Authenticating with OpenID Connect (OIDC)

Learn how to configure GKE on-prem to use an OpenID Connect (OIDC) for authentication to user clusters. This topic covers the process in general to help you understand how to configure any OpenID provider.

For an overview of the GKE on-prem authentication flow, see Authentication. See the following topics to learn how to configure OIDC with other OpenID providers:

• Active Directory Federation Services
• Google

Overview

GKE on-prem supports OIDC as one of the authentication mechanisms for interacting with a user cluster's Kubernetes API server. With OIDC, you can manage access to Kubernetes clusters by using the standard procedures in your organization for creating, enabling, and disabling user accounts.

There are two ways that users can authorize their accounts:

• Use the Google Cloud CLI to initiate the OIDC flow and obtain user authorization through a browser-based consent page.

• Use Google Cloud console to initiate the OIDC authentication flow.

Before you begin

• This topic assumes you are familiar with the following authentication and OpenID concepts:

  • OAuth 2.0
  • OpenID Connect
  • scopes
  • claims.

• Headless systems are unsupported. A browser-based authentication flow is used to prompt you for consent and authorize your user account.

• To authenticate through the Google Cloud console, each cluster that you want to configure for OIDC authentication must be registered with Google Cloud.

Personas

This topic refers to three personas:

• Organization administrator: This person chooses an OpenID provider and registers client applications with the provider.

• Cluster administrator: This person creates one or more user clusters and creates authentication configuration files for developers who use the clusters.

• Developer: This person runs workloads on one or more clusters and uses OIDC to authenticate.

Choosing an OpenID provider

This section is for organization administrators.

You can use any OpenID provider of your choice. For a list of certified providers, see OpenID Certification.

For the following OpenID providers, see specific configuration steps in the following topics:

• Active Directory Federated Services

• Google

Creating redirect URLs

This section is for organization administrators.

You must create redirect URLs for both the gcloud CLI and Google Cloud console that the OpenID provider can use to return ID tokens.

gcloud CLI redirect URL

Google Cloud CLI is installed on each developer's local machine and includes the gcloud CLI. You can specify a port number greater than 1024 to use for the redirect URL:

http://localhost:[PORT]/callback

where [PORT] is your port number.

When you configure your OpenID provider, specify http://localhost:[PORT]/callback as one of your redirect URLs. How you do this depends on your provider.

Google Cloud console redirect URL

The redirect URL for Google Cloud console is:

https://console.cloud.google.com/kubernetes/oidc

When you configure your OIDC provider, specify https://console.cloud.google.com/kubernetes/oidc as one of your redirect URLs. How you do this depends on your provider.

Registering your client applications with the OpenID provider

This section is for organization administrators.

Before your developers can use the gcloud CLI or Google Cloud console with your OpenID provider, you need to register those two clients with the OpenID provider. Registration includes these steps:

• Learn the provider's issuer URI. This is where the gcloud CLI or Google Cloud console sends authentication requests.

• Give the provider the redirect URL for the gcloud CLI.

• Give the provider the redirect URL for Google Cloud console. This is https://console.cloud.google.com/kubernetes/oidc.

• Establish a single client ID. This is the ID that the provider uses to identify both the gcloud CLI and Google Cloud console.

• Establish a single client secret. The gcloud CLI and Google Cloud console both use this secret to authenticate to the OpenID provider.

• Establish a custom scope that the gcloud CLI or Google Cloud console can use to request the user's security groups.

• Establish a custom claim name that the provider will use to return the user's security groups.

How you perform these steps depends on your OpenID provider.

Populating the oidc specification in GKE on-prem configuration file

This section is for cluster administrators.

Before you create a user cluster, you generate a GKE on-prem configuration file using gkectl create-config. The configuration includes the following oidc specification. You must populate oidc with the values specific to your provider:

oidc:
  issuerurl:
  kubectlredirecturl:
  clientid:
  clientsecret:
  username:
  usernameprefix:
  group:
  groupprefix:
  scopes:
  extraparams:
  usehttpproxy:
  capath:

