How can I retrieve a user's public IP address via Amazon API Gateway + Lambda (node)

Update for HTTP APIs

Adding @Elijah's comment. The format for HTTP APIs will be

event['requestContext']['http']['sourceIp']

Edit

A better way is actually to check

event['requestContext']['identity']['sourceIp']

You can also get the User-Agent from the same object

event['requestContext']['identity']['userAgent']

See Cesar's comment below. Headers are easily spoofed and the user can set X-Forwarded-For to anything. AFAIK the sourceIp above is retrieved from the TCP connection.

Original answer

As of September 2017, you can create a method in API Gateway with Lambda Proxy integration, this will give you access to

events['headers']['X-Forwarded-For']

Which will look something like 1.1.1.1,214.25.52.1

The first ip 1.1.1.1 is your user's public ip address.

Here is a simple demonstration of using API Gateway's $context.identity.sourceIp in a Lambda function.

API Mapping template:

{
    "sourceIP" : "$context.identity.sourceIp"
}

Lambda function:

'use strict';
console.log('Loading function');
exports.handler = (event, context, callback) => {
    console.log('SourceIP =', event.identity.sourceIP);
    callback(null, event.identity.sourceIP);
};

In the API Gateway, it's the value

$context.identity.sourceIp

http://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-mapping-template-reference.html#context-variable-reference

You can pass that through to your Lambda via a mapping template.