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TLS bootstrapping

In a Kubernetes cluster, the components on the worker nodes - kubelet and kube-proxy - need to communicate with Kubernetes master components, specifically kube-apiserver. In order to ensure that communication is kept private, not interfered with, and ensure that each component of the cluster is talking to another trusted component, we strongly recommend using client TLS certificates on nodes.

The normal process of bootstrapping these components, especially worker nodes that need certificates so they can communicate safely with kube-apiserver, can be a challenging process as it is often outside of the scope of Kubernetes and requires significant additional work. This in turn, can make it challenging to initialize or scale a cluster.

In order to simplify the process, beginning in version 1.4, Kubernetes introduced a certificate request and signing API to simplify the process. The proposal can be found here.

This document describes the process of node initialization, how to set up TLS client certificate bootstrapping for kubelets, and how it works.

Initialization Process

When a worker node starts up, the kubelet does the following:

  1. Look for its kubeconfig file
  2. Retrieve the URL of the API server and credentials, normally a TLS key and signed certificate from the kubeconfig file
  3. Attempt to communicate with the API server using the credentials.

Assuming that the kube-apiserver successfully validates the kubelet’s credentials, it will treat the kubelet as a valid node, and begin to assign pods to it.

Note that the above process depends upon:

All of the following are responsibilities of whoever sets up and manages the cluster:

  1. Creating the CA key and certificate
  2. Distributing the CA certificate to the master nodes, where kube-apiserver is running
  3. Creating a key and certificate for each kubelet; strongly recommended to have a unique one, with a unique CN, for each kubelet
  4. Signing the kubelet certificate using the CA key
  5. Distributing the kubelet key and signed certificate to the specific node on which the kubelet is running

The TLS Bootstrapping described in this document is intended to simplify, and partially or even completely automate, steps 3 onwards, as these are the most common when initializing or scaling a cluster.

Bootstrap Initialization

In the bootstrap initialization process, the following occurs:

  1. kubelet begins
  2. kubelet sees that it does not have a kubeconfig file
  3. kubelet searches for and finds a bootstrap-kubeconfig file
  4. kubelet reads its bootstrap file, retrieving the URL of the API server and a limited usage “token”
  5. kubelet connects to the API server, authenticates using the token
  6. kubelet now has limited credentials to create and retrieve a certificate signing request (CSR)
  7. kubelet creates a CSR for itself
  8. CSR is approved in one of two ways:
    • If configured, kube-controller-manager automatically approves the CSR
    • If configured, an outside process, possibly a person, approves the CSR using the Kubernetes API or via kubectl
  9. Certificate is created for the kubelet
  10. Certificate is issued to the kubelet
  11. kubelet retrieves the certificate
  12. kubelet creates a proper kubeconfig with the key and signed certificate
  13. kubelet begins normal operation
  14. Optional: if configured, kubelet automatically requests renewal of the certificate when it is close to expiry
  15. The renewed certificate is approved and issued, either automatically or manually, depending on configuration.

The rest of this document describes the necessary steps to configure TLS Bootstrapping, and its limitations.

Configuration

To configure for TLS bootstrapping and optional automatic approval, you must configure options on the following components:

In addition, you need your Kubernetes Certificate Authority (CA).

Certificate Authority

As without bootstrapping, you will need a Certificate Authority (CA) key and certificate. As without bootstrapping, these will be used to sign the kubelet certificate. As before, it is your responsibility to distribute them to master nodes.

For the purposes of this document, we will assume these have been distributed to master nodes at /var/lib/kubernetes/ca.pem (certificate) and /var/lib/kubernetes/ca-key.pem (key). We will refer to these as “Kubernetes CA certificate and key”.

All Kubernetes components that use these certificates - kubelet, kube-apiserver, kube-controller-manager - assume the key and certificate to be PEM-encoded.

kube-apiserver configuration

The kube-apiserver has several requirements to enable TLS bootstrapping:

Recognizing client certificates

This is normal for all client certificate authentication. If not already set, 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, for example --client-ca-file=/var/lib/kubernetes/ca.pem.

