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Authenticating

This page provides an overview of authenticating.

• Users in Kubernetes
• Authentication strategies
• Anonymous requests
• User impersonation
• client-go credential plugins

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. Normal 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 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:

• Username: a string which identifies the end user. Common values might be kube-admin or jane@example.com.
• UID: a string which identifies the end user and attempts to be more consistent and unique than username.
• Groups: a set of strings which associate users with a set of commonly grouped users.
• Extra fields: a map of strings to list of strings which holds additional information authorizers may find useful.

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:

• service account tokens for service accounts
• at least one other method for user authentication.

When multiple authenticator modules are enabled, the first 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.

Integrations with other authentication protocols (LDAP, SAML, Kerberos, alternate x509 schemes, etc) can be accomplished using an authenticating proxy or the authentication webhook.

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 certificate 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 Managing Certificates 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 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

Bootstrap Tokens

This feature is currently in beta.

To allow for streamlined bootstrapping for new clusters, Kubernetes includes a dynamically-managed Bearer token type called a Bootstrap Token. These tokens are stored as Secrets in the kube-system namespace, where they can be dynamically managed and created. Controller Manager contains a TokenCleaner controller that deletes bootstrap tokens as they expire.

The tokens are of the form [a-z0-9]{6}.[a-z0-9]{16}. The first component is a Token ID and the second component is the Token Secret. You specify the token in an HTTP header as follows:

Authorization: Bearer 781292.db7bc3a58fc5f07e

You must enable the Bootstrap Token Authenticator with the --enable-bootstrap-token-auth flag on the API Server. You must enable the TokenCleaner controller via the --controllers flag on the Controller Manager. This is done with something like --controllers=*,tokencleaner. kubeadm will do this for you if you are using it to bootstrap a cluster.

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. The user names and group can be used (and are used by kubeadm) to craft the appropriate authorization policies to support bootstrapping a cluster.

Please see Bootstrap Tokens for in depth documentation on the Bootstrap Token authenticator and controllers along with how to manage these tokens with kubeadm.

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 is a csv file with a minimum of 3 columns: password, user name, user id. In Kubernetes version 1.6 and later, you can specify an optional fourth column containing comma-separated group names. If you have more than one group, you must enclose the fourth column value in double quotes (“). See the following example:

password,user,uid,"group1,group2,group3"

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

A service account is an automatically enabled authenticator that uses signed bearer tokens to verify requests. The plugin takes two optional flags:

• --service-account-key-file A file containing a PEM encoded key for signing bearer tokens. If unspecified, the API server’s TLS private key will be used.
• --service-account-lookup If enabled, tokens which are deleted from the API will be revoked.

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: apps/v1 # this apiVersion is relevant as of Kubernetes 1.9
kind: Deployment
metadata:
  name: nginx-deployment
  namespace: default
spec:
  replicas: 3
  template:
    metadata:
    # ...
    spec:
      serviceAccountName: bob-the-bot
      containers:
      - name: nginx
        image: nginx:1.14.2

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

Check an associated secret:

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)
  namespace: ZGVmYXVsdA==
  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.

1. Login to your identity provider
2. Your identity provider will provide you with an access_token, id_token and a refresh_token
3. When using kubectl, use your id_token with the --token flag or add it directly to your kubeconfig
4. kubectl sends your id_token in a header called Authorization to the API server
5. The API server will make sure the JWT signature is valid by checking against the certificate named in the configuration
6. Check to make sure the id_token hasn’t expired
7. Make sure the user is authorized
8. Once authorized the API server returns a response to kubectl
9. kubectl provides feedback to the user

Since all of the data needed to validate who you are is in the id_token, Kubernetes doesn’t need to “phone home” to the identity provider. In a model where every request is stateless this provides a very scalable solution for authentication. It does offer a few challenges:

1. Kubernetes has no “web interface” to trigger the authentication process. There is no browser or interface to collect credentials which is why you need to authenticate to your identity provider first.
2. The id_token can’t be revoked, it’s like a certificate so it should be short-lived (only a few minutes) so it can be very annoying to have to get a new token every few minutes.
3. There’s no easy way to authenticate to the Kubernetes dashboard without using the kubectl proxy command or a reverse proxy that injects the id_token.

