ApplicationLoadBalancerFunction doesn't play nicely with healthchecks

[aidansteele]

Hi,

Background:

Target groups with Lambda targets don't have health checks turned on by default. If you turn on health checks for an ASP.Net Core app using the ApplicationLoadBalancerFunction.FunctionHandlerAsync handler, health checks will fail and the Lambda will emit exception stack traces.

The Lambda request payload for health checks is documented here as looking like:

{
    "requestContext": {
        "elb": {
            "targetGroupArn": "arn:aws:elasticloadbalancing:region:123456789012:targetgroup/my-target-group/6d0ecf831eec9f09"
        }
    },
    "httpMethod": "GET",  
    "path": "/",  
    "queryStringParameters": {},  
    "headers": {
        "user-agent": "ELB-HealthChecker/2.0"
    },  
    "body": "",  
    "isBase64Encoded": false
}

Problem:

This line of code is what parses the scheme:

aws-lambda-dotnet/Libraries/src/Amazon.Lambda.AspNetCoreServer/ApplicationLoadBalancerFunction.cs

Line 63 in 6414d0b

requestFeatures.Scheme = GetSingleHeaderValue(lambdaRequest, "x-forwarded-proto");

After this line runs for the above request payload, requestFeatures.Scheme is still null. This causes a null reference exception here:

https://github.com/aspnet/Hosting/blob/1da228d28fdfcfb2d77241c4e0413efec1beeafd/src/Microsoft.AspNetCore.Hosting/Internal/HostingRequestStartingLog.cs#L37

The properties after Scheme probably fail too, but this is the first to fail.

Solution:

Given that ASP.Net Core appears to treat these properties as non-null, we should probably populate them unconditionally. Any value will be better than no value. Any thoughts about how you'd best like to approach this? I'd be happy to submit a PR fixing it with unit test coverage.

[normj]

That is an interesting one. I'm afraid to just default the value to https in case there is a situation that is not correct. I wonder if we should look to see if the user-agent starts with "ELB-HealthChecker" and if so default schema to https.

[normj]

PR is submitted to handle ELB Health Check: #452

[normj]

Version 3.0.4 is out with this fix.

[normj]

Closing as the fix was shipped.