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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

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.

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:

You need the cfssl tool. You can download cfssl from https://github.com/cloudflare/cfssl/releases.

Some steps in this page use the jq tool. If you don't have jq, you can install it via your operating system's software sources, or fetch it from https://stedolan.github.io/jq/.

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.

Requesting a certificate

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

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": "my-pod.my-namespace.pod.cluster.local",
  "key": {
    "algo": "ecdsa",
    "size": 256
  }
}
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 output similar to:

2022/02/01 11:45:32 [INFO] generate received request
2022/02/01 11:45:32 [INFO] received CSR
2022/02/01 11:45:32 [INFO] generating key: ecdsa-256
2022/02/01 11:45:32 [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 CertificateSigningRequest object to send to the Kubernetes API

Generate a CSR manifest (in YAML), and send it to the API server. You can do that 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: example.com/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. You are also requesting a certificate with the "digital signature", "key encipherment", and "server auth" key usages, signed by an example example.com/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, 01 Feb 2022 11:49:15 -0500
Requesting User:        yourname@example.com
Signer:                 example.com/serving
Status:                 Pending
Subject:
        Common Name:    my-pod.my-namespace.pod.cluster.local
        Serial Number:
Subject Alternative Names:
        DNS Names:      my-pod.my-namespace.pod.cluster.local
                        my-svc.my-namespace.svc.cluster.local
        IP Addresses:   192.0.2.24
                        10.0.34.2
Events: <none>

Get the CertificateSigningRequest approved

Approving the certificate signing request is either done by an automated approval process or on a one off basis by a cluster administrator. If you're authorized to approve a certificate request, you can do that manually using kubectl; for example:

kubectl certificate approve my-svc.my-namespace

certificatesigningrequest.certificates.k8s.io/my-svc.my-namespace approved

You should now see the following:

kubectl get csr

NAME                  AGE   SIGNERNAME            REQUESTOR              REQUESTEDDURATION   CONDITION
my-svc.my-namespace   10m   example.com/serving   yourname@example.com   <none>              Approved

This means the certificate request has been approved and is waiting for the requested signer to sign it.

Sign the CertificateSigningRequest

Next, you'll play the part of a certificate signer, issue the certificate, and upload it to the API.

A signer would typically watch the CertificateSigningRequest API for objects with its signerName, check that they have been approved, sign certificates for those requests, and update the API object status with the issued certificate.

Create a Certificate Authority

You need an authority to provide the digital signature on the new certificate.

First, create a signing certificate by running the following:

cat <<EOF | cfssl gencert -initca - | cfssljson -bare ca
{
  "CN": "My Example Signer",
  "key": {
    "algo": "rsa",
    "size": 2048
  }
}
EOF

You should see output similar to:

2022/02/01 11:50:39 [INFO] generating a new CA key and certificate from CSR
2022/02/01 11:50:39 [INFO] generate received request
2022/02/01 11:50:39 [INFO] received CSR
2022/02/01 11:50:39 [INFO] generating key: rsa-2048
2022/02/01 11:50:39 [INFO] encoded CSR
2022/02/01 11:50:39 [INFO] signed certificate with serial number 263983151013686720899716354349605500797834580472

This produces a certificate authority key file (ca-key.pem) and certificate (ca.pem).

Issue a certificate

tls/server-signing-config.json 
{
    "signing": {
        "default": {
            "usages": [
                "digital signature",
                "key encipherment",
                "server auth"
            ],
            "expiry": "876000h",
            "ca_constraint": {
                "is_ca": false
            }
        }
    }
}

Use a server-signing-config.json signing configuration and the certificate authority key file and certificate to sign the certificate request:

kubectl get csr my-svc.my-namespace -o jsonpath='{.spec.request}' | \
  base64 --decode | \
  cfssl sign -ca ca.pem -ca-key ca-key.pem -config server-signing-config.json - | \
  cfssljson -bare ca-signed-server

You should see the output similar to:

2022/02/01 11:52:26 [INFO] signed certificate with serial number 576048928624926584381415936700914530534472870337

This produces a signed serving certificate file, ca-signed-server.pem.

