, the request is rejected.

### NamespaceLifecycle

This plug-in enforces that a `Namespace` that is undergoing termination cannot have new objects created in it,
and ensures that requests in a non-existent `Namespace` are rejected. This plug-in also prevents deletion of
three system reserved namespaces `default`, `kube-system`, `kube-public`.

A `Namespace` deletion kicks off a sequence of operations that remove all objects (pods, services, etc.) in that
namespace.  In order to enforce integrity of that process, we strongly recommend running this plug-in.

### NodeRestriction

This plug-in limits the `Node` and `Pod` objects a kubelet can modify. In order to be limited by this admission plugin,
kubelets must use credentials in the `system:nodes` group, with a username in the form `system:node:<nodeName>`.
Such kubelets will only be allowed to modify their own `Node` API object, and only modify `Pod` API objects that are bound to their node.
Future versions may add additional restrictions to ensure kubelets have the minimal set of permissions required to operate correctly.

### OwnerReferencesPermissionEnforcement

This plug-in protects the access to the `metadata.ownerReferences` of an object
so that only users with "delete" permission to the object can change it.
This plug-in also protects the access to `metadata.ownerReferences[x].blockOwnerDeletion`
of an object, so that only users with "update" permission to the `finalizers`
subresource of the referenced *owner* can change it.

### PersistentVolumeLabel

This plug-in automatically attaches region or zone labels to PersistentVolumes
as defined by the cloud provider, e.g. GCE and AWS.
It helps ensure the Pods and the PersistentVolumes mounted are in the same
region and/or zone.
If the plug-in doesn't support automatic labelling your PersistentVolumes, you
may need to add the labels manually to prevent pods from mounting volumes from
a different zone.

### PodNodeSelector

This plug-in defaults and limits what node selectors may be used within a namespace by reading a namespace annotation and a global configuration.

#### Configuration File Format
PodNodeSelector uses the admission config file `--admission-control-config-file` to set configuration options for the behavior of the backend.

Note that the configuration file format will move to a versioned file in a future release.

This file may be json or yaml and has the following format:

```yaml
podNodeSelectorPluginConfig:
 clusterDefaultNodeSelector: <node-selectors-labels>
 namespace1: <node-selectors-labels>
 namespace2: <node-selectors-labels>
```

#### Configuration Annotation Format
PodNodeSelector uses the annotation key `scheduler.alpha.kubernetes.io/node-selector` to assign node selectors to namespaces.

```yaml
apiVersion: v1
kind: Namespace
metadata:
  annotations:
    scheduler.alpha.kubernetes.io/node-selector: <node-selectors-labels>
  name: namespace3
```

### PersistentVolumeClaimResize

This plug-in implements additional validations for checking incoming `PersistentVolumeClaim` resize requests.
**Note:** Support for volume resizing is available as an alpha feature. Admins must set the feature gate `ExpandPersistentVolumes`
to `true` to enable resizing.
{: .note}

After enabling the `ExpandPersistentVolumes` feature gate, enabling the `PersistentVolumeClaimResize` admission
plug-in is recommended, too. This plug-in prevents resizing of all claims by default unless a claim's `StorageClass`
 explicitly enables resizing by setting `allowVolumeExpansion` to `true`.

For example: all `PersistentVolumeClaim`s created from the following `StorageClass` support volume expansion:

```yaml
kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: gluster-vol-default
provisioner: kubernetes.io/glusterfs
parameters:
  resturl: "http://192.168.10.100:8080"
  restuser: ""
  secretNamespace: ""
  secretName: ""
allowVolumeExpansion: true
```

For more information about persistent volume claims, see ["PersistentVolumeClaims"](/docs/concepts/storage/persistent-volumes/#persistentvolumeclaims).

### PodPreset

This plug-in injects a pod with the fields specified in a matching PodPreset.
See also [PodPreset concept](/docs/concepts/workloads/pods/podpreset/) and
[Inject Information into Pods Using a PodPreset](/docs/tasks/inject-data-application/podpreset)
for more information.

### PodSecurityPolicy

This plug-in acts on creation and modification of the pod and determines if it should be admitted
based on the requested security context and the available Pod Security Policies.

For Kubernetes < 1.6.0, the API Server must enable the extensions/v1beta1/podsecuritypolicy API
extensions group (`--runtime-config=extensions/v1beta1/podsecuritypolicy=true`).

See also [Pod Security Policy documentation](/docs/concepts/policy/pod-security-policy/)
for more information.

### PodTolerationRestriction

This plug-in first verifies any conflict between a pod's tolerations and its
namespace's tolerations, and rejects the pod request if there is a conflict.
It then merges the namespace's tolerations into the pod's tolerations.
The resulting tolerations are checked against the namespace's whitelist of
tolerations. If the check succeeds, the pod request is admitted otherwise
rejected.

If the pod's namespace does not have any associated default or whitelist of
tolerations, then the cluster-level default or whitelist of tolerations are used
instead if specified.

Tolerations to a namespace are assigned via the
`scheduler.alpha.kubernetes.io/defaultTolerations` and
`scheduler.alpha.kubernetes.io/tolerationsWhitelist`
annotation keys.

### Priority

The priority admission controller uses the `priorityClassName` field and populates the integer value of the priority. If the priority class is not found, the Pod is rejected.

### ResourceQuota

This plug-in will observe the incoming request and ensure that it does not violate any of the constraints
enumerated in the `ResourceQuota` object in a `Namespace`.  If you are using `ResourceQuota`
objects in your Kubernetes deployment, you MUST use this plug-in to enforce quota constraints.

See the [resourceQuota design doc](https://git.k8s.io/community/contributors/design-proposals/resource-management/admission_control_resource_quota.md) and the [example of Resource Quota](/docs/concepts/policy/resource-quotas/) for more details.

It is strongly encouraged that this plug-in is configured last in the sequence of admission control plug-ins.  This is
so that quota is not prematurely incremented only for the request to be rejected later in admission control.

### SecurityContextDeny

This plug-in will deny any pod that attempts to set certain escalating [SecurityContext](/docs/user-guide/security-context) fields. This should be enabled if a cluster doesn't utilize [pod security policies](/docs/user-guide/pod-security-policy) to restrict the set of values a security context can take.

### ServiceAccount

This plug-in implements automation for [serviceAccounts](/docs/user-guide/service-accounts).
We strongly recommend using this plug-in if you intend to make use of Kubernetes `ServiceAccount` objects.


## Is there a recommended set of plug-ins to use?

Yes.
For Kubernetes >= 1.6.0, we strongly recommend running the following set of admission control plug-ins (order matters):

```shell
--admission-control=NamespaceLifecycle,LimitRanger,ServiceAccount,PersistentVolumeLabel,DefaultStorageClass,ResourceQuota,DefaultTolerationSeconds
```

For Kubernetes >= 1.4.0, we strongly recommend running the following set of admission control plug-ins (order matters):

```shell
--admission-control=NamespaceLifecycle,LimitRanger,ServiceAccount,DefaultStorageClass,ResourceQuota
```

For Kubernetes >= 1.2.0, we strongly recommend running the following set of admission control plug-ins (order matters):

```shell
--admission-control=NamespaceLifecycle,LimitRanger,ServiceAccount,ResourceQuota
```

For Kubernetes >= 1.0.0, we strongly recommend running the following set of admission control plug-ins (order matters):

```shell
--admission-control=NamespaceLifecycle,LimitRanger,SecurityContextDeny,ServiceAccount,PersistentVolumeLabel,ResourceQuota
```
