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0094-kubernetes-node-joining.md

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Hugo Hervieux (hugo.hervieux@goteleport.com)
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RFD 0094 - Kubernetes node joining

Required approvers

  • Engineering: @r0mant && @gus
  • Product: @klizhentas && @xinding33
  • Security: @reedloden

What

This RFD discusses various methods for Teleport proxies to join a Teleport cluster in a same-cluster Kubernetes context and covers implementation details of the chosen approach.

Why

We want to refactor the teleport-cluster Helm chart to be a reference delivery method for Teleport. This involves deploying separately Teleport auth and proxy nodes. Proxy nodes have to register against the Teleport auth nodes without user interaction and without compromising the cluster security (no long-lived tokens). Proxy nodes can be automatically created or deleted (by a Kubernetes HPA for example).

User Motivation:

  • As new Teleport adopter, I want to deploy a working, secure and highly available Teleport cluster on Kubernetes, with a single Helm command and as few values as possible so that I can start using Teleport as fast as possible.
  • As a Teleport power user, I want to easily join my other co-hosted Teleport services (app, db, discovery, windows, Machine ID, ...) on Kubernetes so that I have less join automation to build and maintain.

Details

The first section of this RFD explores three different approaches on how to have proxies automatically join the cluster.

The next sections discuss implementation details and security/ux considerations of the chosen approach.

Exploring different approaches

Approach 1 - A new join method

In this approach we introduce a new kube join method relying on Kubernetes tokens to establish trust (like the ec2 and iam ones). Teleport tokens using this method can be long-lived with less security risk because they are not secret and rely on Kubernetes as a trusted third party.

graph LR
    helmChart
    serviceAccount
    token
    deployment
    pod
    cluster
    kubernetes

    helmChart --> serviceAccount
    helmChart --> deployment
    helmChart --> |create a new Kubernetes join token| cluster

    serviceAccount --> |on-demand| token
    token --> |mounted and bound to| pod
    deployment --> pod

    pod --> |join with SA token| cluster
    cluster --> |check SA token| kubernetes
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This approach is basically extending RFD 0050.

This brings the following challenges:

  • helmChart has to create a Teleport join token for its service account:
    • this can be done through the Teleport operator and a TeleportToken resource (we are due to implement tokens anyway to reach parity between the operator and the Terraform provider)
    • this can be done through a bootstrap resource on cluster creation
  • Kubernetes only supports (read enabled by default) short-lived renewed token after 1.20 (it depends on the Kubernetes distribution, but starting with 1.20 everyone must support it). For prior versions, users might have to use the legacy non-bound tokens produced by Kubernetes.

Approach 2 - MachineID-generated certs

In this approach, proxy certificates are delivered by MachineID.

flowchart LR
    cluster[Teleport cluster]
    tBot
    stateA
    podA
    stateB
    podB

    subgraph statefulset/deployment
    podA
    podB
    end

    cluster <-->|part of cluster| tBot
    tBot --> stateA
    tBot --> stateB
    stateA --- |uses| podA
    stateB --- |uses| podB
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This approach brings the following challenges:

  • each proxy requires multiple certs (one with the default teleport node role, the other with the proxy role)
  • teleport does not support joining from certs (unless we manually build a valid state)
  • tbot has to dynamically discover how many pods want certs and which certs they need (we don't have a streaming destination API yet)
  • pods should have a way to access their specific state:
    • we can't mount a different secret in each pod
    • it can be done through admission webhooks
    • pods can read secrets (this works OK for Teleport nodes, but cannot be generalized with other workloads like access plugin because this requires being able to read all certs from the namespace)
    • pods can request a cert to tBot over HTTP
  • tbot has to validate the identity of the pod requesting the certificate
  • tbot has to join the cluster initially, which is almost the same problem that we're trying to address (almost because we have less constraints due to autoscaling). The easiest approach would be to bundle tbot with auth nodes and use the local admin socket to bootstrap it, like what we're doing with the operator

Approach 3 - Operator/MachineID-generated short-lived tokens

In this approach, short-lived static tokens are generated by the Teleport operator and injected in containers using mutation webhooks.

sequenceDiagram
    StatefulSet->>Kubernetes: New pod
    Kubernetes->>+Operator: Mutation webhook
    Operator->>Teleport Cluster:New token, ttl = 2min
    Teleport Cluster->>Operator: Token
    Operator->>Kubernetes: Create secret with token
    Operator->>-Kubernetes: Mutated pod with secret mount
    Kubernetes->>Proxy: Start
    Note over Operator: Wait for token expiry
    Operator->>Kubernetes: Delete secret
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The operator internally relies on MachineID to get its identity, this solution is basically an automated version of MachineID token generation.

