Bài 10: NetworkPolicies & CKAD Exam Strategy

1. NetworkPolicy

Mặc định, tất cả Pods trong cluster có thể communicate với nhau. NetworkPolicy giới hạn traffic dựa trên labels.

Default: All pods can talk to all pods (no restriction)

After applying default-deny-all:
  Pod A ──✗──► Pod B (blocked)
  Pod A ──✗──► Pod C (blocked)

After applying allow policy:
  Pod A (app=frontend) ──✓──► Pod B (app=backend, port 3000)
  Pod A ──✗──► Pod C (app=database) (still blocked)

2. NetworkPolicy Syntax

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: backend-policy
  namespace: production
spec:
  podSelector:         # Applies to these pods (empty = all pods in ns)
    matchLabels:
      app: backend
  policyTypes:
  - Ingress            # Controls inbound traffic
  - Egress             # Controls outbound traffic
  ingress:
  - from:
    - podSelector:     # Allow from pods with this label
        matchLabels:
          app: frontend
    - namespaceSelector: # Allow from pods in these namespaces
        matchLabels:
          name: production
    ports:
    - protocol: TCP
      port: 3000
  egress:
  - to:
    - podSelector:
        matchLabels:
          app: database
    ports:
    - protocol: TCP
      port: 5432

Exam tip — AND vs OR trong NetworkPolicy:
from: [{podSelector}, {namespaceSelector}] = OR (pod from either selector)
from: [{podSelector + namespaceSelector}] in SAME item = AND (pod matching both)
Đây là một trong những câu hỏi trap nhất của CKAD.

3. Common Patterns

Pattern 1: Default deny all ingress
---
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: default-deny-ingress
spec:
  podSelector: {}     # Empty = match ALL pods
  policyTypes:
  - Ingress
  # No ingress rules = deny all ingress

Pattern 2: Default deny all (both ingress + egress)
---
spec:
  podSelector: {}
  policyTypes:
  - Ingress
  - Egress
  # No rules = deny all

Pattern 3: Allow all ingress (override deny)
---
spec:
  podSelector: {}
  policyTypes:
  - Ingress
  ingress:
  - {}  # Empty rule = allow all ingress

Pattern	policyTypes	Rules	Effect
Deny all ingress	[Ingress]	Không có ingress rules	Block all inbound
Deny all egress	[Egress]	Không có egress rules	Block all outbound
Allow specific	[Ingress]	ingress rules listed	Allow matching only
Allow DNS egress	[Egress]	to port 53 (UDP+TCP)	Allow DNS queries

Exam tip: NetworkPolicy chỉ hoạt động nếu CNI plugin hỗ trợ (Calico, Cilium, Weave). Flannel không hỗ trợ NetworkPolicy! Ingress/Egress rules là additive — nếu nhiều policies apply đến cùng Pod, Kubernetes OR tất cả rules lại.

4. CKAD Exam Strategy

Thông tin exam:
- 2 giờ, ~15-20 tasks thực hành (performance-based)
- Mỗi task có value % khác nhau (ưu tiên task cao điểm trước)
- Pass score: 66%
- Được dùng docs: kubernetes.io/docs + helm.sh/docs

Keyboard shortcuts quan trọng:
  k = kubectl (export alias k=kubectl trong exam, đã set sẵn)
  CTRL+R = search command history
  CTRL+A = go to beginning of line

Workflow cho mỗi task:

1. ĐỌC KỸ task description (đặc biệt để ý namespace!)
2. Switch context nếu cần:
   kubectl config use-context cluster-name
3. Set namespace shortcut:
   export ns=target-namespace
   alias kn="kubectl -n $ns"
4. Dùng --dry-run=client -o yaml để generate YAML:
   kubectl run pod --image=nginx --dry-run=client -o yaml > pod.yaml
5. Edit YAML, apply, verify:
   kubectl apply -f pod.yaml
   kubectl get pods -n $ns

