k3s/docs/proposals/kubelet-tls-bootstrap.md

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If you are using a released version of Kubernetes, you should refer to the docs that go with that version.

Documentation for other releases can be found at releases.k8s.io.

Kubelet TLS bootstrap

Author: George Tankersley (george.tankersley@coreos.com)

Preface

This document describes a method for a kubelet to bootstrap itself into a TLS-secured cluster. Crucially, it automates the provision and distribution of signed certificates.

Overview

When a kubelet runs for the first time, it must be given TLS assets or generate them itself. In the first case, this is a burden on the cluster admin and a significant logistical barrier to secure Kubernetes rollouts. In the second, the kubelet must self-sign its certificate and forfeits many of the advantages of a PKI system. Instead, we propose that the kubelet generate a private key and a CSR for submission to a cluster-level certificate signing process.

Preliminaries

We assume the existence of a functioning control plane. The apiserver should be configured for TLS initially or possess the ability to generate valid TLS credentials for itself. If secret information is passed in the request (e.g. auth tokens supplied with the request or included in ExtraInfo) then all communications from the node to the apiserver must take place over a verified TLS connection.

Each node is additionally provisioned with the following information:

1. Location of the apiserver
2. Any CA certificates necessary to trust the apiserver's TLS certificate
3. Access tokens (if needed) to communicate with the CSR endpoint

These should not change often and are thus simple to include in a static provisioning script.

API Changes

CertificateSigningRequest Object

We introduce a new API object to represent PKCS#10 certificate signing requests. It will be accessible under:

/api/vX/certificaterequests/mycsr

It will have the following structure:

// Describes a certificate signing request
type CertificateSigningRequest struct {
        api.TypeMeta         `json:",inline"`
        api.ObjectMeta       `json:"metadata,omitempty"`

        // Specifies the behavior of the CSR
        Spec CertificateSigningRequestSpec

        // Most recently observed status of the CSR
        Status CertificateSigningRequestStatus
}

type CertificateSigningRequestSpec struct {
        // Raw PKCS#10 CSR data
        CertificateRequest []byte

        // Fingerprint of the public key that signed the CSR
        Fingerprint string

        // Subject fields from the CSR
        Subject pkix.Name

        // DNS SANs from the CSR
        Hostnames []string

        // IP SANs from the CSR
        IPAddresses []string

        // Extra information the node wishes to send with the request
        ExtraInfo []string
}

type CertificateSigningRequestStatus struct {
        // Indicates whether CSR has a response yet. Default is Unknown. Status
        // is True for approval and False for rejections.
        Status api.ConditionStatus

        // If CSR was rejected, these contain the reason why (if any was supplied).
        Reason string
        Message string

        // If CSR was approved, this contains the issued certificate.
        Certificate []byte
}

We also introduce CertificateSigningRequestList to allow listing all the CSRs in the cluster:

type CertificateSigningRequestList struct {
        api.TypeMeta
        api.ListMeta

        Items []CertificateSigningRequest
}

Certificate Request Process

Node intialization

When the kubelet executes it checks a location on disk for TLS assets (currently /var/run/kubernetes/kubelet.{key,crt} by default). If it finds them, it proceeds. If there are no TLS assets, the kubelet generates a keypair and self-signed certificate. We propose the following optional fallback behavior:

1. Generate a keypair
2. Generate a CSR for that keypair with CN set to the hostname (or --hostname-override value) and DNS/IP SANs supplied with whatever values the host knows for itself.
3. Post the CSR to the CSR API endpoint.
4. Set a watch on the CSR object to be notified of approval or rejection.

Controller response

The apiserver must first validate the signature on the raw CSR data and reject requests featuring invalid CSRs. It then persists the CertificateSigningRequests and exposes the List of all CSRs for an administrator to approve or reject. The apiserver should watch for updates the Status field of any CertificateSigningRequest. When a CSR is approved (signified by Status changing from Unknown to True) the apiserver should generate and sign the certificate, then update the CertificateSigningRequestStatus with the new data.

Manual CSR approval

An administrator using kubectl or another API client can query the CertificateSigningRequestList and update the status of CertificateSigningRequests. The default Status is Unknown, indicating that there has been no decision so far. A Status of True indicates that the admin has approved the request and the apiserver should issue the certificate. A Status of False indicates that the admin has denied the request. An admin may also supply Reason and Message fields to explain the rejection.

kube-apiserver support (CA assets)

So that the apiserver can handle certificate issuance on its own, it will need access to CA signing assets. This could be as simple as a private key and a config file or as complex as a PKCS#11 client and supplementary policy system. For now, we will add flags for a signing key, a certificate, and a basic config file.

kubectl support

To support manual CSR inspection and approval, we will add support for listing, inspecting, and approving/rejecting CertificateSigningRequests to kubectl. The interface will be similar to salt-key.

Specifically, the admin will have the ability to retrieve the full list of active CSRs, inspect their contents, and set their statuses to one of:

1. approved if the apiserver should issue the cert
2. rejected if the apiserver should not issue the cert

The suggested commands are kubectl get certificates, kubectl approve <csr> and kubectl reject <csr>. For the reject subcommand, the admin will also be able to supply Reason and Message fields via additional flags.

Security Considerations

Endpoint Access Control

The ability to post CSRs to the signing endpoint should be controlled. As a simple solution we propose that each node be provisioned with an auth token (possibly static across the cluster) that is scoped via ABAC to only allow access to the CSR endpoint.

Expiration & Revocation

The node is responsible for monitoring its own certificate expiration date. When the certificate is close to expiration, the kubelet should begin repeating this flow until it successfully obtains a new certificate. If the expiring certificate has not been revoked then it may do so using the same keypair unless the cluster policy (see "Future Work") requires fresh keys.

Revocation is for the most part an unhandled problem in Go, requiring each application to produce its own logic around a variety of parsing functions. For now, our suggested best practice is to issue only short-lived certificates. In the future it may make sense to add CRL support to the apiserver's client cert auth.

Future Work

• revocation UI in kubectl and CRL support at the apiserver
• supplemental policy (e.g. cluster CA only issues 30-day certs for hostnames *.k8s.example.com, each new cert must have fresh keys, ...)
• fully automated provisioning (using a handshake protocol or external list of authorized machines)