Debugging service connectivity in Kubernetes - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Check pod connectivity with kubectl exec

You want to verify if a pod named webapp-1 can reach the service backend on port 8080. Which command will correctly test this connectivity?

A kubectl exec -it webapp-1 -- ping backend 8080

B kubectl exec webapp-1 curl backend:8080

C kubectl exec webapp-1 -- nc -z backend 8080

D kubectl exec webapp-1 -- curl backend:8080

💡 Hint

Remember to use the correct syntax for kubectl exec with commands.

Option D correctly uses kubectl exec with the pod name, the -- separator, and the command curl backend:8080 to test HTTP connectivity. Option D misses the -- separator, causing a syntax error. Option D uses ping with a port, which is invalid. Option D uses nc but nc may not be installed and the syntax is incomplete.

🧠 Conceptual

intermediate

Understanding Kubernetes Service Types

Which Kubernetes service type exposes the service on a cluster-internal IP only, making it reachable only from within the cluster?

A NodePort

B ClusterIP

C LoadBalancer

D ExternalName

💡 Hint

Think about which service type does not expose ports outside the cluster nodes.

ClusterIP exposes the service on an internal IP accessible only within the cluster. NodePort and LoadBalancer expose services externally. ExternalName maps to an external DNS name.

❓ Troubleshoot

advanced

Diagnosing DNS resolution failure inside a pod

A pod cannot resolve the service name database. Which command inside the pod will help verify if DNS resolution is working?

A nslookup database

B curl database

C ping -c 1 database

D dig database

💡 Hint

Use a command that queries DNS directly.

The nslookup command queries DNS and shows if the name resolves. curl tests HTTP but fails if DNS fails silently. ping may not be installed or blocked. dig is not always installed in minimal containers.

🔀 Workflow

advanced

Order the steps to debug a Kubernetes service connectivity issue

Put these steps in the correct order to debug why a pod cannot connect to a service:

A 1,2,3,4

B 2,1,3,4

C 3,1,2,4

D 1,3,2,4

💡 Hint

Start from DNS, then endpoints, then connectivity, then logs.

First verify DNS resolution (step 1), then check if the service has endpoints (step 2), then test actual connectivity (step 3), and finally check application logs (step 4).

✅ Best Practice

expert

Identify the cause of intermittent service connectivity failures

A Kubernetes service intermittently fails to connect to its pods. Which is the most likely cause?

A The service type is ClusterIP instead of NodePort.

B The pod's container image is outdated.

C Pods are crashing and restarting frequently causing endpoints to be temporarily missing.

D The cluster nodes have insufficient CPU resources.

💡 Hint

Think about what causes endpoints to disappear temporarily.

If pods crash and restart often, the service endpoints list changes, causing intermittent connectivity failures. Service type or image version does not cause intermittent connectivity. Insufficient CPU may cause slowness but not intermittent connection drops.

Practice

1. What is the primary command to check if a Kubernetes service has endpoints assigned?

easy

A. kubectl describe nodes

B. kubectl get pods

C. kubectl get endpoints

D. kubectl get configmaps

Solution

1. Step 1: Understand service connectivity basics

   Services route traffic to endpoints, so checking endpoints shows if pods are linked.

2. Step 2: Use the correct command to list endpoints

   kubectl get endpoints lists endpoints for services, showing if pods are ready.

3. Final Answer:

   kubectl get endpoints -> Option C

4. Quick Check:

   Check endpoints = kubectl get endpoints [OK]

Hint: Use 'kubectl get endpoints' to verify service pod connections [OK]

Common Mistakes:

• Using 'kubectl get pods' which shows pods but not service endpoints
• Checking nodes or configmaps which are unrelated to service endpoints
• Confusing 'kubectl describe svc' with listing endpoints

2. Which of the following commands correctly tests DNS resolution inside a Kubernetes pod named web-123?

easy

A. kubectl exec web-123 -- nslookup myservice

B. kubectl exec web-123 nslookup myservice

C. kubectl exec -it web-123 nslookup myservice

D. kubectl exec web-123 -- curl myservice

Solution

1. Step 1: Understand kubectl exec syntax

   The correct syntax to run a command inside a pod is kubectl exec [pod] -- [command].

