How to publish to SNS from Lambda within VPC using VPC Endpoint?

Question

I have set up a VPC with 3 subnets, this to have access to a private RDS instance from my Lambda functions. The RDS <-> Lambda connection works fine, however now I'm not able to publish to SNS.

I found the announcement of VPC Endpoint support for SNS (incl. this blog post https://aws.amazon.com/blogs/security/securing-messages-published-to-amazon-sns-with-aws-privatelink/) and have added a VPC Endpoint Interface with these properties:

Service name: com.amazonaws.eu-west-1.sns
VPC: same as Lambda functions and other services
Subnets: all included in my VPC (have also tested toggling them individually)
Security Groups: all VPC security groups selected

All the services are in the eu-west-1 region. I know the code that publish to SNS is correct, as it works when run in a non-VPC environment. The ARN I'm publishing to has remained unchanged: arn:aws:sns:eu-west-1:962446592636:whatever.

I'm aware that a NAT server could be set up to avoid this issue, but I'd prefer to use VPC Endpoints if possible to reduce costs.

John Rotenstein John Rotenstein · Accepted Answer · 2018-05-05T09:35:57

It works for me!

I did the following:

Created an Amazon SNS topic and subscribed to it
Created an AWS Lambda function with no VPC configuration, which sends a message to the SNS topic
Tested the Lambda function -- message received
Created a VPC with a two private subnets
Created a Service Endpoint for SNS in the private subnets, with a Security Group allowing All TCP from 0.0.0.0/0 (for testing purposes)
Modified the Lambda function to use the private subnets
Tested the Lambda function -- message received

So, everything worked fine. I didn't have to modify any Lambda code.

My Lambda code:

def lambda_handler(event, context):
    import boto3

    client = boto3.client('sns', region_name='ap-southeast-2')
    response = client.publish(
        TopicArn='arn:aws:sns:ap-southeast-2:123456789012:stack',
        Message='From Lambda'
        )

    return

How to publish to SNS from Lambda within VPC using VPC Endpoint?

1 Answers