CKA Practice Question: Cluster Architecture, Installation and Configuration
A new Kubernetes administrator runs 'kubeadm join --token <token> <control-plane-ip>:6443 --discovery-token-ca-cert-hash sha256:<hash>' on a worker node. The join fails with 'error execution phase preflight: couldn't validate the identity of the API Server'. What is the most likely cause?
⚠ Common exam trap
CNCF often tests the distinction between token expiration and CA hash mismatch, where candidates confuse a token-related error with a TLS validation error, but the specific phrase 'couldn't validate the identity of the API Server' directly points to the CA certificate hash being incorrect.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The --discovery-token-ca-cert-hash value is incorrect
The error 'couldn't validate the identity of the API Server' indicates that the CA certificate hash provided with --discovery-token-ca-cert-hash does not match the actual hash of the API server's CA certificate. This hash is used to verify the API server's identity during the TLS bootstrap process. An incorrect hash value will cause the preflight check to fail, as the worker node cannot confirm it is connecting to the legitimate control plane.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The --discovery-token-ca-cert-hash value is incorrect
Why this is correct
The --discovery-token-ca-cert-hash parameter provides a critical security measure by ensuring the joining node can securely verify the identity of the control plane's Certificate Authority. If this hash value is incorrect, the joining node cannot trust the CA certificate presented by the API server during the TLS handshake. This leads to the 'couldn't validate the identity' error, as the cryptographic proof of authenticity fails, preventing the secure establishment of communication.
- ✗
The token has expired
Why it's wrong here
A kubeadm join token is a temporary credential used for initial authentication, typically valid for 24 hours by default. If this token were to expire, the kubeadm command would typically return an error explicitly stating that the 'token is invalid or has expired.' This is distinct from an identity validation failure, which occurs even if the token is valid but the control plane's CA certificate cannot be trusted.
- ✗
The kubelet is not running on the worker node
Why it's wrong here
The kubelet is the primary agent that runs on each node, responsible for registering the node with the cluster and managing pods. If the kubelet service were not running on the worker node, kubeadm would likely fail during its preflight checks, reporting an error such as 'kubelet service not found' or indicating that the service is inactive. This issue would manifest earlier in the join process as a local service problem, not a remote identity validation failure.
- ✗
The API server is not reachable on port 6443
Why it's wrong here
Initial connectivity to the Kubernetes API server on the control plane is a fundamental requirement for any kubeadm join operation. If the API server were unreachable on its default port 6443, due to network issues or firewall rules, the kubeadm command would immediately report a connection error. Such errors typically manifest as 'connection refused,' 'connection timed out,' or 'no route to host,' indicating a failure to establish a network session, rather than a problem with identity validation after a connection is made.
Go deeper
Related to this question
Learn chapter
Kubernetes Architecture Overview
Key term
Network Policies
A Kubernetes resource that controls how pods communicate with each other and with other network endpoints, acting as a firewall for pod-to-pod traffic.
Key term
Ingress Resources
Ingress Resources are Kubernetes API objects that manage external access to services inside a cluster, typically HTTP and HTTPS traffic, by defining rules for routing requests based on hostnames and paths.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CKA practice question is part of Courseiva's free CNCF certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CKA exam.