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Kubernetes authentication through dex

Overview

This document covers setting up the Kubernetes OpenID Connect token authenticator plugin with dex.

Token responses from OpenID Connect providers include a signed JWT called an ID Token. ID Tokens contain names, emails, unique identifiers, and in dex's case, a set of groups that can be used to identify the user. OpenID Connect providers, like dex, publish public keys; the Kubernetes API server understands how to use these to verify ID Tokens.

The authentication flow looks like:

  1. OAuth2 client logs a user in through dex.
  2. That client uses the returned ID Token as a bearer token when talking to the Kubernetes API.
  3. Kubernetes uses dex's public keys to verify the ID Token.
  4. A claim designated as the username (and optionally group information) will be associated with that request.

Username and group information can be combined with Kubernetes authorization plugins, such as roles based access control (RBAC), to enforce policy.

Configuring the OpenID Connect plugin

Configuring the API server to use the OpenID Connect authentcation plugin requires:

  • Deploying an API server with specific flags.
  • Dex is running on HTTPS.
    • Custom CA files must be accessible by the API server (likely through volume mounts).
  • Dex is accessible to both your browser and the Kubernetes API server.

Use the following flags to point your API server(s) at dex. dex.example.com should be replaced by whatever DNS name or IP address dex is running under.

--oidc-issuer-url=https://dex.example.com:32000
--oidc-client-id=example-app
--oidc-ca-file=/etc/kubernetes/ssl/openid-ca.pem
--oidc-username-claim=email
--oidc-groups-claim=groups

Additional notes:

  • The API server configured with OpenID Connect flags doesn't require dex to be available upfront.
    • Other authenticators, such as client certs, can still be used.
    • Dex doesn't need to be running when you start your API server.
  • Kubernetes only trusts ID Tokens issued to a single client.
  • If a claim other than "email" is used for username, for example "sub", it will be prefixed by "(value of --oidc-issuer-url)#". This is to namespace user controlled claims which may be used for privilege escalation.

Deploying dex on Kubernetes

The dex repo contains scripts for running dex on a Kubernetes cluster with authentication through GitHub. The dex service is exposed using a node port on port 32000. This likely requires a custom /etc/hosts entry pointed at one of the cluster's workers.

There are many different ways to spin up a Kubernetes development cluster, each with different host requirements and support for API server reconfiguration. At this time, this guide does not have copy-pastable examples, but can recommend the following methods for spinning up a cluster:

To run dex on Kubernetes perform the following steps:

  1. Generate TLS assets for dex.
  2. Spin up a Kubernetes cluster with the appropriate flags and CA volume mount.
  3. Create a secret containing your GitHub OAuth2 client credentials.
  4. Deploy dex.

The TLS assets can be created using the following command:

$ cd examles/k8s
$ ./gencert.sh

The created ssl/ca.pem must then be mounted into your API server deployment. Once the cluster is up and correctly configured, use kubectl to add the serving certs as secrets.

$ kubectl create secret tls dex.example.com.tls --cert=ssl/cert.pem --key=ssl/key.pem

Then create a secret for the GitHub OAuth2 client.

$ kubectl create secret \
    generic github-client \
    --from-literal=client-id=$GITHUB_CLIENT_ID \
    --from-literal=client-secret=$GITHUB_CLIENT_SECRET

Finally, create the dex deployment, configmap, and node port service.

$ kubectl create -f dex.yaml

Caveats: No health checking is configured because dex does its own TLS termination complicating the setup. This is a known issue and can be tracked here.

Logging into the cluster

The example app can be used to log into the cluster. Choose the GitHub option and grant access to dex to view your profile.

$ ./bin/example-app --issuer https://dex.example.com:32000 --issuer-root-ca examples/k8s/ssl/ca.pem

The printed ID Token can then be used as a bearer token to authenticate against the API server.

$ token='(id token)'
$ curl -H "Authorization: Bearer $token" -k https://( API server host ):443/api/v1/nodes