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GEP-16: Dynamic kubeconfig generation for Shoot clusters

Table of Contents

Summary

This GEP introduces new Shoot subresource called AdminKubeconfigRequest allowing for users to dynamically generate a short-lived kubeconfig that can be used to access the Shoot cluster as cluster-admin.

Motivation

Today, when access to the created Shoot clusters is needed, a kubeconfig with static token credentials is used. This static token is in the system:masters group, granting it cluster-admin privileges. The kubeconfig is generated when the cluster is reconciled, stored in ShootState and replicated in the Project’s namespace in a Secret. End-users can fetch the secret and use the kubeconfig inside it.

There are several problems with this approach:

  • The token in the kubeconfig does not have any expiration, so end-users have to request a kubeconfig credential rotation if they want revoke the token.
  • There is no user identity in the token. e.g. if user Joe gets the kubeconfig from the Secret, user in that token would be system:cluster-admin and not Joe when accessing the Shoot cluster with it. This makes auditing events in the cluster almost impossible.

Goals

  • Add a Shoot subresource called adminkubeconfig that would produce a kubeconfig used to access that Shoot cluster.

  • The kubeconfig is not stored in the API Server, but generated for each request.

  • In the AdminKubeconfigRequest send to that subresource, end-users can specify the expiration time of the credential.

  • The identity (user) in the Gardener cluster would be part of the identity (x509 client certificate). E.g if Joe authenticates against the Gardener API server, the generated certificate for Shoot authentication would have the following subject:

    • Common Name: Joe
    • Organisation: system:masters
  • The maximum validity of the certificate can be enforced by setting a flag on the gardener-apiserver.

  • Deprecate and remove the old {shoot-name}.kubeconfig secrets in each Project namespace.

Non-Goals

  • Generate OpenID Connect kubeconfigs

Proposal

The gardener-apiserver would serve a new shoots/adminkubeconfig resource. It can only accept CREATE calls and accept AdminKubeconfigRequest. A AdminKubeconfigRequest would have the following structure:

apiVersion: authentication.gardener.cloud/v1alpha1
kind: AdminKubeconfigRequest
spec:
  expirationSeconds: 3600

Where expirationSeconds is the validity of the certificate in seconds. In this case it would be 1 hour. The maximum validity of a AdminKubeconfigRequest is configured by --shoot-admin-kubeconfig-max-expiration flag in the gardener-apiserver.

When such request is received, the API server would find the ShootState associated with that cluster and generate a kubeconfig. The x509 client certificate would be signed by the Shoot cluster’s CA and the user used in the subject’s common name would be from the User.Info used to make the request.

apiVersion: authentication.gardener.cloud/v1alpha1
kind: AdminKubeconfigRequest
spec:
  expirationSeconds: 3600
status:
  expirationTimestamp: "2021-02-22T09:06:51Z"
  kubeConfig: # this is normally base64-encoded, but decoded for the example
    apiVersion: v1
    clusters:
    - cluster:
        certificate-authority-data: LS0tLS1....
        server: https://api.shoot-cluster
      name: shoot-cluster-a
    contexts:
    - context:
        cluster: shoot-cluster-a
        user: shoot-cluster-a
      name: shoot-cluster-a
    current-context: shoot-cluster-a
    kind: Config
    preferences: {}
    users:
    - name: shoot-cluster-a
      user:
        client-certificate-data: LS0tLS1CRUd...
        client-key-data: LS0tLS1CRUd...

New feature gate called AdminKubeconfigRequest enables the above mentioned API in the gardener-apiserver. The old {shoot-name}.kubeconfig is kept, but deprecated and will be removed in the future.

In order to get the server’s address used in the kubeconfig, the Shoot’s status should be updated with new entries:

apiVersion: core.gardener.cloud/v1beta1
kind: Shoot
metadata:
  name: crazy-botany
  namespace: garden-dev
spec: {}
status:
  advertisedAddresses:
  - name: external
    url: https://api.shoot-cluster.external.foo
  - name: internal
    url: https://api.shoot-cluster.internal.foo
  - name: ip
    url: https://1.2.3.4

This is needed, because the Gardener API server might not know on which IP address the API server is advertised on (e.g. DNS is disabled).

If there are multiple entries, each would be added in a separate cluster in the kubeconfig and a context with the same name would be added added as well. The current context would be selected as the first entry in the advertisedAddresses list (.status.advertisedAddresses[0]).

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