Understanding Why You Can't Access Kubernetes Services by Name Outside of a Pod

Learn why you can't access your Kubernetes services by name outside a pod and get insights into how Kubernetes networking works.
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Understanding Why You Can't Access Kubernetes Services by Name Outside of a Pod

As you delve into Kubernetes and its networking features, you may encounter a common sticking point: accessing services by name from outside of a pod. You can easily do this when you’re within the cluster or in a pod itself, but trying to do so from your host machine (or any point outside of a pod) can lead to frustrating moments. In this guide, we’ll explore the reasons behind this limitation and what you need to know about Kubernetes networking.

The Scenario: What Are We Trying to Achieve?

Imagine you have set up an Nginx pod in your default namespace and exposed it via a ClusterIP service. Here's how you might have done it:

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Once you've done this, you can verify that your service is up and running by checking its details with:

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Inside the Cluster vs. Outside the Pod

Inside the Cluster: You can access the service using its internal cluster IP.

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Inside a Pod: You can access the service using its service name:

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Both of these approaches yield a 200 OK response.

From the Host or Outside the Pod: This is where the fun stops. Attempting to access the service by name:

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Or even using the fully qualified domain name:

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Results in an error that states: wget: unable to resolve host address ‘nginx-service’.



The Underlying Cause: CoreDNS and DNS Resolution

How Kubernetes Handles DNS

The key to understanding this issue lies in how Kubernetes manages its internal networking and DNS resolution.

CoreDNS: The magic service hostnames and pod hostnames are provided by the “Cluster DNS” service, which is usually powered by CoreDNS. This service automatically assigns DNS entries to all services within the Kubernetes cluster.

Injection into Pods: Kubernetes automatically injects a resolv.conf targeting the internal CoreDNS service into all pods. This allows for easy DNS resolution when you are within the cluster or inside a pod.

Resolving Issues on the Host: However, when you’re “outside a pod”, which typically means you are running a command directly from the host operating system, your system doesn’t have access to the internal DNS entries because your host doesn’t include the same resolv.conf settings that pods do.



Key Takeaways

Cluster Services: The ability to resolve Kubernetes services by name is confined to the environments that are integrated into the Kubernetes networking stack, namely the pods.

External Access Requires Other Mechanisms: To interact with services from outside the cluster, you’d typically employ other types of services like NodePort or LoadBalancer, which expose services externally.

Networking Understanding: Building a strong foundational understanding of Kubernetes networking and DNS resolution is crucial for successful deployment and management of your cluster.



In summary, the limitation you’re facing is tied to how Kubernetes handles DNS and networking between its internal components and external entities. By leveraging either NodePort or LoadBalancer services, you can effectively expose your services beyond the confines of the Kubernetes cluster networking framework.

Now that you have a clearer picture, you can better navigate Kubernetes networking challenges with confidence!