Upload the signed certificate

Finally, populate the signed certificate in the API object's status:

kubectl get csr my-svc.my-namespace -o json | \
  jq '.status.certificate = "'$(base64 ca-signed-server.pem | tr -d '\n')'"' | \
  kubectl replace --raw /apis/certificates.k8s.io/v1/certificatesigningrequests/my-svc.my-namespace/status -f -

Once the CSR is approved and the signed certificate is uploaded, run:

kubectl get csr

The output is similar to:

NAME                  AGE   SIGNERNAME            REQUESTOR              REQUESTEDDURATION   CONDITION
my-svc.my-namespace   20m   example.com/serving   yourname@example.com   <none>              Approved,Issued

Download the certificate and use it

Now, as the requesting user, 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 populate server.crt and server-key.pem in a Secret that you could later mount into a Pod (for example, to use with a webserver that serves HTTPS).

kubectl create secret tls server --cert server.crt --key server-key.pem

secret/server created

Finally, you can populate ca.pem into a ConfigMap and use it as the trust root to verify the serving certificate:

kubectl create configmap example-serving-ca --from-file ca.crt=ca.pem

configmap/example-serving-ca created

Approving CertificateSigningRequests

A Kubernetes administrator (with appropriate permissions) can manually approve (or deny) CertificateSigningRequests 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:

  1. 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.
  2. 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.

For more information on certificate approval and access control, read the Certificate Signing Requests reference page.

Configuring your cluster to provide signing

This page assumes that a signer is set up 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

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:

  • 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

  1. Distribute the new CA certificates and private keys (for example: ca.crt, ca.key, front-proxy-ca.crt, and front-proxy-ca.key) to all your control plane nodes in the Kubernetes certificates directory.

  2. Update the --root-ca-file flag for the kube-controller-manager to include both old and new CA, then restart the kube-controller-manager.

    Any ServiceAccount created after this point will get Secrets that include both old and new CAs.

  3. Wait for the controller manager to update ca.crt in the service account Secrets to include both old and new CA certificates.

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

  4. Restart all pods using in-cluster configurations (for example: kube-proxy, CoreDNS, etc) so they can use the updated certificate authority data from Secrets that link to ServiceAccounts.

    • Make sure CoreDNS, kube-proxy and other Pods using in-cluster configurations are working as expected.

  5. Append the both old and new CA to the file against --client-ca-file and --kubelet-certificate-authority flag in the kube-apiserver configuration.

  6. Append the both old and new CA to the file against --client-ca-file flag in the kube-scheduler configuration.

  7. 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

  8. Update the --root-ca-file flag for the Cloud Controller Manager to include both old and new CA, then restart the cloud-controller-manager.

  9. Follow the steps below 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 Pods in your cluster trust both old and new CAs, there will be a momentarily disconnection after which pods' Kubernetes clients reconnect to the new API server. The new API server uses a certificate signed by the new CA.

      • Restart the kube-scheduler to use and trust the new CAs.
      • Make sure control plane components logs no TLS errors.

    4. Annotate any DaemonSets and Deployments to trigger pod replacement in a safer rolling fashion.

    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

     Depending on how you use StatefulSets you may also need to perform similar rolling replacement.

  10. 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 -

  11. Verify the cluster functionality.

    1. Check the logs from control plane components, along with the kubelet and the kube-proxy. Ensure those components are not reporting any TLS errors; see looking at the logs for more details.

    2. Validate logs from any aggregated api servers and pods using in-cluster config.

  12. 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 Secret, so that no Pods are relying on the old cluster CA.

    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. On each node, restart the kubelet by removing the old CA from file against the clientCAFile flag and from the kubelet kubeconfig file. You should carry this out as a rolling update.

      If your cluster lets you make this change, you can also roll it out by replacing nodes rather than reconfiguring them.