From 54908094cfa1c4bcae877c6d5c757aa1dd6b9983 Mon Sep 17 00:00:00 2001 From: derekwaynecarr Date: Thu, 28 May 2015 14:50:10 -0400 Subject: [PATCH] Document admission control plug-ins --- docs/admission_controllers.md | 112 ++++++++++++++++++++++++++++++++++ 1 file changed, 112 insertions(+) create mode 100644 docs/admission_controllers.md diff --git a/docs/admission_controllers.md b/docs/admission_controllers.md new file mode 100644 index 00000000000..50a91819b67 --- /dev/null +++ b/docs/admission_controllers.md @@ -0,0 +1,112 @@ +# 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: + +```shell +--admission_control=NamespaceLifecycle,NamespaceExists,LimitRanger,SecurityContextDeny,ServiceAccount,ResourceQuota +```