The answer is to verify that the instance has a public IP address, check the security group inbound rules for port 22, and review the network ACL (NACL) inbound rules for ephemeral ports. This is correct because even if a security group allows SSH traffic, NACLs are stateless and must explicitly permit both inbound traffic on port 22 and outbound return traffic on ephemeral ports (typically 1024-65535); if either rule is missing, the connection will silently drop. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of the layered security model—security groups are stateful and automatically allow return traffic, while NACLs require explicit bidirectional rules. A common trap is assuming a security group allow rule alone guarantees connectivity, forgetting that NACLs operate at the subnet level and can override that permission. Memory tip: think "SG is stateful, NACL is not—check both directions for SSH to work."
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which THREE steps should a DevOps engineer take to troubleshoot an EC2 instance that cannot be reached via SSH? (Choose three.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the network ACL inbound rules for the subnet.
Option A is correct because network ACLs (NACLs) are stateless firewall rules applied at the subnet level. If the inbound rule for ephemeral ports or port 22 is not explicitly allowed, SSH traffic will be dropped even if the security group permits it. Checking NACL inbound rules is a fundamental step in troubleshooting connectivity issues.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Check the network ACL inbound rules for the subnet.
Why this is correct
NACLs control traffic at the subnet level.
Related concept
Read the scenario before looking for a memorised answer.
✗
Verify that the corporate firewall allows SSH to the instance.
Create an AMI from the instance and launch a new one.
Why it's wrong here
This is a recovery step, not troubleshooting.
✓
Check the security group inbound rules for port 22.
Why this is correct
Security group rules control traffic to the instance.
Related concept
Read the scenario before looking for a memorised answer.
✓
Verify that the instance has a public IP address.
Why this is correct
Without a public IP, SSH from internet is impossible.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often overlook network ACLs and focus only on security groups, or they mistake a recovery action (creating an AMI) for a troubleshooting step, when the correct approach is to systematically verify the layered network controls (NACLs, security groups, and public IP assignment).
Detailed technical explanation
How to think about this question
Security groups are stateful, meaning that if an inbound rule allows traffic, the outbound return traffic is automatically allowed regardless of outbound rules. In contrast, NACLs are stateless and require explicit rules for both inbound and outbound traffic, including ephemeral ports (typically 1024-65535) for the return path. A common misconfiguration is allowing inbound SSH (port 22) in the NACL but forgetting to allow outbound ephemeral ports, causing the SSH handshake to fail.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Incident and Event Response — This question tests Incident and Event Response — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Check the network ACL inbound rules for the subnet. — Option A is correct because network ACLs (NACLs) are stateless firewall rules applied at the subnet level. If the inbound rule for ephemeral ports or port 22 is not explicitly allowed, SSH traffic will be dropped even if the security group permits it. Checking NACL inbound rules is a fundamental step in troubleshooting connectivity issues.
What should I do if I get this DOP-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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