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[WIP] Add document explaining certificates and PKI for Kubernetes #8232

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[WIP] Add document explaining certificates and PKI for Kubernetes #8232

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Add document explaining certificates and PKI for Kubernetes
liztio Apr 30, 2018
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Docs review feedback
liztio Aug 2, 2018
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copyedits for clarity, template, partial hugo markdown
Aug 10, 2018
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136 changes: 136 additions & 0 deletions docs/reference/certificates.md
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---
title: PKI Certificates and Requirements
reviewers:
- sig-cluster-lifecycle
content_template: templates/concept
---

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Kubernetes requires PKI certificates for authentication over TLS.
If you install Kubernetes with [kubeadm][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.

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## How certificates are used by your cluster

Kubernetes requires PKI for the following operations:

* Client certificates for the kubelet to authenticate to the API server
* Server certificate for the API server endpoint
* Client certificates for administrators of the cluster to authenticate to the API server
* Client certificates for the API server to talk to the kubelets
* Client certificate for the API server to talk to etcd
* Client certificate/kubeconfig for the controller manager to talk to the API server
* Client certificate/kubeconfig for the scheduler to talk to the API server.
* Client and server certificates for the [front-proxy][proxy]

etcd also implements mutual TLS to authenticate clients and peers.

## Where certificates are stored

Kubernetes stores certificates by default in `/etc/kubernetes/pki`. All paths in this documentation are relative to that directory.

## Configure certificates manually

If you don't want [kubeadm][kubeadm] to generate the required certificates, you can create them in either of the following ways.

### Single root CA

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

Required CAs:

| path | Default CN | description |
|------------------------|---------------------------|----------------------------------|
| ca.crt,key | kubernetes-ca | Kubernetes general CA |
| etcd/ca.crt,key | etcd-ca | For all etcd-related functions |
| front-proxy-ca.crt,key | kubernetes-front-proxy-ca | For the [front-end proxy][proxy] |

### All certificates

If you don't wish to copy these private keys to your API servers, you can generate all certificates yourself.

Required certificates:

| Default CN | Parent CA | O (in Subject) | kind | hosts (SAN) |
|-------------------------------|---------------------------|----------------|----------------------------------------|---------------------------------------------|
| kube-etcd | etcd-ca | | server, client [<sup>1</sup>][etcdbug] | `localhost`, `127.0.0.1` |
| kube-etcd-peer | etcd-ca | | peer | `<hostname>`, `<Host_IP>`, `localhost`, `127.0.0.1` |
| kube-etcd-healthcheck-client | etcd-ca | | client | |
| kube-apiserver-etcd-client | etcd-ca | system:masters | client | |
| kube-apiserver | kubernetes-ca | | server | `<hostname>`, `<Host_IP>`, `<advertise_IP>`, `[1]` |
| kube-apiserver-kubelet-client | kubernetes-ca | system:masters | client | |
| front-proxy-client | kubernetes-front-proxy-ca | | client | |

[1]: `kubernetes`, `kubernetes.default`, `kubernetes.default.svc`, `kubernetes.default.svc.cluster`, `kubernetes.default.svc.cluster.local`

where `kind` is one or more of the [x509 key usage][usage] types:

| kind | TLS role |
|--------|---------------------------------------------------------------------------------------------------|
| server | Digital Signature, Key Encipherment, TLS Web Server Authentication |
| peer | Digital Signature, Key Encipherment, TLS Web Server Authentication, TLS Web Client Authentication |
| client | Digital Signature, Key Encipherment, TLS Web Client Authentication |

### Certificate paths

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

| Default CN | recommend key path | recommended cert path | command | key argument | cert argument |
|------------------------------|------------------------------|-----------------------------|----------------|------------------------------|-------------------------------------------|
| etcd-ca | | etcd/ca.crt | kube-apiserver | | --etcd-cafile |
| etcd-client | apiserver-etcd-client.crt | apiserver-etcd-client.crt | kube-apiserver | --etcd-certfile | --etcd-keyfile |
| kubernetes-ca | | ca.crt | kube-apiserver | --client-ca-file | |
| kube-apiserver | apiserver.crt | apiserver.key | kube-apiserver | --tls-cert-file | --tls-private-key |
| apiserver-kubelet-client | apiserver-kubelet-client.crt | | kube-apiserver | --kubelet-client-certificate | |
| front-proxy-client | front-proxy-client.key | front-proxy-client.crt | kube-apiserver | --proxy-client-cert-file | --proxy-client-key-file |
| | | | | | |
| etcd-ca | | etcd/ca.crt | etcd | | --trusted-ca-file, --peer-trusted-ca-file |
| kube-etcd | | etcd/server.crt | etcd | | --cert-file |
| kube-etcd-peer | etcd/peer.key | etcd/peer.crt | etcd | --peer-key-file | --peer-cert-file |
| etcd-ca | | etcd/ca.crt | etcdctl[2] | | --cacert |
| kube-etcd-healthcheck-client | etcd/healthcheck-client.key | etcd/healthcheck-client.crt | etcdctl[2] | --key | --cert |

[2]: For a liveness probe, if self-hosted

## Configure certificates for user accounts

You must manually configure thesee administrator account and service accounts:

| filename | credential name | Default CN | O (in Subject) |
|-------------------------|----------------------------|--------------------------------|----------------|
| admin.conf | default-admin | kubernetes-admin | system:masters |
| kubelet.conf | default-auth | system:node:<nodename> | system:nodes |
| controller-manager.conf | default-controller-manager | system:kube-controller-manager | |
| scheduler.conf | default-manager | system:kube-scheduler | |

1. For each config, generate an x509 cert/key pair with the given CN and O.

1. Run `kubectl` as follows for each config:

```shell
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:

| filename | command | comment |
|-------------------------|-------------------------|-----------------------------------------------------------------------|
| admin.conf | kubectl | Lets user administer the cluster |
| kubelet.conf | kubelet | One required for each node in the cluster. |
| controller-manager.conf | kube-controller-manager | Must be added to manifest in `manifests/kube-controller-manager.yaml` |
| scheduler.conf | kube-scheduler | Must be added to manifest in `manifests/kube-scheduler.yaml` |

[usage]: https://tools.ietf.org/html/rfc5280#section-4.2.1.3
[kubeadm]: /docs/reference/setup-tools/kubeadm/kubeadm/
[proxy]: /docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/

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