CKA Troubleshooting Practice Question
A pod is in 'ImagePullBackOff' state. You run 'kubectl describe pod' and see the event: 'Failed to pull image "nginx:latest": rpc error: code = Unknown desc = Error response from daemon: Get https://registry-1.docker.io/v2/: dial tcp: lookup registry-1.docker.io on 8.8.8.8:53: no such host'. What is the MOST likely cause?
⚠ Common exam trap
Watch out — candidates often confuse DNS resolution failures with authentication or image tag issues, but the specific 'no such host' error in the event message directly points to a DNS problem on the node.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
DNS resolution is failing on the node
The error message 'no such host' when trying to resolve 'registry-1.docker.io' on DNS server 8.8.8.8:53 indicates that the node's DNS resolution is failing. Kubernetes nodes rely on DNS to resolve container registry hostnames; if the node cannot resolve the registry's domain, the container runtime cannot pull the image, resulting in ImagePullBackOff. This is a node-level DNS issue, not a container runtime or image tag problem.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The container runtime is not installed
Why it's wrong here
If the container runtime (such as containerd or CRI-O) were not installed or running on the node, the Kubelet would be unable to communicate with it via the CRI gRPC socket. Consequently, the node itself would transition to a 'NotReady' status, and pods scheduled to it would fail immediately with a 'ContainerRuntimeDown' error rather than initiating an image pull sequence that results in 'ImagePullBackOff'.
- ✗
The image tag is incorrect
Why it's wrong here
Specifying an invalid or non-existent image tag allows the Kubelet to successfully resolve the registry's IP address and establish a connection. However, the remote registry will reject the request during the manifest retrieval phase, returning an HTTP 404 status code or a 'manifest unknown' error in the pod's events, which is distinct from a network resolution failure.
- ✗
The registry requires authentication
Why it's wrong here
When a private registry requires authentication and the pod lacks the appropriate 'imagePullSecrets', the registry responds with an HTTP 401 Unauthorized or 403 Forbidden status. The resulting `kubectl describe` output will explicitly log an 'unauthorized: authentication required' or 'access denied' error message, proving that the registry host was successfully reached but rejected the credentials.
- ✓
DNS resolution is failing on the node
Why this is correct
The presence of a 'no such host' or 'lookup failed' error in the pod's event log indicates that the node's local resolver cannot translate the registry's domain name into an IP address. Because the Kubelet cannot resolve the external registry's FQDN, it fails to initiate the TCP handshake, ultimately causing the container pull to fail and triggering the 'ImagePullBackOff' state.
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Related to this question
Learn chapter
Kubernetes Architecture Overview
Key term
kubectl Command Reference
kubectl is the command-line tool used to interact with and manage Kubernetes clusters by sending commands to the Kubernetes API.
Key term
Container Runtime
A container runtime is software that runs containers by using the host operating system's kernel to isolate processes, manage filesystem layers, and handle networking.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CKA practice question is part of Courseiva's free CNCF certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CKA exam.