Why can't an AWS lambda function inside a public subnet in a VPC connect to the internet?

Question

I've followed the tutorial here to create a VPC with public and private subnets.

Then I set up an AWS lambda function inside the public subnet to test if it could connect to the outside internet.

Here's my lambda function written in python3

import requests

def lambda_handler(event, context):
    r = requests.get('http://www.google.com')
    print(r)

The function above failed to fetch the content of http://www.google.com when I set it inside the public subnet in a VPC.

Here's the error message:

"errorMessage": "HTTPConnectionPool(host='www.google.com', port=80): Max retries exceeded with url: / (Caused by NewConnectionError(': Failed to establish a new connection: [Errno 110] Connection timed out',))", "errorType": "ConnectionError",

I don't understand why.

The route table of the public subnet looks like this:

The GET request to http://www.google.com should match igw-XXXXXXXXX target. Why can't the internet-gateway(igw) deliver the request to http://www.google.com and get back the website content?

This article says that I must set the lambda function inside the private subnet in order to have internet access.

If your Lambda function needs to access private VPC resources (for example, an Amazon RDS DB instance or Amazon EC2 instance), you must associate the function with a VPC. If your function also requires internet access (for example, to reach a public AWS service endpoint), your function must use a NAT gateway or instance.

But it doesn't explain why I can't set the lambda function inside the public subnet.

Answer 1

Lambda functions connected to a VPC public subnet cannot typically access the internet.

To access the internet from a public subnet you need a public IP or you need to route via a NAT that itself has a public IP. You also need an Internet Gateway (IGW). However:

1. Lambda functions do not, and cannot, have public IP addresses, and
2. the default route target in a VPC public subnet is the IGW, not a NAT

So, because the Lambda function only has a private IP and its traffic is routed to the IGW rather than to a NAT, all packets to the internet from the Lambda function will be dropped at the IGW.

Should I Configure my Lambda Function for VPC Access?

If your Lambda function does not need to reach private resources inside your VPC (e.g. an RDS database or Elasticsearch cluster) then do not configure the Lambda function to connect to the VPC.

If your Lambda function does need to reach private resources inside your VPC, then configure the Lambda function to connect to private subnets (and only private subnets).

NAT or Not?

If the Lambda function only needs access to resources in the VPC (e.g. an RDS database in a private subnet) then you don't need to route through NAT.

If the Lambda function only needs access to resources in the VPC and access to AWS services that are all available via private VPC Endpoint then you don't need to route through NAT. Use VPC Endpoints.

If your Lambda function needs to reach endpoints on the internet then ensure a default route from the Lambda function's private subnets to a NAT instance or NAT Gateway in a public subnet. And configure an IGW, if needed, without which internet access is not possible.

Be aware that NAT gateway charges per hour and per GB processed so it's worth understanding how to reduce data transfer costs for NAT gateway.

Best Practices

When configuring Lambda functions for VPC access, it is an HA best practice to configure multiple (private) subnets across different Availability Zones (AZs).

Intermittent Connectivity

Be sure that all the subnets you configure for your Lambda function are private subnets. It is a common mistake to configure, for example, 1 private subnet and 1 public subnet. This will result in your Lambda function working OK sometimes and failing at other times without any obvious cause.

For example, the Lambda function may succeed 5 times in a row, and then fail with a timeout (being unable to access some internet resource or AWS service). This happens because the first launch was in a private subnet, launches 2-5 reused the same Lambda function execution environment in the same private subnet (the so-called "warm start"), and then launch 6 was a "cold start" where the AWS Lambda service deployed the Lambda function in a public subnet where the Lambda function has no route to the internet.