Configuring the API Server

To enable the plugin, configure the following flags on the API server:

Importantly, the API server is not an OAuth2 client, rather it can only be configured to trust a single issuer. This allows the use of public providers, such as Google, without trusting credentials issued to third parties. Admins who wish to 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.

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, CloudFoundry UAA, or Tremolo Security’s OpenUnison.

For an identity provider to work with Kubernetes it must:

1. Support OpenID connect discovery; not all do.
2. Run in TLS with non-obsolete ciphers
3. Have a CA signed certificate (even if the CA is not a commercial CA or is self signed)

A note about requirement #3 above, requiring a CA signed certificate. If you deploy your own identity provider (as opposed to one of the cloud providers like Google or Microsoft) you MUST have your identity provider’s web server certificate signed by a certificate with the CA flag set to TRUE, even if it is self signed. This is due to GoLang’s TLS client implementation being very strict to the standards around certificate validation. If you don’t have a CA handy, you can use this script from the CoreOS team to create a simple CA and a signed certificate and key pair. Or you can use this similar script that generates SHA256 certs with a longer life and larger key size.

Setup instructions for specific systems:

• UAA
• Dex
• OpenUnison

Using kubectl

Option 1 - OIDC Authenticator

The first option is to use the kubectl oidc authenticator, which sets the id_token as a bearer token for all requests and refreshes the token once it expires. After you’ve logged into your provider, use kubectl to add your id_token, refresh_token, client_id, and client_secret to configure the plugin.

Providers that don’t return an id_token as part of their refresh token response aren’t supported by this plugin and should use “Option 2” below.

kubectl config set-credentials USER_NAME \
   --auth-provider=oidc \
   --auth-provider-arg=idp-issuer-url=( issuer url ) \
   --auth-provider-arg=client-id=( your client id ) \
   --auth-provider-arg=client-secret=( your client secret ) \
   --auth-provider-arg=refresh-token=( your refresh token ) \
   --auth-provider-arg=idp-certificate-authority=( path to your ca certificate ) \
   --auth-provider-arg=id-token=( your id_token )

As an example, running the below command after authenticating to your identity provider:

kubectl config set-credentials mmosley  \
        --auth-provider=oidc  \
        --auth-provider-arg=idp-issuer-url=https://oidcidp.tremolo.lan:8443/auth/idp/OidcIdP  \
        --auth-provider-arg=client-id=kubernetes  \
        --auth-provider-arg=client-secret=1db158f6-177d-4d9c-8a8b-d36869918ec5  \
        --auth-provider-arg=refresh-token=q1bKLFOyUiosTfawzA93TzZIDzH2TNa2SMm0zEiPKTUwME6BkEo6Sql5yUWVBSWpKUGphaWpxSVAfekBOZbBhaEW+VlFUeVRGcluyVF5JT4+haZmPsluFoFu5XkpXk5BXqHega4GAXlF+ma+vmYpFcHe5eZR+slBFpZKtQA= \
        --auth-provider-arg=idp-certificate-authority=/root/ca.pem \
        --auth-provider-arg=id-token=eyJraWQiOiJDTj1vaWRjaWRwLnRyZW1vbG8ubGFuLCBPVT1EZW1vLCBPPVRybWVvbG8gU2VjdXJpdHksIEw9QXJsaW5ndG9uLCBTVD1WaXJnaW5pYSwgQz1VUy1DTj1rdWJlLWNhLTEyMDIxNDc5MjEwMzYwNzMyMTUyIiwiYWxnIjoiUlMyNTYifQ.eyJpc3MiOiJodHRwczovL29pZGNpZHAudHJlbW9sby5sYW46ODQ0My9hdXRoL2lkcC9PaWRjSWRQIiwiYXVkIjoia3ViZXJuZXRlcyIsImV4cCI6MTQ4MzU0OTUxMSwianRpIjoiMm96US15TXdFcHV4WDlHZUhQdy1hZyIsImlhdCI6MTQ4MzU0OTQ1MSwibmJmIjoxNDgzNTQ5MzMxLCJzdWIiOiI0YWViMzdiYS1iNjQ1LTQ4ZmQtYWIzMC0xYTAxZWU0MWUyMTgifQ.w6p4J_6qQ1HzTG9nrEOrubxIMb9K5hzcMPxc9IxPx2K4xO9l-oFiUw93daH3m5pluP6K7eOE6txBuRVfEcpJSwlelsOsW8gb8VJcnzMS9EnZpeA0tW_p-mnkFc3VcfyXuhe5R3G7aa5d8uHv70yJ9Y3-UhjiN9EhpMdfPAoEB9fYKKkJRzF7utTTIPGrSaSU6d2pcpfYKaxIwePzEkT4DfcQthoZdy9ucNvvLoi1DIC-UocFD8HLs8LYKEqSxQvOcvnThbObJ9af71EwmuE21fO5KzMW20KtAeget1gnldOosPtz1G5EwvaQ401-RPQzPGMVBld0_zMCAwZttJ4knw

