February 2020

How Pods access Kubernetes DNS in Docker EE, part two

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This is part two of a two-part blog about Kubernetes DNS resolution and network access by Pods in Kubernetes. In part one we looked at internal Kubernetes DNS and how DNS resolution is configured for containers. In this part, we look at how network traffic gets from the containers in user workload Pods to Pods providing DNS functionality. We’re using Kubernetes running under Docker EE UCP (Docker Enterprise Edition Universal Control Plane) in this example. You can find more information about Docker EE here. Docker EE uses the Calico network plugin for Kubernetes, so some of the details are specific to Calico.

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How Pods access Kubernetes DNS in Docker EE, part one

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Service discovery is one of the important benefits of using a container/Pod orchestrator. When you create a Service in Kubernetes, controllers running behind the scenes create an entry in Kubernetes DNS records. Then other applications deployed in the cluster can look up the Service using its name. Kubernetes also configures routing within the cluster to send traffic for the Service to the Service’s ephemeral endpoint Pods.

Understanding Kubernetes DNS configuration and related traffic flow will help you troubleshoot problems accessing the cluster’s DNS from Pods. This is part one of a two-part deep-dive into how Kubernetes does this under the hood. In part one of this blog, we will look at how Kubernetes sets up DNS resolution for containers in Pods. In part two, we will look at how network traffic flows from containers in Pods for user workloads to the Pods providing DNS functionality. We’re going to use Kubernetes running under Docker Enterprise Edition for our examples in this blog.

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