• issuerurl: Required. URL of your OpenID provider, such as "https://example.com/adfs". Client applications, like the gcloud CLI and Google Cloud console, send authorization requests to this URL. The Kubernetes API server uses this URL to discover public keys for verifying tokens. Must use HTTPS.
• kubectlredirecturl: Required. The redirect URL that you configured previously for the gcloud CLI.
• clientid: Required. ID for the client application that makes authentication requests to the OpenID provider. Both the gcloud CLI and Google Cloud console use this ID.
• clientsecret: Optional. Secret for the client application. Both the gcloud CLI and Google Cloud console use this secret.
• username: Optional. JWT claim to use as the username. The default is sub, which is expected to be a unique identifier of the end user. You can choose other claims, such as email or name, depending on the OpenID provider. However, claims other than email are prefixed with the issuer URL to prevent naming clashes with other plugins.
• usernameprefix: Optional. Prefix prepended to username claims to prevent clashes with existing names. If you do not provide this field, and username is a value other than email, the prefix defaults to issuerurl#. You can use the value - to disable all prefixing.
• group: Optional. JWT claim that the provider will use to return your security groups.
• groupprefix: Optional. Prefix prepended to group claims to prevent clashes with existing names. For example, given a group foobar and a prefix gid-, gid-foobar. By default, this value is empty, and there is no prefix.
• scopes: Optional. Additional scopes to send to the OpenID provider as a comma-delimited list.
  • For authentication with Microsoft Azure or Okta, pass in offline_access.

• extraparams: Optional. Additional key-value parameters to send to the OpenID provider as a comma-delimited list.
  • For a list of authentication parameters, see Authentication URI parameters
  • If you are authorizing a group, pass in resource=token-groups-claim.
  • If your authorization server prompts for consent, pass in prompt=consent.
• usehttpproxy: Optional. Specifies whether to deploy a reverse proxy in the cluster to allow Google Cloud console access to the on-premises OIDC provider for authenticating users. Value must be a string: "true" or "false". If your identity provider is not reachable over the public internet, and you wish to authenticate using Google Cloud console, then this field must be set to "true". If left blank, this field defaults to "false".
• capath: Optional. Path to the certificate for the certificate authority (CA) that issued your identity provider's web certificate. This value might not be necessary. For example, if your identity provider's certificate was issued by a well-known public CA, then you would not need to provide a value here. However, if usehttpproxy is "true", then this value must be provided, even for a well-known public CA.

Example: Authorizing users and groups

Many providers encode user-identifying properties, such as email and user IDs, in a token. However, these properties have implicit risks for authentication policies:

• User IDs can make policies difficult to read and audit.
• Emails can create both an availability risk (if a user changes their primary email) and potentially a security risk (if an email can be re-assigned).

Therefore, it's a best practice to use group policies, as a group ID can be both persistent and easier to audit.

Suppose your provider creates identity tokens that include the following fields:

{
  'iss': 'https://server.example.com'
  'sub': 'u98523-4509823'
  'groupList': ['developers@example.corp', 'us-east1-cluster-admins@example.corp']
  ...
}

Given this token format, you'd populate your configuration file's oidc specification like so:

issueruri: 'https://server.example.com'
username: 'sub'
usernameprefix: 'uid-'
group: 'groupList'
groupprefix: 'gid-'
extraparams: 'resource=token-groups-claim'
...

After you've created the user cluster, you could then use Kubernetes role-based access control (RBAC) to grant privileged access to the authenticated users. For example, you could create a ClusterRole that grants its users read-only access to the cluster's Secrets, and create a ClusterRoleBinding resource to bind the role to the authenticated group:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: secret-reader
rules:
- apiGroups: [""]
  # The resource type for which access is granted
  resources: ["secrets"]
  # The permissions granted by the ClusterRole
  verbs: ["get", "watch", "list"]

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: read-secrets-admins
subjects:
  # Allows anyone in the "us-east1-cluster-admins" group to
  # read Secrets in any namespace within this cluster.
- kind: Group
  name: gid-us-east1-cluster-admins # Name is case sensitive
  apiGroup: rbac.authorization.k8s.io
  # Allows this specific user to read Secrets in any
  # namespace within this cluster
- kind: User
  name: uid-u98523-4509823
  apiGroup: rbac.authorization.k8s.io
roleRef:
  kind: ClusterRole
  name: secret-reader
  apiGroup: rbac.authorization.k8s.io

Creating and distributing the authentication configuration file

This section is for cluster administrators.

After you create a user cluster, you create an authentication configuration file for that cluster. You can configure multiple clusters in a single authentication configuration file. You must provide each authentication configuration file to the users who want to authenticate with each of those clusters.