Which would produce the below configuration:

users:
- name: mmosley
  user:
    auth-provider:
      config:
        client-id: kubernetes
        client-secret: 1db158f6-177d-4d9c-8a8b-d36869918ec5
        id-token: eyJraWQiOiJDTj1vaWRjaWRwLnRyZW1vbG8ubGFuLCBPVT1EZW1vLCBPPVRybWVvbG8gU2VjdXJpdHksIEw9QXJsaW5ndG9uLCBTVD1WaXJnaW5pYSwgQz1VUy1DTj1rdWJlLWNhLTEyMDIxNDc5MjEwMzYwNzMyMTUyIiwiYWxnIjoiUlMyNTYifQ.eyJpc3MiOiJodHRwczovL29pZGNpZHAudHJlbW9sby5sYW46ODQ0My9hdXRoL2lkcC9PaWRjSWRQIiwiYXVkIjoia3ViZXJuZXRlcyIsImV4cCI6MTQ4MzU0OTUxMSwianRpIjoiMm96US15TXdFcHV4WDlHZUhQdy1hZyIsImlhdCI6MTQ4MzU0OTQ1MSwibmJmIjoxNDgzNTQ5MzMxLCJzdWIiOiI0YWViMzdiYS1iNjQ1LTQ4ZmQtYWIzMC0xYTAxZWU0MWUyMTgifQ.w6p4J_6qQ1HzTG9nrEOrubxIMb9K5hzcMPxc9IxPx2K4xO9l-oFiUw93daH3m5pluP6K7eOE6txBuRVfEcpJSwlelsOsW8gb8VJcnzMS9EnZpeA0tW_p-mnkFc3VcfyXuhe5R3G7aa5d8uHv70yJ9Y3-UhjiN9EhpMdfPAoEB9fYKKkJRzF7utTTIPGrSaSU6d2pcpfYKaxIwePzEkT4DfcQthoZdy9ucNvvLoi1DIC-UocFD8HLs8LYKEqSxQvOcvnThbObJ9af71EwmuE21fO5KzMW20KtAeget1gnldOosPtz1G5EwvaQ401-RPQzPGMVBld0_zMCAwZttJ4knw
        idp-certificate-authority: /root/ca.pem
        idp-issuer-url: https://oidcidp.tremolo.lan:8443/auth/idp/OidcIdP
        refresh-token: q1bKLFOyUiosTfawzA93TzZIDzH2TNa2SMm0zEiPKTUwME6BkEo6Sql5yUWVBSWpKUGphaWpxSVAfekBOZbBhaEW+VlFUeVRGcluyVF5JT4+haZmPsluFoFu5XkpXk5BXq
      name: oidc

Once your id_token expires, kubectl will attempt to refresh your id_token using your refresh_token and client_secret storing the new values for the refresh_token and id_token in your .kube/config.

