The Lambda function, running within a VPC with no internet access, cannot reach the public AWS Secrets Manager endpoint. The most secure method to enable this communication is by creating an interface VPC endpoint for Secrets Manager. This endpoint, powered by AWS PrivateLink, creates an Elastic Network Interface (ENI) with a private IP address inside the VPC. All traffic from the Lambda function to the Secrets Manager service is then routed securely through this private endpoint, remaining entirely within the AWS network and never traversing the public internet. This approach maintains the security posture of the isolated VPC.

A. A NAT gateway requires an internet gateway and routes traffic over the public internet, which is less secure than using a private endpoint.

B. Gateway VPC endpoints are a different technology and are only supported for Amazon S3 and Amazon DynamoDB, not for AWS Secrets Manager.

D. An internet gateway provides direct, two-way internet access, which is the least secure option and contradicts the requirement of a private, isolated VPC.

1. AWS Secrets Manager User Guide, "Use AWS Secrets Manager with VPC endpoints": "To allow your Lambda rotation function to access Secrets Manager and the database or service without that traffic leaving the Amazon network, we recommend you configure interface VPC endpoints. Interface endpoints send traffic to AWS services using AWS PrivateLink." This directly confirms that an interface endpoint is the recommended and secure solution.

2. Amazon VPC User Guide, "Interface VPC endpoints (AWS PrivateLink)": This document lists the AWS services available through interface endpoints, which includes AWS Secrets Manager. It states, "An interface endpoint is an elastic network interface with a private IP address that serves as an entry point for traffic destined to a supported AWS service..."

3. Amazon VPC User Guide, "Gateway VPC endpoints": This official documentation explicitly states, "A gateway endpoint is a gateway that you specify as a target for a route in your route table for traffic destined to a supported AWS service. The following AWS services are supported: Amazon S3, DynamoDB." This confirms that a gateway endpoint cannot be used for Secrets Manager.

4. AWS Lambda Developer Guide, "Configuring VPC access for a Lambda function": "When you connect a function to a VPC, it doesn't have access to the internet unless your VPC provides it... To grant your function internet access, you can configure a NAT gateway... To access AWS services without using a NAT gateway, you can configure an interface VPC endpoint." This explains the networking context and presents the interface endpoint as the secure alternative to a NAT gateway.