Which of the following troubleshooting steps should be performed?
Check inbound and outbound security groups, looking for DENY rules
Check inbound and outbound Network ACL rules, looking for DENY rules
Review the rejected packet reason codes in the VPC Flow Logs
Use AWS X-Ray to trace the end-to-end application flow
Explanations:
Security groups in AWS do not have explicit DENY rules; by default, all inbound traffic is denied unless explicitly allowed. Thus, if the security groups have valid ALLOW rules for the required traffic, there should be no issue here.
Network ACLs (NACLs) operate at the subnet level and have both ALLOW and DENY rules. If there are DENY rules in the NACLs associated with the subnets of the two EC2 instances, this could prevent communication between them, even if the security groups are correctly configured. Therefore, checking NACLs is essential for troubleshooting.
VPC Flow Logs can provide insights into the traffic patterns, but they do not directly indicate reasons for rejected packets. They show whether the packets were allowed or denied but do not specify the reasons, making them less useful for immediate troubleshooting compared to checking ACLs.
AWS X-Ray is primarily used for tracing and analyzing requests in distributed applications, helping to debug issues in the application layer rather than network connectivity issues. This option does not address the fundamental networking aspect that is causing the EC2 instances to be unable to communicate.
I estimate that the answer is:
Check inbound and outbound Network ACL rules, looking for DENY rules