PKI certificates and requirements

Kubernetes requires PKI certificates for authentication over TLS. If you install Kubernetes with kubeadm, the certificates that your cluster requires are automatically generated. You can also generate your own certificates -- for example, to keep your private keys more secure by not storing them on the API server. This page explains the certificates that your cluster requires.

How certificates are used by your cluster

Kubernetes requires PKI for the following operations:

Server certificates

• Server certificate for the API server endpoint
• Server certificate for the etcd server
• Server certificates for each kubelet (every node runs a kubelet)
• Optional server certificate for the front-proxy

Client certificates

• Client certificates for each kubelet, used to authenticate to the API server as a client of the Kubernetes API
• Client certificate for each API server, used to authenticate to etcd
• Client certificate for the controller manager to securely communicate with the API server
• Client certificate for the scheduler to securely communicate with the API server
• Client certificates, one for each node, for kube-proxy to authenticate to the API server
• Optional client certificates for administrators of the cluster to authenticate to the API server
• Optional client certificate for the front-proxy

Kubelet's server and client certificates

To establish a secure connection and authenticate itself to the kubelet, the API Server requires a client certificate and key pair.

In this scenario, there are two approaches for certificate usage:

• Shared Certificates: The kube-apiserver can utilize the same certificate and key pair it uses to authenticate its clients. This means that the existing certificates, such as apiserver.crt and apiserver.key, can be used for communicating with the kubelet servers.

• Separate Certificates: Alternatively, the kube-apiserver can generate a new client certificate and key pair to authenticate its communication with the kubelet servers. In this case, a distinct certificate named kubelet-client.crt and its corresponding private key, kubelet-client.key are created.

etcd also implements mutual TLS to authenticate clients and peers.

Where certificates are stored

If you install Kubernetes with kubeadm, most certificates are stored in /etc/kubernetes/pki. All paths in this documentation are relative to that directory, with the exception of user account certificates which kubeadm places in /etc/kubernetes.

Configure certificates manually

If you don't want kubeadm to generate the required certificates, you can create them using a single root CA or by providing all certificates. See Certificates for details on creating your own certificate authority. See Certificate Management with kubeadm for more on managing certificates.

Single root CA

You can create a single root CA, controlled by an administrator. This root CA can then create multiple intermediate CAs, and delegate all further creation to Kubernetes itself.

Required CAs:

On top of the above CAs, it is also necessary to get a public/private key pair for service account management, sa.key and sa.pub. The following example illustrates the CA key and certificate files shown in the previous table:

/etc/kubernetes/pki/ca.crt
/etc/kubernetes/pki/ca.key
/etc/kubernetes/pki/etcd/ca.crt
/etc/kubernetes/pki/etcd/ca.key
/etc/kubernetes/pki/front-proxy-ca.crt
/etc/kubernetes/pki/front-proxy-ca.key

All certificates

If you don't wish to copy the CA private keys to your cluster, you can generate all certificates yourself.

Required certificates:

where kind maps to one or more of the x509 key usage, which is also documented in the .spec.usages of a CertificateSigningRequest type:

Certificate paths

Certificates should be placed in a recommended path (as used by kubeadm). Paths should be specified using the given argument regardless of location.

Same considerations apply for the service account key pair:

The following example illustrates the file paths from the previous tables you need to provide if you are generating all of your own keys and certificates:

/etc/kubernetes/pki/etcd/ca.key
/etc/kubernetes/pki/etcd/ca.crt
/etc/kubernetes/pki/apiserver-etcd-client.key
/etc/kubernetes/pki/apiserver-etcd-client.crt
/etc/kubernetes/pki/ca.key
/etc/kubernetes/pki/ca.crt
/etc/kubernetes/pki/apiserver.key
/etc/kubernetes/pki/apiserver.crt
/etc/kubernetes/pki/apiserver-kubelet-client.key
/etc/kubernetes/pki/apiserver-kubelet-client.crt
/etc/kubernetes/pki/front-proxy-ca.key
/etc/kubernetes/pki/front-proxy-ca.crt
/etc/kubernetes/pki/front-proxy-client.key
/etc/kubernetes/pki/front-proxy-client.crt
/etc/kubernetes/pki/etcd/server.key
/etc/kubernetes/pki/etcd/server.crt
/etc/kubernetes/pki/etcd/peer.key
/etc/kubernetes/pki/etcd/peer.crt
/etc/kubernetes/pki/etcd/healthcheck-client.key
/etc/kubernetes/pki/etcd/healthcheck-client.crt
/etc/kubernetes/pki/sa.key
/etc/kubernetes/pki/sa.pub

Configure certificates for user accounts

You must manually configure these administrator accounts and service accounts:

1. For each configuration, generate an x509 certificate/key pair with the given Common Name (CN) and Organization (O).

2. Run kubectl as follows for each configuration:

   KUBECONFIG=<filename> kubectl config set-cluster default-cluster --server=https://<host ip>:6443 --certificate-authority <path-to-kubernetes-ca> --embed-certs
   KUBECONFIG=<filename> kubectl config set-credentials <credential-name> --client-key <path-to-key>.pem --client-certificate <path-to-cert>.pem --embed-certs
   KUBECONFIG=<filename> kubectl config set-context default-system --cluster default-cluster --user <credential-name>
   KUBECONFIG=<filename> kubectl config use-context default-system
   

These files are used as follows:

The following files illustrate full paths to the files listed in the previous table:

/etc/kubernetes/admin.conf
/etc/kubernetes/super-admin.conf
/etc/kubernetes/kubelet.conf
/etc/kubernetes/controller-manager.conf
/etc/kubernetes/scheduler.conf