Creating the authentication configuration file

To create the authentication configuration file in the current directory, run the following gkectl command:

gkectl create-login-config --kubeconfig [USER_CLUSTER_KUBECONFIG]

where [USER_CLUSTER_KUBECONFIG] is the path of your user cluster's kubeconfig file. When you ran gkectl create cluster to create your user cluster, your kubeconfig file was created.

Result: Your authentication configuration file, named kubectl-anthos-config.yaml, is created in the current directory.

Adding multiple clusters to the authentication configuration file

You can store the authentication configuration details for multiple clusters within a single authentication configuration file.

You can use the following command to merge additional user cluster authentication details into an existing authentication configuration file. Given an existing authentication configuration file, you can either merge or combine additional user cluster authentication details:

• Merge the additional user cluster authentication details into that existing authentication configuration file.
• Combine the additional user cluster authentication details into a new file.

For example, you might need to manage both the anthos-config-1cluster.yaml and anthos-config-3clusters.yaml authentication configuration files to accommodate the access needs of the multiple user groups in your organization.

To add additional user clusters to your existing authentication configuration file:

1. Ensure that each cluster has a unique name. If your clusters have the same names, you cannot combine them into the same authentication configuration file. Note that after a cluster is created, that cluster cannot be renamed.

2. Run the following gkectl command to merge or combine configuration details:

   gkectl create-login-config --kubeconfig [USER_CLUSTER_KUBECONFIG] \
   --merge-from [IN_AUTH_CONFIG_FILE] --output [OUT_AUTH_CONFIG_FILE]

   where

   • [USER_CLUSTER_KUBECONFIG] specifies the kubeconfig file of the user cluster that you want to add.

   • [IN_AUTH_CONFIG_FILE] specifies the path of the existing authentication configuration file that you want to merge with the additional cluster information.

   • [OUT_AUTH_CONFIG_FILE] specifies the path of the file where you want to store the merged authentication configuration:

     • Specify the same file as [IN_AUTH_CONFIG_FILE] to merge the additional cluster information into that existing file.
     • Specify a new path and filename to combine the authentication configuration details into a new file.

Distributing the authentication configuration file

To enable your users to authenticate against your user clusters, you must provide them with access to one or more of the authentication configuration files that you created. Note that the following steps use the default file name and the location that are expected by the gcloud CLI. For information about using alternate file names and locations, see Custom configuration.

Consider distributing the authentication configuration files by:

• Hosting the file at an accessible URL. If you include the --login-config flag in the gcloud anthos auth login command, the gcloud CLI obtains the authentication configuration file from that location.

  Consider hosting the file on a secure host. See the --login-config-cert flag of the gcloud CLI for more information about using PEM certificates for secure HTTPS access.

• Manually providing the file to each user. After users download the file, you must instruct them about how to store the file in the default location and with the default filename that the gcloud CLI expects.

  For example, users can run the following commands to store the authentication configuration file with the default kubectl-anthos-config.yaml filename and in the default location:

  Linux

  mkdir -p  $HOME/.config/google/anthos/
  cp [AUTH_CONFIG_FILE] $HOME/.config/google/anthos/kubectl-anthos-config.yaml

  where [AUTH_CONFIG_FILE] is the name of your authentication configuration file. For example kubectl-anthos-config.yaml.

  macOS

  mkdir -p  $HOME/Library/Preferences/google/anthos/
  cp [AUTH_CONFIG_FILE] $HOME/Library/Preferences/google/anthos/kubectl-anthos-config.yaml

  where [AUTH_CONFIG_FILE] is the name of your authentication configuration file. For example kubectl-anthos-config.yaml.

  Windows

  md "%APPDATA%\google\anthos"
  copy [AUTH_CONFIG_FILE] "%APPDATA%\google\anthos\kubectl-anthos-config.yaml"

  where [AUTH_CONFIG_FILE] is the name of your authentication configuration file. For example kubectl-anthos-config.yaml.

• Using your internal tools to push the authentication configuration file onto each user's machine. For example, you could use your tooling to push files using the default kubectl-anthos-config.yaml filename into their default locations on each user's machine:

  Linux

  $HOME/.config/google/anthos/kubectl-anthos-config.yaml

  macOS

  $HOME/Library/Preferences/google/anthos/kubectl-anthos-config.yaml

  Windows

  %APPDATA%\google\anthos\kubectl-anthos-config.yaml

Custom configuration

The gcloud CLI expects the authentication configuration file to be stored in the default location and with the default filename kubectl-anthos-config.yaml as mentioned in the prior section. However, you have the option to rename or store your authentication configuration file in an alternate location. If the file's name and location differ from the default, you must append the --login-config flag to each command that you run when you authenticate with the cluster. The extra command flag passes in the custom path and filename. To learn more about the command flag, see Authenticating through the gcloud CLI.