2. Step 2: Identify the command to test DNS

   nslookup tests DNS resolution, so kubectl exec web-123 -- nslookup myservice is correct.

3. Final Answer:

   kubectl exec web-123 -- nslookup myservice -> Option A

4. Quick Check:

   Correct exec syntax + nslookup = kubectl exec web-123 -- nslookup myservice [OK]

Hint: Use '--' before command in kubectl exec to run inside pod [OK]

Common Mistakes:

• Omitting '--' which causes command to fail
• Using '-it' without need for interactive shell
• Using curl instead of nslookup for DNS test

3. You run kubectl describe svc myservice and see no endpoints listed. What will be the output of kubectl get endpoints myservice?

medium

A. Error from server (NotFound): endpoints "myservice" not found

B. NAME ENDPOINTS AGE
myservice 10.0.0.5:80,10.0.0.6:80 10m

C. NAME ENDPOINTS AGE
myservice 127.0.0.1:80 10m

D. NAME ENDPOINTS AGE
myservice <none> 10m

Solution

1. Step 1: Interpret service describe output

   No endpoints means no pods are linked to the service, so endpoints list is empty.

2. Step 2: Predict endpoints output

   kubectl get endpoints myservice will show the service name with <none> under ENDPOINTS.

3. Final Answer:

   NAME ENDPOINTS AGE myservice <none> 10m -> Option D

4. Quick Check:

   No endpoints = <none> shown [OK]

Hint: No endpoints in describe means endpoints show <none> [OK]

Common Mistakes:

• Assuming endpoints will list IPs even if none exist
• Expecting an error when endpoints exist but are empty
• Confusing endpoints with pod IPs

4. A pod cannot reach a service by its DNS name. You run kubectl exec pod1 -- nslookup myservice and get a timeout. What is the most likely cause?

medium

A. The pod is missing the DNS policy or DNS is misconfigured

B. The service has no endpoints, so DNS resolves but no response

C. The service selector labels do not match any pods

D. The pod is running in a different namespace without DNS search path

Solution

1. Step 1: Analyze DNS timeout symptom

   A DNS timeout means the pod cannot resolve the service name, indicating DNS issues.

2. Step 2: Identify DNS misconfiguration causes

   Missing DNS policy or broken DNS config in pod causes nslookup timeout, unlike no endpoints which still resolve DNS.

3. Final Answer:

   The pod is missing the DNS policy or DNS is misconfigured -> Option A

4. Quick Check:

   DNS timeout = DNS config issue [OK]

Hint: DNS timeout means DNS config or policy problem, not endpoints [OK]

Common Mistakes:

• Confusing DNS resolution failure with no endpoints
• Assuming label mismatch causes DNS timeout instead of no response
• Ignoring namespace DNS search path issues

5. You have a service myservice in namespace prod. A pod in namespace dev tries to connect using curl myservice but fails. Which is the best way to debug this connectivity issue?

hard

A. Run kubectl describe pod -n prod myservice to check pod details

B. Run kubectl exec -n dev pod -- curl myservice.prod.svc.cluster.local to test full DNS name

C. Run kubectl get svc -n dev myservice to check service in dev namespace

D. Run kubectl exec -n prod pod -- curl myservice to test from the service namespace

Solution

1. Step 1: Understand cross-namespace service access

   Pods in different namespaces must use the full DNS name including namespace to reach a service.

2. Step 2: Test connectivity using full DNS name from the pod in dev namespace

   Running kubectl exec -n dev pod -- curl myservice.prod.svc.cluster.local tests correct DNS and connectivity.

3. Final Answer:

   Run kubectl exec -n dev pod -- curl myservice.prod.svc.cluster.local to test full DNS name -> Option B

4. Quick Check:

   Cross-namespace access needs full DNS name [OK]

Hint: Use full DNS name with namespace for cross-namespace service access [OK]

Common Mistakes:

• Trying to curl service without namespace from another namespace
• Checking service in wrong namespace
• Describing pod instead of testing connectivity