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Users in Kubernetes

All Kubernetes clusters have two categories of users: service accounts managed by Kubernetes, and normal users.

Normal users are assumed to be managed by an outside, independent service. An admin distributing private keys, a user store like Keystone or Google Accounts, even a file with a list of usernames and passwords. In this regard, Kubernetes does not have objects which represent normal user accounts. Regular users cannot be added to a cluster through an API call.

In contrast, service accounts are users managed by the Kubernetes API. They are bound to specific namespaces, and created automatically by the API server or manually through API calls. Service accounts are tied to a set of credentials stored as Secrets, which are mounted into pods allowing in cluster processes to talk to the Kubernetes API.

API requests are tied to either a normal user or a service account, or are treated as anonymous requests. This means every process inside or outside the cluster, from a human user typing kubectl on a workstation, to kubelets on nodes, to members of the control plane, must authenticate when making requests to the the API server, or be treated as an anonymous user.

Authentication strategies

Kubernetes uses client certificates, bearer tokens, an authenticating proxy, or HTTP basic auth to authenticate API requests through authentication plugins. As HTTP requests are made to the API server, plugins attempt to associate the following attributes with the request:

All values are opaque to the authentication system and only hold significance when interpreted by an authorizer.

You can enable multiple authentication methods at once. You should usually use at least two methods:

When multiple are enabled, the first authenticator module to successfully authenticate the request short-circuits evaluation. The API server does not guarantee the order authenticators run in.

The system:authenticated group is included in the list of groups for all authenticated users.

X509 Client Certs

Client certificate authentication is enabled by passing the --client-ca-file=SOMEFILE option to API server. The referenced file must contain one or more certificates authorities to use to validate client certificates presented to the API server. If a client certificate is presented and verified, the common name of the subject is used as the user name for the request. As of Kubernetes 1.4, client certificates can also indicate a user’s group memberships using the certificate’s organization fields. To include multiple group memberships for a user, include multiple organization fields in the certificate.

For example, using the openssl command line tool to generate a certificate signing request:

openssl req -new -key jbeda.pem -out jbeda-csr.pem -subj "/CN=jbeda/O=app1/O=app2"

This would create a CSR for the username “jbeda”, belonging to two groups, “app1” and “app2”.

See APPENDIX for how to generate a client cert.

Static Token File

The API server reads bearer tokens from a file when given the --token-auth-file=SOMEFILE option on the command line. Currently, tokens last indefinitely, and the token list cannot be changed without restarting API server.

The token file format is implemented in plugin/pkg/auth/authenticator/token/tokenfile/... and is a csv file with a minimum of 3 columns: token, user name, user uid, followed by optional group names. Note, if you have more than one group the column must be double quoted e.g.

token,user,uid,"group1,group2,group3"

Putting a Bearer Token in a Request

When using bearer token authentication from an http client, the API server expects an Authorization header with a value of Bearer THETOKEN. The bearer token must be a character sequence that can be put in an HTTP header value using no more than the encoding and quoting facilities of HTTP. For example: if the bearer token is 31ada4fd-adec-460c-809a-9e56ceb75269 then it would appear in an HTTP header as shown below.

Authorization: Bearer 31ada4fd-adec-460c-809a-9e56ceb75269

Static Password File

Basic authentication is enabled by passing the --basic-auth-file=SOMEFILE option to API server. Currently, the basic auth credentials last indefinitely, and the password cannot be changed without restarting API server. Note that basic authentication is currently supported for convenience while we finish making the more secure modes described above easier to use.

The basic auth file format is implemented in plugin/pkg/auth/authenticator/password/passwordfile/... and is a csv file with 3 columns: password, user name, user id.

password,user,uid

When using basic authentication from an http client, the API server expects an Authorization header with a value of Basic BASE64ENCODED(USER:PASSWORD).

Service Account Tokens

Service accounts are an automatically enabled authenticator that uses signed bearer tokens to verify requests. The plugin takes two optional flags:

Service accounts are usually created automatically by the API server and associated with pods running in the cluster through the ServiceAccount Admission Controller. Bearer tokens are mounted into pods at well known locations, and allow in cluster processes to talk to the API server. Accounts may be explicitly associated with pods using the serviceAccountName field of a PodSpec.

NOTE: serviceAccountName is usually omitted because this is done automatically.

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: nginx-deployment
  namespace: default
spec:
  replicas: 3
  template:
    metadata:
    # ...
    spec:
      containers:
      - name: nginx
        image: nginx:1.7.9
        serviceAccountName: bob-the-bot

Service account bearer tokens are perfectly valid to use outside the cluster and can be used to create identities for long standing jobs that wish to talk to the Kubernetes API. To manually create a service account, simply use the kubectl create serviceaccount (NAME) command. This creates a service account in the current namespace and an associated secret.

