ANS-C01 Network Management and Operations Practice Question
Network Topology
A network engineer is troubleshooting an issue where an EC2 instance launched in VPC vpc-0abcd1234efgh5678 cannot resolve DNS names for other instances using their private DNS names. The VPC has DHCP options set with domain-name-servers=AmazonProvidedDNS. What is the most likely cause?
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
✓
The VPC attribute 'enableDnsHostnames' is set to false
For an EC2 instance to resolve private DNS names using the Amazon-provided DNS server, the VPC must have both enableDnsSupport and enableDnsHostnames set to true. The DHCP options are correctly set to AmazonProvidedDNS, so the most likely cause is that enableDnsHostnames is false. Option B is incorrect because VPC Flow Logs are used for monitoring network traffic, not DNS resolution. Option C is incorrect because security groups by default allow all outbound traffic, and DNS traffic (UDP/53) is not blocked unless explicitly denied. Option D is incorrect because the DHCP options are already set to AmazonProvidedDNS as stated in the stem.
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 VPC attribute 'enableDnsHostnames' is set to false
Why this is correct
Without hostnames, instances cannot resolve private DNS names.
- ✗
VPC Flow Logs are not enabled
Why it's wrong here
Flow logs do not affect DNS resolution.
- ✗
The security group on the instances blocks DNS traffic
Why it's wrong here
DNS traffic uses UDP/53, but the issue is with hostname assignment.
- ✗
The DHCP options set is not using AmazonProvidedDNS
Why it's wrong here
The DHCP options set is correct according to the scenario.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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