How to get the original URL called in AWS Lambda?

Question

I have a lambda setup with the proxy integration in API Gateway.

I can call my lambda with a URL like https://7kbw9fcfa4.execute-api.us-east-1.amazonaws.com/dev/foo The path here is /dev/foo.

However if I set up CloudFront in front of that, the URL becomes https://a45ex7tnds5r5o.cloudfront.net/foo and the path is /foo.

In both cases I have the same keys in the event variable:

How can I get reliably the actual URL that the client sees in the browser? (both when using Cloudfront and when using API Gateway directly)

Have you tried looking at origin header? Also Did you enable headers from CloudFront to API Gateway. If you don't whitelist the headers, you will not get them from CloudFront. — Kannaiyan
@Kannaiyan I whitelisted all headers except the Host header because it is recommended not to forward it (else it messes up the Cloudfront -> API Gateway connection). The Origin header doesn't seem to be useful as it contains only the domain name, not the path (see developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Origin). — Matthieu Napoli

Tom Tom · Accepted Answer · 2018-10-01T04:36:05

You could configure your CloudFront distribution to use a custom header that would be passed to the Lambda function. Then you could determine whether to use path or requestContext.path based on if that CloudFront-only header is present.