5. --dry-run Pattern

# Generate YAML templates nhanh hơn viết tay:

Pod:
  kubectl run nginx --image=nginx --dry-run=client -o yaml > pod.yaml

Deployment:
  kubectl create deployment myapp --image=nginx --replicas=3 \
    --dry-run=client -o yaml > deploy.yaml

Service (ClusterIP):
  kubectl create service clusterip myapp --tcp=80:8080 \
    --dry-run=client -o yaml > svc.yaml

ConfigMap:
  kubectl create configmap myconfig --from-literal=k=v \
    --dry-run=client -o yaml > cm.yaml

Secret:
  kubectl create secret generic mysecret --from-literal=pass=secret \
    --dry-run=client -o yaml > secret.yaml

Job:
  kubectl create job myjob --image=busybox -- echo hello \
    --dry-run=client -o yaml > job.yaml

CronJob:
  kubectl create cronjob mycron --image=busybox --schedule="*/1 * * * *" \
    -- echo hello --dry-run=client -o yaml > cron.yaml

6. Essential kubectl Shortcuts

Lệnh đầy đủ	Short form
kubectl get pods	k get po
kubectl get deployments	k get deploy
kubectl get services	k get svc
kubectl get namespaces	k get ns
kubectl get persistentvolumeclaims	k get pvc
kubectl get configmaps	k get cm
kubectl get serviceaccounts	k get sa
kubectl get networkpolicies	k get netpol
kubectl describe pod mypod	k describe po mypod
kubectl delete pod mypod --force	k delete po mypod --force

7. Final CKAD Cheat Sheet

Domain	Key Topics	% Weight
App Design & Build	Multi-container, Init Containers, Jobs, CronJobs, volumes	20%
App Deployment	Rolling updates, rollbacks, Helm, Kustomize	20%
App Observability	Probes (liveness/readiness/startup), logs, debug	15%
App Env/Config/Security	ConfigMaps, Secrets, SecurityContext, SA, Resources, QoS	25%
Services & Networking	Services, Ingress, NetworkPolicies	20%

8. Practice Questions

Q1: You apply a NetworkPolicy with podSelector: {} and policyTypes: [Ingress] but no ingress rules. What happens?

• A) All ingress traffic is allowed (no rules = no restriction)
• B) All ingress traffic to ALL pods in the namespace is denied ✓
• C) All pods are deleted
• D) Only external ingress is denied; internal pod-to-pod traffic is allowed

Explanation: podSelector: {} matches ALL pods in the namespace. policyTypes: [Ingress] says this policy controls ingress. Having no ingress rules means zero traffic is allowed. This is the "default deny all ingress" pattern. Pod-to-pod traffic within the cluster is also denied because NetworkPolicy controls all ingress, regardless of source.

Q2: In a NetworkPolicy, what is the difference between these two from clauses?
Clause A: from: [{podSelector: {app: web}}, {namespaceSelector: {env: prod}}]
Clause B: from: [{podSelector: {app: web}, namespaceSelector: {env: prod}}]

• A) They are identical
• B) Clause A: allow from pods with app=web OR from any pod in env=prod namespace. Clause B: allow only from pods with app=web AND in env=prod namespace ✓
• C) Clause A uses AND logic, Clause B uses OR logic
• D) Clause B is invalid YAML syntax

Explanation: In NetworkPolicy, when podSelector and namespaceSelector are in SEPARATE list items (separated by -), they use OR logic. When they are in the SAME list item (same indentation level, no -), they use AND logic. This is a critical distinction and a common exam trap.

Q3: During the CKAD exam, you need to create a Deployment with a specific Pod spec. What is the fastest approach?

• A) Write the entire YAML from memory
• B) Search kubernetes.io docs and copy-paste example YAML
• C) Use kubectl create deployment --dry-run=client -o yaml to generate a template, then edit ✓
• D) Use helm to deploy a chart with default values

Explanation: The --dry-run=client -o yaml pattern generates valid YAML without creating resources. You redirect to a file, edit only the fields that differ, then apply. This is faster than manual YAML authoring and less likely to have syntax errors. Combining with > file.yaml lets you make precise edits.