Initial bootstrap authentication

In order for the bootstrapping kubelet to connect to kube-apiserver and request a certificate, it must first authenticate to the server. You can use any authenticator that can authenticate the kubelet.

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

Bootstrap tokens are a simpler and more easily managed method to authenticate kubelets, and do not require any additional flags when starting kube-apiserver. Using bootstrap tokens is currently beta as of Kubernetes version 1.12.

Whichever method you choose, the requirement is that the kubelet be able to authenticate as a user with the rights to:

  1. create and retrieve CSRs
  2. be automatically approved to request node client certificates, if automatic approval is enabled.

A kubelet authenticating using bootstrap tokens is authenticated as a user in the group system:bootstrappers, which is the standard method to use.

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. For example, you could disable a particular bootstrap group’s access when you are done provisioning the nodes.

Bootstrap tokens

Bootstrap tokens are described in detail here. These are tokens that are stored as secrets in the Kubernetes cluster, and then issued to the individual kubelet. You can use a single token for an entire cluster, or issue one per worker node.

The process is two-fold:

  1. Create a Kubernetes secret with the token ID, secret and scope(s).
  2. Issue the token to the kubelet

From the kubelet’s perspective, one token is like another and has no special meaning. From the kube-apiserver’s perspective, however, the bootstrap token is special. Due to its Type, namespace and name, kube-apiserver recognizes it as a special token, and grants anyone authenticating with that token special bootstrap rights, notably treating them as a member of the system:bootstrappers group. This fulfills a basic requirement for TLS bootstrapping.

The details for creating the secret are available here.

If you want to use bootstrap tokens, you must enable it on kube-apiserver with the flag:

--enable-bootstrap-token-auth=true

Token authentication file

kube-apiserver has an ability to accept tokens as authentication. These 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.

Authorize kubelet to create CSR

Now that the bootstrapping node is authenticated as part of the system:bootstrappers group, it needs to be authorized to create a certificate signing request (CSR) as well as retrieve it when done. Fortunately, Kubernetes ships with a ClusterRole with precisely these (and just these) permissions, system:node-bootstrapper.

To do this, you just need to create a ClusterRoleBinding that binds the system:bootstrappers group to the cluster role system:node-bootstrapper.

# enable bootstrapping nodes to create CSR
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: create-csrs-for-bootstrapping
subjects:
- kind: Group
  name: system:bootstrappers
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: system:node-bootstrapper
  apiGroup: rbac.authorization.k8s.io

kube-controller-manager configuration

While the apiserver receives the requests for certificates from the kubelet and authenticates those requests, the controller-manager is responsible for issuing actual signed certificates.

The controller-manager performs this function via a certificate-issuing control loop. 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.

In order for the controller-manager to sign certificates, it needs the following:

Access to key and certificate

As described earlier, you need to create a Kubernetes CA key and certificate, and distribute it to the master nodes. These will be used by the controller-manager to sign the kubelet certificates.

Since these signed certificates will, in turn, be used by the kubelet to authenticate as a regular kubelet to kube-apiserver, it is important that the CA provided to the controller-manager at this stage also be trusted by kube-apiserver for authentication. This is provided to kube-apiserver with the flag --client-ca-file=FILENAME (for example, --client-ca-file=/var/lib/kubernetes/ca.pem), as described in the kube-apiserver configuration section.

To provide the Kubernetes CA key and certificate to kube-controller-manager, use the following flags:

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

for example:

--cluster-signing-cert-file="/var/lib/kubernetes/ca.pem" --cluster-signing-key-file="/var/lib/kubernetes/ca-key.pem"

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

--experimental-cluster-signing-duration

Approval

In order to approve CSRs, you need to tell the controller-manager that it is acceptable to approve them. This is done by granting RBAC permissions to the correct group.

