You’ve been hired to enhance the overall security posture for a large e-commerce site. They have a well-architected, multi-tier application running in a VPC that uses ELBs in front of both the web and the app tier with static assets served directly from S3. They are using a combination of RDS and DynamoDB for their dynamic data and then archiving nightly into S3 for further processing with EMR. They are concerned because they found questionable log entries and a flood of superfluous requests for accessing the resources. You suspect that someone is performing a DDoS attack. How would you mitigate this kind of attack in the easiest and most cost-efficient way?

Answer options:

A.Recommend that they lease space at a DirectConnect partner location and establish a 1G DirectConnect connection to their VPC. Then they would establish Internet connectivity into their space, filter the traffic in hardware Web Application Firewall (WAF) and then pass the traffic through the DirectConnect connection into their application running in their VPC.B.Add previously identified host file source IPs as an explicit INBOUND DENY NACL to the web tier subnet.
C.Enable AWS WAF to protect the application from the DDoS attack.
D.Remove all but TLS 1 & 2 from the web tier ELB and enable Advanced Protocol Filtering. This will enable the ELB itself to perform WAF functionality.

Answer – C
In such scenarios where you are designing a solution to prevent the DDoS attack, you can use a Web Application Firewall (WAF).
AWS WAF is a web application firewall that helps protect your web applications from common web exploits that could affect application availability, compromise security, or consume excessive resources. AWS WAF gives you control over which traffic to allow or block your web applications by defining customizable web security rules. You can use AWS WAF to create custom rules that block common attack patterns, such as SQL injection or cross-site scripting, and rules designed for your specific application. New rules can be deployed within minutes, letting you respond quickly to changing traffic patterns. 
Option A is incorrect because, although this option could work, the setup is very complex and is not a cost-effective solution.
Option B is incorrect because (a) even though blocking certain IPs will mitigate the risk, the attacker could maneuver the IP address and circumvent the IP check by NACL, and (b) it does not prevent the attack from the new source of threat.
Option C is CORRECT because you can use AWS WAF web access control lists (web ACLs) to help minimize the effects of a distributed denial of service (DDoS) attack. Reference can be found in https://docs.aws.amazon.com/waf/latest/developerguide/what-is-aws-waf.html.
Option D is incorrect because there is no such thing as the Advanced Protocol Filtering feature for ELB.For more information on WAF, please visit the below URL-
https://aws.amazon.com/waf/