The answer is that the web server on the instance is not responding to health check requests on port 80. When you see an ALB health check timeout even when security group allows ALB traffic, the Target.Timeout reason directly indicates that the load balancer sent a request but received no response within the timeout window—this is a layer-4 connectivity failure, not a firewall issue. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your ability to distinguish between network-layer access controls and application-layer readiness; a common trap is assuming that an open security group guarantees a healthy target, but the web server process itself must be actively listening on the configured port. Remember that security groups only control traffic permission, not whether the service is actually running. Memory tip: "Timeout means the server isn't talking, not that the door is locked."
DOP-C02 Resilient Cloud Solutions Practice Question
This DOP-C02 practice question tests your understanding of resilient cloud solutions. 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.
Network Topology
A DevOps engineer runs the above command and sees that one target is unhealthy with reason 'Target.Timeout'. The target is an EC2 instance running a web server on port 80. The security group for the instance allows inbound traffic on port 80 from the ALB's security group. What is the most likely cause of the health check failure?
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 web server on the instance is not responding to health check requests on port 80.
Option B is correct. The 'Target.Timeout' reason indicates that the health check request timed out, meaning the instance is not responding within the timeout period. The most common cause is that the web server is not running or is not listening on the correct port. Option A is wrong because the security group already allows traffic from the ALB. Option C is wrong because the health check is from the ALB, not the internet. Option D is wrong because the health check is to the instance's IP, not a public IP.
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 instance's network ACL is blocking inbound traffic from the internet.
Why it's wrong here
Health checks come from the ALB's private IPs, not the internet.
✓
The web server on the instance is not responding to health check requests on port 80.
Why this is correct
Timeout indicates no response from the web server.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
✗
The security group for the ALB does not allow outbound traffic to the instance.
Why it's wrong here
The ALB security group does not need to allow outbound; it's the instance SG that matters.
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.
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The web server on the instance is not responding to health check requests on port 80. — Option B is correct. The 'Target.Timeout' reason indicates that the health check request timed out, meaning the instance is not responding within the timeout period. The most common cause is that the web server is not running or is not listening on the correct port. Option A is wrong because the security group already allows traffic from the ALB. Option C is wrong because the health check is from the ALB, not the internet. Option D is wrong because the health check is to the instance's IP, not a public IP.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This DOP-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DOP-C02 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.