Enable both Windows authentication and Anonymous authentication in an ASP.NET Core app

IIS will act as a reverse proxy and will be responsible for setting and transmitting to Kestrel the Windows identity of the user. So first, set up IIS to allow both Windows and Anonymous Authentication:

Then, you need to change your web.config to ask IIS to transmit the Windows identity (in case one is found) to your ASP.NET Core application like that: https://stackoverflow.com/a/42163175/6827240

At this point, if you create a controller action with an "[Authorize]" attribute, HttpContext.User.Identity.Name; should have the value of the Windows identity used by your client. I replied to something similar here: NTLM authentication on specific route in ASP.NET Core

The good thing is that a standard controller action will still work if your client doesn't pass along Windows identity token, while a protected one (using [Authorize] tag) will fail.

PS: I like to use curl.exe in verbose mode to see what is happening in terms of authorization protocol (Negotiate protocol, NTLM tokens ...)

I have a similar scenario for an ASP.NET Core 2.0 application (use Windows Authentication throughout the app except a single controller) and Daboul's explanation was not enough.

I had to set up a custom middleware as indicated here since anonymous takes precedence.

The middleware

public class NtlmAndAnonymousSetupMiddleware
{
    private readonly RequestDelegate next;

    public NtlmAndAnonymousSetupMiddleware(RequestDelegate next)
    {
        this.next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        if (context.User.Identity.IsAuthenticated || context.Request.Path.ToString().StartsWith("/Anonymous"))
        {
            await next(context);
            return;
        }

        await context.ChallengeAsync("Windows");
    }

}

and its usage in Startup.cs:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    app.UseMiddleware<NtlmAndAnonymousSetupMiddleware>();

    // other code here
}

So, the middleware accept anonymous requests for AnonymousController only and will provide a challenge if Windows Authentication info is not provided.

Anonymous controller

Since the middleware makes the differece between what is anonymous and requires authentication, this will look just like any ordinary controller:

[Route("Anonymous")]
public class AnonymousController : Controller
{
    [HttpGet("Echo")]
    public string Echo(string data)
    {
        return data;
    }
}

Tests

(all done on a Windows machine)

1. Chrome + access non-anonymous controller action => works fine (both @User.Identity.Name and @Context.User.Identity.Name return the correct user

2. Chrome + anonymous action => works directly

3. Firefox (which does not directly transfer NTLM ticket from OS) + non-anonymous => a modal asks for user/pass => if provided correctly, it works fine

4. Firefox + anonymous action => works directly

In case anyone wonders, I modified @Alexei's answer to use Attributes rather than request path in Netcore 3.X

First create the class and get the endpoints metadata

public class NtlmAndAnonymousSetupMiddleware
{
    private readonly RequestDelegate next;

    public NtlmAndAnonymousSetupMiddleware(RequestDelegate next)
    {
        this.next = next;
    }

    public async Task Invoke(HttpContext context)
    {

        if (context.User.Identity.IsAuthenticated || HasAnonymousAttribute(context))
        {
            await next(context);
            return;
        }

        await context.ChallengeAsync("Windows");
    }

    private bool HasAnonymousAttribute(HttpContext context)
    {
        var endpoint = context.GetEndpoint();
        var retVal = (endpoint?.Metadata?.GetMetadata<IAllowAnonymous>() != null);

        return retVal;
    }
}

Then modify public void Configure(IApplicationBuilder app, IWebHostEnvironment env)

        app.UseAuthentication();
        app.UseAuthorization();
        app.UseMiddleware<NtlmAndAnonymousSetupMiddleware>();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapRazorPages();
            endpoints.MapControllers();
        });