This CKA practice question tests your understanding of services & networking. 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 developer created a Deployment with 3 replicas and a ClusterIP Service named 'app-service' on port 80 targeting port 8080 on the pods. Pod logs show that the container is listening on 8080, but curl from another pod in the same namespace to http://app-service:80 fails with 'Connection refused'. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Service selector does not match the pod labels.
The most likely cause is that the Service's selector does not match the pod labels. A ClusterIP Service routes traffic to pods based on label selectors; if the selector does not match the labels on the pods (e.g., the pods have labels like 'app: myapp' but the Service selector is 'app: frontend'), the endpoints controller will not populate the Service's endpoints, and traffic will be dropped, resulting in a 'Connection refused' error.
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 selector does not match the pod labels.
Why this is correct
If labels don't match, the Service has no endpoints, causing connection refused.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The container port is 8080 but the Service targetPort is 80.
Why it's wrong here
The Service targetPort is 8080, which matches the container port.
✗
The Service type should be NodePort for inter-pod communication.
Why it's wrong here
ClusterIP is sufficient for inter-pod communication within the cluster.
✗
The DNS resolution for 'app-service' is failing.
Why it's wrong here
DNS resolution would result in a different error (name resolution failure).
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'port' and 'targetPort' and assume a mismatch causes 'Connection refused', but the actual issue is a selector mismatch, which is a common misconfiguration in CKA troubleshooting scenarios.
Detailed technical explanation
How to think about this question
Under the hood, when a Service is created, the kube-controller-manager's endpoints controller watches for pods matching the Service's label selector and creates Endpoint objects. If the selector doesn't match, the Endpoints list remains empty, and kube-proxy on each node will not install iptables or IPVS rules for that Service, causing connections to be refused. A real-world scenario where this matters is when a developer accidentally uses a different label key (e.g., 'version' vs 'app') or a typo in the selector value, leading to silent traffic drops.
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 CKA 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CKA question in full detail.
Services & Networking — This question tests Services & Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Service selector does not match the pod labels. — The most likely cause is that the Service's selector does not match the pod labels. A ClusterIP Service routes traffic to pods based on label selectors; if the selector does not match the labels on the pods (e.g., the pods have labels like 'app: myapp' but the Service selector is 'app: frontend'), the endpoints controller will not populate the Service's endpoints, and traffic will be dropped, resulting in a 'Connection refused' error.
What should I do if I get this CKA 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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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