- A
The service is not exposed externally
Why wrong: ClusterIP is internal; that doesn't cause DNS failure.
- B
CoreDNS is not running or misconfigured
DNS resolution for service names relies on CoreDNS; if it's down, names won't resolve.
- C
The service port does not match the container port
Why wrong: Port mismatch would cause connection refused, not DNS resolution failure.
- D
The selector does not match any pod labels
Why wrong: If the selector doesn't match, the service would have no endpoints, but DNS would still resolve.
Question 255 of 997
CoreDNS and Kubernetes Service DNS Resolution
This KCNA practice question tests your understanding of kubernetes fundamentals. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
A service of type ClusterIP is created but pods cannot reach it using the service name. The pods are in the same namespace. What is a likely cause?
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
✓
CoreDNS is not running or misconfigured
When a ClusterIP service cannot be reached by its DNS name from pods in the same namespace, the most common cause is that CoreDNS (the cluster DNS resolver) is not running or is misconfigured. Kubernetes relies on CoreDNS to resolve service names to ClusterIP addresses; if CoreDNS is down or its configuration is incorrect, DNS queries for the service name will fail, even though the service itself and its endpoints are healthy.
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.
- ✗
The service is not exposed externally
Why it's wrong here
ClusterIP is internal; that doesn't cause DNS failure.
- ✓
CoreDNS is not running or misconfigured
Why this is correct
DNS resolution for service names relies on CoreDNS; if it's down, names won't resolve.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The service port does not match the container port
Why it's wrong here
Port mismatch would cause connection refused, not DNS resolution failure.
- ✗
The selector does not match any pod labels
Why it's wrong here
If the selector doesn't match, the service would have no endpoints, but DNS would still resolve.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The exam tests the distinction between DNS resolution failures and connectivity failures, trapping candidates who confuse a selector mismatch or port mismatch with a DNS issue, even though those would still allow the service name to resolve.
Detailed technical explanation
How to think about this question
CoreDNS runs as a Deployment in the kube-system namespace, typically with a ClusterIP service named kube-dns. It implements the Kubernetes DNS specification (RFC 1035-style records) and watches the API server for Service and EndpointSlice changes. A common subtle failure is when CoreDNS pods are running but the ConfigMap (Corefile) has a syntax error or the upstream DNS server is unreachable, causing NXDOMAIN or SERVFAIL responses for service names. In real-world scenarios, misconfigured pod DNS policies (e.g., dnsPolicy: None) or network policies blocking UDP port 53 can also prevent service name resolution.
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 practitioner preparing for the KCNA exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Visual reference
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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FAQ
Questions learners often ask
What does this KCNA question test?
Kubernetes Fundamentals — This question tests Kubernetes Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: CoreDNS is not running or misconfigured — When a ClusterIP service cannot be reached by its DNS name from pods in the same namespace, the most common cause is that CoreDNS (the cluster DNS resolver) is not running or is misconfigured. Kubernetes relies on CoreDNS to resolve service names to ClusterIP addresses; if CoreDNS is down or its configuration is incorrect, DNS queries for the service name will fail, even though the service itself and its endpoints are healthy.
What should I do if I get this KCNA question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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Last reviewed: Jul 4, 2026
This KCNA 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 KCNA exam.
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