Скачать или смотреть Why Can’t I Reach a Pod in Another Namespace? Understanding Kubernetes DNS with Calico

Discover why accessing a pod across namespaces in Kubernetes fails using the DNS format. Learn how to troubleshoot and resolve issues with `Calico` networking in your Kubernetes setup.
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This video is based on the question https://stackoverflow.com/q/67133241/ asked by the user 'Don Rhummy' ( https://stackoverflow.com/u/857025/ ) and on the answer https://stackoverflow.com/a/67133414/ provided by the user 'Don Rhummy' ( https://stackoverflow.com/u/857025/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Why You Can't Reach a Pod in Another Namespace

Kubernetes is a powerful orchestration tool that allows you to run and manage containerized applications at scale. However, as users delve deeper into its networking capabilities, they often encounter puzzling issues, one of which is the challenge of reaching a pod in another namespace. Many users wonder why they can’t access a pod using a specific DNS format, leading to a frustrating experience when trying to ensure smooth communication between services across namespaces.

In this post, we will explore a common scenario involving pod communication across namespaces, particularly focusing on the underlying networking, why it fails, and how to resolve it effectively.

Scenario Breakdown

The Setup

Imagine you have the following setup in your Kubernetes environment:

Namespace One:

Created a namespace and a pod named rest using the NGINX image.

The command to create the pod looks like this:

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

Namespace Two:

Similarly, created another namespace and a pod named call-rest that will attempt to reach the first pod.

The command to create this pod is:

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

Getting the Pod IP:

Next, you retrieve the IP address of the rest pod:

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

Attempting Access:

Then, you try to reach the rest pod from call-rest using a specific format that Kubernetes uses for DNS resolution:

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

Unfortunately, this call times out, leaving you puzzled about the networking issue at hand.

The Cause of the Problem

Networking with Calico

The root of this issue lies in the choice of the network plugin—in this case, Calico. When using Calico, pods are assigned cluster-wide IP addresses rather than IPs that are scoped within each namespace. This diverges from the behavior seen when using the default Flannel networking.

Default Behavior with Flannel:

When using Flannel, the DNS format you attempted would work correctly because it handled the namespace isolation differently, enabling inter-namespace communication via the generated DNS names.

Calico’s Approach:

In contrast, with Calico managing networking, you need to refer to pods by their IP addresses directly without the namespace-based DNS format—this is why your original attempt fails.

The Solution

To successfully communicate with the rest pod from the call-rest pod using Calico, you can directly reference the pod’s IP address without the DNS format.

Correct Command

You can access the rest pod using:

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

Summary of Key Points

Use direct IP addresses for pod-to-pod communication when using Calico.

Ensure you fully understand the networking plugin's behavior in your Kubernetes environment.

Reachability issues often stem from how DNS is handled across namespaces, particularly with different CNI (Container Network Interface) plugins.

Conclusion

Understanding the nuances of Kubernetes networking is crucial for anyone deploying applications in a multi-namespace setup. By diving into this scenario, we've illuminated the challenges presented by different network plugins like Calico and illustrated how to adapt your approach accordingly. Remember, when working with Calico, bypass the DNS format and utilize the direct IP approach for inter-namespace communication. Happy Kubernetes-ing!