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Agent plugin: WorkloadAttestor "k8s"

The k8s plugin generates kubernetes-based selectors for workloads calling the agent. It does so by retrieving the workload's pod ID from its cgroup membership, then querying the kubelet for information about the pod.

The plugin can talk to the kubelet via the insecure read-only port or the secure port. Both X509 client authentication and bearer token (e.g. service account token) authentication to the secure port is supported.

Verifying the certificate presented by the kubelet over the secure port is optional. The default is to verify, based on the certificate file passed via kubelet_ca_path. skip_kubelet_verification can be set to disable verification.

The agent will contact the kubelet using the node name obtained via the node_name_env or node_name configurables. If a node name is not obtained, the kubelet is contacted over 127.0.0.1 (requires host networking to be enabled). In the latter case, the hostname is used to perform certificate server name validation against the kubelet certificate.

Note kubelet authentication via bearer token requires that the kubelet be started with the --authentication-token-webhook flag. See Kubelet authentication/authorization for details.

Note The kubelet uses the TokenReview API to validate bearer tokens. This requires reachability to the Kubernetes API server. Therefore API server downtime can interrupt workload attestation. The --authentication-token-webhook-cache-ttl kubelet flag controls how long the kubelet caches TokenReview responses and may help to mitigate this issue. A large cache ttl value is not recommended however, as that can impact permission revocation.

Configuration Description
kubelet_read_only_port The kubelet read-only port. This is mutually exlusive with kubelet_secure_port.
kubelet_secure_port The kubelet secure port. It defaults to 10250 unless kubelet_read_only_port is set.
kubelet_ca_path The path on disk to a file containing CA certificates used to verify the kubelet certificate. Required unless skip_kubelet_verification is set. Defaults to the cluster CA bundle /run/secrets/kubernetes.io/serviceaccount/ca.crt.
skip_kubelet_verification If true, kubelet certificate verification is skipped
token_path The path on disk to the bearer token used for kubelet authentication. Defaults to the service account token /run/secrets/kubernetes.io/serviceaccount/token
certificate_path The path on disk to client certificate used for kubelet authentication
private_key_path The path on disk to client key used for kubelet authentication
node_name_env The environment variable used to obtain the node name. Defaults to MY_NODE_NAME.
node_name The name of the node. Overrides the value obtained by the environment variable specified by node_name_env.
Selector Value
k8s:ns The workload's namespace
k8s:sa The workload's service account
k8s:container-image The image of the workload's container
k8s:container-name The name of the workload's container
k8s:node-name The name of the workload's node
k8s:pod-label A label given to the the workload's pod
k8s:pod-owner The name of the workload's pod owner
k8s:pod-owner-uid The UID of the workload's pod owner
k8s:pod-uid The UID of the workload's pod

Examples

To use the kubelet read-only port:

WorkloadAttestor "k8s" {
  plugin_data {
    kubelet_read_only_port = 10255
  }
}

To use the secure kubelet port, verify via /run/secrets/kubernetes.io/serviceaccount/ca.crt, and authenticate via the default service account token:

WorkloadAttestor "k8s" {
  plugin_data {
  }
}

To use the secure kubelet port, skip verification, and authenticate via the default service account token:

WorkloadAttestor "k8s" {
  plugin_data {
    skip_kubelet_verification = true
  }
}

To use the secure kubelet port, skip verification, and authenticate via some other token:

WorkloadAttestor "k8s" {
  plugin_data {
    skip_kubelet_verification = true
    token_path = "/path/to/token"
  }
}

To use the secure kubelet port, verify the kubelet certificate, and authenticate via an X509 client certificate:

WorkloadAttestor "k8s" {
  plugin_data {
    kubelet_ca_path = "/path/to/kubelet-ca.pem"
    certificate_path = "/path/to/cert.pem"
    private_key_path = "/path/to/key.pem"
  }
}
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