Option 2 - Use the --token Option

The kubectl command lets you pass in a token using the --token option. Simply copy and paste the id_token into this option:

kubectl --token=eyJhbGciOiJSUzI1NiJ9.eyJpc3MiOiJodHRwczovL21sYi50cmVtb2xvLmxhbjo4MDQzL2F1dGgvaWRwL29pZGMiLCJhdWQiOiJrdWJlcm5ldGVzIiwiZXhwIjoxNDc0NTk2NjY5LCJqdGkiOiI2RDUzNXoxUEpFNjJOR3QxaWVyYm9RIiwiaWF0IjoxNDc0NTk2MzY5LCJuYmYiOjE0NzQ1OTYyNDksInN1YiI6Im13aW5kdSIsInVzZXJfcm9sZSI6WyJ1c2VycyIsIm5ldy1uYW1lc3BhY2Utdmlld2VyIl0sImVtYWlsIjoibXdpbmR1QG5vbW9yZWplZGkuY29tIn0.f2As579n9VNoaKzoF-dOQGmXkFKf1FMyNV0-va_B63jn-_n9LGSCca_6IVMP8pO-Zb4KvRqGyTP0r3HkHxYy5c81AnIh8ijarruczl-TK_yF5akjSTHFZD-0gRzlevBDiH8Q79NAr-ky0P4iIXS8lY9Vnjch5MF74Zx0c3alKJHJUnnpjIACByfF2SCaYzbWFMUNat-K1PaUk5-ujMBG7yYnr95xD-63n8CO8teGUAAEMx6zRjzfhnhbzX-ajwZLGwGUBT4WqjMs70-6a7_8gZmLZb2az1cZynkFRj2BaCkVT3A2RrjeEwZEtGXlMqKJ1_I2ulrOVsYx01_yD35-rw get nodes

Webhook Token Authentication

Webhook authentication is a hook for verifying bearer tokens.

• --authentication-token-webhook-config-file a configuration file describing how to access the remote webhook service.
• --authentication-token-webhook-cache-ttl how long to cache authentication decisions. Defaults to two minutes.

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

# Kubernetes API version
apiVersion: v1
# kind of the API object
kind: Config
# 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 POSTs a JSON-serialized authentication.k8s.io/v1beta1 TokenReview object containing the token to the remote service. 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 POST 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 status 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.

• --requestheader-username-headers Required, case-insensitive. Header names to check, in order, for the user identity. The first header containing a value is used as the username.
• --requestheader-group-headers 1.6+. Optional, case-insensitive. “X-Remote-Group” is suggested. Header names to check, in order, for the user’s groups. All values in all specified headers are used as group names.
• --requestheader-extra-headers-prefix 1.6+. Optional, case-insensitive. “X-Remote-Extra-” is suggested. Header prefixes to look for to determine extra information about the user (typically used by the configured authorization plugin). Any headers beginning with any of the specified prefixes have the prefix removed. The remainder of the header name is lowercased and percent-decoded and becomes the extra key, and the header value is the extra value.

Note: Prior to 1.11.3 (and 1.10.7, 1.9.11), the extra key could only contain characters which were legal in HTTP header labels.

For example, with this configuration:

--requestheader-username-headers=X-Remote-User
--requestheader-group-headers=X-Remote-Group
--requestheader-extra-headers-prefix=X-Remote-Extra-

this request:

GET / HTTP/1.1
X-Remote-User: fido
X-Remote-Group: dogs
X-Remote-Group: dachshunds
X-Remote-Extra-Acme.com%2Fproject: some-project
X-Remote-Extra-Scopes: openid
X-Remote-Extra-Scopes: profile

would result in this user info:

name: fido
groups:
- dogs
- dachshunds
extra:
  acme.com/project:
  - some-project
  scopes:
  - openid
  - profile

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. WARNING: do not reuse a CA that is used in a different context unless you understand the risks and the mechanisms to protect the CA’s usage.

• --requestheader-client-ca-file Required. PEM-encoded certificate bundle. A valid client certificate must be presented and validated against the certificate authorities in the specified file before the request headers are checked for user names.
• --requestheader-allowed-names Optional. List of Common Name values (CNs). If set, a valid client certificate with a CN in the specified list must be presented before the request headers are checked for user names. If empty, any CN is allowed.

Anonymous requests

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.

In 1.5.1-1.5.x, anonymous access is disabled by default, and can be enabled by passing the --anonymous-auth=true option to the API server.

