Troubleshooting: Instances Not Receiving Hostnames in AWS VPC

Answer - B.

You need to set both values for instances to receive DNS hostnames.

This is also given in the AWS Documentation.

If both attributes are set to true, the following occurs:

Your instance receives a public DNS hostname.

The Amazon-provided DNS server can resolve Amazon-provided private DNS hostnames.

For more information on using VPC DNS, please refer to the below URL:

https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-dns.html

To ensure that instances in an AWS VPC receive hostnames from the AWS DNS, you need to ensure that the enableDnsHostname attribute is set to true for your VPC. However, if you have already set this attribute and instances are still not receiving hostnames when launched, there could be an underlying issue that needs to be addressed.

Here are the possible reasons why instances in your VPC are not receiving hostnames from the AWS DNS, even when the enableDnsHostname attribute is set to true:

A. The Auto-Assign Public IP is not set for the Subnet in which the Instance is launched: If the Auto-Assign Public IP is not enabled for the subnet in which your instances are launched, then the instances will not be able to communicate with the internet, which is required for DNS resolution. To fix this, you need to enable the Auto-Assign Public IP option for your subnet.

B. Enable "DNSSupport" and "DNSHostnames" for the VPC: If you have not enabled the "DNSSupport" and "DNSHostnames" options for your VPC, then the instances will not receive hostnames from the AWS DNS. To enable these options, go to the VPC console, select your VPC, click on Actions, and then click on Edit DNS resolution. Then, select "Yes" for both "Enable DNS resolution" and "Enable DNS hostnames" options, and click on Save.

C. You need to configure a Route 53 private hosted zone first: If you want to use a private DNS hostname for your instances, then you need to configure a Route 53 private hosted zone first. A private hosted zone is used to resolve domain names within a VPC or a set of VPCs without exposing the domain names to the internet. You can create a private hosted zone in the Route 53 console, and then associate it with your VPC.

D. You need to configure a Route 53 public hosted zone first: If you want to use a public DNS hostname for your instances, then you need to configure a Route 53 public hosted zone first. A public hosted zone is used to resolve domain names that are publicly accessible over the internet. You can create a public hosted zone in the Route 53 console, and then configure the necessary DNS records for your instances.

In summary, if your instances are not receiving hostnames from the AWS DNS, even when the enableDnsHostname attribute is set to true, you need to check the Auto-Assign Public IP option for your subnet, enable "DNSSupport" and "DNSHostnames" options for your VPC, and configure either a private or public Route 53 hosted zone depending on the type of DNS hostname you want to use.