Why Missing Outbound NACL Rules for Ephemeral Ports Cause Connection Timeouts
An application runs on EC2 instances in private subnets behind an Application Load Balancer (ALB). Security groups allow inbound HTTPS (443) from the ALB’s security group to the instance security group, and outbound from instances is set to allow ephemeral ports.
Despite this, clients see connection timeouts. After reviewing network ACLs, you find the NACL associated with the instance subnet has an inbound allow for destination port 443, but it does not have a corresponding outbound allow for ephemeral ports.
What is the most likely reason the traffic fails, and what should be updated?
Quick Answer
The failure traces back to a fundamental difference between security groups and network ACLs: security groups are stateful, so once inbound traffic is allowed, the corresponding return traffic is automatically permitted without needing an explicit outbound rule, but NACLs are stateless and evaluate inbound and outbound traffic as completely separate rule sets. In this scenario the security groups are configured correctly and aren't the problem, but the NACL only has an inbound allow for port 443 and no outbound allow for the ephemeral port range that response traffic actually uses to get back to the client, so even though the request reaches the instance, the reply is blocked on its way out and the connection times out. This is why simply confirming the security group rules look right isn't enough when troubleshooting NACL-related connectivity issues; both directions have to be checked independently, and a rule that looks correct for inbound traffic says nothing about whether the matching outbound path is open. The fix is to add an outbound NACL rule permitting the ephemeral port range back to the client. Any time symptoms include inbound traffic reaching an instance but responses failing, with security groups already correctly configured, the stateless nature of NACLs and a missing outbound rule for ephemeral ports is the pattern to check first.
⚠ Common exam trap
Watch out — candidates often assume security groups alone handle all traffic filtering, forgetting that NACLs are stateless and require explicit outbound rules for return traffic, especially for ephemeral ports.
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
✓
NACLs are stateless, so you must update the NACL to allow the return (outbound) ephemeral port range; security groups alone cannot override a blocked NACL.
Network ACLs are stateless, meaning they do not automatically allow return traffic. Even though the security group allows inbound HTTPS from the ALB, the NACL blocks the response traffic because it lacks an outbound rule for ephemeral ports (typically 1024-65535). Since NACLs are evaluated before security groups, a missing outbound allow rule causes the connection to time out.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
NACLs are stateless, so you must update the NACL to allow the return (outbound) ephemeral port range; security groups alone cannot override a blocked NACL.
Why this is correct
Stateless NACLs require both inbound and outbound rules. Missing outbound for ephemeral ports will block return traffic even if SG rules are correct.
- ✗
NACLs are stateful and automatically track connections; the fix is to add a new inbound rule to the security group for client source ports.
Why it's wrong here
NACLs are stateless; connection tracking occurs at the security group level only. Adding SG inbound rules won’t unblock outbound traffic blocked by the NACL.
When this WOULD be correct
This option would be correct if the question stated that NACLs are stateful and the security group was blocking return traffic. For example: 'An application uses a stateful firewall (like a security group) and clients see timeouts; what is the fix?'
- ✗
The issue is caused by ALB health checks; configure a new target group health check on port 80 so traffic can be routed.
Why it's wrong here
Health checks may affect routing, but the symptom points to NACL blocking return packets. Changing health check ports won’t address NACL stateless behavior.
When this WOULD be correct
In a scenario where an ALB target group health check is failing because the health check path or port is not configured correctly (e.g., the application listens on port 8080 but health check targets port 80), updating the health check to the correct port would resolve the issue.
- ✗
Because instances are in private subnets, add a NAT gateway so return traffic can reach the internet over dynamic routing.
Why it's wrong here
NAT affects internet egress, not east-west return paths inside the VPC subnet. The observed NACL rule gap is within the subnet path.
When this WOULD be correct
In a scenario where EC2 instances in private subnets need to download updates from the internet, and the outbound NACL blocks ephemeral ports, adding a NAT gateway would be correct to allow outbound traffic and its return traffic.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The SAA-C03 exam frequently reuses these exact scenarios with slightly different constraints.
