Alright, let's get your Lambda function sending messages to SNS through the VPC endpoint you've already set up. Since you've got the endpoint created, we just need to tie up a few configuration loose ends to make the traffic flow correctly. Here's the step-by-step breakdown:

1. Double-Check Your VPC Endpoint's Core Settings #

First, confirm the basics are in place:

• Private DNS must be enabled: This is critical. When you created the com.amazonaws.eu-west-1.sns endpoint, AWS should have enabled private DNS by default, but it's worth verifying. With private DNS on, your Lambda's calls to the standard SNS endpoint (like sns.eu-west-1.amazonaws.com) will resolve to the VPC endpoint's private IPs instead of going out to the public internet. If this is off, your Lambda might still try to hit the public SNS endpoint, which won't work if your Lambda is in a private subnet without a NAT gateway.
• Endpoint subnets overlap with Lambda's: Make sure the VPC endpoint is deployed in at least one of the subnets your Lambda function uses. This reduces cross-subnet traffic overhead and ensures the route can resolve properly.

2. Update the Lambda Execution Role's Permissions #

Your Lambda's IAM role needs explicit permission to publish to your target SNS topic. Add a policy like this to the role:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "sns:Publish",
      "Resource": "arn:aws:sns:eu-west-1:YOUR_ACCOUNT_ID:YOUR_SNS_TOPIC_NAME"
    }
  ]
}

Replace YOUR_ACCOUNT_ID and YOUR_SNS_TOPIC_NAME with your actual values. You don't need special permissions for the VPC endpoint itself—Lambda can access VPC resources by default as long as networking rules allow it.

3. Verify Route Tables for Lambda's Subnets #

AWS should have automatically added a route to your Lambda subnets' route tables when you created the endpoint, but let's confirm:

• Go to the VPC console, find the route tables associated with your Lambda's subnets.
• Look for a route where the destination is the SNS service prefix list (looks like pl-xxxxxx) and the target is your VPC endpoint ID (vpce-xxxxxx).
• If that route's missing, add it manually. This tells traffic bound for SNS to go through the VPC endpoint instead of the internet.

4. Lock Down Security Group Rules #

Security groups are often the culprit here—make sure both sides allow the right traffic:

• Lambda's security group: Allow outbound HTTPS (TCP port 443) traffic to either the VPC endpoint's security group or your VPC's private IP range (since the endpoint lives inside your VPC).
• VPC endpoint's security group: Allow inbound HTTPS (TCP port 443) traffic from your Lambda's security group. This lets Lambda initiate connections to the SNS endpoint inside your VPC.

5. Test with a Simple Lambda Function #

Write a quick test function to validate everything works. Here's a Python example:

import boto3

def lambda_handler(event, context):
    sns_client = boto3.client('sns', region_name='eu-west-1')
    try:
        publish_response = sns_client.publish(
            TopicArn='arn:aws:sns:eu-west-1:YOUR_ACCOUNT_ID:YOUR_SNS_TOPIC_NAME',
            Message='Test message from Lambda via VPC Endpoint'
        )
        return {
            'statusCode': 200,
            'body': f"Success! Message ID: {publish_response['MessageId']}"
        }
    except Exception as e:
        return {
            'statusCode': 500,
            'body': f"Error publishing to SNS: {str(e)}"
        }

Run a test and check CloudWatch Logs for errors:

• If you see EndpointConnectionError, it's a networking issue (route table or security group problem).
• If you see AccessDenied, double-check your Lambda's IAM permissions.

Troubleshooting Quick Hits #

• No NAT gateway needed: Since we're using the VPC endpoint, your Lambda doesn't need a NAT gateway to reach SNS—traffic stays entirely within your VPC.
• Same region only: Your SNS topic must be in eu-west-1, matching your VPC endpoint's region. Cross-region SNS calls via VPC endpoints aren't supported this way.
• Private DNS fix: If you forgot to enable private DNS, go to your VPC endpoint's settings, check "Enable private DNS name", and save the changes. This will take a minute to propagate, but it's essential for using the standard SNS API endpoint.

内容的提问来源于stack exchange，提问作者Mickel