AWS ALB returning 502 from Lambda instead of custom HTTP status

Question

I've created a Lambda that check on a DynamoDB table the existence of a record matching host and request path and, if found, returns a redirect to the matching URL.

My Lambda returns this response but the ALB returns 502.

{
    "statusCode": 301,
    "statusDescription": null,
    "headers": {
        "Location": "https://www.my-target.co.uk/"
    },
    "multiValueHeaders": null,
    "body": "Redirecting to https://www.my-target.co.uk/",
    "isBase64Encoded": false
}

This is the log I find in CloudWatch for the Lambda function

START RequestId: 8b5a28f2-c56d-4418-a7b9-66ebe0ba2470 Version: $LATEST
[Information] EMG.ApplicationLoadBalancerRequestHandler: Received: GET / 
[Information] EMG.ApplicationLoadBalancerRequestHandler: Processing: my-test.net / 
[Information] EMG.RedirectManagers.RedirectTableRedirectManager: Fetching item: my-test.net / from table redirect-table 
[Information] EMG.ApplicationLoadBalancerRequestHandler: Found: https://www.target.co.uk/ Permanent 
END RequestId: 8b5a28f2-c56d-4418-a7b9-66ebe0ba2470
REPORT RequestId: 8b5a28f2-c56d-4418-a7b9-66ebe0ba2470  Duration: 69.59 ms  Billed Duration: 100 ms Memory Size: 128 MB Max Memory Used: 39 MB

This is the response I get

HTTP/1.1 502
status: 502
Server: awselb/2.0
Date: Thu, 15 Aug 2019 19:13:58 GMT
Content-Type: text/html
Content-Length: 138
Connection: keep-alive
<html>
 <head><title>502 Bad Gateway</title></head>
 <body bgcolor="white">
 <center><h1>502 Bad Gateway</h1></center>
 </body>
 </html>

I couldn't find anything saying we can't return non-200 responses from Lambda, so I really have no idea...

You can also find this question in the relevant GitHub repo: https://github.com/aws/aws-lambda-dotnet/issues/507

Kralizek · Accepted Answer · 2019-11-25T08:15:45.690

Apparently I was missing a required property of the returned HTTP response.

Here is the relevant part of the AWS documentation (emphasis mine)

The response from your Lambda function must include the Base64 encoding status, status code, status description, and headers. You can omit the body. The statusDescription header must contain the status code and reason phrase, separated by a single space.

Changing my code so that the response of the Lambda complied to the requirement fixed the issue.

{
    "statusCode": 301,
    "statusDescription": "301 Found",
    "headers": {
        "Location": "https://www.my-target.co.uk/"
    },
    "multiValueHeaders": null,
    "body": "Redirecting to https://www.my-target.co.uk/",
    "isBase64Encoded": false
}

For future folks, the linked AWS documentation has now changed to remove the instructions around *statusDescription*, so I don't think this answer is still relevant. — htxryan, Aug 05 '22 at 18:21

AWS ALB returning 502 from Lambda instead of custom HTTP status

1 Answers1