Kubelet authentication/authorization

Overview

A kubelet's HTTPS endpoint exposes APIs which give access to data of varying sensitivity, and allow you to perform operations with varying levels of power on the node and within containers.

This document describes how to authenticate and authorize access to the kubelet's HTTPS endpoint.

Kubelet authentication

By default, requests to the kubelet's HTTPS endpoint that are not rejected by other configured authentication methods are treated as anonymous requests, and given a username of system:anonymous and a group of system:unauthenticated.

To disable anonymous access and send 401 Unauthorized responses to unauthenticated requests:

• start the kubelet with the --anonymous-auth=false flag

To enable X509 client certificate authentication to the kubelet's HTTPS endpoint:

• start the kubelet with the --client-ca-file flag, providing a CA bundle to verify client certificates with
• start the apiserver with --kubelet-client-certificate and --kubelet-client-key flags
• see the apiserver authentication documentation for more details

To enable API bearer tokens (including service account tokens) to be used to authenticate to the kubelet's HTTPS endpoint:

• ensure the authentication.k8s.io/v1 API group is enabled in the API server
• start the kubelet with the --authentication-token-webhook and --kubeconfig flags
• the kubelet calls the TokenReview API on the configured API server to determine user information from bearer tokens

Kubelet authorization

Any request that is successfully authenticated (including an anonymous request) is then authorized. The default authorization mode is AlwaysAllow, which allows all requests.

There are many possible reasons to subdivide access to the kubelet API:

• anonymous auth is enabled, but anonymous users' ability to call the kubelet API should be limited
• bearer token auth is enabled, but arbitrary API users' (like service accounts) ability to call the kubelet API should be limited
• client certificate auth is enabled, but only some of the client certificates signed by the configured CA should be allowed to use the kubelet API

To subdivide access to the kubelet API, delegate authorization to the API server:

• ensure the authorization.k8s.io/v1 API group is enabled in the API server
• start the kubelet with the --authorization-mode=Webhook and the --kubeconfig flags
• the kubelet calls the SubjectAccessReview API on the configured API server to determine whether each request is authorized

The kubelet authorizes API requests using the same request attributes approach as the apiserver.

The verb is determined from the incoming request's HTTP verb:

HTTP verb	request verb
POST	create
GET, HEAD	get
PUT	update
PATCH	patch
DELETE	delete

The resource and subresource is determined from the incoming request's path:

Kubelet API	resource	subresource
/stats/*	nodes	stats
/metrics/*	nodes	metrics
/logs/*	nodes	log
/spec/*	nodes	spec
/checkpoint/*	nodes	checkpoint
all others	nodes	proxy

The namespace and API group attributes are always an empty string, and the resource name is always the name of the kubelet's Node API object.

When running in this mode, ensure the user identified by the --kubelet-client-certificate and --kubelet-client-key flags passed to the apiserver is authorized for the following attributes:

• verb=*, resource=nodes, subresource=proxy
• verb=*, resource=nodes, subresource=stats
• verb=*, resource=nodes, subresource=log
• verb=*, resource=nodes, subresource=spec
• verb=*, resource=nodes, subresource=metrics

Fine-grained authorization

When the feature gate KubeletFineGrainedAuthz is enabled kubelet performs a fine-grained check before falling back to the proxy subresource for the /pods, /runningPods, /configz and /healthz endpoints. The resource and subresource are determined from the incoming request's path:

Kubelet API	resource	subresource
/stats/*	nodes	stats
/metrics/*	nodes	metrics
/logs/*	nodes	log
/pods	nodes	pods, proxy
/runningPods/	nodes	pods, proxy
/healthz	nodes	healthz, proxy
/configz	nodes	configz, proxy
all others	nodes	proxy

When the feature-gate KubeletFineGrainedAuthz is enabled, ensure the user identified by the --kubelet-client-certificate and --kubelet-client-key flags passed to the API server is authorized for the following attributes:

• verb=*, resource=nodes, subresource=proxy
• verb=*, resource=nodes, subresource=stats
• verb=*, resource=nodes, subresource=log
• verb=*, resource=nodes, subresource=metrics
• verb=*, resource=nodes, subresource=configz
• verb=*, resource=nodes, subresource=healthz
• verb=*, resource=nodes, subresource=pods

If RBAC authorization is used, enabling this gate also ensure that the builtin system:kubelet-api-admin ClusterRole is updated with permissions to access all the above mentioned subresources.