The answer is that Private DNS is not enabled for the VPC Interface Endpoint. When you create an interface endpoint for a service like Amazon ECR, enabling the Private DNS option automatically associates a Route 53 private hosted zone with your VPC, which allows the default private DNS name (such as *.dkr.ecr.us-east-1.amazonaws.com) to resolve to the endpoint’s elastic network interface IP addresses. Without this setting, the EC2 instance will resolve that name to the public IP of the ECR service, bypassing the endpoint entirely and causing authentication failures due to network path mismatches. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how VPC endpoints interact with DNS resolution—a common trap is assuming that simply creating the endpoint is enough, when in fact the private hosted zone must be explicitly enabled. Remember the memory tip: “No private DNS, no private path—your traffic goes out to the internet instead.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
When PrivateDnsEnabled is false, the private DNS name for the service is not automatically resolved to the endpoint's IPs, causing authentication failures.
B
The endpoint service name is incorrect; it should be com.amazonaws.us-east-1.ecr.api
Why wrong: com.amazonaws.us-east-1.ecr.dkr is the correct service name for the Docker registry API.
C
The endpoint is not in the 'available' state
Why wrong: The state is 'available', so it is functioning.
D
The endpoint type should be Gateway, not Interface
Why wrong: ECR Docker registry requires an Interface endpoint, not a Gateway endpoint.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that 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.
Detailed technical explanation
How to think about this question
Under the hood, enabling Private DNS for a VPC Interface Endpoint creates a Route 53 private hosted zone (e.g., *.dkr.ecr.us-east-1.amazonaws.com) that is associated with the VPC, overriding public DNS resolution. The EC2 instance's DNS resolver (the Amazon-provided DNS server at the VPC's +2 base address) will then return the endpoint's private IP, ensuring traffic stays within the VPC and uses the endpoint's security groups and network ACLs. In real-world scenarios, forgetting to enable Private DNS is a common misconfiguration that leads to authentication failures because the Docker client attempts to reach the registry via the public internet, which may be blocked by security groups or route tables.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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