There are two distinct sets of permissions:

To enable the kubelet to request and receive a new certificate, create a ClusterRoleBinding that binds the group in which the bootstrapping node is a member system:bootstrappers to the ClusterRole that grants it permission, system:certificates.k8s.io:certificatesigningrequests:nodeclient:

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

To enable the kubelet to renew its own client certificate, create a ClusterRoleBinding that binds the group in which the fully functioning node is a member system:nodes to the ClusterRole that grants it permission, system:certificates.k8s.io:certificatesigningrequests:selfnodeclient:

# Approve renewal CSRs for the group "system:nodes"
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: auto-approve-renewals-for-nodes
subjects:
- kind: Group
  name: system:nodes
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: system:certificates.k8s.io:certificatesigningrequests:selfnodeclient
  apiGroup: rbac.authorization.k8s.io

Note: Kubernetes Below 1.8: If you are running an earlier version of Kubernetes, notably a version below 1.8, then the cluster roles referenced above do not ship by default. You will have to create them yourself in addition to the ClusterRoleBindings listed.

To create the ClusterRoles:

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

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.

kubelet configuration

Finally, with the master nodes properly set up and all of the necessary authentication and authorization in place, we can configure the kubelet.

The kubelet requires the following configuration to bootstrap:

The bootstrap kubeconfig should be in a path available to the kubelet, for example /var/lib/kubelet/bootstrap-kubeconfig.

Its format is identical to a normal kubeconfig file. A sample file might look as follows:

apiVersion: v1
kind: Config
clusters:
- cluster:
    certificate-authority: /var/lib/kubernetes/ca.pem
    server: https://my.server.example.com:6443
  name: bootstrap
contexts:
- context:
    cluster: bootstrap
    user: kubelet-bootstrap
  name: bootstrap
current-context: bootstrap
preferences: {}
users:
- name: kubelet-bootstrap
  user:
    token: 07401b.f395accd246ae52d

The important elements to note are:

The format of the token does not matter, as long as it matches what kube-apiserver expects. In the above example, we used a bootstrap token. As stated earlier, any valid authentication method can be used, not just tokens.

Because the bootstrap kubeconfig is a standard kubeconfig, you can use kubectl to generate it. To create the above example file:

kubectl config --kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig set-cluster bootstrap --server='https://my.server.example.com:6443' --certificate-authority=/var/lib/kubernetes/ca.pem
kubectl config --kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig set-credentials kubelet-bootstrap --token=07401b.f395accd246ae52d
kubectl config --kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig set-context bootstrap --user=kubelet-bootstrap --cluster=bootstrap
kubectl config --kubeconfig=/var/lib/kubelet/bootstrap-kubeconfig use-context bootstrap

To indicate to the kubelet to use the bootstrap kubeconfig, use the following kubelet flag:

--bootstrap-kubeconfig="/var/lib/kubelet/bootstrap-kubeconfig" --kubeconfig="/var/lib/kubelet/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.

Client and Serving Certificates

All of the above relate to kubelet client certificates, specifically, the certificates a kubelet uses to authenticate to kube-apiserver.

A kubelet also can use serving certificates. The kubelet itself exposes an https endpoint for certain features. To secure these, the kubelet can do one of:

The client certificate provided by TLS bootstrapping is signed, by default, for client auth only, and thus cannot be used as serving certificates, or server auth.

However, you can enable its server certificate, at least partially, via certificate rotation.

Certificate Rotation

Kubernetes v1.8 and higher kubelet implements beta features for enabling rotation of its client and/or serving certficates. 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 both to 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.

Other authenticating components

All of TLS bootstrapping described in this document relates to the kubelet. However, other components may need to communicate directly with kube-apiserver. Notable is kube-proxy, which is part of the Kubernetes control plane and runs on every node, but may also include other components such as monitoring or networking.

Like the kubelet, these other components also require a method of authenticating to kube-apiserver. You have several options for generating these credentials:

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>.

Limits

Although Kubernetes supports running control plane master components like kube-apiserver and kube-controller-manager in containers, and even as Pods in a kubelet, as of this writing, you cannot both TLS Bootstrap a kubelet and run master plane components on it.

The reason for this limitation is that the kubelet attempts to bootstrap communication with kube-apiserver before starting any pods, even static ones define on disk and referenced via the kubelet option --pod-manifest-path=<PATH>. Trying to do both TLS Bootstrapping and master components in kubelet leads to a race condition: kubelet needs to communicate to kube-apiserver to request certificates, yet requires those certificates to be available to start kube-apiserver.

An issue is open referencing this here.

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