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TLS bootstrapping

This document describes how to set up TLS client certificate bootstrapping for kubelets. Kubernetes 1.4 introduced an API for requesting certificates from a cluster-level Certificate Authority (CA). The original intent of this API is to enable provisioning of TLS client certificates for kubelets. The proposal can be found here.

kube-apiserver configuration

The API server should be configured with an authenticator that can authenticate tokens as a user in the system:bootstrappers group.

This group will later be used in the controller-manager configuration to scope approvals in the default approval controller. As this feature matures, you should ensure tokens are bound to a Role Based Access Control (RBAC) policy which limits requests (using the bootstrap token) strictly to client requests related to certificate provisioning. With RBAC in place, scoping the tokens to a group allows for great flexibility (e.g. you could disable a particular bootstrap group’s access when you are done provisioning the nodes).

While any authentication strategy can be used for the kubelet’s initial bootstrap credentials, the following two authenticators are recommended for ease of provisioning.

  1. Bootstrap Tokens - beta
  2. Token authentication file

Using bootstrap tokens is currently beta and will simplify the management of bootstrap token management especially in a HA scenario.

Token authentication file

Tokens are arbitrary but should represent at least 128 bits of entropy derived from a secure random number generator (such as /dev/urandom on most modern Linux systems). There are multiple ways you can generate a token. For example:

head -c 16 /dev/urandom | od -An -t x | tr -d ' '

will generate tokens that look like 02b50b05283e98dd0fd71db496ef01e8.

The token file should look like the following example, where the first three values can be anything and the quoted group name should be as depicted:

02b50b05283e98dd0fd71db496ef01e8,kubelet-bootstrap,10001,"system:bootstrappers"

Add the --token-auth-file=FILENAME flag to the kube-apiserver command (in your systemd unit file perhaps) to enable the token file. See docs here for further details.

Client certificate CA bundle

Add the --client-ca-file=FILENAME flag to the kube-apiserver command to enable client certificate authentication, referencing a certificate authority bundle containing the signing certificate (e.g. --client-ca-file=/var/lib/kubernetes/ca.pem).

kube-controller-manager configuration

The API for requesting certificates adds a certificate-issuing control loop to the Kubernetes Controller Manager. This takes the form of a cfssl local signer using assets on disk. Currently, all certificates issued have one year validity and a default set of key usages.

Signing assets

You must provide a Certificate Authority in order to provide the cryptographic materials necessary to issue certificates. This CA should be trusted by kube-apiserver for authentication with the --client-ca-file=FILENAME flag. The management of the CA is beyond the scope of this document but it is recommended that you generate a dedicated CA for Kubernetes. Both certificate and key are assumed to be PEM-encoded.

The kube-controller-manager flags are:

--cluster-signing-cert-file="/etc/path/to/kubernetes/ca/ca.crt" --cluster-signing-key-file="/etc/path/to/kubernetes/ca/ca.key"

The validity duration of signed certificates can be configured with flag:

--experimental-cluster-signing-duration

SubjectAccessReview Approval Controller

The csrapproving controller that ships as part of kube-controller-manager and is enabled by default. The controller uses the SubjectAccessReview API to determine if a given user is authorized to request a CSR, then approves based on the authorization outcome. To prevent conflicts with other approvers, the builtin approver doesn’t explicitly deny CSRs. It only ignores unauthorized requests. The controller also prunes expired certificates as part of garbage collection.

The controller categorizes CSRs into three subresources:

  1. nodeclient - a request by a user for a client certificate with O=system:nodes and CN=system:node:(node name).
  2. selfnodeclient - a node renewing a client certificate with the same O and CN. A node can use its existing client certificate to authenticate this request.

The following RBAC ClusterRoles represent the nodeclient and selfnodeclient, capabilities.

