- A
Use iptables on each node to allow incoming connections to port 10250 only from the cluster's CIDR (e.g., 10.0.0.0/8).
Restricting source IPs to the cluster network reduces exposure while allowing legitimate kubelet API calls from the control plane and other nodes.
- B
Configure a Calico GlobalNetworkPolicy to block inbound traffic to port 10250 on all nodes.
Why wrong: Calico network policies operate at the pod level, not on host ports like kubelet. This would not affect host network traffic.
- C
Change the kubelet port to a non-standard port (e.g., 10260) and update all kubelet configurations.
Why wrong: Changing the port requires updating kube-apiserver and other components, causing potential disruption and complexity.
- D
Set the --anonymous-auth flag to false on each kubelet and restart them one by one.
Why wrong: Disabling anonymous auth does not block network access; authenticated requests from compromised nodes could still reach the kubelet.
CKS Cluster Setup and Hardening Practice Question
This CKS practice question tests your understanding of cluster setup and hardening. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
You are a security engineer for a financial services company running a Kubernetes cluster on-premises. The cluster uses kubeadm for bootstrapping and Calico for network policy. Recently, a compliance audit revealed that all nodes in the cluster have the kubelet port 10250 open to the public network, allowing unauthenticated access to the kubelet API. This poses a severe security risk. The cluster has 10 worker nodes and 3 control plane nodes. You need to remediate this without disrupting running workloads. The nodes are behind a corporate firewall, but the internal network is considered untrusted. You have access to the node's iptables and can modify configuration files. Which course of action best secures the kubelet port while maintaining cluster functionality?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"best"Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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
Use iptables on each node to allow incoming connections to port 10250 only from the cluster's CIDR (e.g., 10.0.0.0/8).
Option A is correct because using iptables to restrict access to port 10250 to only the cluster's internal CIDR (e.g., 10.0.0.0/8) directly addresses the audit finding by blocking unauthenticated public access while allowing necessary kubelet API communication from within the cluster. This approach does not disrupt running workloads, as it only modifies firewall rules without restarting kubelet or changing its configuration. It leverages the existing network infrastructure (iptables) and is consistent with the principle of least privilege for an untrusted internal network.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use iptables on each node to allow incoming connections to port 10250 only from the cluster's CIDR (e.g., 10.0.0.0/8).
Why this is correct
Restricting source IPs to the cluster network reduces exposure while allowing legitimate kubelet API calls from the control plane and other nodes.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Configure a Calico GlobalNetworkPolicy to block inbound traffic to port 10250 on all nodes.
Why it's wrong here
Calico network policies operate at the pod level, not on host ports like kubelet. This would not affect host network traffic.
- ✗
Change the kubelet port to a non-standard port (e.g., 10260) and update all kubelet configurations.
Why it's wrong here
Changing the port requires updating kube-apiserver and other components, causing potential disruption and complexity.
- ✗
Set the --anonymous-auth flag to false on each kubelet and restart them one by one.
Why it's wrong here
Disabling anonymous auth does not block network access; authenticated requests from compromised nodes could still reach the kubelet.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse Kubernetes network policies (like Calico) with host-level firewall rules, assuming they can block node ports, when in fact network policies only apply to pod-to-pod traffic and cannot control access to the kubelet's host network interface.
Detailed technical explanation
How to think about this question
The kubelet API on port 10250 serves as a critical control plane endpoint for node management, including pod lifecycle operations and metrics; by default, kubeadm clusters enable anonymous authentication, but even with it disabled, the port remains reachable from any network source. In a real-world scenario, an attacker on the internal network could exploit this to execute commands on nodes or exfiltrate data, making iptables-based CIDR restriction a robust defense-in-depth measure that complements Kubernetes RBAC. This approach aligns with the CIS Kubernetes Benchmark recommendation to restrict kubelet access to trusted networks using host-level firewalls.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A junior network technician can log in to a core router but cannot reach the enable prompt or configuration mode. The AAA server is authenticating the login — but the authorisation policy only grants privilege level 1, not 15. Authentication (who you are) is working; authorisation (what you can do) is not.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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Cluster Setup and Hardening — study guide chapter
Learn the concepts, then practise the questions
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Cluster Setup and Hardening practice questions
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FAQ
Questions learners often ask
What does this CKS question test?
Cluster Setup and Hardening — This question tests Cluster Setup and Hardening — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use iptables on each node to allow incoming connections to port 10250 only from the cluster's CIDR (e.g., 10.0.0.0/8). — Option A is correct because using iptables to restrict access to port 10250 to only the cluster's internal CIDR (e.g., 10.0.0.0/8) directly addresses the audit finding by blocking unauthenticated public access while allowing necessary kubelet API communication from within the cluster. This approach does not disrupt running workloads, as it only modifies firewall rules without restarting kubelet or changing its configuration. It leverages the existing network infrastructure (iptables) and is consistent with the principle of least privilege for an untrusted internal network.
What should I do if I get this CKS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jun 11, 2026
This CKS 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 CKS exam.
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