X-Forwarded-Host header missing in dotnet 8 isolated

[paulverbeke]

The X-Forwarded-Host header provided by Azure Front Door is missing in a fresh new function in dotnet 8 isolated.
Is working fine in dotnet 6 in-process but stops working when migrating to 8 isolated.

Investigative information

Please provide the following:

• Timestamp:
• Function App version: 4
• Function App name:
• Function name(s) (as appropriate):
• Invocation ID:
• Region: France Central

Repro steps

1. create an Azure Function in dotnet 8 isolated
2. in a function testforwardheader, output the value of the header X-Forwarded-Host req.Headers.TryGetValue("X-Forwarded-Host", out StringValues value)
3. publish the function behind a Front Door with redirect rules to this app from the url subdomain.myfrontdoordomain.com/myfunction.
4. call the function using subdomain.myfrontdoordomain.com/myfunction/api/testforwardheader

Expected behavior

in step 4 you should get the host requested by client, so subdomain.myfrontdoordomain.com

Actual behavior

in step 4 you get an empty string

Known workarounds

Someone found a workaround. They were able to find and restore the value from the bindingContext.BindingData using custom middleware.

Workaround

using Microsoft.AspNetCore.HttpOverrides;
using Microsoft.Azure.Functions.Worker;
using Microsoft.Azure.Functions.Worker.Middleware;
using Microsoft.Extensions.Hosting;
using Newtonsoft.Json;

var builder = new HostBuilder();

var host = builder
    .ConfigureFunctionsWebApplication(
        app =>
        {
            app.UseMiddleware<ForwardedForHeaderMiddleware>();
        })
    .Build();

await host.RunAsync();

public class ForwardedForHeaderMiddleware : IFunctionsWorkerMiddleware
{
    public async Task Invoke(FunctionContext context, FunctionExecutionDelegate next)
    {
        await CorrectForwardedForAsync(context);

        await next(context);
    }

    private static async Task CorrectForwardedForAsync(FunctionContext context)
    {
        var val = GetForwardedFor(context);
        var req = await context.GetHttpRequestDataAsync();
        req?.Headers.TryAddWithoutValidation(ForwardedHeadersDefaults.XForwardedForHeaderName, val);
    }

    private static string? GetForwardedFor(FunctionContext context)
    {
        var hdr = context.BindingContext.BindingData["Headers"];
        dynamic obj = JsonConvert.DeserializeObject(hdr.ToString());

        var val = obj[ForwardedHeadersDefaults.XForwardedForHeaderName]?.Value as string;
        return val;
    }
}

Related information

written in C#

Someone else had this problem

I know that the Front Door will send X-Forwarded-Host and X-Forwarded-Proto which we were able to use with the older Swashbuckle library which we used for In-Process Function Apps.

[fabiocav]

@paulverbeke are you using the ASP.NET Core integration for your tests? In the HTTP proxying mode, all headers should be forwarded and will be processed by the worker.

[paulverbeke]

@fabiocav thanks for getting back quickly. Yes we are using ASP.NET Core integration. But I couldn't find anything online regarding "HTTP proxying mode". Is there something I need to enable or specify somewhere ?

EDIT: I just tested the starting Azure Functions template without ASP.NET Core integration (with HttpRequestData/HttpResponseData) and the header is available.
Is it expected behavior that ASP.NET Core integration is causing header to be dropped ? If so why the default Visual Studio template is showing an example function with ASP.NET Core integration without this drawback be documented somewhere ?