The answer is failing NLB target group health checks causing connection failures. When health checks fail, the Network Load Balancer marks the instance as unhealthy and stops forwarding traffic to it, which explains why tcpdump shows SYN packets from the NLB’s private IPs reaching the instance at 10.0.1.5, yet no internet HTTPS connection can be established. This scenario tests your understanding of how NLB health checks interact with target group registration and traffic flow, a common trap on the AWS Certified Advanced Networking Specialty ANS-C01 exam where engineers mistakenly blame security groups or route tables. The key insight is that NLB does not preserve source IP by default, so the security group allowing inbound HTTPS from 0.0.0.0/0 is not the issue—the health check failure is. Memory tip: “Healthy check, traffic flows; failing check, traffic goes.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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.
Exhibit
Refer to the exhibit.
[root@ip-10-0-1-5 ~]# tcpdump -i eth0 -n port 443
09:32:15.123456 IP 203.0.113.5.34567 > 10.0.1.5.443: Flags [S], seq 12345, win 65535, options [mss 1460], length 0
09:32:15.123456 IP 10.0.1.5.443 > 203.0.113.5.34567: Flags [S.], seq 54321, ack 12346, win 65535, options [mss 1460], length 0
09:32:15.123456 IP 203.0.113.5.34567 > 10.0.1.5.443: Flags [.], ack 54322, win 65535, length 0
A security engineer runs tcpdump on an EC2 instance (10.0.1.5) and sees the output above. The instance is in a private subnet with a security group that allows inbound HTTPS from 0.0.0.0/0. The instance is behind a Network Load Balancer (NLB) that has a public IP. The engineer is unable to establish an HTTPS connection from the internet. What is the most likely cause?
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 NLB's target group health checks are failing, causing the NLB to consider the instance unhealthy.
The tcpdump output shows the NLB's health check traffic (SYN packets from the NLB's private IPs) reaching the instance, but the engineer cannot establish an HTTPS connection from the internet. If the target group health checks are failing, the NLB marks the instance as unhealthy and stops forwarding traffic to it, which would prevent any internet HTTPS connections from reaching the instance. This is the most likely cause because the security group allows inbound HTTPS from 0.0.0.0/0, and the NLB does not preserve source IP by default, so the security group is not blocking traffic from the internet.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 NLB is not preserving the source IP, so the security group is blocking the traffic.
Why it's wrong here
The source IP seen is the NLB's IP, but the security group allows all inbound, so it's not blocking.
✓
The NLB's target group health checks are failing, causing the NLB to consider the instance unhealthy.
Why this is correct
The tcpdump shows the handshake from the NLB, but if health checks fail, the NLB stops sending traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The instance's operating system firewall is blocking the connection.
Why it's wrong here
The tcpdump shows successful handshake, so OS firewall is not blocking.
✗
The security group is blocking inbound traffic from the NLB.
Why it's wrong here
The security group allows all inbound HTTPS, so it is not blocking.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that security group rules are the primary cause of connectivity issues with NLBs, but the real trap is that candidates overlook how NLB health checks directly control traffic forwarding, and a failing health check silently drops all traffic even when security groups and routes are correctly configured.
Trap categories for this question
Command / output trap
The tcpdump shows successful handshake, so OS firewall is not blocking.
Detailed technical explanation
How to think about this question
NLB health checks are sent from the NLB's private subnet IPs (e.g., 10.0.0.1, 10.0.0.2) to the target instance's health check port (default: same as traffic port). If the instance fails to respond with a SYN-ACK within the health check interval and threshold, the NLB marks it unhealthy and stops routing traffic. In a private subnet with a security group allowing 0.0.0.0/0, the health check traffic should succeed unless the instance's application (e.g., HTTPS server) is not listening or the health check path/port is misconfigured.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The NLB's target group health checks are failing, causing the NLB to consider the instance unhealthy. — The tcpdump output shows the NLB's health check traffic (SYN packets from the NLB's private IPs) reaching the instance, but the engineer cannot establish an HTTPS connection from the internet. If the target group health checks are failing, the NLB marks the instance as unhealthy and stops forwarding traffic to it, which would prevent any internet HTTPS connections from reaching the instance. This is the most likely cause because the security group allows inbound HTTPS from 0.0.0.0/0, and the NLB does not preserve source IP by default, so the security group is not blocking traffic from the internet.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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