k3s/docs/admin/admission-controllers.md

PLEASE NOTE: This document applies to the HEAD of the source tree only. If you are using a released version of Kubernetes, you almost certainly want the docs that go with that version.

Documentation for specific releases can be found at releases.k8s.io.

Admission Controllers

Table of Contents

• Admission Controllers
  • What are they?
  • Why do I need them?
  • How do I turn on an admission control plug-in?
  • What does each plug-in do?
    • AlwaysAdmit
    • AlwaysDeny
    • DenyExecOnPrivileged
    • ServiceAccount
    • SecurityContextDeny
    • ResourceQuota
    • LimitRanger
    • NamespaceExists
    • NamespaceAutoProvision (deprecated)
    • NamespaceLifecycle
  • Is there a recommended set of plug-ins to use?

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.

AlwaysDeny

Rejects all requests. Used for testing.

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

SecurityContextDeny

This plug-in will deny any pod with a 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.

See the resourceQuota design doc.

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.

See the limitRange design doc.

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 (order matters):

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