ANS-C01 Network Design Practice Question
Network Topology
A company has created a VPC Interface Endpoint for Amazon ECR (Docker registry API) as shown in the exhibit. However, an EC2 instance in the same VPC is unable to authenticate with the ECR registry using the private DNS name. What is the most likely cause?
⚠ Common exam trap
Candidates often confuse the endpoint service name for ECR (dkr vs. api) or assume a Gateway endpoint can be used for ECR, but the core issue is the Private DNS toggle, which is a subtle but critical setting for Interface endpoints.
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
✓
Private DNS is not enabled for the endpoint
The most likely cause is that Private DNS is not enabled for the VPC Interface Endpoint. When Private DNS is enabled, the endpoint automatically associates a Route 53 private hosted zone with the VPC, allowing the default private DNS name (e.g., *.dkr.ecr.us-east-1.amazonaws.com) to resolve to the endpoint's elastic network interface IP addresses. Without this, the EC2 instance will resolve the private DNS name to the public IP of the ECR service, bypassing the endpoint and failing authentication due to network path issues.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Private DNS is not enabled for the endpoint
Why this is correct
When PrivateDnsEnabled is false, the private DNS name for the service is not automatically resolved to the endpoint's IPs, causing authentication failures.
- ✗
The endpoint service name is incorrect; it should be com.amazonaws.us-east-1.ecr.api
Why it's wrong here
com.amazonaws.us-east-1.ecr.dkr is the correct service name for the Docker registry API.
- ✗
The endpoint is not in the 'available' state
Why it's wrong here
The state is 'available', so it is functioning.
- ✗
The endpoint type should be Gateway, not Interface
Why it's wrong here
ECR Docker registry requires an Interface endpoint, not a Gateway endpoint.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services 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 ANS-C01 exam.