DOP-C02 Resilient Cloud Solutions Practice Question
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?
⚠ Common exam trap
DOP-C02 often tests troubleshooting of load balancer health checks, and candidates might overlook security group rules, assuming the application is at fault because local curl works.
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 security group for the instance does not allow inbound traffic from the ALB on port 80.
The 'Target.Timeout' health check reason indicates that the load balancer's health check request timed out, meaning it could not establish a connection or receive a response within the timeout period. Since the instance responds to curl locally, the application is running, but the ALB cannot reach it. The most likely cause is that the instance's security group does not allow inbound traffic from the ALB on port 80, blocking the health check requests.
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.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
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