In 1.6+, anonymous access is enabled by default if an authorization mode other than AlwaysAllow is used, and can be disabled by passing the --anonymous-auth=false option to the API server. Starting in 1.6, the ABAC and RBAC authorizers require explicit authorization of the system:anonymous user or the system:unauthenticated group, so legacy policy rules that grant access to the * user or * group do not include anonymous users.

User impersonation

A user can act as another user through impersonation headers. These let requests manually override the user info a request authenticates as. For example, an admin could use this feature to debug an authorization policy by temporarily impersonating another user and seeing if a request was denied.

Impersonation requests first authenticate as the requesting user, then switch to the impersonated user info.

• A user makes an API call with their credentials and impersonation headers.
• API server authenticates the user.
• API server ensures the authenticated users have impersonation privileges.
• Request user info is replaced with impersonation values.
• Request is evaluated, authorization acts on impersonated user info.

The following HTTP headers can be used to performing an impersonation request:

• Impersonate-User: The username to act as.
• Impersonate-Group: A group name to act as. Can be provided multiple times to set multiple groups. Optional. Requires “Impersonate-User”
• Impersonate-Extra-( extra name ): A dynamic header used to associate extra fields with the user. Optional. Requires “Impersonate-User”. In order to be preserved consistently, ( extra name ) should be lower-case, and any characters which aren’t legal in HTTP header labels MUST be utf8 and percent-encoded.

Note: Prior to 1.11.3 (and 1.10.7, 1.9.11), ( extra name ) could only contain characters which were legal in HTTP header labels.

An example set of headers:

Impersonate-User: jane.doe@example.com
Impersonate-Group: developers
Impersonate-Group: admins
Impersonate-Extra-dn: cn=jane,ou=engineers,dc=example,dc=com
Impersonate-Extra-acme.com%2Fproject: some-project
Impersonate-Extra-scopes: view
Impersonate-Extra-scopes: development

When using kubectl set the --as flag to configure the Impersonate-User header, set the --as-group flag to configure the Impersonate-Group header.

kubectl drain mynode

Error from server (Forbidden): User "clark" cannot get nodes at the cluster scope. (get nodes mynode)

Set the --as and --as-group flag:

kubectl drain mynode --as=superman --as-group=system:masters

node/mynode cordoned
node/mynode drained

To impersonate a user, group, or set extra fields, the impersonating user must have the ability to perform the “impersonate” verb on the kind of attribute being impersonated (“user”, “group”, etc.). For clusters that enable the RBAC authorization plugin, the following ClusterRole encompasses the rules needed to set user and group impersonation headers:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: impersonator
rules:
- apiGroups: [""]
  resources: ["users", "groups", "serviceaccounts"]
  verbs: ["impersonate"]

Extra fields are evaluated as sub-resources of the resource “userextras”. To allow a user to use impersonation headers for the extra field “scopes,” a user should be granted the following role:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: scopes-impersonator
rules:
# Can set "Impersonate-Extra-scopes" header.
- apiGroups: ["authentication.k8s.io"]
  resources: ["userextras/scopes"]
  verbs: ["impersonate"]

The values of impersonation headers can also be restricted by limiting the set of resourceNames a resource can take.

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: limited-impersonator
rules:
# Can impersonate the user "jane.doe@example.com"
- apiGroups: [""]
  resources: ["users"]
  verbs: ["impersonate"]
  resourceNames: ["jane.doe@example.com"]

# Can impersonate the groups "developers" and "admins"
- apiGroups: [""]
  resources: ["groups"]
  verbs: ["impersonate"]
  resourceNames: ["developers","admins"]

# Can impersonate the extras field "scopes" with the values "view" and "development"
- apiGroups: ["authentication.k8s.io"]
  resources: ["userextras/scopes"]
  verbs: ["impersonate"]
  resourceNames: ["view", "development"]

client-go credential plugins

k8s.io/client-go and tools using it such as kubectl and kubelet are able to execute an external command to receive user credentials.

This feature is intended for client side integrations with authentication protocols not natively supported by k8s.io/client-go (LDAP, Kerberos, OAuth2, SAML, etc.). The plugin implements the protocol specific logic, then returns opaque credentials to use. Almost all credential plugin use cases require a server side component with support for the webhook token authenticator to interpret the credential format produced by the client plugin.

