---
approvers:
- jbeda
title: Authenticating with Bootstrap Tokens
---

* TOC
{:toc}

## Overview

Bootstrap tokens are a simple bearer token that is meant to be used when
creating new clusters or joining new nodes to an existing cluster.  It was built
to support [`kubeadm`](/docs/admin/kubeadm/), but can be used in other contexts
for users that wish to start clusters without `kubeadm`. It is also built to
work, via RBAC policy, with the [Kubelet TLS
Bootstrapping](/docs/admin/kubelet-tls-bootstrapping/) system.

Bootstrap Tokens are defined with a specific type
(`bootstrap.kubernetes.io/token`) of secrets that lives in the `kube-system`
namespace.  These Secrets are then read by the Bootstrap Authenticator in the
API Server.  Expired tokens are removed with the TokenCleaner controller in the
Controller Manager.  The tokens are also used to create a signature for a
specific ConfigMap used in a "discovery" process through a BootstrapSigner
controller.

Currently, Bootstrap Tokens are **alpha** but there are no large breaking
changes expected.

## Token Format

Bootstrap Tokens take the form of `abcdef.0123456789abcdef`.  More formally,
they must match the regular expression `[a-z0-9]{6}\.[a-z0-9]{16}`.

The first part of the token is the "Token ID" and is considered public
information.  It is used when referring to a token without leaking the secret
part used for authentication. The second part is the "Token Secret" and should
only be shared with trusted parties.

## Enabling Bootstrap Tokens

All features for Bootstrap Tokens are disabled by default in Kubernetes v1.6.

You can enable the Bootstrap Token authenticator with the
`--experimental-bootstrap-token-auth` flag on the API server.  You can enable
the Bootstrap controllers by specifying them with the `--controllers` flag on the
controller manager with something like
`--controllers=*,tokencleaner,bootstrapsigner`.  This is done automatically when
using `kubeadm`.

Tokens are used in an HTTPS call as follows:

```http
Authorization: Bearer 07401b.f395accd246ae52d
```

## Bootstrap Token Secret Format

Each valid token is backed by a secret in the `kube-system` namespace.  You can
find the full design doc
[here](https://github.com/kubernetes/community/blob/{{page.githubbranch}}/contributors/design-proposals/bootstrap-discovery.md).

Here is what the secret looks like.  Note that `base64(string)` indicates the
value should be base64 encoded.  The undecoded version is provided here for
readability.

```yaml
apiVersion: v1
kind: Secret
metadata:
  name: bootstrap-token-07401b
  namespace: kube-system
type: bootstrap.kubernetes.io/token
data:
  description: base64(The default bootstrap token generated by 'kubeadm init'.)
  token-id: base64(07401b)
  token-secret: base64(f395accd246ae52d)
  expiration: base64(2017-03-10T03:22:11Z)
  usage-bootstrap-authentication: base64(true)
  usage-bootstrap-signing: base64(true)
```

The type of the secret must be `bootstrap.kubernetes.io/token` and the name must
be `bootstrap-token-<token id>`.  It must also exist in the `kube-system`
namespace.  `description` is a human readable description that should not be
used for machine readable information.  The Token ID and Secret are included in
the data dictionary.

The `usage-bootstrap-*` members indicate what this secret is intended to be used
for.  A value must be set to `true` to be enabled.

`usage-bootstrap-authentication` indicates that the token can be used to
authenticate to the API server.  The authenticator authenticates as
`system:bootstrap:<Token ID>`.  It is included in the `system:bootstrappers`
group.  The naming and groups are intentionally limited to discourage users from
using these tokens past bootstrapping.

`usage-bootstrap-signing` indicates that the token should be used to sign the
`cluster-info` ConfigMap as described below.

The `expiration` data member lists a time after which the token is no longer
valid.  This is encoded as an absolute UTC time using RFC3339.  The TokenCleaner
controller will delete expired tokens.

## Token Management with `kubeadm`

You can use the `kubeadm` tool to manage tokens on a running cluster.  It will
automatically grab the default admin credentials on a master from a `kubeadm`
created cluster (`/etc/kubernetes/admin.conf`).  You can specify an alternate
kubeconfig file for credentials with the `--kubeconfig` to the following
commands.

* `kubeadm token list` Lists the tokens along with when they expire and what the
  approved usages are.
* `kubeadm token create` Creates a new token.
    * `--description` Set the description on the new token.
    * `--ttl duration` Set expiration time of the token as a delta from "now".
      Default is 0 for no expiration.
    * `--usages` Set the ways that the token can be used.  The default is
      `signing,authentication`.  These are the usages as described above.
* `kubeadm token delete <token id>|<token id>.<token secret>` Delete a token.
  The token can either be identified with just an ID or with the entire token
  value.  Only the ID is used; the token is still deleted if the secret does not
  match.

## ConfigMap Signing

In addition to authentication, the tokens can be used to sign a ConfigMap.  This
is used early in a cluster bootstrap process before the client trusts the API
server.  The signed ConfigMap can be authenticated by the shared token.

The ConfigMap that is signed is `cluster-info` in the `kube-public` namespace.
The typical flow is that a client reads this ConfigMap while unauthenticated and
ignoring TLS errors.  It then validates the payload of the ConfigMap by looking
at a signature embedded in the ConfigMap.

The ConfigMap may look like this:

```yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: cluster-info
  namespace: kube-public
data:
  jws-kubeconfig-07401b: eyJhbGciOiJIUzI1NiIsImtpZCI6IjA3NDAxYiJ9..tYEfbo6zDNo40MQE07aZcQX2m3EB2rO3NuXtxVMYm9U
  kubeconfig: |
    apiVersion: v1
    clusters:
    - cluster:
        certificate-authority-data: <really long certificate data>
        server: https://10.138.0.2:6443
      name: ""
    contexts: []
    current-context: ""
    kind: Config
    preferences: {}
    users: []
```

The `kubeconfig` member of the ConfigMap is a config file with just the cluster
information filled out.  The key thing being communicated here is the
`certificate-authority-data`.  This may be expanded in the future.

The signature is a JWS signature using the "detached" mode.  To validate the
signature, the user should encode the `kubeconfig` payload according to JWS
rules (base64 encoded while discarding any trailing `=`).  That encoded payload
is then used to form a whole JWS by inserting it between the 2 dots.  You can
verify the JWS using the `HS256` scheme (HMAC-SHA256) with the full token (e.g.
`07401b.f395accd246ae52d`) as the shared secret.  Users _must_ verify that HS256
is used.