✓NACLs are stateless, so you must update the NACL to allow the return (outbound) ephemeral port range; security groups alone cannot override a blocked NACL.Correct answer▾
Why this is correct
Stateless NACLs require both inbound and outbound rules. Missing outbound for ephemeral ports will block return traffic even if SG rules are correct.
✗NACLs are stateful and automatically track connections; the fix is to add a new inbound rule to the security group for client source ports.Wrong answer — click to see why▾
Why this is wrong here
NACLs are stateless, not stateful; they do not automatically track connections. The issue is missing outbound ephemeral port rules in the NACL, not security group inbound rules for client source ports.
★ When this WOULD be the correct answer
This option would be correct if the question stated that NACLs are stateful and the security group was blocking return traffic. For example: 'An application uses a stateful firewall (like a security group) and clients see timeouts; what is the fix?'
Why candidates choose this
Candidates may confuse NACLs with security groups, assuming NACLs are stateful like security groups, and think the fix involves adding inbound rules to the security group for client ports.
✗The issue is caused by ALB health checks; configure a new target group health check on port 80 so traffic can be routed.Wrong answer — click to see why▾
Why this is wrong here
The question describes connection timeouts due to missing outbound NACL rules for ephemeral ports, not health check failures. ALB health checks are not mentioned as failing, and changing the health check port does not address the stateless NACL issue.
★ When this WOULD be the correct answer
In a scenario where an ALB target group health check is failing because the health check path or port is not configured correctly (e.g., the application listens on port 8080 but health check targets port 80), updating the health check to the correct port would resolve the issue.
Why candidates choose this
Candidates may confuse connectivity issues with health check misconfigurations, especially when ALB is involved, and assume that adjusting health check settings can fix general traffic flow problems.
✗Because instances are in private subnets, add a NAT gateway so return traffic can reach the internet over dynamic routing.Wrong answer — click to see why▾
Why this is wrong here
The issue is not about internet connectivity; instances are in private subnets but the ALB is in a public subnet and handles internet-facing traffic. A NAT gateway is for outbound internet access from private instances, not for fixing return traffic blocked by a stateless NACL.
★ When this WOULD be the correct answer
In a scenario where EC2 instances in private subnets need to download updates from the internet, and the outbound NACL blocks ephemeral ports, adding a NAT gateway would be correct to allow outbound traffic and its return traffic.
Why candidates choose this
Candidates may confuse network connectivity issues with internet access requirements, assuming private subnets always need a NAT gateway for any traffic flow, even when the traffic is internal to the VPC via an ALB.
Analysis generated from the official SAA-C03blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Same concept, more angles
1 more way this is tested on SAA-C03
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An application runs on EC2 instances in private subnets behind an Application Load Balancer (ALB). Security groups allow inbound HTTPS (443) from the ALB’s security group to the instance security group, and outbound from instances is set to allow ephemeral ports. Despite this, clients see connection timeouts. After reviewing network ACLs, you find the NACL associated with the instance subnet has an inbound allow for destination port 443, but it does not have a corresponding outbound allow for ephemeral ports. What is the most likely reason the traffic fails, and what should be updated?
medium- ✓ A.NACLs are stateless, so you must update the NACL to allow the return (outbound) ephemeral port range; security groups alone cannot override a blocked NACL.
- B.NACLs are stateful and automatically track connections; the fix is to add a new inbound rule to the security group for client source ports.
- C.The issue is caused by ALB health checks; configure a new target group health check on port 80 so traffic can be routed.
- D.Because instances are in private subnets, add a NAT gateway so return traffic can reach the internet over dynamic routing.
Why A: Network ACLs are stateless, meaning they do not automatically allow return traffic. Even though the security group allows inbound HTTPS from the ALB, the NACL blocks the return traffic because it lacks an outbound rule for ephemeral ports (typically 1024-65535). Since NACLs are evaluated before security groups, a missing outbound allow rule causes the response packets to be dropped, resulting in connection timeouts.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SAA-C03 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SAA-C03 exam.