Example use case

In a hypothetical use case, an organization would run an external service that exchanges LDAP credentials for user specific, signed tokens. The service would also be capable of responding to webhook token authenticator requests to validate the tokens. Users would be required to install a credential plugin on their workstation.

To authenticate against the API:

• The user issues a kubectl command.
• Credential plugin prompts the user for LDAP credentials, exchanges credentials with external service for a token.
• Credential plugin returns token to client-go, which uses it as a bearer token against the API server.
• API server uses the webhook token authenticator to submit a TokenReview to the external service.
• External service verifies the signature on the token and returns the user’s username and groups.

Configuration

Credential plugins are configured through kubectl config files as part of the user fields.

apiVersion: v1
kind: Config
users:
- name: my-user
  user:
    exec:
      # Command to execute. Required.
      command: "example-client-go-exec-plugin"

      # API version to use when decoding the ExecCredentials resource. Required.
      #
      # The API version returned by the plugin MUST match the version listed here.
      #
      # To integrate with tools that support multiple versions (such as client.authentication.k8s.io/v1alpha1),
      # set an environment variable or pass an argument to the tool that indicates which version the exec plugin expects.
      apiVersion: "client.authentication.k8s.io/v1beta1"

      # Environment variables to set when executing the plugin. Optional.
      env:
      - name: "FOO"
        value: "bar"

      # Arguments to pass when executing the plugin. Optional.
      args:
      - "arg1"
      - "arg2"
clusters:
- name: my-cluster
  cluster:
    server: "https://172.17.4.100:6443"
    certificate-authority: "/etc/kubernetes/ca.pem"
contexts:
- name: my-cluster
  context:
    cluster: my-cluster
    user: my-user
current-context: my-cluster

Relative command paths are interpreted as relative to the directory of the config file. If KUBECONFIG is set to /home/jane/kubeconfig and the exec command is ./bin/example-client-go-exec-plugin, the binary /home/jane/bin/example-client-go-exec-plugin is executed.

- name: my-user
  user:
    exec:
      # Path relative to the directory of the kubeconfig
      command: "./bin/example-client-go-exec-plugin"
      apiVersion: "client.authentication.k8s.io/v1beta1"

Input and output formats

The executed command prints an ExecCredential object to stdout. k8s.io/client-go authenticates against the Kubernetes API using the returned credentials in the status.

When run from an interactive session, stdin is exposed directly to the plugin. Plugins should use a TTY check to determine if it’s appropriate to prompt a user interactively.

To use bearer token credentials, the plugin returns a token in the status of the ExecCredential.

{
  "apiVersion": "client.authentication.k8s.io/v1beta1",
  "kind": "ExecCredential",
  "status": {
    "token": "my-bearer-token"
  }
}

Alternatively, a PEM-encoded client certificate and key can be returned to use TLS client auth. If the plugin returns a different certificate and key on a subsequent call, k8s.io/client-go will close existing connections with the server to force a new TLS handshake.

If specified, clientKeyData and clientCertificateData must both must be present.

clientCertificateData may contain additional intermediate certificates to send to the server.

{
  "apiVersion": "client.authentication.k8s.io/v1beta1",
  "kind": "ExecCredential",
  "status": {
    "clientCertificateData": "-----BEGIN CERTIFICATE-----\n...\n-----END CERTIFICATE-----",
    "clientKeyData": "-----BEGIN RSA PRIVATE KEY-----\n...\n-----END RSA PRIVATE KEY-----"
  }
}

Optionally, the response can include the expiry of the credential formatted as a RFC3339 timestamp. Presence or absence of an expiry has the following impact:

• If an expiry is included, the bearer token and TLS credentials are cached until the expiry time is reached, or if the server responds with a 401 HTTP status code, or when the process exits.

• If an expiry is omitted, the bearer token and TLS credentials are cached until the server responds with a 401 HTTP status code or until the process exits.

  {
  "apiVersion": "client.authentication.k8s.io/v1beta1",
  "kind": "ExecCredential",
  "status": {
  "token": "my-bearer-token",
  "expirationTimestamp": "2018-03-05T17:30:20-08:00"
  }
  }

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