How to Configure Location used for Redirects

[KentonParton]

Hi all,

I have the following setup:

my-application.example.com --> CloudFront --> Lambda (aws-lambda-web-adapter - FastAPI)

The FastAPI application has a route /playground. When I open my-application.example.com/playground in the browser, it redirects to the lambda URL https://xxx.lambda-url.us-east-1.on.aws/playground/ instead of the Route53 URL. I suspect this is because CloudFront is forwarding a Location of the Lambda URL instead of my-application.example.com.

Is there a way to override this behaviour in aws-lambda-web-adapter or will I need to update Location using Lambda@Edge?

Thanks!

[KentonParton]

Hi @bnusunny, is it possible to achieve the above? Appreciate your help!

[bnusunny]

Sorry, I missed this question. I will take a look in a few days.

[bnusunny]

This is not a Lambda Web Adapter issue. It's generally how FastAPI works behind a proxy or CDN. Here is how to get it working:

1. Add a custom hear to CloudFront Distribution's origin. For example: X-Forwarded-Host=my-application.example.com
2. Add a middleware in FastAPI to rewrite Location in the response based on the custom header's value. Here is an example.

from fastapi import FastAPI, Request
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.types import ASGIApp
from urllib.parse import urlparse, urlunparse


class RewriteRedirectMiddleware(BaseHTTPMiddleware):
    def __init__(self, app: ASGIApp):
        super().__init__(app)

    async def dispatch(self, request: Request, call_next):
        response = await call_next(request)

        if 'Location' in response.headers:
            original_location = response.headers['Location']
            forwarded_host = request.headers.get('X-Forwarded-Host')

            if forwarded_host:
                parsed_url = urlparse(original_location)
                new_url = urlunparse(parsed_url._replace(netloc=forwarded_host))
                response.headers['Location'] = new_url

        return response


app = FastAPI()
app.add_middleware(RewriteRedirectMiddleware)


@app.get("/")
async def root():
    return {"headers": "Hello world!"}


@app.get("/playground/")
async def playground(request: Request):
    return {"headers": request.headers}

[bnusunny]

Lambda Function URL does not support custom domain name. It requires the Host header to match its own URL.