DOP-C02 Resilient Cloud Solutions Practice Question
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?
Answer choices
Why each option matters
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.
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 C is correct because a non-responsive web server on port 80 would cause the timeout. Option A is incorrect because health checks are performed over private IPs, not public IPs. Option B is incorrect because the network ACL applies at the subnet level and would affect all traffic; the ALB's security group is already allowed inbound, and the instance's security group allows inbound from the ALB. Option D is incorrect because the ALB's security group does not need to allow outbound to the instance; traffic flows from the ALB to the instance, and the instance's security group must allow inbound from the ALB.
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 does not have a public IP address.
Why it's wrong here
ALB communicates with instances via private IPs.
- ✗
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.