Add documentation for certificate rotation.

Author: jcbsmpsn

_data/tasks.yml

@@ -110,6 +110,7 @@ toc:
 - title: TLS
   section:
   - docs/tasks/tls/managing-tls-in-a-cluster.md
+  - docs/tasks/tls/certificate-rotation.md
 
 - title: Administer a Cluster
   section:

docs/tasks/tls/certificate-rotation.md

@@ -0,0 +1,82 @@
+---
+approvers:
+- jcbsmpsn
+- mikedanese
+title: Certificate Rotation
+---
+
+{% capture overview %}
+This page shows how to enable and configure certificate rotation for the kubelet.
+{% endcapture %}
+
+{% capture prerequisites %}
+
+* Kubernetes version 1.8.0 or later is required
+
+* Kubelet certificate rotation is beta in 1.8.0 which means it may change without notice.
+
+{% endcapture %}
+
+{% capture steps %}
+
+## 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 1.8 contains [kubelet certificate
+rotation](/docs/tasks/administer-cluster/certificate-rotation/), a beta feature
+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.
+
+## Configuration and determining whether certificate rotation is already enabled
+
+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.  Since certificate rotation is a
+beta feature, the feature flag must also be enabled with
+`--feature-gates=RotateKubeletClientCertificate=true`.
+
+
+The `kube-controller-manager` process accepts an argument
+`--experimental-cluster-signing-duration` 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:
+
+```sh
+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
+`--experimental-cluster-signing-duration` parameter, and the signed certificate
+will be attached to the certificate signing requests.
+
+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 restart itself and 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. 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.
+
+{% endcapture %}
+
+{% include templates/task.md %}