🧠 Kubernetes Service Connectivity: `nslookup` vs `curl`¶

This guide explains the difference between nslookup and curl when testing Kubernetes Services — what each one checks, when to use which, and how to interpret the results during CKAD/CKA exams.

🧩 1. The Two Layers of Service Access¶

When you test a Kubernetes Service from inside a pod, you’re checking two separate layers:

Tool	Layer Tested	Purpose
`nslookup`	DNS (CoreDNS)	Checks if the Service name resolves to a ClusterIP
`curl`	Network + Application	Checks if traffic can reach the Service and the app responds

⚙️ 2. Example Commands¶

✅ DNS Test¶

nslookup nginx-svc.default.svc.cluster.local

Checks: - Can the pod resolve the Service name using CoreDNS? - Is DNS properly configured in the cluster?

Example output:

Name:   nginx-svc.default.svc.cluster.local
Address: 10.96.12.101

✅ DNS resolution works.

✅ Application Test¶

curl nginx-svc.default:80

Checks: - Can the pod reach the Service ClusterIP on port 80? - Are kube-proxy and service routing working? - Are backend pods responding?

Example output (for Nginx):

<html>
<head><title>Welcome to nginx!</title></head>
<body>...</body>
</html>

✅ Application reachable and responding.

🧠 3. Difference in Depth¶

Command	What it Tests	What Success Means
`nslookup svc-name.ns.svc.cluster.local`	CoreDNS DNS resolution	Service name → ClusterIP works
`curl svc-name.ns:port`	Network + application path	Service + Pod connectivity works

Analogy:¶

nslookup = finding someone’s phone number 📞
curl = actually calling them and getting a reply 👋

🧩 4. How Kubernetes Resolves Names¶

Inside any pod, /etc/resolv.conf includes:

search ns.svc.cluster.local svc.cluster.local cluster.local
nameserver 10.96.0.10

When you type:

curl amor.amor:80

CoreDNS automatically expands it to:

amor.amor.svc.cluster.local

So you can use short or full DNS forms interchangeably.

🧰 5. How to Interpret Results in Exams¶

`nslookup` Result	`curl` Result	Meaning
✅ Works	✅ Works	Everything is healthy
❌ Fails	❌ Fails	DNS/CoreDNS issue
✅ Works	❌ Fails	NetworkPolicy issue / wrong target port / no endpoints
❌ Fails	✅ Works	Misconfiguration (rare)

🧩 6. Example CKAD Workflow¶

# Step 1: DNS Resolution Test
kubectl exec testpod -n alpha -- nslookup nginx-svc.alpha.svc.cluster.local

# Step 2: Application Connectivity Test
kubectl exec testpod -n alpha -- curl nginx-svc.alpha:80

Result Meaning:¶

Both ✅ → Service fully functional
nslookup ✅ but curl ❌ → Routing or app issue
Both ❌ → DNS/CoreDNS issue

⚡ 7. Common Exam Scenarios¶

Exam Prompt	You Should Run	Layer Tested
“Verify if service name resolves”	`nslookup svc-name.ns.svc.cluster.local`	DNS
“Check if the pod can access the service”	`curl svc-name.ns:port`	Application
“Service resolves but not reachable”	Both commands	DNS + Network
“Pods can’t reach app even though DNS works”	`curl` only	NetworkPolicy or wrong port

🧠 8. Flowchart for Debugging¶

                ┌──────────────────────────┐
                │ Run nslookup             │
                │ (Check DNS resolution)   │
                └────────────┬─────────────┘
                             │
                  ┌───────────┴───────────┐
                  │                       │
               Works ✅               Fails ❌
                  │                       │
        ┌─────────┴─────────┐        ┌────┴─────────┐
        │ Run curl          │        │ Fix DNS/CoreDNS │
        │ (Check app reach) │        │ or Service name │
        └─────────┬─────────┘        └────────────────┘
                  │
          ┌───────┴────────┐
          │                │
        Works ✅         Fails ❌
          │                │
          │     ┌──────────┴───────────┐
          │     │ Check NetworkPolicy  │
          │     │ Ports / Endpoints    │
          │     └──────────────────────┘

🧭 9. Quick Reference Table¶

Test Goal	Command	What It Proves
Check DNS resolution	`nslookup svc-name.ns.svc.cluster.local`	CoreDNS is resolving names
Check Service routing	`curl svc-name.ns:port`	Service forwards traffic correctly
Check ClusterIP access	`curl <ClusterIP>:<port>`	Network path works (bypass DNS)
Test both layers	Run both commands	Full connectivity verified

❤️ 10. Summary (for CKAD Mindset)¶

Command	Focus	Layer
`nslookup`	“Can I find the service?”	DNS (CoreDNS)
`curl`	“Can I reach and talk to it?”	Network + App

Rule of thumb:

🔹 If nslookup fails → DNS problem.
🔹 If nslookup works but curl fails → NetworkPolicy or wrong port.
🔹 If both work → everything is fine.

🧠 Kubernetes Service Connectivity: nslookup vs curl¶