The correct answer is that the instance’s security group does not allow inbound traffic from the ALB on port 80. A `Target.Timeout` error means the ALB sent a health check request but never received a response within the timeout period, which occurs when the security group silently drops the traffic rather than rejecting it with a reset. Since the application responds locally on port 80, the process is running, ruling out application-level failures; the issue is purely a network-layer block. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding of how security groups act as stateful firewalls that must explicitly permit health check traffic from the ALB’s source IP range or security group. A common trap is confusing timeouts with HTTP errors like 404—remember that a timeout always points to connectivity, not content. Memory tip: “Timeout = Traffic blocked; 404 = File not found.”
DOP-C02 Resilient Cloud Solutions Practice Question
This DOP-C02 practice question tests your understanding of resilient cloud solutions. 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
A DevOps engineer runs the above command and sees that instance i-0abcd1234efgh5678 is unhealthy with reason 'Target.Timeout'. The instance is running and the application on port 80 responds to curl from the instance itself. What is the MOST likely cause?
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 security group for the instance does not allow inbound traffic from the ALB on port 80.
Option C is correct because a timeout suggests the ALB cannot reach the instance, likely due to a security group blocking traffic from the ALB. Option A is wrong if the application responds locally, the process is running. Option B is wrong because a missing index.html would cause HTTP 404, not timeout. Option D is wrong because the health check path is not specified; the default is '/' which typically returns something.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 ALB health check interval is set too high.
Why it's wrong here
Interval affects frequency, not timeout.
✗
The web server process is not running on the instance.
Why it's wrong here
The instance responds locally, so the process is running.
✗
The health check path returns a 404 status code.
Why it's wrong here
A 404 would result in 'unhealthy' with reason 'Application' or 'Failed', not timeout.
✓
The security group for the instance does not allow inbound traffic from the ALB on port 80.
Why this is correct
A timeout typically indicates a network connectivity issue between ALB and instance.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DOP-C02 NAT questions on configuration and troubleshooting.
Resilient Cloud Solutions — This question tests Resilient Cloud Solutions — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The security group for the instance does not allow inbound traffic from the ALB on port 80. — Option C is correct because a timeout suggests the ALB cannot reach the instance, likely due to a security group blocking traffic from the ALB. Option A is wrong if the application responds locally, the process is running. Option B is wrong because a missing index.html would cause HTTP 404, not timeout. Option D is wrong because the health check path is not specified; the default is '/' which typically returns something.
What should I do if I get this DOP-C02 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DOP-C02 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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