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Understanding networking

Kubernetes ensures that pods are able to network with each other, and allocates each pod an IP address from an internal network. This ensures all containers within the pod behave as if they were on the same host. Giving each pod its own IP address means that pods can be treated like physical hosts or virtual machines in terms of port allocation, networking, naming, service discovery, load balancing, application configuration, and migration.

Some cloud platforms offer metadata APIs that listen on the 169.254.169.254 IP address, a link-local IP address in the IPv4 169.254.0.0/16 CIDR block.

This CIDR block is not reachable from the pod network. Pods that need access to these IP addresses must be given host network access by setting the spec.hostNetwork field in the pod spec to true.

If you allow a pod host network access, you grant the pod privileged access to the underlying network infrastructure.

OpenShift Container Platform DNS

If you are running multiple services, such as front-end and back-end services for use with multiple pods, environment variables are created for user names, service IPs, and more so the front-end pods can communicate with the back-end services. If the service is deleted and recreated, a new IP address can be assigned to the service, and requires the front-end pods to be recreated to pick up the updated values for the service IP environment variable. Additionally, the back-end service must be created before any of the front-end pods to ensure that the service IP is generated properly, and that it can be provided to the front-end pods as an environment variable.

For this reason, OpenShift Container Platform has a built-in DNS so that the services can be reached by the service DNS as well as the service IP/port.