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Kubernetes CSR Vault Controller

Kubernetes supports the approval and signing of x509 Certificate Signing Requests. This can be used internally by Kubernetes for things such as Kubelet client certificate rotation. Typically CSRs are signed by the controller-manager with a provided CA and key.

This project replaces that controller with a signer that uses Vault to sign the approved CSR objects. This allows Kubelet certificate rotation, and indeed all in cluster certificate signing to be delegated to Hashicorp Vault.

How it works

Controller

This controller uses much of the same code as the default Kubernetes CSR signer, the only difference is the function that performs the signing. For this is uses the sign-verbatim endpoint provided by the Vault PKI mount to sign the CSR.

Bootstrapping

Before nodes can rotate their own certs they must generate their initial certs, this is done using a bootstrap kubeconfig. Typically this uses a bootstap token, however certs are a valid option and allow for Vault to manage the access for bootstrapping nodes, not Kubernetes. This tool includes a command to generate a bootstrap kubeconfig which can be used to request the initial node certificate.

Another use case is creating node certificates as part of cluster bootstrap. If for example you are using bootkube then you need kublet to be running in order to bring up the tempoary control plane. But Kubelet will not start until it can connect to an APIServer and issue its initial node certs, creating a chicken and egg senareo. However if you generate Kubelets node certs using this tool (with the group system:nodes) on the node you are trying to bootstrap then you can avoid this and get a running Kubelet which can be used to bring up the tempoary control plane. This control plane can then handle the issuing of certificates for further Kubelets.

Requirements

This controller requires Vault 0.10.3 or greater to function. This is because it relies on the ability to specify "key usage" or "extended key usage" when using the sign-verbatim endpoint (hashicorp/vault#4777).

Installing

k8s-vault-csr can run in cluster or standalone. The fastest path is to run in cluster:

  • The default csrsigning controller first need disabling in the controller manager. This can be done through the command line flag --controllers, for example --controllers=-csrsigning
  • If you want to use kubernetes auth in vault then this needs setting up, the signer needs permission to call /pki/sign-verbatim/role where pki and role are the pki mount and role respectively.
  • Deploy kube-vault-signer and RBAC. See deploy.yaml for an example.

Docs

Autogenerated command docs can be found in the docs folder