ANS-C01 AWS PrivateLink Practice Question
A company uses AWS PrivateLink to access a SaaS application hosted in another AWS account. The SaaS provider has created a VPC endpoint service in their account. The consumer has created a VPC endpoint in their VPC. The consumer's VPC has a route table with a local route and a route to a NAT gateway. The VPC endpoint is associated with a security group that allows inbound HTTPS from the consumer's VPC CIDR. The consumer's EC2 instances can resolve the DNS name of the endpoint but cannot connect to the SaaS service. What is the most likely cause?
⚠ Common exam trap
A common mistake is to assume that if DNS resolution works, routing is fine. However, DNS resolves to private IPs, but the route table must have a route directing traffic to the VPC endpoint's ENI. Without that route, traffic may go to the NAT gateway (if present) or be dropped.
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
✓
The consumer's route table does not have a route to the VPC endpoint
For PrivateLink, the consumer's VPC endpoint is accessed via private IP addresses from the endpoint's elastic network interfaces (ENIs) in the consumer's VPC. The consumer's VPC route table must have a route to the endpoint's ENI to direct traffic correctly. The stem states that the security group allows inbound HTTPS from the consumer's VPC CIDR, so option C is not the cause. DNS resolution works because the endpoint's DNS name resolves to the private IPs of the ENIs, but without a matching route in the route table, traffic may be dropped or misrouted. Option D is the most likely cause.
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 consumer's route table routes traffic to the endpoint via the NAT gateway
Why it's wrong here
Incorrect. Traffic to a VPC endpoint is destined to private IP addresses of endpoint ENIs, not to the NAT gateway. The route table should have a route to the endpoint, not via the NAT gateway.
- ✗
The VPC endpoint service is not available in the consumer's Availability Zone
Why it's wrong here
Incorrect. Since DNS resolution works, the endpoint service is available in the consumer's AZ. DNS resolution would fail if the endpoint service were not available in the AZ.
- ✗
The VPC endpoint security group does not allow inbound HTTPS from the consumer's EC2 instances
Why it's wrong here
Incorrect. The stem explicitly states that the security group allows inbound HTTPS from the consumer's VPC CIDR, so this is not the cause of the connection failure.
- ✓
The consumer's route table does not have a route to the VPC endpoint
Why this is correct
Correct. The consumer's route table must contain a route that points traffic destined to the endpoint's service (or the endpoint's CIDR) to the VPC endpoint's ENI. Without this route, traffic may be dropped or misrouted, preventing connectivity despite successful DNS resolution.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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