accessing origin URL from AWS lambda@edge

Question

I'm trying to implement a "proxy" to multiple websites using lambda@edge on AWS Cloudfront.

My setup is roughly:

DNS: *.domain.com -> some_uuid.cloudfront.net (Cloudfront distribution)

Cloudfront: some_uuid.cloudfront.net -> s3 bucket origin

s3 bucket: websites/ (a folder that contains multiple websites)

lambda@edge function: defined as origin-request

My lambda@edge function is quite simple:

1. check if the website resource exists in the s3 bucket.

2. if it does, change the request uri to the resource s3 url.

3. if not, send a request to a backend server to render the resource, store on s3 and return it.

I'm having trouble getting the origin domain of the website. For example, if I try to access "my_website.domain.com" - in my lambda function i don't have this domain info from the request.

I think I can implement another lambda@edge function as viewer request to pass the domain as a header, but if I can, I prefer to avoid that.

Is there any other solution?

Thanks

Answer 1

The primary problem here is that Cloudfront overwrites the host header in the request to the origin host. I was able to workaround this by using a combination of viewer-request L@E and origin-request L@E.

In the viewer request L@E copy the host header into another header like x-forwarded-host.

// viewer-request.js
exports.handler = (event, context, callback) => {
    const request = event.Records[0].cf.request
    request.headers['x-forwarded-host'] = [
      { key: 'X-Forwarded-Host', value: request.headers.host[0].value }
    ]
    return callback(null, request)
}

Configure your cloudfront to whitelist x-forwarded-host host header in the behaviour. This way the Cloudfront also takes into consideration the x-forwarded-host when caching and also passes the header along to the origin-request lambda.

So now in your origin-request lambda you can access the x-forwarded-host header

// origin-request.js
exports.handler = (event, context, callback) => {
    const request = event.Records[0].cf.request;
    const headers = request.headers;
    const requestHost = headers['x-forwarded-host'][0].value;
    console.log(requestHost);
    callback(null, request);
}