Motivation

We got a report from a support case that trying to deploy an EC2 instance with CDK and a basic stack would fail with a KeyError:

2025-01-10 08:31:51 localstack-main  |   File "/opt/code/localstack/.venv/lib/python3.11/site-packages/localstack/services/ec2/resource_providers/aws_ec2_instance.py", line 255, in create
2025-01-10 08:31:51 localstack-main  |     model["PublicIp"] = instance["PublicIpAddress"]
2025-01-10 08:31:51 localstack-main  |                         ~~~~~~~~^^^^^^^^^^^^^^^^^^^
2025-01-10 08:31:51 localstack-main  | KeyError: 'PublicIpAddress'

from aws_cdk import (
    Stack,
    aws_ec2 as ec2,
    CfnOutput,
)
from constructs import Construct

class CdkStack(Stack):

    def __init__(self, scope: Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        # Create a VPC with outbound internet access
        vpc = ec2.Vpc(self, "Vpc", max_azs=3, vpc_name="my-vpc")

        # Create an ec2 instance
        instance = ec2.Instance(
            self,
             "Instance",
            vpc=vpc,
            instance_type=ec2.InstanceType("t3.nano"),
            instance_name="my-instance",
            machine_image=ec2.MachineImage.lookup(
                name="amazonlinux-2023"
            ),
            key_name="my-key"
        )

(Note: To run on AWS, I had to update the stack to use machine_image=ec2.MachineImage.latest_amazon_linux2(), to find the proper AMI, and also create a key pair).

This is due because the PublicIpAddress response field is optional, and is not given to be returned depending on the configuration of the instance (and if the VPC of the instance is the default one or not, see https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-instance-addressing.html#concepts-public-addresses).

This stack did not failed for me, possibly due to the different platform (macOS).

I do not know how to add a test for this, because spawning real instance is pretty difficult as it is very platform dependent, and the fact that we do not return the PublicIpAddress seems to also be dependent on the platform.

I've done a lot of testing on AWS, and with this stack, the instance does not have a Public Address, but it deploys correctly. However, trying to add the following to the stack and deploying again, or deploying from scratch:

CfnOutput(self, "TestOutput", value=instance.instance_public_ip)

See https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ec2.Instance.html#instancepublicip

Will fail in AWS with the following message: Attribute 'PublicIp' does not exist, meaning that the field is actually not attached to the "model", and thus failing.
I believe we can get close to this behavior by conditionally adding the field to the model.

If you have an idea about the testing, please feel free to suggest it.

Changes

• only add the PublicIpAddress and PublicDnsName to the CFN model if those are present