kubernetes/docs/admission_controllers.md

Admission Controllers

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. 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

This plug-in will accept all incoming requests made to the Kubernetes API server.

AlwaysDeny

This plug-in will reject all mutating requests made to the Kubernetes API server. It's largely intended for testing purposes and is not recommended for usage in a real deployment.

DenyExecOnPrivileged

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.

ServiceAccount

This plug-in limits admission of Pod creation requests based on the Pod's ServiceAccount.

1. If the pod does not have a ServiceAccount, it modifies the pod's ServiceAccount to "default".
2. It ensures that the ServiceAccount referenced by a pod exists.
3. If LimitSecretReferences is true, it rejects the pod if the pod references Secret objects which the pods ServiceAccount does not reference.
4. If the pod does not contain any ImagePullSecrets, the ImagePullSecrets of the ServiceAccount are added to the pod.
5. If MountServiceAccountToken is true, it adds a VolumeMount with the pod's ServiceAccount API token secret to containers in the pod.

We strongly recommend using this plug-in if you intend to make use of Kubernetes ServiceAccount objects.

SecurityContextDeny

This plug-in will deny any SecurityContext that defines options that were not available on the Container.

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.

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.

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.

NamespaceExists

This plug-in will observe all incoming requests that attempt to create a resource in a Kubernetes Namespace and reject the request if the Namespace was not previously created. We strongly recommend running this plug-in to ensure integrity of your data.

NamespaceAutoProvision (deprecated)

This plug-in will observe all incoming requests that attempt to create a resource in a Kubernetes Namespace and create a new Namespace if one did not already exist previously.

We strongly recommend NamespaceExists over NamespaceAutoProvision.

NamespaceLifecycle

This plug-in enforces that a Namespace that is undergoing termination cannot have new content created in it.

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

Once NamespaceAutoProvision is deprecated, we anticipate NamespaceLifecycle and NamespaceExists will be merged into a single plug-in that enforces the life-cycle of a Namespace in Kubernetes.

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

Yes.

For Kubernetes 1.0, we strongly recommend running the following set of admission control plug-ins:

--admission_control=NamespaceLifecycle,NamespaceExists,LimitRanger,SecurityContextDeny,ServiceAccount,ResourceQuota