The answer is to verify that the application on the unhealthy instance is configured to respond to the root path '/' with a 200 status code. A 503 error from an ALB health check specifically means the target server is reachable and the TCP connection succeeded, but the application itself returned a 503 HTTP status, indicating it cannot handle the request—often because the health check path is not returning the expected 200. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your understanding that a 503 is an application-level error, not a network or timeout issue, and it’s a common trap to confuse it with a connection failure or wrong port. Remember, the health check is already hitting port 80 and the root path, so the fix is purely about the application’s response logic. Memory tip: “503 means the app is alive but not giving a 200—check your code, not your connection.”
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 one target is unhealthy with a 503 error. The application is a web server running on port 80. The health check is configured to hit the root path '/'. Which action should the engineer take to resolve the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Verify that the application on the unhealthy instance is configured to respond to '/' with a 200 status code
A 503 error indicates the web server is running but cannot handle the request, likely because the application is not responding correctly to the health check path. Option A is wrong because a 503 means the server is reachable. Option C is wrong because the health check is already on port 80. Option D is wrong because a 503 is not a connection timeout.
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.
✗
Change the health check port to 443 and use HTTPS
Why it's wrong here
The health check is already on port 80 and the server is responding.
✓
Verify that the application on the unhealthy instance is configured to respond to '/' with a 200 status code
Why this is correct
The health check expects a 200 response; a 503 means the app is not serving the root path correctly.
Related concept
Static NAT maps one inside address to one outside address.
✗
Increase the health check interval and timeout settings
Why it's wrong here
A 503 is not a timeout; adjusting interval won't fix the response code.
✗
Check the security group rules for the target group to ensure port 80 is open
Why it's wrong here
A 503 indicates the server is reachable, not a security group issue.
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: Verify that the application on the unhealthy instance is configured to respond to '/' with a 200 status code — A 503 error indicates the web server is running but cannot handle the request, likely because the application is not responding correctly to the health check path. Option A is wrong because a 503 means the server is reachable. Option C is wrong because the health check is already on port 80. Option D is wrong because a 503 is not a connection timeout.
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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