--- approvers: - smarterclayton - lavalamp - whitlockjc - caesarxuchao title: Dynamic Admission Control --- * TOC {:toc} ## Overview The [admission controllers documentation](/docs/admin/admission-controllers/) introduces how to use standard, plugin-style admission controllers. However, plugin admission controllers are not flexible enough for all use cases, due to the following: * They need to be compiled into kube-apiserver. * They are only configurable when the apiserver starts up. 1.7 introduces two alpha features, *Initializers* and *External Admission Webhooks*, that address these limitations. These features allow admission controllers to be developed out-of-tree and configured at runtime. This page describes how to use Initializers and External Admission Webhooks. ## Initializers ### What are initializers? *Initializer* has two meanings: * A list of pending pre-initialization tasks, stored in every object's metadata (e.g., "AddMyCorporatePolicySidecar"). * A user customized controller, which actually performs those tasks. The name of the task corresponds to the controller which performs the task. For clarity, we call them *initializer controllers* in this page. Once the controller has performed its assigned task, it removes its name from the list. For example, it may send a PATCH that inserts a container in a pod and also removes its name from `metadata.initializers.pending`. Initializers may make mutations to objects. Objects which have a non-empty initializer list are considered uninitialized, and are not visible in the API unless specifically requested by using the query parameter, `?includeUninitialized=true`. ### When to use initializers? Initializers are useful for admins to force policies (e.g., the [AlwaysPullImages](/docs/admin/admission-controllers/#alwayspullimages) admission controller), or to inject defaults (e.g., the [DefaultStorageClass](/docs/admin/admission-controllers/#defaultstorageclass) admission controller), etc. **Note:** If your use case does not involve mutating objects, consider using external admission webhooks, as they have better performance. ### How are initializers triggered? When an object is POSTed, it is checked against all existing `initializerConfiguration` objects (explained below). For all that it matches, all `spec.initializers[].name`s are appended to the new object's `metadata.initializers.pending` field. An initializer controller should list and watch for uninitialized objects, by using the query parameter `?includeUninitialized=true`. If using client-go, just set [listOptions.includeUninitialized](https://github.com/kubernetes/kubernetes/blob/v1.7.0-rc.1/staging/src/k8s.io/apimachinery/pkg/apis/meta/v1/types.go#L315) to true. For the observed uninitialized objects, an initializer controller should first check if its name matches `metadata.initializers.pending[0]`. If so, it should then perform its assigned task and remove its name from the list. ### Enable initializers alpha feature *Initializers* is an alpha feature, so it is disabled by default. To turn it on, you need to: * Include "Initializers" in the `--admission-control` flag when starting `kube-apiserver`. If you have multiple `kube-apiserver` replicas, all should have the same flag setting. * Enable the dynamic admission controller registration API by adding `admissionregistration.k8s.io/v1alpha1` to the `--runtime-config` flag passed to `kube-apiserver`, e.g. `--runtime-config=admissionregistration.k8s.io/v1alpha1`. Again, all replicas should have the same flag setting. ### Deploy an initializer controller You should deploy an initializer controller via the [deployment API](/docs/api-reference/{{page.version}}/#deployment-v1beta1-apps). ### Configure initializers on the fly You can configure what initializers are enabled and what resources are subject to the initializers by creating `initializerConfiguration` resources. You should first deploy the initializer controller and make sure that it is working properly before creating the `initializerConfiguration`. Otherwise, any newly created resources will be stuck in an uninitialized state. The following is an example `initializerConfiguration`: ```yaml apiVersion: admissionregistration.k8s.io/v1alpha1 kind: InitializerConfiguration metadata: name: example-config initializers: # the name needs to be fully qualified, i.e., containing at least two "." - name: podimage.example.com rules: # apiGroups, apiVersion, resources all support wildcard "*". # "*" cannot be mixed with non-wildcard. - apiGroups: - "" apiVersions: - v1 resources: - pods ``` After you create the `initializerConfiguration`, the system will take a few seconds to honor the new configuration. Then, `"podimage.example.com"` will be appended to the `metadata.initializers.pending` field of newly created pods. You should already have a ready "podimage" initializer controller that handles pods whose `metadata.initializers.pending[0].name="podimage.example.com"`. Otherwise the pods will stuck uninitialized. Make sure that all expansions of the `` tuple in a `rule` are valid. If they are not, separate them in different `rules`. ## External Admission Webhooks ### What are external admission webhooks? External admission webhooks are HTTP callbacks that are intended to receive admission requests and do something with them. What an external admission webhook does is up to you, but there is an [interface](https://github.com/kubernetes/kubernetes/blob/v1.7.0-rc.1/pkg/apis/admission/v1alpha1/types.go) that it must adhere to so that it responds with whether or not the admission request should be allowed. Unlike initializers or the plugin-style admission controllers, external admission webhooks are not allowed to mutate the admission request in any way. Because admission is a high