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Networking

Networking in Kubernetes (KCNA Relevant)

Networking is a core component of Kubernetes, enabling communication between Pods, Services, and external resources. Below are the relevant Networking topics for Kubernetes in the context of the KCNA exam:

1. Kubernetes Networking Basics:

  • Pod-to-Pod Communication:
    • Pods can communicate with each other within a Kubernetes cluster using their IP addresses.
    • Kubernetes assigns each Pod a unique IP address, and Pods on different nodes can communicate with each other over the cluster network.
  • Flat Network Model:
    • Kubernetes assumes that every Pod can communicate with every other Pod in the cluster without NAT (Network Address Translation).

2. Services in Kubernetes:

  • ClusterIP (default):
    • Exposes a service on a cluster-internal IP address. This type of service is only accessible within the Kubernetes cluster.
  • NodePort:
    • Exposes a service on a specific port on each Node's IP address. Allows external access to the service through <NodeIP>:<NodePort>.
  • LoadBalancer:
    • Provisioned by cloud providers to expose services externally, typically using an external load balancer (e.g., AWS ELB, GCP Load Balancer).
  • ExternalName:
    • Maps a service to an external DNS name, allowing Kubernetes to access external services by their DNS names.

3. DNS (Domain Name System):

  • CoreDNS:
    • Kubernetes uses CoreDNS for service discovery. Each Service gets a DNS entry that can be accessed using its name within the cluster.
  • Service Discovery:
    • Pods can access Services using DNS names (e.g., my-service.my-namespace.svc.cluster.local).

4. Network Policies:

  • Network Policies:
    • Allows you to control the communication between Pods. You can define rules to allow or block traffic between Pods based on labels, IP blocks, or namespaces.
  • Ingress and Egress Rules:
    • Ingress: Incoming traffic to Pods.
    • Egress: Outgoing traffic from Pods.
  • Pod Security:
    • Control which Pods can communicate with others, enhancing network isolation and security.

5. Ingress and Egress Controllers:

  • Ingress Controller:
    • Manages HTTP/HTTPS traffic into the cluster. It routes traffic based on domain name, paths, or other rules defined in the Ingress resource.
    • Popular Ingress controllers: NGINX Ingress, Traefik, HAProxy.
  • Egress Controllers:
    • Manage outbound traffic from the cluster to external services. Ensures control and security of traffic leaving the cluster.

6. CNI (Container Network Interface):

  • CNI Plugins:
    • Kubernetes uses CNI plugins to manage networking for containers. Popular CNI plugins include Flannel, Calico, Weave, and Cilium.
  • Networking Model:
    • The CNI ensures that Pods on different nodes can communicate using an overlay network or other networking strategies.
  • Network Overlay:
    • Virtual networks that enable Pod-to-Pod communication across different physical machines or nodes.

7. Load Balancing:

  • Service Load Balancing:
    • Kubernetes Services can automatically distribute traffic to Pods based on service type (e.g., ClusterIP, NodePort, LoadBalancer).
  • Ingress Load Balancing:
    • Ingress Controllers handle the distribution of HTTP(S) traffic across multiple Pods, supporting features like SSL termination, routing, etc.

8. Network Security:

  • mTLS (Mutual TLS) with Service Mesh:
    • Service meshes like Istio can be used to enforce mTLS for secure communication between microservices.
  • Network Isolation:
    • Using Network Policies to isolate services and restrict communication between Pods.
    • Restrict which services can access certain Pods based on labels and namespaces.

9. External Connectivity:

  • Outbound Networking:
    • Pods can access external services outside the cluster, managed through egress rules and NAT configurations.
  • External IPs:
    • Assigning external IP addresses to services (e.g., using LoadBalancer services or NodePort for external access).

10. Troubleshooting Networking Issues:

  • kubectl commands like kubectl get pods -o wide, kubectl describe pod <pod-name>, kubectl logs <pod-name>, and kubectl exec to troubleshoot Pod networking issues.
  • Network Diagnostics Tools like ping, traceroute, and curl to test connectivity between Pods, Services, and external endpoints.
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