The answer is that the application running on the instance is not responding correctly or has crashed. A 502 Bad Gateway error from an ALB specifically means the target closed the connection or failed to respond properly, often because the web server or application process has stopped or is overwhelmed, while the health check itself may still pass if it only tests a basic TCP port. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your ability to distinguish between ALB error codes: 502s point to application-level failures on the target, not network or security issues. A common trap is confusing a 502 with a 504 (gateway timeout) or assuming security group misconfigurations cause 502s, but those typically result in timeouts or refused connections. Remember the mnemonic: 502 = App is sick, 504 = Network is sore.
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
Refer to the exhibit. A DevOps engineer runs the describe-target-health command and receives the output shown. The ALB target group has two instances. One instance is healthy, and the other is unhealthy with a 502 error. What is the most likely cause of the 502 error?
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 application running on the instance is not responding correctly or has crashed.
Option D is correct because a 502 Bad Gateway error from an ALB indicates that the target (EC2 instance) has closed the connection or the application is not responding properly. Common causes include the web server or application crashing, or the target not being able to handle the request. Option A is wrong because security group rules would cause connection timeouts (504) or refused connections, not 502. Option B is wrong because a missing route table would cause network unreachability, not a 502. Option C is wrong because the health check path returning a 404 would result in a 404, not a 502.
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 security group for the instance does not allow inbound traffic on port 80 from the ALB.
Why it's wrong here
If security groups block traffic, the health check would fail with a connection timeout (504) or refused connection, not a 502.
✓
The application running on the instance is not responding correctly or has crashed.
Why this is correct
A 502 Bad Gateway indicates the target closed the connection or the application is faulty.
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.
✗
The instance's route table does not have a route to the internet gateway.
Why it's wrong here
Missing route would cause network unreachability, not a 502.
✗
The health check path is configured to return a 404 status code.
Why it's wrong here
A 404 response is not a 502; the health check would show a different status.
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.
Trap categories for this question
Command / output trap
A 404 response is not a 502; the health check would show a different status.
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 application running on the instance is not responding correctly or has crashed. — Option D is correct because a 502 Bad Gateway error from an ALB indicates that the target (EC2 instance) has closed the connection or the application is not responding properly. Common causes include the web server or application crashing, or the target not being able to handle the request. Option A is wrong because security group rules would cause connection timeouts (504) or refused connections, not 502. Option B is wrong because a missing route table would cause network unreachability, not a 502. Option C is wrong because the health check path returning a 404 would result in a 404, not a 502.
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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