Kubernetes networking refers to the system that enables communication among various resources both within and outside your cluster. It addresses multiple scenarios, including interactions between Pods, connections between Kubernetes Services, and the management of external traffic directed to the cluster.

As a distributed system, Kubernetes networking extends across the physical Nodes of your cluster. It employs a virtual overlay network that creates a flat architecture for connecting cluster resources.

Example of a Kubernetes Networking Diagram

The Kubernetes networking framework is responsible for allocating IP addresses, assigning DNS names, and mapping ports to your Pods and Services. This process is typically automated, meaning you generally won’t need to manage these tasks within your network infrastructure or Node hosts when using Kubernetes.

At a high level, the Kubernetes network model assigns each Pod a unique IP address that is resolvable within the cluster. Pods can communicate with each other using these IP addresses without the need for Network Address Translation (NAT) or additional configurations.

This basic structure is enhanced by the Service model, which allows traffic to be directed to any Pod within a defined set, along with control mechanisms like network policies that restrict unwanted Pod-to-Pod communications.

Differences Between Physical/VM Networking and Kubernetes Networking

Kubernetes networking applies familiar networking concepts to cluster environments, offering a simpler, more consistent, and more automated approach compared to traditional networking models used for physical devices and virtual machines (VMs).

In the past, you would have to manually set up new endpoints with IP addresses, open firewall ports, and configure DNS routes. In contrast, Kubernetes automates this functionality for your cluster’s workloads.

Developers and operators do not need to understand the underlying network implementation to deploy resources successfully and make them accessible. This simplification aids in setup, maintenance, and the ongoing enforcement of security protocols, as all management occurs within Kubernetes itself.

Differences Between Docker Networking and Kubernetes Networking

Kubernetes utilizes a flat networking model designed for distributed systems, allowing all Pods to communicate, even when deployed on different physical Nodes.

Conversely, Docker adopts a different networking approach as a single-host containerization solution. By default, Docker connects all containers to a bridge network linked to the host. You can create additional networks using various types, including bridge, host (directly sharing the host’s network stack), and overlay (for distributed networking in Swarm environments).

Once connected to a shared network, Docker containers can communicate with one another. Each container receives an internal IP address and DNS name for network members to access it. However, Docker does not automatically create port mappings from the host to the containers; these must be configured at container startup.

In summary, while Docker and Kubernetes networking share some similarities, each is tailored to its specific use case. Docker focuses on single-node networking, simplified by the bridged mode, whereas Kubernetes is inherently distributed and requires overlay networking.

This distinction is evident in how communication is restricted between containers: in Docker, you must place containers in different networks to prevent interaction, while in Kubernetes, all Pods are automatically part of a single overlay network, with traffic managed through policy-based controls.

Kubernetes Networking Architecture

As previously mentioned, Kubernetes networking features a fundamentally flat structure with the following characteristics:

  • Each Pod is assigned a unique IP address.
  • Nodes operate a root network namespace that connects the Pod interfaces, enabling all Pods to communicate using their IP addresses, regardless of their scheduled Node.
  • Communication is independent of Network Address Translation (NAT), simplifying the system and enhancing portability.
  • Pods have their own network namespaces and interfaces, with all communications routed through these interfaces.
  • The cluster-level network layer maps Node-level namespaces, ensuring accurate traffic routing across Nodes.
  • There is no need for manual binding of Pod ports to Nodes, although this can be done if necessary by assigning a hostPort to Pods.

These principles make Kubernetes networking predictable and consistent for both users and administrators, ensuring reliable network connectivity for all Pods without requiring manual configuration.

How Kubernetes Allocates Pod IP Addresses

Kubernetes uses Classless Inter-Domain Routing (CIDR) to allocate IP addresses to Pods. This system defines the subnet of IP addresses available for your Pods. Each Pod receives an address from the specified CIDR range applicable to your cluster, which you must define when setting up the cluster’s networking layer.

Many Kubernetes networking plugins also support IP Address Management (IPAM) functions, allowing for manual assignment of IP addresses, prefixes, and pools, which is beneficial for complex networking scenarios.

How DNS Works in Kubernetes Clusters

Kubernetes clusters come equipped with built-in DNS support, with CoreDNS being one of the most widely used providers, typically enabled by default in many distributions.

Kubernetes automatically generates DNS names for Pods and Services in the following format:

  • Pod: pod-ip-address.pod-namespace-name.pod.cluster-domain.example (e.g., 10.244.0.1.my-app.svc.cluster.local)
  • Service: service-name.service-namespace-name.svc.cluster-domain.example (e.g., database.my-app.svc.cluster.local)

Applications within your Pods should generally be configured to communicate with Services using their DNS names, as these names are predictable. In contrast, a Service’s IP address may change if the Service is deleted and recreated.