# 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]( service_accounts.md).
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](security_context.md) 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]( design/admission_control_resource_quota.md).

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]( design/admission_control_limit_range.md).

### 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):

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


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