You have created a custom VPC with public and private subnets. You will also be setting up a VPN connection with your organization to communicate with resources within the VPC. Your organization would need DNS names for some of the on-premise applications to communicate with VPC resources. You have launched a new EC2 instance with Auto-assign public IP as enable. When the instance is ready to use, you found that the Public DNS name is missing. What should be done to enable it?

Answer options:

A.Check whether attributes for DNS Hostnames and DNS Resolution are enabled for VPC.B.Check DHCP options set
C.Set auto-assign public IP to Use Subnet Setting.
D.You cannot have private DNS names for custom VPCs. Setup EC2 instance in default VPC.

Answer: A
By default, both attributes are set to `
true`
 in a default VPC or a VPC created by the VPC wizard. 
 The following rules apply:
If both attributes are set to 
true
, the following occurs:
Instances with a public IP address receive corresponding public DNS hostnames.
The Amazon Route 53 Resolver server can resolve Amazon-provided private DNS hostnames.
If either or both of the attributes is set to 
false
, the following occurs:
Instances with a public IP address do not receive corresponding public DNS hostnames.
The Amazon Route 53 Resolver cannot resolve Amazon-provided private DNS hostnames.
Instances receive custom private DNS hostnames if there is a custom domain name in the DHCP options set. If you are not using the Amazon Route 53 Resolver server, your custom domain name servers must resolve the hostname as appropriate.
Option B is incorrect because the DHCP options set is used if there is a custom domain name created by the user. If you are not using the Amazon Route 53 Resolver server, your custom domain name servers must resolve the hostname as appropriate. So there is no need to check the DHCP options set.
Default DHCP options set: AmazonProvidedDNS
Option C is incorrect because it is mentioned that EC2 instance is enabled with Auto-assign public IP, auto-assign public IP defines whether you can have a public IP address for the EC2 you are launching. If you launch EC2 in the public subnet, you can choose to have a public IP address or not.
Option D is incorrect. Custom VPC provides an option to enable/disable DNS Hostnames as described above.
We provide a DNS server (the Amazon Route 53 Resolver) for your VPC. To use your own DNS server, create a new set of DHCP options for your VPC. For more information, see DHCP options sets for your VPC.Reference:
https://docs.aws.amazon.com/vpc/latest/userguide/vpc-dns.html#vpc-dns-viewing
https://docs.aws.amazon.com/vpc/latest/userguide/VPC_DHCP_Options.html