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TLS

Understand how to protect traffic within your cluster using Transport Layer Security (TLS).

1: Configure Certificate Rotation for the Kubelet
2: Manage TLS Certificates in a Cluster
3: Manual Rotation of CA Certificates

1 - Configure Certificate Rotation for the Kubelet

This page shows how to enable and configure certificate rotation for the kubelet.

FEATURE STATE: Kubernetes v1.19 [stable]

Before you begin

Kubernetes version 1.8.0 or later is required

Overview

The kubelet uses certificates for authenticating to the Kubernetes API. By default, these certificates are issued with one year expiration so that they do not need to be renewed too frequently.

Kubernetes contains kubelet certificate rotation, that will automatically generate a new key and request a new certificate from the Kubernetes API as the current certificate approaches expiration. Once the new certificate is available, it will be used for authenticating connections to the Kubernetes API.

Enabling client certificate rotation

The kubelet process accepts an argument --rotate-certificates that controls if the kubelet will automatically request a new certificate as the expiration of the certificate currently in use approaches.

The kube-controller-manager process accepts an argument --cluster-signing-duration (--experimental-cluster-signing-duration prior to 1.19) that controls how long certificates will be issued for.

Understanding the certificate rotation configuration

When a kubelet starts up, if it is configured to bootstrap (using the --bootstrap-kubeconfig flag), it will use its initial certificate to connect to the Kubernetes API and issue a certificate signing request. You can view the status of certificate signing requests using:

kubectl get csr

Initially a certificate signing request from the kubelet on a node will have a status of Pending. If the certificate signing requests meets specific criteria, it will be auto approved by the controller manager, then it will have a status of Approved. Next, the controller manager will sign a certificate, issued for the duration specified by the --cluster-signing-duration parameter, and the signed certificate will be attached to the certificate signing request.

The kubelet will retrieve the signed certificate from the Kubernetes API and write that to disk, in the location specified by --cert-dir. Then the kubelet will use the new certificate to connect to the Kubernetes API.

As the expiration of the signed certificate approaches, the kubelet will automatically issue a new certificate signing request, using the Kubernetes API. This can happen at any point between 30% and 10% of the time remaining on the certificate. Again, the controller manager will automatically approve the certificate request and attach a signed certificate to the certificate signing request. The kubelet will retrieve the new signed certificate from the Kubernetes API and write that to disk. Then it will update the connections it has to the Kubernetes API to reconnect using the new certificate.

2 - Manage TLS Certificates in a Cluster

Kubernetes provides a certificates.k8s.io API, which lets you provision TLS certificates signed by a Certificate Authority (CA) that you control. These CA and certificates can be used by your workloads to establish trust.

certificates.k8s.io API uses a protocol that is similar to the ACME draft.

Note: Certificates created using the certificates.k8s.io API are signed by a dedicated CA. It is possible to configure your cluster to use the cluster root CA for this purpose, but you should never rely on this. Do not assume that these certificates will validate against the cluster root CA.

Before you begin

You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:

To check the version, enter kubectl version.

Trusting TLS in a Cluster

Trusting the custom CA from an application running as a pod usually requires some extra application configuration. You will need to add the CA certificate bundle to the list of CA certificates that the TLS client or server trusts. For example, you would do this with a golang TLS config by parsing the certificate chain and adding the parsed certificates to the RootCAs field in the tls.Config struct.

You can distribute the CA certificate as a ConfigMap that your pods have access to use.

Requesting a Certificate

The following section demonstrates how to create a TLS certificate for a Kubernetes service accessed through DNS.

Note: This tutorial uses CFSSL: Cloudflare's PKI and TLS toolkit click here to know more.

Download and install CFSSL

The cfssl tools used in this example can be downloaded at https://github.com/cloudflare/cfssl/releases.

Create a Certificate Signing Request

Generate a private key and certificate signing request (or CSR) by running the following command:

cat <<EOF | cfssl genkey - | cfssljson -bare server
{
  "hosts": [
    "my-svc.my-namespace.svc.cluster.local",
    "my-pod.my-namespace.pod.cluster.local",
    "192.0.2.24",
    "10.0.34.2"
  ],
  "CN": "system:node:my-pod.my-namespace.pod.cluster.local",
  "key": {
    "algo": "ecdsa",
    "size": 256
  },
  "names": [
    {
      "O": "system:nodes"
    }
  ]
}
EOF

Where 192.0.2.24 is the service's cluster IP, my-svc.my-namespace.svc.cluster.local is the service's DNS name, 10.0.34.2 is the pod's IP and my-pod.my-namespace.pod.cluster.local is the pod's DNS name. You should see the following output:

2017/03/21 06:48:17 [INFO] generate received request
2017/03/21 06:48:17 [INFO] received CSR
2017/03/21 06:48:17 [INFO] generating key: ecdsa-256
2017/03/21 06:48:17 [INFO] encoded CSR

This command generates two files; it generates server.csr containing the PEM encoded pkcs#10 certification request, and server-key.pem containing the PEM encoded key to the certificate that is still to be created.