This approach brings the following challenges:

  • the operator becomes mandatory, which might not be a possibility for certain users (because of new additional RBAC)
  • the operator enters the critical path, if it is not available no pod can start
  • pods started before the mutation webhook is added will not get their token
  • there is a race condition if the proxy pod does not start before expiry, the failure mode is a pod stuck in crashloop and the only way out is to delete the pod. This is particularly likely to happen on clusters with node autoscaling or capacity issues.

Choosing the approach

The second approach tends to bring more issues than it solves. Even if we added MachineID support for Teleport nodes, this would not be used by customers with their Teleport workload.

The third approach adds new components in the critical path and new complex failure modes.

The first approach seems the more robust, leverages previous work with ProvisionTokens and provides very solid security guarantees for Kubernetes 1.20+. MachineID would also benefit from it as tBot would be easily deployed in a Kubernetes cluster hosting a Teleport cluster.

Implementation details

Note: the term token is confusing in the context of this RFD as it covers both Kubernetes-issued tokens and Teleport join tokens of type kube. In the following doc, "Kubernetes token" and "service account token" refer to a Kubernetes-issued token and "join token" refers to a Teleport join token of type kube.

If this RFD is accepted, the teleport documentation should reflect this nuance and using the term token alone should be avoided.

The feasibility of this approach has been tested against Multiple Kubernetes setups, including:

  • Kubernetes 1.18 on Minikube
  • Kubernetes 1.20 on EKS
  • Kubernetes 1.22 on GKE

The proof of concept code can be found here.

User flow

  • A teleport cluster is running in Kubernetes.
  • An operator (human or automated) wants to deploy a new set of Teleport proxies in the same cluster.
  • The operator creates a new kube join token, allowing the kubernetes service account new-proxies
  • The operator creates a Kubernetes service account new-proxies
  • The operator creates its new pods (with a Deployment, a StatefulSet, or even a DaemonSet) using the new-proxies SA
  • The Teleport proxies uses the mounted Kubernetes token to join the existing Teleport cluster
  • The Teleport cluster validates the token against Kubernetes using the TokenReview API
  • The proxies are part of the cluster

Kubernetes tokens

All Kubernetes tokens contain the service account name and its groups.

Versions 1.20 and above (bound tokens):

  • tokens are signed per pod
  • tokens are automatically renewed by the kubelet
  • token TTL is configurable in the PodSpec
  • tokens are revoked when the pod is deleted

Versions prior 1.20:

  • tokens are signed per service-account
  • tokens are long-lived
  • tokens are revoked when the service account is deleted

Tokens are JWTs, Kubernetes JWKS can be manually fetched and JWT validation can be done offline. Kubernetes also exposes a TokenReview API to validate tokens. This API performs more checks, like revocation, UUID checking pod and service account, ... This API is the sanest way to validate Kubernetes token.

Teleport join tokens

The teleport resource would look like:

kind: token
version: v2
metadata:
  name: proxy-token
  expires: "3000-01-01T00:00:00Z"
spec:
  roles: ["Proxy"]
  k8s:
    allow:
    - service_account: "my-namespace:my-service-account"

The allow[*].service_accountcheck should match exactly the User field of the Kubernetes token stripped of the system:serviceaccounts: prefix. Checking if the system:serviceaccounts prefix is present ensures no Kubernetes user tokens can be used to join the Teleport cluster.

Supporting this token kind does not require changes to the RegisterUsingTokenRequest type, the Kubernetes token should be passed through the IDToken field and the Teleport token name should be passed in the Token field. The join logic would be similar to what is done with GitHub joining.

