Cross-namespace communication in Kubernetes - Mini Project: Build & Apply

Cross-namespace communication

📖 Scenario: You are working with Kubernetes namespaces to organize your applications. You want to allow a pod in one namespace to communicate with a service in another namespace.

🎯 Goal: Set up two namespaces with a service in one namespace and a pod in another namespace that can communicate with that service using the full DNS name.

📋 What You'll Learn

Create two namespaces named frontend and backend

Deploy a simple backend service in the backend namespace

Deploy a pod in the frontend namespace that can access the backend service using the full DNS name

Verify communication by printing the response from the backend service

💡 Why This Matters

🌍 Real World

In real Kubernetes clusters, teams use namespaces to separate environments or projects. Services in one namespace often need to talk to services in another namespace.

💼 Career

Understanding cross-namespace communication is essential for Kubernetes administrators and DevOps engineers to design secure and scalable applications.

1

Create namespaces

Create two namespaces called frontend and backend using kubectl commands.

Kubernetes

# Create the frontend namespace
# Create the backend namespace

Hint

Use kubectl create namespace frontend and kubectl create namespace backend to create the namespaces.

2

Deploy backend service

Deploy a simple backend pod and service in the backend namespace. Use the image hashicorp/http-echo with arguments -text=Hello from backend. Name the pod backend-pod and the service backend-service.

Kubernetes

kubectl create namespace frontend
kubectl create namespace backend
# Deploy backend pod and service in backend namespace

Hint

Use kubectl run with --namespace=backend to create the pod, then kubectl expose pod to create the service.

3

Deploy frontend pod with curl

Deploy a pod named frontend-pod in the frontend namespace using the image curlimages/curl. This pod will be used to test communication to the backend service.

Kubernetes

kubectl create namespace frontend
kubectl create namespace backend
kubectl run backend-pod --image=hashicorp/http-echo --restart=Never --namespace=backend -- -text="Hello from backend"
kubectl expose pod backend-pod --port=5678 --target-port=5678 --name=backend-service --namespace=backend
# Deploy frontend pod in frontend namespace

Hint

Use kubectl run frontend-pod --image=curlimages/curl --restart=Never --namespace=frontend -- sleep 3600 to create a pod that stays running.

4

Test cross-namespace communication

Use kubectl exec to run a curl command inside frontend-pod in the frontend namespace. Access the backend service at http://backend-service.backend.svc.cluster.local:5678 and print the response.

Kubernetes

kubectl create namespace frontend
kubectl create namespace backend
kubectl run backend-pod --image=hashicorp/http-echo --restart=Never --namespace=backend -- -text="Hello from backend"
kubectl expose pod backend-pod --port=5678 --target-port=5678 --name=backend-service --namespace=backend
kubectl run frontend-pod --image=curlimages/curl --restart=Never --namespace=frontend -- sleep 3600
# Run curl inside frontend-pod to access backend-service

Hint

Use kubectl exec -n frontend frontend-pod -- curl -s http://backend-service.backend.svc.cluster.local:5678 to test communication.

Practice

1. What is the main purpose of namespaces in Kubernetes?

easy

A. To group and isolate resources within the cluster

B. To increase the number of nodes in the cluster

C. To speed up container startup time

D. To automatically update container images

Solution

1. Step 1: Understand namespaces role

   Namespaces help organize and isolate resources like pods and services inside a Kubernetes cluster.

2. Step 2: Compare options

   Options B, C, and D describe unrelated cluster features, not namespaces.

3. Final Answer:

   To group and isolate resources within the cluster -> Option A

4. Quick Check:

   Namespaces = resource grouping and isolation [OK]

Hint: Namespaces isolate resources logically inside the cluster [OK]

Common Mistakes:

• Confusing namespaces with nodes
• Thinking namespaces speed up containers
• Assuming namespaces auto-update images

2. Which DNS name format is correct to access a service named webapp in namespace prod from another namespace?

easy

A. svc.webapp.prod.cluster.local

B. prod.webapp.svc.cluster.local

C. webapp.prod.svc.cluster.local

D. webapp.svc.prod.cluster.local

Solution

1. Step 1: Recall Kubernetes DNS format

   The full DNS name for a service is service.namespace.svc.cluster.local.

2. Step 2: Match the format to the service and namespace

   For service webapp in namespace prod, the correct format is webapp.prod.svc.cluster.local.

3. Final Answer:

   webapp.prod.svc.cluster.local -> Option C

4. Quick Check:

   DNS = service.namespace.svc.cluster.local [OK]

Hint: Use service.namespace.svc.cluster.local for cross-namespace access [OK]

Common Mistakes:

• Swapping service and namespace order
• Omitting 'svc' in DNS name
• Using incorrect domain suffix

3. Given this command run in namespace dev:

kubectl exec -n dev pod1 -- curl http://api.prod.svc.cluster.local

What is the expected result if api service exists in prod namespace and network policies allow cross-namespace traffic?

medium

A. The curl command fails with DNS resolution error

B. The pod1 container crashes immediately

C. The curl command times out due to network policy blocking

D. The curl command successfully connects to the api service

Solution

1. Step 1: Understand DNS and network policy

   The DNS name api.prod.svc.cluster.local correctly targets the service in the prod namespace.

2. Step 2: Check network policy effect

   Since network policies allow cross-namespace traffic, the connection is permitted.

3. Final Answer:

   The curl command successfully connects to the api service -> Option D

4. Quick Check:

   Allowed network + correct DNS = successful connection [OK]

Hint: Correct DNS + allowed network = successful cross-namespace call [OK]

Common Mistakes:

• Assuming DNS fails without explicit config
• Ignoring network policy restrictions
• Expecting pod crash on curl failure

4. You try to access db.prod.svc.cluster.local from namespace dev but get a timeout. What is the most likely cause?

medium

A. The service db does not exist in prod

B. Network policies block traffic between dev and prod

C. The pod in dev has no internet access

D. The DNS name is incorrectly formatted

Solution

1. Step 1: Verify service existence

   If the service did not exist, DNS would fail, not timeout.

2. Step 2: Analyze network policy impact

   Timeout usually means packets are blocked by network policies preventing cross-namespace communication.

3. Final Answer:

   Network policies block traffic between dev and prod -> Option B

4. Quick Check:

   Timeout = network block, not DNS error [OK]

Hint: Timeout often means network policy blocks traffic [OK]

Common Mistakes:

• Confusing timeout with DNS failure
• Assuming pod internet access affects cluster DNS
• Misreading DNS format as cause

5. You want to allow a pod in namespace frontend to access a service backend in namespace api. Which steps are needed to enable this cross-namespace communication?

hard

A. Use the full DNS name backend.api.svc.cluster.local and configure network policies to allow traffic from frontend to api

B. Create a service account in frontend and bind it to api namespace

C. Expose the backend service as NodePort and access it via node IP

D. Deploy the backend service in frontend namespace instead

Solution

1. Step 1: Use correct DNS name for cross-namespace access

   Accessing a service in another namespace requires the full DNS name: backend.api.svc.cluster.local.

2. Step 2: Configure network policies

   Network policies must allow traffic from pods in frontend namespace to api namespace services.

3. Final Answer:

   Use the full DNS name backend.api.svc.cluster.local and configure network policies to allow traffic from frontend to api -> Option A

4. Quick Check:

   DNS + network policy = cross-namespace communication [OK]

Hint: Full DNS + network policy allow = cross-namespace access [OK]

Common Mistakes:

• Confusing service accounts with network access
• Using NodePort unnecessarily
• Moving services instead of configuring access