Create a Certificate Signing Request object to send to the Kubernetes API

Generate a CSR yaml blob and send it to the apiserver by running the following command:

cat <<EOF | kubectl apply -f -
apiVersion: certificates.k8s.io/v1
kind: CertificateSigningRequest
metadata:
  name: my-svc.my-namespace
spec:
  request: $(cat server.csr | base64 | tr -d '\n')
  signerName: kubernetes.io/kubelet-serving
  usages:
  - digital signature
  - key encipherment
  - server auth
EOF

Notice that the server.csr file created in step 1 is base64 encoded and stashed in the .spec.request field. We are also requesting a certificate with the "digital signature", "key encipherment", and "server auth" key usages, signed by the kubernetes.io/kubelet-serving signer. A specific signerName must be requested. View documentation for supported signer names for more information.

The CSR should now be visible from the API in a Pending state. You can see it by running:

kubectl describe csr my-svc.my-namespace

Name:                   my-svc.my-namespace
Labels:                 <none>
Annotations:            <none>
CreationTimestamp:      Tue, 21 Mar 2017 07:03:51 -0700
Requesting User:        yourname@example.com
Status:                 Pending
Subject:
        Common Name:    my-svc.my-namespace.svc.cluster.local
        Serial Number:
Subject Alternative Names:
        DNS Names:      my-svc.my-namespace.svc.cluster.local
        IP Addresses:   192.0.2.24
                        10.0.34.2
Events: <none>

Get the Certificate Signing Request Approved

Approving the certificate signing request is either done by an automated approval process or on a one off basis by a cluster administrator. More information on what this involves is covered below.

Download the Certificate and Use It

Once the CSR is signed and approved you should see the following:

kubectl get csr

NAME                  AGE       REQUESTOR               CONDITION
my-svc.my-namespace   10m       yourname@example.com    Approved,Issued

You can download the issued certificate and save it to a server.crt file by running the following:

kubectl get csr my-svc.my-namespace -o jsonpath='{.status.certificate}' \
    | base64 --decode > server.crt

Now you can use server.crt and server-key.pem as the keypair to start your HTTPS server.

Approving Certificate Signing Requests

A Kubernetes administrator (with appropriate permissions) can manually approve (or deny) Certificate Signing Requests by using the kubectl certificate approve and kubectl certificate deny commands. However if you intend to make heavy usage of this API, you might consider writing an automated certificates controller.

Whether a machine or a human using kubectl as above, the role of the approver is to verify that the CSR satisfies two requirements:

The subject of the CSR controls the private key used to sign the CSR. This addresses the threat of a third party masquerading as an authorized subject. In the above example, this step would be to verify that the pod controls the private key used to generate the CSR.
The subject of the CSR is authorized to act in the requested context. This addresses the threat of an undesired subject joining the cluster. In the above example, this step would be to verify that the pod is allowed to participate in the requested service.

If and only if these two requirements are met, the approver should approve the CSR and otherwise should deny the CSR.

A Word of Warning on the Approval Permission

The ability to approve CSRs decides who trusts whom within your environment. The ability to approve CSRs should not be granted broadly or lightly. The requirements of the challenge noted in the previous section and the repercussions of issuing a specific certificate should be fully understood before granting this permission.

A Note to Cluster Administrators

This tutorial assumes that a signer is setup to serve the certificates API. The Kubernetes controller manager provides a default implementation of a signer. To enable it, pass the --cluster-signing-cert-file and --cluster-signing-key-file parameters to the controller manager with paths to your Certificate Authority's keypair.

3 - Manual Rotation of CA Certificates

This page shows how to manually rotate the certificate authority (CA) certificates.

Before you begin

Your Kubernetes server must be at or later than version v1.13. To check the version, enter kubectl version.

For more information about authentication in Kubernetes, see Authenticating.
For more information about best practices for CA certificates, see Single root CA.

Rotate the CA certificates manually

Caution:
Make sure to back up your certificate directory along with configuration files and any other necessary files.
This approach assumes operation of the Kubernetes control plane in a HA configuration with multiple API servers. Graceful termination of the API server is also assumed so clients can cleanly disconnect from one API server and reconnect to another.
Configurations with a single API server will experience unavailability while the API server is being restarted.

Distribute the new CA certificates and private keys (ex: ca.crt, ca.key, front-proxy-ca.crt, and front-proxy-ca.key) to all your control plane nodes in the Kubernetes certificates directory.
Update kube-controller-manager's --root-ca-file to include both old and new CA. Then restart the component.
Any service account created after this point will get secrets that include both old and new CAs.
Note: The files specified by the kube-controller-manager flags --client-ca-file and --cluster-signing-cert-file cannot be CA bundles. If these flags and --root-ca-file point to the same ca.crt file which is now a bundle (includes both old and new CA) you will face an error. To workaround this problem you can copy the new CA to a separate file and make the flags --client-ca-file and --cluster-signing-cert-file point to the copy. Once ca.crt is no longer a bundle you can restore the problem flags to point to ca.crt and delete the copy.

