Troubleshooting VPC Flow Logs for Failed Ping from 203.0.113.12 to 172.31.16.140

Correct Answer - B.

Check https://docs.aws.amazon.com/vpc/latest/userguide/flow-logs-records-examples.html#flow-log-example-security-groups for the flow log record examples.

Option A is incorrect: Because the security group is stateful.

As it allows the incoming traffic, the outbound traffic is automatically allowed.

Option B is CORRECT: Because this ensures that the inbound traffic is accepted.

For the outbound traffic, as the ACL denies it, the message will be rejected.

The configurations align with the flow logs.

Option C is incorrect: In this scenario, the incoming ping message is accepted.

So the ACL rule for the inbound traffic should allow it.

Option D is incorrect: The inbound rule in the security group should allow the traffic since there is an ACCEPT for the incoming message.

The VPC flow logs provide a record of the traffic flow within a VPC. In this scenario, the ping command from the machine with IP address 203.0.113.12 to the EC2 instance with IP address 172.31.16.140 has failed. The VPC flow logs show two entries, which are as follows:

2 123456789010 eni-1235b8ca123456789 203.0.113.12 172.31.16.140 0 0 1 4 336 1432917027 1432917142 ACCEPT OK 2 123456789010 eni-1235b8ca123456789 172.31.16.140 203.0.113.12 0 0 1 4 336 1432917094 1432917142 REJECT OK

Each VPC flow log entry contains the following fields:

1. The version of the VPC flow log record format.
2. The ID of the AWS account that owns the network interface.
3. The ID of the network interface.
4. The source IP address.
5. The destination IP address.
6. The source port.
7. The destination port.
8. The protocol.
9. The number of bytes sent from the source to the destination.
10. The number of bytes sent from the destination to the source.
11. The start time of the traffic flow.
12. The end time of the traffic flow.
13. The action (ACCEPT or REJECT).
14. The reason for the action (OK or NODATA).

From the provided VPC flow logs, it can be observed that the ping request from 203.0.113.12 to 172.31.16.140 was ACCEPTED by the destination EC2 instance, but the reply from 172.31.16.140 to 203.0.113.12 was REJECTED. This indicates that the issue may be related to outbound traffic from the EC2 instance.

Now, let's consider the answer options given:

A. The EC2 security group and the ACL in the EC2 subnet allow the inbound traffic. The security group denies the outbound traffic.

If the EC2 security group denies the outbound traffic, the ping request from 172.31.16.140 to 203.0.113.12 would have been blocked. This does not match the VPC flow log entry that shows the reply was rejected.

B. The EC2 security group and the ACL in the EC2 subnet allow the inbound traffic. The ACL denies the outbound traffic.

If the ACL in the EC2 subnet denies the outbound traffic, the ping request from 172.31.16.140 to 203.0.113.12 would have been blocked. This matches the VPC flow log entry that shows the reply was rejected. Therefore, this is a possible configuration that may result in this result.

C. The ACL in the EC2 subnet denies the inbound traffic. The EC2 security group allows the inbound traffic.

If the ACL in the EC2 subnet denies the inbound traffic, the ping request from 203.0.113.12 to 172.31.16.140 would have been blocked. This does not match the VPC flow log entry that shows the request was accepted.

D. The EC2 security group denies the inbound traffic. The ACL in the EC2 subnet allows both inbound and outbound traffic.

If the EC2 security group denies the inbound traffic, the ping request from 203.0.113.12 to 172.31.16.140 would have been blocked. This does not match the VPC flow log entry that shows the request was accepted.