Can't Access a Pod From Another Pod Using NodePort Service? Here's Why!

Описание: Discover the reason behind your inability to access a pod from another pod in Kubernetes using `NodePort` service and learn the best practices for seamless communication between pods.
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This video is based on the question https://stackoverflow.com/q/62890837/ asked by the user 'mnj' ( https://stackoverflow.com/u/5021151/ ) and on the answer https://stackoverflow.com/a/62890975/ provided by the user 'Arghya Sadhu' ( https://stackoverflow.com/u/1839482/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Can't Access a Pod From Another Pod Using NodePort Service? Here's Why!

In the world of Kubernetes, understanding how services work is crucial for your applications to communicate effectively. One common problem that users encounter is the inability to access a pod from another pod when using a NodePort service. If you've experienced this issue, you’re not alone!

In this guide, we will break down the scenario and explain why this behavior occurs, along with a suggested solution to help you fix the issue.

The Scenario

You are running Kubernetes with the Docker Desktop feature on your PC, and your setup includes:

Two pods running from a deployment manifest

An additional standalone pod created via command line

A service defined as NodePort

Your Kubernetes Configuration

Below is a simplified view of what your Kubernetes setup looks like:

Deployment Configuration

[[See Video to Reveal this Text or Code Snippet]]

Standalone Pod Command

You created another pod with the command:

[[See Video to Reveal this Text or Code Snippet]]

Service Definition

[[See Video to Reveal this Text or Code Snippet]]

The Problem

You attempted to access the nginx-nodeport service from the nginx-standalone pod. Here’s what you did:

SSH into nginx-standalone

Installed curl inside the pod

Ran two curl commands:

[[See Video to Reveal this Text or Code Snippet]]

While the first command succeeded, the second resulted in a connection issue with the message: curl: (7) Failed to connect to nginx-nodeport port 31000: Connection refused.

The Confusion

You might be wondering: Why does accessing the service via the nodePort fail from the pod but succeed from your host PC?

The Explanation

The failure to connect to the NodePort from the nginx-standalone pod is expected behavior in Kubernetes.

Understanding NodePort

A NodePort service is designed to expose a pod externally, making it accessible from outside the Kubernetes cluster.

The nodePort (like 31000 in this case) listens on the node's network interface but does not exist within the pod's network interface.

What's Happening?

When you use curl nginx-nodeport, you are querying the internal service IP which properly routes to your pods.

When using curl nginx-nodeport:31000, you are trying to access the NodePort, which the pod cannot reach as it is not configured to interact with external network interfaces.

The Solution

If you want to enable communication between pods:

Use ClusterIP Services

For pod-to-pod communication within your Kubernetes cluster, you should use the ClusterIP type service instead of NodePort.

How to Change:

Change the service type from NodePort to ClusterIP:

[[See Video to Reveal this Text or Code Snippet]]

Redeploy the service, and now you can access your nginx-nodeport or renamed nginx-clusterip service from any pod within the cluster using just:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Understanding the differences between service types in Kubernetes is essential to troubleshoot and configure your cluster networks effectively. If you’re facing communication issues between your pods, remember to utilize ClusterIP for seamless internal communication.

Now you can keep your applications running smoothly without any hiccups in service access. Happy Kubernetes-ing!