security operation, the external admission webhooks must support TLS. ### When to use admission webhooks? A simple example use case for an external admission webhook is to do semantic validation of Kubernetes resources. Suppose that your infrastructure requires that all `Pod` resources have a common set of labels, and you do not want any `Pod` to be persisted to Kubernetes if those needs are not met. You could write your external admission webhook to do this validation and respond accordingly. ### How are external admission webhooks triggered? Whenever a request comes in, the `GenericAdmissionWebhook` admission plugin will get the list of interested external admission webhooks from `externalAdmissionHookConfiguration` objects (explained below) and call them in parallel. If **all** of the external admission webhooks approve the admission request, the admission chain continues. If **any** of the external admission webhooks deny the admission request, the admission request will be denied, and the reason for doing so will be based on the _first_ external admission webhook denial reason. _This means if there is more than one external admission webhook that denied the admission request, only the first will be returned to the user._ If there is an error encountered when calling an external admission webhook, that request is ignored and will not be used to approve/deny the admission request. **Note:** The admission chain depends solely on the order of the `--admission-control` option passed to `kube-apiserver`. ### Enable external admission webhooks *External Admission Webhooks* is an alpha feature, so it is disabled by default. To turn it on, you need to * Include "GenericAdmissionWebhook" in the `--admission-control` flag when starting the apiserver. If you have multiple `kube-apiserver` replicas, all should have the same flag setting. * Enable the dynamic admission controller registration API by adding `admissionregistration.k8s.io/v1alpha1` to the `--runtime-config` flag passed to `kube-apiserver`, e.g. `--runtime-config=admissionregistration.k8s.io/v1alpha1`. Again, all replicas should have the same flag setting. ### Write a webhook admission controller See [caesarxuchao/example-webhook-admission-controller](https://github.com/caesarxuchao/example-webhook-admission-controller) for an example webhook admission controller. The communication between the webhook admission controller and the apiserver, or more precisely, the GenericAdmissionWebhook admission controller, needs to be TLS secured. You need to generate a CA cert and use it to sign the server cert used by your webhook admission controller. The pem formatted CA cert is supplied to the apiserver via the dynamic registration API `externaladmissionhookconfigurations.clientConfig.caBundle`. For each request received by the apiserver, the GenericAdmissionWebhook admission controller sends an [admissionReview](https://github.com/kubernetes/kubernetes/blob/v1.7.0-rc.1/pkg/apis/admission/v1alpha1/types.go#L27) to the relevant webhook admission controller. The webhook admission controller gathers information like `object`, `oldobject`, and `userInfo`, from `admissionReview.spec`, sends back a response with the body also being the `admissionReview`, whose `status` field is filled with the admission decision. ### Deploy the webhook admission controller See [caesarxuchao/example-webhook-admission-controller deployment](https://github.com/caesarxuchao/example-webhook-admission-controller/tree/master/deployment) for an example deployment. The webhook admission controller should be deployed via the [deployment API](/docs/api-reference/{{page.version}}/#deployment-v1beta1-apps). You also need to create a [service](/docs/api-reference/{{page.version}}/#service-v1-core) as the front-end of the deployment. ### Configure webhook admission controller on the fly You can configure what webhook admission controllers are enabled and what resources are subject to the admission controller via creating externaladmissionhookconfigurations. We suggest that you first deploy the webhook admission controller and make sure it is working properly before creating the externaladmissionhookconfigurations. Otherwise, depending whether the webhook is configured as fail open or fail closed, operations will be unconditionally accepted or rejected. The following is an example `externaladmissionhookconfiguration`: ```yaml apiVersion: admissionregistration.k8s.io/v1alpha1 kind: ExternalAdmissionHookConfiguration metadata: name: example-config externalAdmissionHooks: - name: pod-image.k8s.io rules: - apiGroups: - "" apiVersions: - v1 operations: - CREATE resources: - pods failurePolicy: Ignore clientConfig: caBundle: service: name: namespace: ``` For a request received by the apiserver, if the request matches any of the `rules` of an `externalAdmissionHook`, the `GenericAdmissionWebhook` admission controller will send an `admissionReview` request to the `externalAdmissionHook` to ask for admission decision. The `rule` is similar to the `rule` in `initializerConfiguration`, with two differences: * The addition of the `operations` field, specifying what operations the webhook is interested in; * The `resources` field accepts subresources in the form or resource/subresource. Make sure that all expansions of the `` tuple in a `rule` are valid. If they are not, separate them to different `rules`. You can also specify the `failurePolicy`. In 1.7, the system supports `Ignore` and `Fail` policies, meaning that upon a communication error with the webhook admission controller, the `GenericAdmissionWebhook` can admit or reject the operation based on the configured policy. After you create the `externalAdmissionHookConfiguration`, the system will take a few seconds to honor the new configuration.