Update all service account tokens to include both old and new CA certificates.

If any pods are started before new CA is used by API servers, they will get this update and trust both old and new CAs.

base64_encoded_ca="$(base64 -w0 <path to file containing both old and new CAs>)"

for namespace in $(kubectl get ns --no-headers | awk '{print $1}'); do
    for token in $(kubectl get secrets --namespace "$namespace" --field-selector type=kubernetes.io/service-account-token -o name); do
        kubectl get $token --namespace "$namespace" -o yaml | \
          /bin/sed "s/\(ca.crt:\).*/\1 ${base64_encoded_ca}/" | \
          kubectl apply -f -
    done
done

Restart all pods using in-cluster configs (ex: kube-proxy, coredns, etc) so they can use the updated certificate authority data from ServiceAccount secrets.
- Make sure coredns, kube-proxy and other pods using in-cluster configs are working as expected.
Append the both old and new CA to the file against --client-ca-file and --kubelet-certificate-authority flag in the kube-apiserver configuration.
Append the both old and new CA to the file against --client-ca-file flag in the kube-scheduler configuration.
Update certificates for user accounts by replacing the content of client-certificate-data and client-key-data respectively.
For information about creating certificates for individual user accounts, see Configure certificates for user accounts.
Additionally, update the certificate-authority-data section in the kubeconfig files, respectively with Base64-encoded old and new certificate authority data
Follow below steps in a rolling fashion.
1. Restart any other aggregated api servers or webhook handlers to trust the new CA certificates.
2. Restart the kubelet by update the file against clientCAFile in kubelet configuration and certificate-authority-data in kubelet.conf to use both the old and new CA on all nodes.
  If your kubelet is not using client certificate rotation update client-certificate-data and client-key-data in kubelet.conf on all nodes along with the kubelet client certificate file usually found in /var/lib/kubelet/pki.
3. Restart API servers with the certificates (apiserver.crt, apiserver-kubelet-client.crt and front-proxy-client.crt) signed by new CA. You can use the existing private keys or new private keys. If you changed the private keys then update these in the Kubernetes certificates directory as well.
  Since the pod trusts both old and new CAs, there will be a momentarily disconnection after which the pod's kube client will reconnect to the new API server that uses the certificate signed by the new CA.
  - Restart Scheduler to use the new CAs.
  - Make sure control plane components logs no TLS errors.
  Note: To generate certificates and private keys for your cluster using the openssl command line tool, see Certificates (openssl). You can also use cfssl.
4. Annotate any Daemonsets and Deployments to trigger pod replacement in a safer rolling fashion.
  Example:
```
for namespace in $(kubectl get namespace -o jsonpath='{.items[*].metadata.name}'); do
    for name in $(kubectl get deployments -n $namespace -o jsonpath='{.items[*].metadata.name}'); do
        kubectl patch deployment -n ${namespace} ${name} -p '{"spec":{"template":{"metadata":{"annotations":{"ca-rotation": "1"}}}}}';
    done
    for name in $(kubectl get daemonset -n $namespace -o jsonpath='{.items[*].metadata.name}'); do
        kubectl patch daemonset -n ${namespace} ${name} -p '{"spec":{"template":{"metadata":{"annotations":{"ca-rotation": "1"}}}}}';
    done
done
```
  Note: To limit the number of concurrent disruptions that your application experiences, see configure pod disruption budget.

If your cluster is using bootstrap tokens to join nodes, update the ConfigMap cluster-info in the kube-public namespace with new CA.

base64_encoded_ca="$(base64 -w0 /etc/kubernetes/pki/ca.crt)"

kubectl get cm/cluster-info --namespace kube-public -o yaml | \
    /bin/sed "s/\(certificate-authority-data:\).*/\1 ${base64_encoded_ca}/" | \
    kubectl apply -f -

Verify the cluster functionality.
1. Validate the logs from control plane components, along with the kubelet and the kube-proxy are not throwing any tls errors, see looking at the logs.
2. Validate logs from any aggregated api servers and pods using in-cluster config.
Once the cluster functionality is successfully verified:
1. Update all service account tokens to include new CA certificate only.
  - All pods using an in-cluster kubeconfig will eventually need to be restarted to pick up the new SA secret for the old CA to be completely untrusted.
2. Restart the control plane components by removing the old CA from the kubeconfig files and the files against --client-ca-file, --root-ca-file flags resp.
3. Restart kubelet by removing the old CA from file against the clientCAFile flag and kubelet kubeconfig file.