Exam AWS Certified Solutions Architect - Professional SAP-C02 topic 1 question 90 discussion

I would go with option B. Source will be public IP like 198.51.100.2.

https://aws.amazon.com/premiumsupport/knowledge-center/vpc-analyze-inbound-traffic-nat-gateway/ Refer Reason 1 Run the query below. filter (dstAddr like 'xxx.xxx' and srcAddr like 'public IP') | stats sum(bytes) as bytesTransferred by srcAddr, dstAddr | limit 10 Note: You can use just the first two octets in the search filter to analyze all network interfaces in the VPC. In the example above, replace xxx.xxx with the first two octets of your VPC classless inter-domain routing (CIDR). Also, replace public IP with the public IP that you're seeing in the VPC flow log entry. Query results show traffic on the NAT gateway private IP from the public IP, but not traffic on other private IPs in the VPC. These results confirm that the incoming traffic was unsolicited. However, if you do see traffic on the private instance's IP, then follow the steps under Reason #2.

Step-by-Step Approach: You already have this log entry: srcaddr = 198.51.100.2 dstaddr = 203.0.x.x action = ACCEPT Now, query CloudWatch Logs for the reverse flow: srcaddr = 203.0.x.x (your EC2 instance) dstaddr = 198.51.100.2 (the public IP) If you find outbound traffic from your instance to the public IP before the inbound traffic, then the connection is solicited (i.e. it's a reply). If you do not find any outbound flow to that public IP, then the traffic is unsolicited — a potentially unexpected or malicious inbound attempt that should have been blocked.

The NAT gateway allows outbound internet traffic from private instances but does not accept unsolicited inbound connections. If 198.51.100.2 is contacting the private instance, we need to determine if this is a response to an existing outbound request from the private instance.

The NAT gateway allows outbound internet traffic from private instances but does not accept unsolicited inbound connections. If 198.51.100.2 is contacting the private instance, we need to determine if this is a response to an existing outbound request from the private instance.

we need to check if the request starts from ec2 instances outbound, not the other way round.

Correct answer is D. This is for NAT traffic analysis, so the focus is outbound. VPC Flow Logs are published to CloudWatch Logs, not CloudTrail1. This immediately eliminates options A and C. To determine if the traffic is unsolicited inbound connections: We need to check if the private EC2 instance (starting with 203.0) initiated the connection to 198.51.100.2 If the source IP is from the VPC (203.0) and the destination is 198.51.100.2, this indicates the connection was initiated from inside the VPC This would mean the ACCEPT traffic is a response to an outbound request, not unsolicited inbound traffic.

The answer is B. This is not about an IP but about a port. If the packet from outside to inside has the source port which is well known and the destination port dynamic, it means that the connection was initiated from inside, if the packet from outside to inside has a source port dynamic and destination port well known, it means that the traffic was originated from outside :)

Why Option B is Problematic: // Example CloudWatch Logs Insights Query for Option B fields @timestamp, sourceAddress, destinationAddress, action, bytes | filter destinationAddress like "203.0" | filter sourceAddress like "198.51.100.2" | stats sum(bytes) by sourceAddress, destinationAddress 1. Incorrect Traffic Direction - It looks for traffic where source = 198.51.100.2 (internet) and destination = 203.0.x.x (VPC) This only shows successful inbound connections (ACCEPT) It doesn't reveal whether these connections were solicited or unsolicited 2. Missing Context - Doesn't show the initial outbound connection that would indicate a solicited response - Cannot differentiate between legitimate responses and actual unsolicited connections - Lacks the temporal relationship between outbound and inbound flows

D, we need to see if the internal origin was first used

destination address set as "like 203.0" and the source address set as "like 198.51.100.2"

Of course it is D. What useful info will you get from B? You need to check original request which in case of NAT is always EC2, not something in the internet.

I vote B. Because the network traffic to check is unsolicited inbound connection. IT is initiated from the internet to internal EC2. The source is public IP address and the target is internal IP.

To determine whether the traffic represents unsolicited inbound connections from the internet, use the Amazon CloudWatch console. Select the log group that contains the NAT gateway’s elastic network interface and the private instance’s elastic network interface. Run a query to filter with the destination address set as “like 203.0” and the source address set as “like 198.51.100.2”. This approach helps you analyze the VPC flow logs to identify if the inbound traffic to the private EC2 instance is expected return traffic or unsolicited. The stats command can be used to filter the sum of bytes transferred by the source address and the destination address, providing insight into the traffic patterns and ensuring network security.

the solution architect want to check if it's unsolicited traffic or not, so we need to check the if the request is sent by us. which means 198.51.100.2 should be the destination.

B, CloudWatch & destination address 203.0

The question is "Solutions architect must determine whether the traffic represents unsolicited inbound connections from the internet". The NAT gateway does not allow any inbound traffic from an internet other than response to a traffic it sent out to internet which came from a VPC resource (eg, EC2). So to find out if the inbound traffic to NAT Gateway from internet IP 198.51.100.2 is unsolicit or not, check the vpc flowlog to see if there was an original request from source IP 203.0 to destination 198.51.100.2. This is what option D says.