$ kubectl create serviceaccount jenkins
serviceaccount "jenkins" created
$ kubectl get serviceaccounts jenkins -o yaml
apiVersion: v1
kind: ServiceAccount
metadata:
  # ...
secrets:
- name: jenkins-token-1yvwg

The created secret holds the public CA of the API server and a signed JSON Web Token (JWT).

$ kubectl get secret jenkins-token-1yvwg -o yaml
apiVersion: v1
data:
  ca.crt: (APISERVER'S CA BASE64 ENCODED)
  token: (BEARER TOKEN BASE64 ENCODED)
kind: Secret
metadata:
  # ...
type: kubernetes.io/service-account-token

Note: values are base64 encoded because secrets are always base64 encoded.

The signed JWT can be used as a bearer token to authenticate as the given service account. See above for how the token is included in a request. Normally these secrets are mounted into pods for in-cluster access to the API server, but can be used from outside the cluster as well.

Service accounts authenticate with the username system:serviceaccount:(NAMESPACE):(SERVICEACCOUNT), and are assigned to the groups system:serviceaccounts and system:serviceaccounts:(NAMESPACE).

WARNING: Because service account tokens are stored in secrets, any user with read access to those secrets can authenticate as the service account. Be cautious when granting permissions to service accounts and read capabilities for secrets.

OpenID Connect Tokens

OpenID Connect is a flavor of OAuth2 supported by some OAuth2 providers, notably Azure Active Directory, Salesforce, and Google. The protocol’s main extension of OAuth2 is an additional field returned with the access token called an ID Token. This token is a JSON Web Token (JWT) with well known fields, such as a user’s email, signed by the server.

To identify the user, the authenticator uses the id_token (not the access_token) from the OAuth2 token response as a bearer token. See above for how the token is included in a request.

To enable the plugin, pass the following required flags:

Importantly, the API server is not an OAuth2 client, rather it can only be configured to trust a single client. This allows the use of public providers, such as Google, without trusting credentials issued to third parties. Admins who wish utilize multiple OAuth clients should explore providers which support the azp (authorized party) claim, a mechanism for allowing one client to issue tokens on behalf of another.

The plugin also accepts the following optional flags:

And experimental flags:

Kubernetes does not provide an OpenID Connect Identity Provider. You can use an existing public OpenID Connect Identity Provider (such as Google, or others). Or, you can run your own Identity Provider, such as CoreOS dex, Keycloak or CloudFoundry UAA.

The provider needs to support OpenID connect discovery; not all do.

Setup instructions for specific systems:

Webhook Token Authentication

Webhook authentication is a hook for verifying bearer tokens.

The configuration file uses the kubeconfig file format. Within the file “users” refers to the API server webhook and “clusters” refers to the remote service. An example would be:

# clusters refers to the remote service.
clusters:
  - name: name-of-remote-authn-service
    cluster:
      certificate-authority: /path/to/ca.pem         # CA for verifying the remote service.
      server: https://authn.example.com/authenticate # URL of remote service to query. Must use 'https'.

# users refers to the API server's webhook configuration.
users:
  - name: name-of-api-server
    user:
      client-certificate: /path/to/cert.pem # cert for the webhook plugin to use
      client-key: /path/to/key.pem          # key matching the cert

# kubeconfig files require a context. Provide one for the API server.
current-context: webhook
contexts:
- context:
    cluster: name-of-remote-authn-service
    user: name-of-api-sever
  name: webhook

When a client attempts to authenticate with the API server using a bearer token as discussed above, the authentication webhook queries the remote service with a review object containing the token. Kubernetes will not challenge a request that lacks such a header.

Note that webhook API objects are subject to the same versioning compatibility rules as other Kubernetes API objects. Implementers should be aware of looser compatibility promises for beta objects and check the “apiVersion” field of the request to ensure correct deserialization. Additionally, the API server must enable the authentication.k8s.io/v1beta1 API extensions group (--runtime-config=authentication.k8s.io/v1beta1=true).

The request body will be of the following format:

{
  "apiVersion": "authentication.k8s.io/v1beta1",
  "kind": "TokenReview",
  "spec": {
    "token": "(BEARERTOKEN)"
  }
}

The remote service is expected to fill the TokenAccessReviewStatus field of the request to indicate the success of the login. The response body’s “spec” field is ignored and may be omitted. A successful validation of the bearer token would return:

{
  "apiVersion": "authentication.k8s.io/v1beta1",
  "kind": "TokenReview",
  "status": {
    "authenticated": true,
    "user": {
      "username": "janedoe@example.com",
      "uid": "42",
      "groups": [
        "developers",
        "qa"
      ],
      "extra": {
        "extrafield1": [
          "extravalue1",
          "extravalue2"
        ]
      }
    }
  }
}

An unsuccessful request would return:

{
  "apiVersion": "authentication.k8s.io/v1beta1",
  "kind": "TokenReview",
  "status": {
    "authenticated": false
  }
}

HTTP status codes can be used to supply additional error context.

