Networking and VPC configurations

Lambda functions always run inside VPCs owned by the Lambda service. As with customer-owned VPCs, this allows the service to apply network access and security rules to everything within the VPC. These VPCs are not visible to customers, the configurations are maintained automatically, and monitoring is managed by the service.

When you use some AWS services, they create resources that are only accessible from within your customer VPC. To access these resources with Lambda, your Lambda function must also be configured for access to the same VPC.

By default, Lambda functions have access to the public internet. This is not the case after they have been configured with access to one of your VPCs. If you continue to need access to resources on the internet, set up a NAT instance or Amazon NAT Gateway. Alternatively, you can also use VPC endpoints to enable private communications between your VPC and supported AWS services.

The high availability of the Lambda service depends upon access to multiple Availability Zones within the Region where your code runs. When you create a Lambda function without a VPC configuration, it’s automatically available in all Availability Zones within the Region. When you set up VPC access, you choose which Availability Zones the Lambda function can use. As a result, to provide continued high availability, ensure that the function has access to at least two Availability Zones.

The Lambda service uses a Network Function Virtualization platform to provide NAT capabilities from the Lambda VPC to customer VPCs. This configures the required elastic network interfaces (ENIs) at the point where Lambda functions are created or updated. It also enables ENIs from your account to be shared across multiple execution environments, which allows Lambda to make more efficient use of a limited network resource when functions scale.

Since ENIs are an exhaustible resource and there is a soft limit of 250 ENIs per Region, you should monitor elastic network interface usage if you are configuring Lambda functions for VPC access. Generally, if you increase concurrency limits in Lambda, you should evaluate if you need an elastic network interface increase. If the limit is reached, this causes invocations of VPC-enabled Lambda functions to be throttled.

Most serverless services can be used without further VPC configuration, while most instance-based services require VPC configuration:

To learn more, about how VPC networking works with Lambda functions, see this Compute Blog post.