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Discover the causes and solutions to Kubernetes pod DNS issues when `apt-get` fails to resolve addresses. Learn how to diagnose network problems efficiently.
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Troubleshooting Kubernetes Pod DNS Issues: Cannot Access Network After Update

Kubernetes is a powerful tool for managing containerized applications, but sometimes things can go awry. One common issue users encounter is when their pods cannot access the network, leading to failed operations like running apt-get commands to update packages. In this blog, we will delve into a particular case where a Kubernetes pod successfully receives DNS configurations yet cannot reach external networks. We’ll explore the steps to diagnose and resolve this problem effectively.

The Problem: Network Access Failure

Consider the following scenario: you are trying to update your Kubernetes pods using the apt-get command, and the process fails due to "Temporary failure resolving" various external URLs, such as security.debian.org and http.debian.net. Despite the fact that you can successfully ping the kube-dns service (10.96.0.10 in this case), you are unable to reach the broader internet or Kubernetes services.

This situation prompts the question: Why are the pods unable to access the network?

Diagnosing the Issue

Understanding DNS Resolution

You've confirmed that your DNS configuration appears to be working correctly, as evidenced by the ability to ping the Kubernetes DNS server (kube-dns). However, the key task is to examine whether the DNS can resolve service names such as kubernetes successfully.

Basic Steps to Diagnose the DNS Issue:

Check Current DNS Settings: Look into your /etc/resolv.conf to confirm the configuration of nameservers and search domains.

Ping Kubernetes Services: Use ping to check whether the pods can reach the kube-dns and other internal services.

Test DNS Resolution: Using tools like nslookup or dig, test the resolution of common service names, such as kubernetes. As noted in the example, a nslookup command may return failure, indicating DNS issues.

Example Command Output

In your case, running nslookup kubernetes inside a busybox pod leads to a failure:

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This suggests that the DNS service itself may have problems or that network policies could restrict the pod's ability to reach the DNS service.

Steps to Resolve

1. Verify CoreDNS Functionality

Since the CoreDNS service is vital for the DNS resolution process in Kubernetes, you'll want to ensure it's functioning properly. Here’s what you can do:

Get the CoreDNS logs: Use the command below to check the logs from the CoreDNS pods to see if there are any error messages that could indicate problems.

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

Check CoreDNS Version: Validate the version of CoreDNS running in your cluster:

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

This command will let you run CoreDNS commands to debug issues. Note: Only the coredns binary is executable, not a shell.

2. Inspect Network Policies

If your cluster is using network policies, you need to ensure that your policies allow pods to communicate with the external world:

Check existing policies: Run the following command to see all network policies in your namespace:

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

3. Review Security Group Rules (if applicable)

If your Kubernetes cluster is hosted on a cloud service like AWS or GCP, verify that the security groups or firewall rules allow outbound traffic over the necessary ports (usually port 53 for DNS).

Conclusion

Network access issues in Kubernetes can stem from a variety of causes, including DNS misconfigurations, network policies, or external firewall rules. By thoroughly checking each potential issue—from DNS resolution failures to valid network policies—you can efficiently resolve the inabilit