Teleport should validate on token creation if it can access the TokenReview API by reviewing its own Kubernetes token. This will allow to fail fast on broken or not supported setups.

Supporting external cluster joining

The token spec could be extended in a second phase to support trusting other Kubernetes apiservers than the one where the auth is running. This will imply adding fields to the ProvisionToken to specify how Teleport should authenticate to Kubernetes. As a large portion of clusters are hosted by cloud providers, we will likely need to support the biggest cloud authentication methods.

The token would look like this:

kind: token
version: v2
metadata:
  name: proxy-token
  expires: "3000-01-01T00:00:00Z"
spec:
  roles: ["Proxy"]
  k8s:
    allow:
      - service_account: "my-namespace:my-service-account"
    # empty authType defaults to "inCluster" for compatibility
    authType: "inCluster|kubeconfig|gcp|aws|azure|rancher|..."
    kubeconfig:
      certificate-authority-data: base64
      server: http://1.2.3.4
      # either set token or user && client-*
      token: my-secret-token
      user: client-sa-name
      client-certificate-data: base64
      client-key-data: base64
    gcp:
      # optional SA account key/name
      # if they are empty, teleport should try to use ambient credentials (see google.DefaultTokenSource)
      service-account-key: base64
      service-account-name: "foo@sa-project.iam.gserviceaccount.com"
      # mandatory fields to identify the cluster
      project: cluster-project
      location: us-east1
      cluster-name: my-cluster
    aws:
      # optional SA account id/secret
      # if they are empty, Teleport should try to use ambient credentials
      key-id: ASDFGHJK
      key-secret: QWERTYUIOP
      # mandatory
      region: us-east-1
      account-id: 1234567890
      cluster-name: my-cluster
    azure:
    # [...]
    rancher:
    # [...]

For cloud providers like GCP or AWS, users will have to make sure the cloud service-account is mapped to a Kubernetes group bound to a ClusterRole allowing to use the TokenReview API.

As this setup can be complex, Teleport should validate on token creation it has access to the TokenReview API. It can do so by reviewing its own Kubernetes token.

Security considerations

Bound tokens mounted through projected volumes offer a solid level of security as those tokens are short-lived, automatically renewed and are revoked on the pod deletion. Those tokens are the pod's identity from a Kubernetes point of view.

Legacy tokens offer less security guarantees as they are long-lived and not automatically rotated. Joining using them is still an improvement over static tokens because the Kubernetes-issued token doesn't need to leave the cluster by design: human operators and the CI/CD system should never be in contact with the token. Legacy tokens are mostly a user experience improvement rather than a security improvement.

The Teleport Helm charts should always favor using bound tokens. Kubernetes support for bound tokens must be detected at the deploy time and the chart must adapt its behaviour to maximize security based on Kubernetes capabilities.

To mitigate token stealing or replaying MiTM attack, the joining Teleport nodes should be configured with the ca_pins, as for regular tokens.

Introducing the feature does not add new critical components that could cause availability issues. Creating new pods already requires a healthy apiserver. If the apiserver were to fail, this would not affect already joined proxies.

Implications of adding the feature

While joining Teleport proxy nodes is the main motivation of this proposal, other Teleport nodes would also benefit from this additional join method.

Deploying nodes in the same Kubernetes cluster will become easier. This can be used to join all kinds of Teleport service (kubernetes, app, db, windows desktop, discovery), but also to join tbot and offer an easier Machine ID integration in Kubernetes. This would pave the way for a better experience for users hosting access plugins next to the Teleport control plane.

graph LR
    subgraph cluster helm chart
    cluster
    tBot

    subgraph accessPlugin chart
    accessPlugin
    end

    cluster --- |joined using the 'kube' method| tBot
    tBot --> |store certs| secret
    secret --- |mounted| accessPlugin

    accessPlugin --> |connects with its MachineID certs| cluster
    end
Loading

Joining nodes from outside the Kubernetes cluster would imply generating and exporting a Kubernetes token from the service account. While this pattern is feasible, it does not provide much added value compared to generating a static token from Teleport directly.