Installing the gcloud CLI

This section is for both cluster administrators and developers.

Each developer or user who needs to authenticate with a cluster, must install Google Cloud CLI on their own machine. The Anthos authentication commands have been integrated into the gcloud CLI as the anthos-auth component.

• If you have an old version of "Anthos Plugin for Kubectl", you must uninstall that plugin before installing the gcloud CLI and anthos-auth component.

• If you have an existing version of the gcloud CLI, install the latest version and the anthos-auth component.

Removing old plugins

You must uninstall the old plugin before you can use the anthos-auth component of gcloud CLI. You can check whether one of the past kubectl based plugins exists on your machine by running the following command:

kubectl anthos version

• If the command responds with Error: unknown command "anthos" for "kubectl", no plugin was found and you can skip to the next section.

• If a 1.0beta version of the plugin was found, you must locate the plugin binary and delete it. Run the following command to list the location and then use that location to remove the binary from your machine:

  kubectl plugin list

Installing gcloud CLI and the gcloud CLI

To install the gcloud CLI you must first first install gcloud CLI:

1. Install gcloud CLI but skip the gcloud init command.

2. Run the following commands to install the anthos-auth component:

   gcloud components update
   gcloud components install anthos-auth

3. Verify that the gcloud CLI was installed successfully by running either of the following commands:

   gcloud anthos auth
   gcloud anthos auth login

   Result: Each command should respond with details about the required arguments and available options.

Obtaining the authentication configuration file

This section is for developers.

Your administrator is responsible for creating your authentication configuration file and then providing it to you. For more details, see Distributing the authentication configuration file.

By default, the gcloud CLI uses a default filename and storage location for your authentication configuration file. If you were manually provided the file and need to save it on your machine, use the defaults to simplify your gcloud authentication commands.

Use the following commands to copy the authentication configuration file to the default location:

mkdir -p  $HOME/.config/google/anthos/
cp [AUTH_CONFIG_FILE] $HOME/.config/google/anthos/kubectl-anthos-config.yaml

mkdir -p  $HOME/Library/Preferences/google/anthos/
cp [AUTH_CONFIG_FILE] $HOME/Library/Preferences/google/anthos/kubectl-anthos-config.yaml

md "%APPDATA%\google\anthos"
copy [AUTH_CONFIG_FILE] "%APPDATA%\google\anthos\kubectl-anthos-config.yaml"

If you choose to use a different filename or location, you have the option to include the --login-config flag with each of your authentication requests. See the following section for details about using the gcloud anthos auth login command.

Authenticating with user clusters

This section is for developers.

Now that gcloud CLI is installed on your machine and the authentication configuration file has been provided to you by your administrator, you can use either the gcloud CLI or the Google Cloud console to authenticate with your clusters.

Authenticating through the gcloud CLI

Run gcloud commands to authenticate with your clusters:

1. Run the gcloud anthos auth login command to initiate the authentication flow:

   gcloud anthos auth login \
    --cluster [CLUSTER_NAME] \
    --user [USER_NAME] \
    --login-config [AUTH_CONFIG_FILE_PATH] \
    --login-config-cert [CA_CERT_PEM_FILE] \
    --kubeconfig [USER_CLUSTER_KUBECONFIG]

   where:

   • [CLUSTER_NAME] (optional) specifies the name of your user cluster. If this flag is omitted, you are prompted to choose from the user clusters that are specified in your authentication configuration file.

   • [USER_NAME] (optional) specifies the username for the credentials stored in the kubeconfig file. The default value is [CLUSTER_NAME]-anthos-default-user.

   • [AUTH_CONFIG_FILE_PATH] (optional) specifies the custom path or URL to where your authentication configuration file is stored or hosted. You can omit this parameter, if the file is in the default location. Example: --login-config /path/to/custom/authentication-config.yaml

   • [CA_CERT_PEM_FILE] (optional) specifies the path to a PEM certificate file from your CA. If your authentication configuration file is hosted securely, you can use an HTTPS connection to access the file. Example: --login-config-cert my-cert.pem

   • [USER_CLUSTER_KUBECONFIG] (optional) specifies the custom path to your user cluster's kubeconfig file. The OIDC ID tokens that are returned by your OpenID provider are stored in the kubeconfig file.

