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.
Network Topology
An EC2 instance is in 'running' state according to the CLI output, but the application hosted on it is unreachable. The DevOps engineer checks the security group and finds it allows inbound HTTP traffic from 0.0.0.0/0. The instance has a public IP. What is the MOST likely issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The instance's OS firewall (e.g., iptables) is blocking the traffic.
Option D is correct because the instance's operating system may have a firewall (e.g., iptables) blocking inbound traffic. Option A is wrong because the instance has a public IP. Option B is wrong because security groups are stateful and allow return traffic. Option C is wrong because the security group allows all inbound HTTP.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The network ACL is blocking inbound HTTP traffic.
Why it's wrong here
Network ACLs are stateless and would need explicit rules; but the security group is more likely the issue.
✗
The instance does not have a public IP address assigned.
Why it's wrong here
The instance has a public IP.
✗
The security group is attached to the instance but does not allow inbound HTTP.
The instance's OS firewall (e.g., iptables) is blocking the traffic.
Why this is correct
OS-level firewalls can block traffic even if security groups allow it.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related DOP-C02 ACL questions on filtering logic and placement.
Incident and Event Response — This question tests Incident and Event Response — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The instance's OS firewall (e.g., iptables) is blocking the traffic. — Option D is correct because the instance's operating system may have a firewall (e.g., iptables) blocking inbound traffic. Option A is wrong because the instance has a public IP. Option B is wrong because security groups are stateful and allow return traffic. Option C is wrong because the security group allows all inbound HTTP.
What should I do if I get this DOP-C02 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related DOP-C02 ACL questions on filtering logic and placement.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Question Discussion
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