The answer is to check the security groups for the EC2 instances to ensure they allow traffic from the ALB on the listener port. This is correct because a 503 error from an Application Load Balancer indicates that the ALB is healthy but cannot establish a connection to its targets, often due to a network access control layer blocking the traffic. Even when the ALB itself reports as healthy, if the target’s security group does not permit inbound traffic from the ALB’s security group or CIDR range on the listener port, the ALB will fail to forward requests and return a 503. On the AWS Certified DevOps Engineer Professional DOP-C02 exam, this scenario tests your ability to distinguish between ALB health and target reachability, a common trap where candidates focus on the ALB’s status instead of the target-side configuration. A useful memory tip is “503 means the ALB can’t talk to the target—check the security group, not the ALB health.”
DOP-C02 Monitoring and Logging Practice Question
This DOP-C02 practice question tests your understanding of monitoring and logging. 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.
A DevOps engineer is troubleshooting a production issue where an Application Load Balancer (ALB) is returning 503 errors. The ALB targets are EC2 instances in an Auto Scaling group behind the ALB. The engineer checks the ALB access logs in Amazon S3 and finds that the ALB is healthy. However, the 503 errors persist. Which configuration should the engineer check next?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the security groups for the EC2 instances to ensure they allow traffic from the ALB on the listener port.
Option C is correct because 503 errors are often caused by the ALB being unable to establish a connection to the targets due to security groups blocking traffic. Checking the target group's health check settings and the security groups for the EC2 instances is the logical next step. Option A is wrong because the ALB is healthy per the logs. Option B is wrong because the issue is not about SSL certificates. Option D is wrong because AWS Shield Advanced is a DDoS protection service, not relevant to 503 errors.
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.
✗
Enable AWS Shield Advanced to protect the ALB from DDoS attacks.
Why it's wrong here
DDoS protection would not resolve a 503 error due to misconfiguration.
✗
Verify that the SSL certificate associated with the ALB is not expired.
Why it's wrong here
An expired SSL certificate would cause HTTPS errors, not 503.
✓
Check the security groups for the EC2 instances to ensure they allow traffic from the ALB on the listener port.
Why this is correct
Security groups blocking traffic from the ALB can cause 503 errors.
Related concept
Static NAT maps one inside address to one outside address.
✗
Check the ALB's target group health check settings and verify that the health check path is correct.
Why it's wrong here
The ALB is reported as healthy, so health check settings are likely correct.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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.
Monitoring and Logging — This question tests Monitoring and Logging — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Check the security groups for the EC2 instances to ensure they allow traffic from the ALB on the listener port. — Option C is correct because 503 errors are often caused by the ALB being unable to establish a connection to the targets due to security groups blocking traffic. Checking the target group's health check settings and the security groups for the EC2 instances is the logical next step. Option A is wrong because the ALB is healthy per the logs. Option B is wrong because the issue is not about SSL certificates. Option D is wrong because AWS Shield Advanced is a DDoS protection service, not relevant to 503 errors.
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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