ALB is not propagating response headers correctly

Question

I have a lambda target behind an ALB. My lambda is a python lambda.

def handler(event, context):
response = {
    "statusCode": 200,
    "statusDescription": "200 OK",
    "isBase64Encoded": False,
    "headers": {
        "Content-Type": "text/html; charset=utf-8"
    }
}

On hitting my url using curl though, I receive a

< HTTP/1.1 200 OK
< Server: awselb/2.0
< Date: Sat, 06 Apr 2019 04:46:50 GMT
< Content-Type: application/octet-stream
< Content-Length: 0
< Connection: keep-alive

Note Content-Type is an octet-stream, which causes browsers to download the response as a file instead of displaying it. I tried adding additional headers "Foo":"Bar" to the response and they don't show up in the curl response. ALB seems to be eating my lambda supplied headers. How can I fix this?

Answer 1

Turns out I had multivalue headers turned on for my target group. With that setting turned on, my lambdas needs to return a response with field multiValueHeaders set instead of headers. So my lambda code needed to be:

def handler(event, context):
response = {
    "statusCode": 200,
    "statusDescription": "200 OK",
    "isBase64Encoded": False,
    "multiValueHeaders": {
        "Content-Type": ["text/html; charset=utf-8"]
    }
}

More information on AWS' release blog post.