mirror of https://github.com/k3s-io/k3s
113 lines
4.9 KiB
Markdown
113 lines
4.9 KiB
Markdown
# Admission Controllers
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## What are they?
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An admission control plug-in is a piece of code that intercepts requests to the Kubernetes
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API server prior to persistence of the object, but after the request is authenticated
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and authorized. The plug-in code is in the API server process
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and must be compiled into the binary in order to be used at this time.
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Each admission control plug-in is run in sequence before a request is accepted into the cluster. If
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any of the plug-ins in the sequence reject the request, the entire request is rejected immediately
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and an error is returned to the end-user.
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Admission control plug-ins may mutate the incoming object in some cases to apply system configured
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defaults. In addition, admission control plug-ins may mutate related resources as part of request
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processing to do things like increment quota usage.
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## Why do I need them?
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Many advanced features in Kubernetes require an admission control plug-in to be enabled in order
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to properly support the feature. As a result, a Kubernetes API server that is not properly
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configured with the right set of admission control plug-ins is an incomplete server and will not
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support all the features you expect.
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## How do I turn on an admission control plug-in?
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The Kubernetes API server supports a flag, ```admission_control``` that takes a comma-delimited,
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ordered list of admission control choices to invoke prior to modifying objects in the cluster.
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## What does each plug-in do?
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### AlwaysAdmit
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Use this plugin by itself to pass-through all requests.
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### AlwaysDeny
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Rejects all requests. Used for testing.
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### DenyExecOnPrivileged
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This plug-in will intercept all requests to exec a command in a pod if that pod has a privileged container.
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If your cluster supports privileged containers, and you want to restrict the ability of end-users to exec
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commands in those containers, we strongly encourage enabling this plug-in.
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### ServiceAccount
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This plug-in implements automation for [serviceAccounts]( service_accounts.md).
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We strongly recommend using this plug-in if you intend to make use of Kubernetes ```ServiceAccount``` objects.
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### SecurityContextDeny
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This plug-in will deny any pod with a [SecurityContext](security_context.md) that defines options that were not available on the ```Container```.
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### ResourceQuota
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This plug-in will observe the incoming request and ensure that it does not violate any of the constraints
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enumerated in the ```ResourceQuota``` object in a ```Namespace```. If you are using ```ResourceQuota```
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objects in your Kubernetes deployment, you MUST use this plug-in to enforce quota constraints.
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See the [resourceQuota design doc]( design/admission_control_resource_quota.md).
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It is strongly encouraged that this plug-in is configured last in the sequence of admission control plug-ins. This is
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so that quota is not prematurely incremented only for the request to be rejected later in admission control.
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### LimitRanger
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This plug-in will observe the incoming request and ensure that it does not violate any of the constraints
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enumerated in the ```LimitRange``` object in a ```Namespace```. If you are using ```LimitRange``` objects in
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your Kubernetes deployment, you MUST use this plug-in to enforce those constraints.
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See the [limitRange design doc]( design/admission_control_limit_range.md).
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### NamespaceExists
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This plug-in will observe all incoming requests that attempt to create a resource in a Kubernetes ```Namespace```
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and reject the request if the ```Namespace``` was not previously created. We strongly recommend running
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this plug-in to ensure integrity of your data.
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### NamespaceAutoProvision (deprecated)
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This plug-in will observe all incoming requests that attempt to create a resource in a Kubernetes ```Namespace```
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and create a new ```Namespace``` if one did not already exist previously.
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We strongly recommend ```NamespaceExists``` over ```NamespaceAutoProvision```.
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### NamespaceLifecycle
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This plug-in enforces that a ```Namespace``` that is undergoing termination cannot have new content created in it.
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A ```Namespace``` deletion kicks off a sequence of operations that remove all content (pods, services, etc.) in that
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namespace. In order to enforce integrity of that process, we strongly recommend running this plug-in.
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Once ```NamespaceAutoProvision``` is deprecated, we anticipate ```NamespaceLifecycle``` and ```NamespaceExists``` will
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be merged into a single plug-in that enforces the life-cycle of a ```Namespace``` in Kubernetes.
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## Is there a recommended set of plug-ins to use?
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Yes.
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For Kubernetes 1.0, we strongly recommend running the following set of admission control plug-ins (order matters):
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```shell
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--admission_control=NamespaceLifecycle,NamespaceExists,LimitRanger,SecurityContextDeny,ServiceAccount,ResourceQuota
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```
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