Amazon Web Services Updated ANS-C01 Exam Questions and Answers by kareem

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

Option C is the optimal solution as it involves deploying the EC2 instances in the private subnets, which provides additional security benefits. Additionally, creating an S3 gateway endpoint in the VPC will enable the EC2 instances to communicate with Amazon S3 directly, without incurring data transfer costs. This is because the S3 gateway endpoint uses Amazon's private network to transfer data between the VPC and S3, which is not charged for data transfer. Furthermore, specifying the route table of the private subnets during endpoint creation will create routes to Amazon S3, which is required for the EC2 instances to communicate with S3.

Ref: https://docs.aws.amazon.com/vpc/lat est/userguide/vpc-nat-gateway.html

"A NAT gateway can support up to 55,000 simultaneous connections to each unique destination. This limit also applies if you create approximately 900 connections per second to a single destination (about 55,000 connections per minute). If the destination IP address, the destination port, or the protocol (TCP/UDP/ICMP) changes, you can create an additional 55,000 connections. For more than 55,000 connections, there is an increased chance of connection errors due to port allocation errors. These errors can be monitored by viewing the ErrorPortAllocation CloudWatch metric for your NAT gateway. For more information, see Monitoring NAT Gateways Using Amazon CloudWatch."

Access logs is an optional feature of Elastic Load Balancing that is disabled by default. After you enable access logs for your load balancer, Elastic Load Balancing captures the logs and stores them in the Amazon S3 bucket that you specify as compressed files. You can disable access logs at any time. https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-access-logs.html