Authenticating Proxy

The API server can be configured to identify users from request header values, such as X-Remote-User. It is designed for use in combination with an authenticating proxy, which sets the request header value. In order to prevent header spoofing, the authenticating proxy is required to present a valid client certificate to the API server for validation against the specified CA before the request headers are checked.

Keystone Password

Keystone authentication is enabled by passing the --experimental-keystone-url=<AuthURL> option to the API server during startup. The plugin is implemented in plugin/pkg/auth/authenticator/password/keystone/keystone.go and currently uses basic auth to verify used by username and password.

For details on how to use keystone to manage projects and users, refer to the Keystone documentation. Please note that this plugin is still experimental, under active development, and likely to change in subsequent releases.

Please refer to the discussion, blueprint and proposed changes for more details.

Anonymous requests

Anonymous access is enabled by default, and can be disabled by passing --anonymous-auth=false option to the API server during startup.

When enabled, requests that are not rejected by other configured authentication methods are treated as anonymous requests, and given a username of system:anonymous and a group of system:unauthenticated.

For example, on a server with token authentication configured, and anonymous access enabled, a request providing an invalid bearer token would receive a 401 Unauthorized error. A request providing no bearer token would be treated as an anonymous request.

If you rely on authentication alone to authorize access, either change to use an authorization mode other than AlwaysAllow, or set --anonymous-auth=false.

Plugin Development

We plan for the Kubernetes API server to issue tokens after the user has been (re)authenticated by a bedrock authentication provider external to Kubernetes. We also plan to make it easy to develop modules that interface between Kubernetes and a bedrock authentication provider (e.g. github.com, google.com, enterprise directory, kerberos, etc.)

APPENDIX

Creating Certificates

When using client certificate authentication, you can generate certificates using an existing deployment script or manually through easyrsa or openssl.

Using an Existing Deployment Script

Using an existing deployment script is implemented at cluster/saltbase/salt/generate-cert/make-ca-cert.sh.

Execute this script with two parameters. The first is the IP address of API server. The second is a list of subject alternate names in the form IP:<ip-address> or DNS:<dns-name>.

The script will generate three files: ca.crt, server.crt, and server.key.

Finally, add the following parameters into API server start parameters:

easyrsa

easyrsa can be used to manually generate certificates for your cluster.

  1. Download, unpack, and initialize the patched version of easyrsa3.

      curl -L -O https://storage.googleapis.com/kubernetes-release/easy-rsa/easy-rsa.tar.gz
  tar xzf easy-rsa.tar.gz
  cd easy-rsa-master/easyrsa3
  ./easyrsa init-pki

  2. Generate a CA. (--batch set automatic mode. --req-cn default CN to use.)

      ./easyrsa --batch "--req-cn=${MASTER_IP}@`date +%s`" build-ca nopass

  3. Generate server certificate and key. (build-server-full [filename]: Generate a keypair and sign locally for a client or server)

      ./easyrsa --subject-alt-name="IP:${MASTER_IP}" build-server-full kubernetes-master nopass
  4. Copy pki/ca.crt, pki/issued/kubernetes-master.crt, and pki/private/kubernetes-master.key to your directory.

  5. Fill in and add the following parameters into the API server start parameters:

      --client-ca-file=/yourdirectory/ca.crt
  --tls-cert-file=/yourdirectory/server.cert
  --tls-private-key-file=/yourdirectory/server.key

openssl

openssl can also be use to manually generate certificates for your cluster.

  1. Generate a ca.key with 2048bit:

      openssl genrsa -out ca.key 2048

  2. According to the ca.key generate a ca.crt (use -days to set the certificate effective time):

      openssl req -x509 -new -nodes -key ca.key -subj "/CN=${MASTER_IP}" -days 10000 -out ca.crt

  3. Generate a server.key with 2048bit

      openssl genrsa -out server.key 2048

  4. According to the server.key generate a server.csr:

      openssl req -new -key server.key -subj "/CN=${MASTER_IP}" -out server.csr

  5. According to the ca.key, ca.crt and server.csr generate the server.crt:

      openssl x509 -req -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt -days 10000

  6. View the certificate.

      openssl x509  -noout -text -in ./server.crt

Finally, do not forget to fill out and add the same parameters into the API server start parameters.

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