How to choose Elastic IP when my aws lambda function execute

Question

I want to select specific Elastic IP my own when my lambda function executed.

my service has to respond to several situations, and by user's attributes. Could I write code in a lambda function, that can choose specific my own elastic IP?

I had searched for this. but old information says it cannot do. but recently I heard about it is possible by using Network Load Balancer or Application Load Balancer. But I don't know how to use this for the problem.

Answer 1

No. You cannot associate an Elastic IP (EIP) address with an AWS Lambda function.

Well, actually you can, but I wouldn't recommend it. When a Lambda function is associated with a VPC, it connects via an Elastic Network Interface (ENI). It is possible to attach an EIP to an ENI. This also grants access to the Internet if it is attached to a public subnet.

So why avoid it? Because Lambda might create additional ENIs, especially if the Lambda function is frequently invoked and run in parallel. This means it will not have a consistent ENI.

An alternative method is:

• Attach the AWS Lambda function to a private subnet
• Put a NAT Gateway in a public subnet
• Associate an Elastic IP address with the NAT Gateway
• All traffic from the Lambda function to the Internet will then come from the NAT Gateway's EIP (however, I don't think you can change that EIP)

Answer 2

Looking at @John Rotenstein's reply: for small systems, with limited calls to the same lambda adding an EIP to the ENI for a lambda could work - if you put a queue in front of the lambda to handle the requests and limit the concurrency of the lambda to 1. That's cheaper than a NAT Gateway (saves around $30) per month. For larger systems, this may not be an issue and you may need the concurrency to be more than one - in that case the NAT gateway is the only way out.