This CKAD practice question tests your understanding of services and networking. 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 have deployed a microservices application in a Kubernetes cluster. One of the services, 'payment-service', needs to be accessed by other services within the cluster via a stable DNS name. You create a Service of type ClusterIP named 'payment' with selector app=payment. However, when you try to curl http://payment from another Pod, the connection times out. You verify that the Pods backing 'payment-service' are running and ready, and the Endpoints object lists the correct Pod IPs. You also confirm that the Pods are listening on port 8080, and the Service defines targetPort: 8080. The cluster uses a standard CNI plugin (Calico) and DNS is provided by CoreDNS. What is the most likely cause of the timeout?
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 Pods are listening on 127.0.0.1 only, not on 0.0.0.0
The most likely cause is that the Pods are listening only on 127.0.0.1 (localhost), which means they only accept connections from within the same Pod. When the Service sends traffic to the Pod via its cluster IP, the connection arrives on the Pod's network interface (e.g., eth0), not on loopback. Since the application is not bound to 0.0.0.0, it rejects or ignores the incoming packets, causing a timeout. This is a classic misconfiguration where the application's listen address is too restrictive.
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 name 'payment' is not resolvable by DNS
Why it's wrong here
You can curl http://payment; DNS resolves if CoreDNS is working.
✓
The Pods are listening on 127.0.0.1 only, not on 0.0.0.0
Why this is correct
Pods listening on localhost are not reachable from other Pods via service IP.
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 targetPort in the Service does not match the containerPort in the Pod spec
Why it's wrong here
Given endpoints exist, targetPort matches; otherwise endpoints would not be created.
✗
The Service type should be NodePort instead of ClusterIP
Why it's wrong here
ClusterIP is appropriate for internal access; NodePort is for external.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume DNS or port mismatches are the issue, but the timeout (not 'connection refused' or 'could not resolve') points to the application not accepting traffic on the correct network interface, which is a subtle but critical detail in Kubernetes networking.
Detailed technical explanation
How to think about this question
When a Pod's application binds to 127.0.0.1, it only listens on the loopback interface, which is isolated to the Pod's network namespace. Kubernetes Services use iptables or IPVS rules to forward traffic to the Pod's IP on its primary network interface (e.g., eth0). The kernel's routing table ensures packets destined for the Pod's IP arrive on eth0, but if the application is not listening on that interface, the TCP SYN packet is met with a RST or is silently dropped, leading to a connection timeout. This is a common pitfall, especially when developers hardcode 'localhost' in configuration files.
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 CKAD 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 CKAD question in full detail.
Services and Networking — This question tests Services and Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Pods are listening on 127.0.0.1 only, not on 0.0.0.0 — The most likely cause is that the Pods are listening only on 127.0.0.1 (localhost), which means they only accept connections from within the same Pod. When the Service sends traffic to the Pod via its cluster IP, the connection arrives on the Pod's network interface (e.g., eth0), not on loopback. Since the application is not bound to 0.0.0.0, it rejects or ignores the incoming packets, causing a timeout. This is a classic misconfiguration where the application's listen address is too restrictive.
What should I do if I get this CKAD 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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