Overview

AWS Lambda is a serverless computing platform that allows you to run code without provisioning or managing servers. One common requirement for Lambda functions is the ability to interact with external internet resources, such as APIs and databases, or send out requests to web services. In this article, we'll explore how AWS Lambda connects to the Internet, along with technical explanations, examples, and considerations for optimal configuration.

How AWS Lambda Connects to the Internet

Default Internet Access

By default, AWS Lambda enables internet access for outgoing requests when not associated with a Virtual Private Cloud (VPC). Without attaching the Lambda function to a VPC, it can freely communicate with any external service on the web. This default setup is simple and sufficient for many use cases, such as calling public APIs, accessing cloud services, or gathering data from the internet.

Communication Through a VPC

When a Lambda function needs to access resources within a VPC, it must be configured to connect to the VPC. While this grants the Lambda function access to private resources (such as RDS databases or EC2 instances) within the VPC, external internet access becomes complicated and, by default, is not enabled.

To enable internet access for a Lambda function within a VPC, the following components must be configured:

1. Subnets: Attach the Lambda functions to private subnets. To ensure routing to the internet, at least one of these subnets should have a private IP address range.
2. NAT Gateway (or NAT Instance): A NAT Gateway must be configured in a public subnet with an elastic IP address, allowing subnets within the VPC to initiate outbound traffic to the internet while blocking inbound traffic initiated by the internet.
3. Route Table: Update the route table associated with the private subnets to direct all internet-bound traffic to the NAT Gateway.

Example Scenario

Let's consider a Lambda function that needs to access both a private RDS database inside a VPC and call an external API to fetch data. Here's how you’d set it up:

1. Create a VPC: Design your VPC with at least one public and one private subnet.
2. Deploy a NAT Gateway: Set this up in the public subnet.
3. Create a Lambda function: Assign it to a private subnet within the VPC.
4. Modify the Route Table: Route all outbound traffic from the private subnet through the NAT Gateway.
5. Update Security Groups and NACLs: Ensure that security groups and network ACLs permit outgoing traffic.

This configuration allows the Lambda function to interact seamlessly with both the private RDS database and any external web service using the internet.

Key Considerations

• Cost: Using a NAT Gateway entails additional costs, as AWS charges per hour of operation and per GB of data processed. Consider optimizing the use of internet calls within your Lambda function to manage expenses.
• Latency and Performance: Make sure the NAT Gateway is properly scaled, particularly for high-throughput applications, to avoid throttling and optimize performance.
• Security: Although NAT Gateways inherently block incoming internet traffic, additional security best practices should be followed, such as restricting outbound requests using security groups and monitoring with AWS CloudTrail and VPC Flow Logs.

Summary Table

Conclusion

Configuring AWS Lambda for internet access involves a balance of functionality and security considerations. By default, Lambda can access the internet seamlessly, but when located within a VPC, additional components such as NAT Gateways become necessary. Proper setup ensures that Lambda functions can effectively communicate with external resources while maintaining the security and integrity of your AWS environment.