RequireLocalPort extension for IEndpointConventionBuilder

Background and Motivation

I’ve found myself in a situation where I needed endpoints responding on different ports. Mainly one for public requests and another for internal requests (health checks for example). To do this I resorted to the .RequireHost extension method and did something similar to this

app.MapHealthChecks( "/healthz" )
    .RequireHost( "*:9001" )
    .AllowAnonymous();

app.MapGet( "/", () =>
{
   // ...
} )
.RequireHost( "*:8080" );

Locally things work properly and so I was surprised when deployed behind a reverse proxy I wasn’t able to reach the public endpoint, but was able to reach the health check (internally).

Turns out that RequireHost uses the Host header, which behind a reverse proxy that may be the public host/port and not the one we’re listening to. It took me a while to figure this out and it also took me quite a while to figure out how to create a filter to look at the HttpContext.Connection.LocalPort instead.

Proposed API

In addition to better documentation on RequireHost, to clarify whoever is using it, I’d suggest having a RequireLocalPort extension. If I’m not mistaken, previous versions of UseHealthChecks had an optional port argument.

namespace Microsoft.AspNetCore.Builder;

public static class EndpointConventionBuilderLocalExtensions
{
    public static IEndpointConventionBuilder RequireLocalPort( this IEndpointConventionBuilder builder, int port )
        => builder.AddEndpointFilter( async ( context, next ) =>
        {
            if ( context.HttpContext.Connection.LocalPort != port )
            {
                return Results.NotFound();
            }

            return await next( context );
        } );
}

Usage Examples

The code above would be replaced with

app.MapHealthChecks( "/healthz" )
    .RequireLocalPort( 9001 )
    .AllowAnonymous();

app.MapGet( "/", () =>
{
   // ...
} )
.RequireLocalPort( 8080 );

[JamesNK]

Setting the host header seems like a simple solution.

I don’t have any more thoughts off the top of my head. It’s been a long time since I wrote host matcher 😄

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