containerPort

ContainerPort is a field used in container specifications within Kubernetes to declare the network port on which a container listens. It is defined inside a container’s ports section and is typically expressed as an integer value, often accompanied by an optional name. While not strictly required for a container to run, specifying containerPort helps with documentation, tooling, and integration with Kubernetes Services.

In a Pod specification, you may declare multiple containers, each with its own containerPort. A named port can be used to simplify service configuration, for example:

- containerPort: 8080

name: web

This naming allows a Service to reference the port by name (targetPort: web) instead of a numeric value.

ContainerPort and Services: In Kubernetes, Services route traffic to Pods by selecting them and forwarding to a targetPort on the container. The targetPort can be a numeric port or a named port defined in the Pod’s containerPort. If targetPort is omitted in a Service spec, it defaults to the Service’s port value. The containerPort value serves as documentation and a mapping reference for how the container is expected to receive traffic.

Difference from hostPort: containerPort is an internal port within the container’s network namespace. HostPort would expose a port on the node’s external interface, which is less common and can create port conflicts. containerPort does not by itself expose a port on the node.

Best practices: declare containerPort when possible, use meaningful port names for easier service configuration, and ensure the Service’s targetPort aligns with the container’s listening port.