# A ClusterRole which instructs the CSR approver to approve a user requesting
# node client credentials.
kind: ClusterRole
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: approve-node-client-csr
rules:
- apiGroups: ["certificates.k8s.io"]
  resources: ["certificatesigningrequests/nodeclient"]
  verbs: ["create"]
---
# A ClusterRole which instructs the CSR approver to approve a node renewing its
# own client credentials.
kind: ClusterRole
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: approve-node-client-renewal-csr
rules:
- apiGroups: ["certificates.k8s.io"]
  resources: ["certificatesigningrequests/selfnodeclient"]
  verbs: ["create"]

As of 1.8, equivalent roles to the ones listed above are automatically created as part of the default RBAC roles. For 1.8 clusters admins are recommended to bind node bootstrap identities to the following roles instead of creating their own:

For example, to grant these permissions to identities attached to bootstrap tokens, an admin would create a ClusterRoleBinding targeting the system:bootstrappers group:

# Approve all CSRs for the group "system:bootstrappers"
kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: auto-approve-csrs-for-group
subjects:
- kind: Group
  name: system:bootstrappers
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: approve-node-client-csr
  apiGroup: rbac.authorization.k8s.io

To let all nodes renew their own credentials, an admin can create a ClusterRoleBinding targeting node identities:

kind: ClusterRoleBinding
apiVersion: rbac.authorization.k8s.io/v1
metadata:
  name: node1-client-cert-renewal
subjects:
- kind: Group
  name: system:nodes
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: approve-node-client-renewal-csr
  apiGroup: rbac.authorization.k8s.io

kubelet configuration

To request a client certificate from kube-apiserver, the kubelet first needs a path to a kubeconfig file that contains the credentials for the identity that it will use to bootstrap its individual node identity.

If you are using a bootstrap token, you can use kubectl config set-cluster, set-credentials, and set-context to build this kubeconfig. Provide the name kubelet-bootstrap to kubectl config set-credentials and include --token=<token-value> as follows:

kubectl config set-credentials kubelet-bootstrap --token=${BOOTSTRAP_TOKEN} --kubeconfig=bootstrap.kubeconfig

When starting the kubelet, if the file specified via --kubeconfig does not exist, the bootstrap kubeconfig specified via --bootstrap-kubeconfig is used to request a client certificate from the API server. On approval of the certificate request and receipt back by the kubelet, a kubeconfig file referencing the generated key and obtained certificate is written to the path specified by --kubeconfig. The certificate and key file will be placed in the directory specified by --cert-dir.

Note: The following flags are required to enable this bootstrapping when starting the kubelet:

--bootstrap-kubeconfig="/path/to/bootstrap/kubeconfig"

Additionally, in 1.7 the kubelet implements beta features for enabling rotation of both its client and/or serving certs. These can be enabled through the respective RotateKubeletClientCertificate and RotateKubeletServerCertificate feature flags on the kubelet and are enabled by default.

RotateKubeletClientCertificate causes the kubelet to rotate its client certificates by creating new CSRs as its existing credentials expire. To enable this feature pass the following flag to the kubelet:

--rotate-certificates

RotateKubeletServerCertificate causes the kubelet to both request a serving certificate after bootstrapping its client credentials and to rotate that certificate. To enable this feature pass the following flag to the kubelet:

--rotate-server-certificates
Note: The CSR approving controllers implemented in core Kubernetes do not approve node serving certificates for security reasons. To use RotateKubeletServerCertificate operators need to run a custom approving controller, or manually approve the serving certificate requests.

kubectl approval

CSRs can be approved outside of the approval flows builtin to the controller manager.

The signing controller does not immediately sign all certificate requests. Instead, it waits until they have been flagged with an “Approved” status by an appropriately-privileged user. This flow is intended to allow for automated approval handled by an external approval controller or the approval controller implemented in the core controller-manager. However cluster administrators can also manually approve certificate requests using kubectl. An administrator can list CSRs with kubectl get csr and describe one in detail with kubectl describe csr <name>. An administrator can approve or deny a CSR with kubectl certificate approve <name> and kubectl certificate deny <name>.