Exam AWS Certified SysOps Administrator - Associate topic 1 question 253 discussion

https://repost.aws/knowledge-center/connect-s3-vpc-endpoint#:~:text=An%20outbound%20rule%20allowing%20traffic%20to%20the%20ID%20of%20the%20prefix%20list%20associated%20with%20the%20gateway%20VPC%20endpoint.

It’s C - subnet route table must have an entry pointing to the VPC gateway prefix.

Selected Answer: A

WNhay Not D>>While this could be part of a broader solution to ensure that the S3 bucket accepts uploads from the instances in the private subnet, this step alone does not solve the connectivity issue. The EC2 instance must be able to reach the S3 bucket, which is a matter of network configuration, not bucket policy.

It should be C

C makes the most sense here.

When an Amazon S3 gateway endpoint is configured in a VPC, the private subnets within the VPC need to have their route tables updated to route the S3 traffic to the gateway endpoint, instead of the internet. Without the appropriate route table updates, the EC2 instance in the private subnet will not be able to communicate with the S3 bucket, even if the instance has the necessary S3 permissions. Option A is incorrect because the issue is not related to the IAM role permissions, but rather the routing configuration. Option B is incorrect because allowing outbound traffic to 0.0.0.0/0 on port 80 is not necessary to solve this problem. The issue is with the routing to the S3 gateway endpoint, not the internet access. Option D is incorrect because the S3 bucket policy is not the issue here. The problem is with the routing, not the permissions on the S3 bucket.

Why Option C is Essential: Directing Traffic to S3: Without the correct route in the subnet’s route table, the instance cannot send traffic to S3 because it doesn’t know that it should use the gateway endpoint. Using the Gateway Endpoint: The S3 gateway endpoint enables access to S3 without internet. For this to work, the route table must have a route for the S3 prefix list pointing to the endpoint. Why Option D Alone is Insufficient: Access Control vs. Network Path: S3 Bucket Policy: Controls who (or what) can access the bucket and perform specific actions (e.g., s3:PutObject). While necessary for access control, it does not configure the network path needed for the EC2 instance to reach S3. Network Configuration: Ensures that there is a valid route for the traffic from the EC2 instance to S3.

To resolve the issue where a user cannot upload a file to an Amazon S3 bucket from an Amazon EC2 instance in a private subnet without outbound internet access, even with an S3 gateway endpoint configured, you should: C. Update the EC2 subnet route table to include the S3 prefix list destination routes to the S3 gateway endpoint. The primary step needed here is to ensure that the route table associated with the private subnet where the EC2 instance resides correctly routes S3 traffic to the S3 gateway endpoint, enabling secure, private connectivity to S3.

WHY NOT A or D. the issue is related to network connectivity, not permissions.

The answer is C: Each subnet route table must have a route that sends traffic destined for the service to the gateway endpoint using the prefix list for the service https://docs.aws.amazon.com/vpc/latest/privatelink/gateway-endpoints.html

A - you need access to allow upload Put object. why not D ? well this is a potential cause, however, when you create S3 Gateway Endpoint you can associate your subnets and that creates a route automatically. https://docs.aws.amazon.com/vpc/latest/privatelink/vpc-endpoints-s3.html#create-gateway-endpoint-s3:~:text=For%20Route%20tables%2C%20select%20the%20route%20tables%20to%20be%20used%20by%20the%20endpoint.%20We%20automatically%20add%20a%20route%20that%20points%20traffic%20destined%20for%20the%20service%20to%20the%20endpoint%20network%20interface. Why not C ? Once traffic is routed via the S3 Gateway endpoint, then the private subnet CIDR is irrelevant.

Why not A? Its actually A and C.

When you create a gateway endpoint, you select the VPC route tables for the subnets that you enable. The following route is automatically added to each route table that you select. The destination is a prefix list for the service owned by AWS and the target is the gateway endpoint.

I tackled a similar issue in work this week and I still couldn't confidently answer this. I click-ops a dummy network my findings: routes are by default, VPC -> local prefix list would apply to a SG. Endpoint policy by default allows absolutely everything. By default an s3 bucket has no policy.

In the question As the SysOps admin already configures an Amazon S3 gateway endpoint in a VPC then Updating the EC2 subnet route table to include the S3 prefix list destination routes to the S3 gateway endpoint is not required. So just Update the S3 bucket policy to allow s3:PutObject access from the private subnet CIDR block.

"For Route tables, select the route tables to be used by the endpoint. We automatically add a route that points traffic destined for the service to the endpoint network interface." https://docs.aws.amazon.com/vpc/latest/privatelink/vpc-endpoints-s3.html#associate-route-tables-s3 So route should be already enable by default, Change need to bedone in s3 bucket policy