AWS Certified Advanced Networking Specialty Practice Exam

If EC2 instances are not receiving DNS hostnames, despite enableDnsHostname being true, what might be the issue?

1. The instances are not in a public subnet.

2. The enableDnsSupport attribute is not set to true.

3. The security group settings are incorrect.

4. The NAT gateway needs to be configured.

The correct answer is: The enableDnsSupport attribute is not set to true.

The correct choice highlights the critical role of the enableDnsSupport attribute. In Amazon EC2, for instances to receive DNS hostnames, the enableDnsSupport setting must be true at the VPC level. This setting allows instances within that VPC to utilize the Amazon-provided DNS for name resolution. If this attribute is not set to true, regardless of the enableDnsHostname setting being true for individual instances, the DNS hostnames will not be assigned. This distinction is important because it emphasizes that both attributes – enableDnsHostname and enableDnsSupport – are necessary for instances to correctly receive DNS hostnames. While enableDnsHostname must be true for an instance to provide a public DNS hostname, the global setting of enableDnsSupport determines whether DNS functionality is available in the first place. If enableDnsSupport is false, instances won't be able to resolve hostnames, leading to the issue described in the question. In contrast, the other options address different networking scenarios. If instances are not in a public subnet, they may not have public IP addresses, which could affect accessibility but does not directly correlate to DNS hostname receipt. Incorrect security group settings could restrict traffic but would not prevent DNS hostnames themselves from being issued. Similarly, the need for a NAT gateway