     Use this flag if your kubeconfig file resides in a location other than the default. If this flag is omitted, a new kubeconfig file is created in the default location. Example: --kubeconfig /path/to/custom.kubeconfig

   Examples:

   • Authenticate to specific cluster:

     gcloud anthos auth login --cluster my-production-cluster
     

   • Use a prompt to select which cluster to authenticate with:

     gcloud anthos auth login
     

     Result:

     Please use the --cluster flag to specify a cluster from the list below:
     Source: $HOME/.config/google/anthos/kubectl-anthos-config.yaml
     1. Cluster: test-cluster ServerIP: https://192.168.0.1:6443
     2. Cluster: test-cluster-2 ServerIP: https://192.168.0.2:6443
     3. Cluster: my-production-cluster ServerIP: https://192.168.0.3:6443
     

   • Use a hosted authentication configuration file:

     gcloud anthos auth login \
      --cluster my-production-cluster \
      --login-config HTTPS://my-secure-server/kubectl-anthos-config.yaml \
      --login-config-cert my-cert.pem
     

2. Enter your credentials in the browser-based consent screen that opens.

3. Verify that authentication was successful by running one of the kubectl commands to retrieve details about your cluster. For example:

   kubectl get nodes --kubeconfig [USER_CLUSTER_KUBECONFIG]

Result: Your kubeconfig file now contains an ID token that your kubectl commands will use to authenticate with the Kubernetes API server on your user cluster.

Using SSH to authenticate from a remote machine

Suppose you want to SSH into a remote machine and authenticate to a user cluster from the remote machine. To do this, your authentication configuration file must be on the remote machine, and you must be able to reach your Open ID provider from your local machine.

On your local machine, run the following command:

ssh [USER_NAME]@[REMOTE_MACHINE] -L [LOCAL_PORT]:localhost:[REMOTE_PORT]

where:

• [USER_NAME] and [REMOTE_MACHINE] are the standard values used to log in with SSH.

• [LOCAL_PORT] is an open port of your choice on your local machine that you will use to access the remote machine.

• [REMOTE_PORT] is the port you configured for your OIDC redirect URL. You can find this in the kubectlRedirectURI field of your authentication configuration file.

In your SSH shell, run the following command to initiate authentication:

gcloud anthos auth login --login-config [AUTH_CONFIG_FILE]

where [AUTH_CONFIG_FILE] is the path of your authentication configuration file on the remote machine.

On your local machine, in a browser, go to http://localhost:[LOCAL_PORT]/login and complete the OIDC login flow.

Now the kubeconfig file on your remote machine has the token that you need to access the user cluster.

In your SSH shell, verify that you have access to the user cluster:

kubectl --kubeconfig [USER_CLUSTER_KUBECONFIG] get nodes

Authenticating through the Google Cloud console

Initiate the authentication flow from the Kubernetes clusters page in the Google Cloud console:

1. Open the Google Cloud console:

   Visit the Kubernetes clusters page

2. Locate your GKE on-prem cluster in the list and then click Login.

3. Select Authenticate with the Identity Provider configured for the cluster, and then click LOGIN.

   You are redirected to your identity provider, where you might need to log in or consent to Google Cloud console accessing your account. Then you are redirected back to the Kubernetes clusters page in Google Cloud console.

Troubleshooting your OIDC configuration

Review the following behaviors and errors to help resolve your OIDC issues:

Invalid configuration

If Google Cloud console cannot read the OIDC configuration from your cluster, the LOGIN button will be disabled.

Invalid provider configuration

If your identity provider configuration is invalid, you will see an error screen from your identity provider after you click LOGIN. Follow the provider-specific instructions to correctly configure the provider or your cluster.

Invalid permissions

If you complete the authentication flow, but still don't see the details of the cluster, make sure you granted the correct RBAC permissions to the account that you used with OIDC. Note that this might be a different account from the one you use to access Google Cloud console.

Error: missing 'RefreshToken' field in 'OAuth2Token' in credentials struct

You might get this error if the authorization server prompts for consent, but the required authentication parameter wasn't provided. Provide the prompt=consent parameter to GKE on-prem configuration file's oidc: extraparams field, and regenerate the client authentication file with the --extra-params prompt=consent flag.

What's next

• Learn more about scopes and claims.