ANS-C01 Exam Dumps : Amazon AWS Certified Advanced Networking - Specialty

The most operationally efficient way to restrict inbound traffic to the ALB to come from CloudFront is to use the AWS managed prefix list for CloudFront. A prefix list is a collection of CIDR blocks that can be used to configure security groups and network ACLs. AWS provides a managed prefix list for CloudFront that is automatically updated when CloudFront IP ranges change. By adding an inbound rule to the ALB’s security group to allow the AWS managed prefix list for CloudFront, the network engineer can ensure that only CloudFront can access the ALB at the network layer. This solution does not require any additional configuration or maintenance. Option B is less efficient because network ACLs are stateless and require rules for both inbound and outbound traffic. Option C is not a network layer solution, but an application layer solution that requires the ALB to inspect the HTTP headers and reject requests that do not have the custom header. Option D is also not a network layer solution, but a web layer solution that requires AWS WAF to filter the traffic based on the CloudFront IP set. This solution also requires an AWS Lambda function to update the CloudFront IP set, which adds complexity and cost

Understanding the Issue: The IPv6-only EC2 instance cannot communicate with IPv4-only services because IPv6 and IPv4 are not directly compatible. To bridge this gap, DNS64 and NAT64 are used together. However, AWS NAT gateways do not natively support NAT64, but you can use DNS64 to translate IPv4 DNS records (A records) into IPv6-compatible addresses (AAAA records).

DNS64 for IPv6-Only Subnets: DNS64 is a service that synthesizes AAAA records for IPv4-only services. This allows IPv6-only clients to resolve IPv4 addresses as IPv6-compatible addresses, enabling communication through the NAT gateway.

NAT Gateway with Route Table Updates: The NAT gateway enables outbound communication from private subnets to the internet. Updating the route tables for IPv6-only subnets to send traffic through the NAT gateway ensures that the IPv6 EC2 instance can reach IPv4 services.