Troubleshooting Ping Issues in AWS VPC
Question
You've set up an EC2 Instance in a VPC.
You are trying to ping the instance but are not able to do so.
You have verified the following. a.
Internet gateway attached to the VPC b.Route tables added for the Internet gateway c.
Public IP address assigned to the Instance You have enabled VPC flow logs and can see a rejection request for the outgoing traffic. 2 123456789111 eni-3456b8ca 54.0.113.12 172.31.16.140 0 0 1 4 336 1432917027 1432917142 ACCEPT OK 2 123456789111 eni-3456b8ca 172.31.16.140 54.0.113.12 0 0 1 4 336 1432917094 1432917142 REJECT OK What can be done to ensure that the ping request will work?
Answers
Explanations
Click on the arrows to vote for the correct answer
A. B. C. D.Answer - B.
Option A is incorrect since it is the outbound traffic that is causing the issue.
Options C and D are incorrect since the Security Groups are stateful.
Since the ICMP incoming is being accepted, it means that the outgoing for the NACL is the issue.
Since the outgoing traffic is being rejected that means that the NACL outbound rules are not allowing the traffic to flow.
For more information on NACLs, please refer to the below URL.
https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_ACLs.htmlBased on the provided information, the VPC is properly configured with an internet gateway and route tables for the gateway, and the EC2 instance has a public IP address assigned. However, the VPC flow logs show that the outgoing traffic from the instance is being rejected, which is causing the ping request to fail.
To ensure that the ping request works, we need to identify the source of the rejection and then take the appropriate action to allow the traffic.
The VPC flow logs show that the outgoing traffic from the instance is being rejected. The log entry shows that the traffic is being rejected by the VPC network access control list (NACL). The second field in the log entry (123456789111) is the NACL ID. Therefore, we need to check the rules in the NACL associated with the instance's subnet to see if there are any rules that are blocking the traffic.
Option A suggests ensuring that the NACL allows inbound ICMP requests. This would not help in this case because the VPC flow logs show that the outgoing traffic from the instance is being rejected, not the incoming traffic.
Option B suggests ensuring that the NACL allows outbound ICMP response. This is a valid option because the VPC flow logs show that the outgoing traffic from the instance is being rejected, which means that the NACL is blocking the outgoing traffic. Allowing outbound ICMP response in the NACL should resolve the issue.
Option C suggests ensuring that the security group allows inbound ICMP request. This would not help in this case because the VPC flow logs show that the outgoing traffic from the instance is being rejected, not the incoming traffic.
Option D suggests ensuring that the security group allows outbound ICMP response. This would not help in this case because security groups are stateful, which means that if they allow outgoing traffic, they automatically allow incoming traffic in response to that outgoing traffic. Therefore, allowing outbound ICMP response in the security group would not have any effect on the outgoing traffic being rejected by the NACL.
Therefore, the correct answer is B - Ensure that the NACL allows outbound ICMP response.
