Set dummy IP address in integration test with Asp.Net Core TestServer

You can write middleware to set custom IP Address since this property is writable:

public class FakeRemoteIpAddressMiddleware
{
    private readonly RequestDelegate next;
    private readonly IPAddress fakeIpAddress = IPAddress.Parse("127.168.1.32");

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

    public async Task Invoke(HttpContext httpContext)
    {
        httpContext.Connection.RemoteIpAddress = fakeIpAddress;

        await this.next(httpContext);
    }
}

Then you can create StartupStub class like this:

public class StartupStub : Startup
{
    public StartupStub(IConfiguration configuration) : base(configuration)
    {
    }

    public override void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        app.UseMiddleware<FakeRemoteIpAddressMiddleware>();
        base.Configure(app, env);
    }
}

And use it to create a TestServer:

new TestServer(new WebHostBuilder().UseStartup<StartupStub>());

As per this answer in ASP.NET Core, is there any way to set up middleware from Program.cs?

It's also possible to configure the middleware from ConfigureServices, which allows you to create a custom WebApplicationFactory without the need for a StartupStub class:

public class CustomWebApplicationFactory : WebApplicationFactory<Startup>
{
    protected override IWebHostBuilder CreateWebHostBuilder()
    {
        return WebHost
            .CreateDefaultBuilder<Startup>(new string[0])
            .ConfigureServices(services =>
            {
                services.AddSingleton<IStartupFilter, CustomStartupFilter>();
            });
    }
}


public class CustomStartupFilter : IStartupFilter
{
    public Action<IApplicationBuilder> Configure(Action<IApplicationBuilder> next)
    {
        return app =>
        {
            app.UseMiddleware<FakeRemoteIpAddressMiddleware>();
            next(app);
        };
    }
}

Using WebHost.CreateDefaultBuilder can mess up with your app configuration.

And there's no need to change Product code just to accommodate for testing, unless absolutely necessary.

The simplest way to add your own middleware, without overriding Startup class methods, is to add the middleware through a IStartupFilter‍ as suggested by Elliott's answer.

But instead of using WebHost.CreateDefaultBuilder, just use

base.CreateWebHostBuilder().ConfigureServices...

    public class CustomWAF : WebApplicationFactory<Startup>
    {
        protected override IWebHostBuilder CreateWebHostBuilder()
        {
            return base.CreateWebHostBuilder().ConfigureServices(services =>
            {
                services.AddSingleton<IStartupFilter, CustomStartupFilter>();
            });
        }
    }