The answer is to deregister the compromised EC2 instance from the target group and then stop it for forensic analysis. This is correct because deregistering triggers connection draining, which allows existing sessions to complete while the Application Load Balancer immediately stops routing new traffic to the instance, preserving application availability for the remaining healthy instances. Stopping the instance, rather than terminating it, freezes the volatile memory and disk state, which is essential for a proper forensic investigation. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how to isolate compromised EC2 instances for forensics without disrupting production traffic—a common trap is to terminate the instance or simply detach it from the Auto Scaling group, both of which destroy evidence or fail to stop traffic cleanly. Remember the key sequence: deregister first to cut traffic, then stop to preserve the crime scene.
DOP-C02 Incident and Event Response Practice Question
This DOP-C02 practice question tests your understanding of incident and event response. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
A company runs a critical application on Amazon EC2 instances behind an Application Load Balancer. During a security incident, the security team needs to isolate a compromised instance for forensic analysis without affecting the application's availability. What is the MOST effective action to take?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Deregister the instance from the target group and stop the instance for forensic analysis.
Deregistering the instance from the target group removes it from the Application Load Balancer's routing, ensuring no new traffic is sent to it while existing connections drain (connection draining). Stopping the instance preserves its memory and disk state for forensic analysis without impacting application availability, as the remaining healthy instances continue to serve traffic.
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.
✓
Deregister the instance from the target group and stop the instance for forensic analysis.
Why this is correct
Isolates the instance while maintaining availability.
Related concept
Read the scenario before looking for a memorised answer.
✗
Modify the security group of the instance to deny all inbound and outbound traffic.
Why it's wrong here
May not isolate the instance from internal network traffic if other security groups allow it.
✗
Terminate the compromised instance immediately to prevent further damage.
Why it's wrong here
Termination loses forensic data and is irreversible.
✗
Change the subnet route table to route traffic away from the compromised instance.
Why it's wrong here
Affects all instances in the subnet, causing potential disruption.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse network-level isolation (security groups or route tables) with application-level isolation (target group deregistration), failing to recognize that the ALB continues to route traffic to a registered instance regardless of its security group or subnet routing.
Detailed technical explanation
How to think about this question
Connection draining (deregistration delay) in ALB target groups allows in-flight requests to complete before the instance is fully removed; the default delay is 300 seconds but can be adjusted. Stopping an EC2 instance transitions it to the 'stopped' state, preserving the EBS root volume and instance store (if any) for later forensic attachment to a separate analysis instance, while the instance's private IP and ENI remain allocated until termination. This approach aligns with the AWS Well-Architected Framework's incident response pillar, emphasizing isolation without data loss.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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: Deregister the instance from the target group and stop the instance for forensic analysis. — Deregistering the instance from the target group removes it from the Application Load Balancer's routing, ensuring no new traffic is sent to it while existing connections drain (connection draining). Stopping the instance preserves its memory and disk state for forensic analysis without impacting application availability, as the remaining healthy instances continue to serve traffic.
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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