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*** 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
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