APIServer Admission Plugins

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Overview

Similar to the kube-apiserver, the gardener-apiserver comes with a few in-tree managed admission plugins. If you want to get an overview of the what and why of admission plugins then this document might be a good start.

This document lists all existing admission plugins with a short explanation of what it is responsible for.

`ClusterOpenIDConnectPreset`, `OpenIDConnectPreset`

(both enabled by default)

These admission controllers react on CREATE operations for Shoots. If the Shoot does not specify any OIDC configuration (.spec.kubernetes.kubeAPIServer.oidcConfig=nil), then it tries to find a matching ClusterOpenIDConnectPreset or OpenIDConnectPreset, respectively. If there are multiple matches, then the one with the highest weight “wins”. In this case, the admission controller will default the OIDC configuration in the Shoot.

`ControllerRegistrationResources`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for ControllerRegistrations. It validates that there exists only one ControllerRegistration in the system that is primarily responsible for a given kind/type resource combination. This prevents misconfiguration by the Gardener administrator/operator.

`CustomVerbAuthorizer`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for Projects. It validates whether the user is bound to a RBAC role with the modify-spec-tolerations-whitelist verb in case the user tries to change the .spec.tolerations.whitelist field of the respective Project resource. Usually, regular project members are not bound to this custom verb, allowing the Gardener administrator to manage certain toleration whitelists on Project basis.

`DeletionConfirmation`

(enabled by default)

This admission controller reacts on DELETE operations for Projects and Shoots and ShootStates. It validates that the respective resource is annotated with a deletion confirmation annotation, namely confirmation.gardener.cloud/deletion=true. Only if this annotation is present it allows the DELETE operation to pass. This prevents users from accidental/undesired deletions.

`ExposureClass`

(enabled by default)

This admission controller reacts on Create operations for Shoots. It mutates Shoot resources which have an ExposureClass referenced by merging both their shootSelectors and/or tolerations into the Shoot resource.

`ExtensionValidator`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for BackupEntrys, BackupBuckets, Seeds, and Shoots. For all the various extension types in the specifications of these objects, it validates whether there exists a ControllerRegistration in the system that is primarily responsible for the stated extension type(s). This prevents misconfigurations that would otherwise allow users to create such resources with extension types that don’t exist in the cluster, effectively leading to failing reconciliation loops.

`ExtensionLabels`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for BackupBuckets, BackupEntrys, CloudProfiles, Seeds, SecretBindings and Shoots. For all the various extension types in the specifications of these objects, it adds a corresponding label in the resource. This would allow extension admission webhooks to filter out the resources they are responsible for and ignore all others. This label is of the form <extension-type>.extensions.gardener.cloud/<extension-name> : "true". For example, an extension label for provider extension type aws, looks like provider.extensions.gardener.cloud/aws : "true".

`ProjectValidator`

(enabled by default)

This admission controller reacts on CREATE operations for Projects. It prevents creating Projects with a non-empty .spec.namespace if the value in .spec.namespace does not start with garden-.

⚠️ This admission plugin will be removed in a future release and its business logic will be incorporated into the static validation of the gardener-apiserver.

`ResourceQuota`

(enabled by default)

This admission controller enables object count ResourceQuotas for Gardener resources, e.g. Shoots, SecretBindings, Projects, etc.

⚠️ In addition to this admission plugin, the ResourceQuota controller must be enabled for the Kube-Controller-Manager of your Garden cluster.

`ResourceReferenceManager`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for CloudProfiles, Projects, SecretBindings, Seeds, and Shoots. Generally, it checks whether referred resources stated in the specifications of these objects exist in the system (e.g., if a referenced Secret exists). However, it also has some special behaviours for certain resources:

CloudProfiles: It rejects removing Kubernetes or machine image versions if there is at least one Shoot that refers to them.
Projects: It sets the .spec.createdBy field for newly created Project resources, and defaults the .spec.owner field in case it is empty (to the same value of .spec.createdBy).
Shoots: It sets the gardener.cloud/created-by=<username> annotation for newly created Shoot resources.

`SeedValidator`

(enabled by default)

This admission controller reacts on DELETE operations for Seeds. Rejects the deletion if Shoot(s) reference the seed cluster.

`ShootDNS`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for Shoots. It tries to assign a default domain to the Shoot. It also validates the DNS configuration (.spec.dns) for shoots.

