Kubernetes Apiserver Authentication, Authorization, and Proxying to Extension Apiservers

3. The Kube API server authorizes the requested URL using any configured authorization method (e.g. RBAC) kube-apiserver / aggregator -> aggregated apiserver: note: 4. The aggregator opens a connection to the aggregated API server using `--proxy-client-cert-file`/`--proxy-client-key-file` client certificate/key to secure the channel 5. The aggregator sends the user info from step 1 to the aggregated API server as http headers, as defined by the following flags: * `--requestheader-username-headers` * `--requestheader-group-headers` * `--requestheader-extra-headers-prefix` aggregated apiserver -> aggregated apiserver: authentication note: 6. The aggregated apiserver authenticates the incoming request using the auth proxy authentication method: * verifies the request has a recognized auth proxy client certificate * pulls user info from the incoming request's http headers By default, it pulls the configuration information for this from a configmap in the kube-system namespace that is published by the kube-apiserver, containing the info from the `--requestheader-...` flags provided to the kube-apiserver (CA bundle to use, auth proxy client certificate names to allow, http header names to use, etc) aggregated apiserver -> kube-apiserver / aggregator: authorization note: 7. The aggregated apiserver authorizes the incoming request by making a SubjectAccessReview call to the kube-apiserver aggregated apiserver -> aggregated apiserver: admission note: 8. For mutating requests, the aggregated apiserver runs admission checks. by default, the namespace lifecycle admission plugin ensures namespaced resources are created in a namespace that exists in the kube-apiserver -----END----- --> ### Kubernetes Apiserver Authentication and Authorization A request to an API path that is served by an extension apiserver begins the same way as all API requests: communication to the Kubernetes apiserver. This path already has been registered with the Kubernetes apiserver by the extension apiserver. The user communicates with the Kubernetes apiserver, requesting access to the path. The Kubernetes apiserver uses standard authentication and authorization configured with the Kubernetes apiserver to authenticate the user and authorize access to the specific path. For an overview of authenticating to a Kubernetes cluster, see ["Authenticating to a Cluster"](/docs/reference/access-authn-authz/authentication/). For an overview of authorization of access to Kubernetes cluster resources, see ["Authorization Overview"](/docs/reference/access-authn-authz/authorization/). Everything to this point has been standard Kubernetes API requests, authentication and authorization. The Kubernetes apiserver now is prepared to send the request to the extension apiserver. ### Kubernetes Apiserver Proxies the Request The Kubernetes apiserver now will send, or proxy, the request to the extension apiserver that registered to handle the request. In order to do so, it needs to know several things: 1. How should the Kubernetes apiserver authenticate to the extension apiserver, informing the extension apiserver that the request, which comes over the network, is coming from a valid Kubernetes apiserver? 2. How should the Kubernetes apiserver inform the extension apiserver of the username and group for which the original request was authenticated? In order to provide for these two, you must configure the Kubernetes apiserver using several flags. #### Kubernetes Apiserver Client Authentication The Kubernetes apiserver connects to the extension apiserver over TLS, authenticating itself using a client certificate. You must provide the following to the Kubernetes apiserver upon startup, using the provided flags: * private key file via `--proxy-client-key-file` * signed client certificate file via `--proxy-client-cert-file` * certificate of the CA that signed the client certificate file via `--requestheader-client-ca-file` * valid Common Name values (CNs) in the signed client certificate via `--requestheader-allowed-names`

This section details how the Kubernetes apiserver handles requests for API paths served by extension apiservers. It covers the authentication and authorization processes within the Kubernetes apiserver, as well as how the apiserver proxies requests to the extension apiserver. It also outlines the configuration required for the Kubernetes apiserver to authenticate itself to the extension apiserver using client certificates and to pass user information from the original request.