`ShootNodeLocalDNSEnabledByDefault`

(disabled by default)

This admission controller reacts on CREATE operations for Shoots. If enabled, it will enable node local dns within the shoot cluster (for more information, see NodeLocalDNS Configuration) by setting spec.systemComponents.nodeLocalDNS.enabled=true for newly created Shoots. Already existing Shoots and new Shoots that explicitly disable node local dns (spec.systemComponents.nodeLocalDNS.enabled=false) will not be affected by this admission plugin.

`ShootQuotaValidator`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for Shoots. It validates the resource consumption declared in the specification against applicable Quota resources. Only if the applicable Quota resources admit the configured resources in the Shoot then it allows the request. Applicable Quotas are referred in the SecretBinding that is used by the Shoot.

`ShootResourceReservation`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for Shoots. It injects the Kubernetes.Kubelet.KubeReserved setting for kubelet either as global setting for a shoot or on a per worker pool basis. If the admission configuration (see this example) for the ShootResourceReservation plugin contains useGKEFormula: false (the default), then it sets a static default resource reservation for the shoot.

If useGKEFormula: true is set, then the plugin injects resource reservations based on the machine type similar to GKE’s formula for resource reservation into each worker pool. Already existing resource reservations are not modified; this also means that resource reservations are not automatically updated if the machine type for a worker pool is changed. If a shoot contains global resource reservations, then no per worker pool resource reservations are injected.

`ShootVPAEnabledByDefault`

(disabled by default)

This admission controller reacts on CREATE operations for Shoots. If enabled, it will enable the managed VerticalPodAutoscaler components (for more information, see Vertical Pod Auto-Scaling) by setting spec.kubernetes.verticalPodAutoscaler.enabled=true for newly created Shoots. Already existing Shoots and new Shoots that explicitly disable VPA (spec.kubernetes.verticalPodAutoscaler.enabled=false) will not be affected by this admission plugin.

`ShootTolerationRestriction`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for Shoots. It validates the .spec.tolerations used in Shoots against the whitelist of its Project, or against the whitelist configured in the admission controller’s configuration, respectively. Additionally, it defaults the .spec.tolerations in Shoots with those configured in its Project, and those configured in the admission controller’s configuration, respectively.

`ShootValidator`

(enabled by default)

This admission controller reacts on CREATE, UPDATE and DELETE operations for Shoots. It validates certain configurations in the specification against the referred CloudProfile (e.g., machine images, machine types, used Kubernetes version, …). Generally, it performs validations that cannot be handled by the static API validation due to their dynamic nature (e.g., when something needs to be checked against referred resources). Additionally, it takes over certain defaulting tasks (e.g., default machine image for worker pools, default Kubernetes version).

`ShootManagedSeed`

(enabled by default)

This admission controller reacts on UPDATE and DELETE operations for Shoots. It validates certain configuration values in the specification that are specific to ManagedSeeds (e.g. the nginx-addon of the Shoot has to be disabled, the Shoot VPA has to be enabled). It rejects the deletion if the Shoot is referred to by a ManagedSeed.

`ManagedSeedValidator`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for ManagedSeedss. It validates certain configuration values in the specification against the referred Shoot, for example Seed provider, network ranges, DNS domain, etc. Similar to ShootValidator, it performs validations that cannot be handled by the static API validation due to their dynamic nature. Additionally, it performs certain defaulting tasks, making sure that configuration values that are not specified are defaulted to the values of the referred Shoot, for example Seed provider, network ranges, DNS domain, etc.

`ManagedSeedShoot`

(enabled by default)

This admission controller reacts on DELETE operations for ManagedSeeds. It rejects the deletion if there are Shoots that are scheduled onto the Seed that is registered by the ManagedSeed.

`ShootDNSRewriting`

(disabled by default)

This admission controller reacts on CREATE operations for Shoots. If enabled, it adds a set of common suffixes configured in its admission plugin configuration to the Shoot (spec.systemComponents.coreDNS.rewriting.commonSuffixes) (for more information, see DNS Search Path Optimization). Already existing Shoots will not be affected by this admission plugin.

`NamespacedCloudProfileValidator`

(enabled by default)

This admission controller reacts on CREATE and UPDATE operations for NamespacedCloudProfiles. It primarily validates if the referenced parent CloudProfile exists in the system. In addition, the admission controller ensures that the NamespacedCloudProfile only configures new machine types, and does not overwrite those from the parent CloudProfile.