dschenkelman/Extensions.cs

## Extensions.cs
namespace Owin.IpFilter
{
    using System;
    using System.Net;

    public static class Extensions
    {
        public static IAppBuilder UseIpFiltering(this IAppBuilder appBuilder, Func<IPAddress, bool> rejectRequest)
        {
            appBuilder.Use(typeof(IpFilterMiddleware), rejectRequest);
            return appBuilder;
        }
    }
}

## IpFilterMiddleware.cs
namespace Owin.IpFilter
{
    using System;
    using System.Collections.Generic;
    using System.Globalization;
    using System.IO;
    using System.Net;
    using System.Text;
    using System.Threading.Tasks;

    using AppFunc = System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>;

    public class IpFilterMiddleware
    {
        private const string Html404 = "<!doctype html><html><head><meta charset=\"utf-8\"><title>404 Not Found</title></head><body>The resource cannot be found.</body>" + "</html>";

        private readonly AppFunc nextMiddleware;

        private readonly Func<IPAddress, bool> rejectRequest;

        public IpFilterMiddleware(AppFunc nextMiddleware, Func<IPAddress, bool> rejectRequest)
        {
            if (nextMiddleware == null)
            {
                throw new ArgumentNullException("nextMiddleware");
            }

            this.nextMiddleware = nextMiddleware;
            this.rejectRequest = rejectRequest;
        }

        public Task Invoke(IDictionary<string, object> environment)
        {
            var remoteAddress = IPAddress.Parse((string)environment["server.RemoteIpAddress"]).MapToIPv4();

            if (this.rejectRequest(remoteAddress))
            {
                var responseStream = (Stream)environment["owin.ResponseBody"];
                var responseHeaders =
                    (IDictionary<string, string[]>)environment["owin.ResponseHeaders"];

                var responseBytes = Encoding.UTF8.GetBytes(Html404);

                responseHeaders["Content-Type"] = new[] { "text/html" };
                responseHeaders["Content-Length"] = new[] { responseBytes.Length.ToString(CultureInfo.InvariantCulture) };

                environment["owin.ResponseStatusCode"] = 404;

                return responseStream.WriteAsync(responseBytes, 0, responseBytes.Length);
            }

            return this.nextMiddleware(environment);
        }
    }
}

## Startup.cs
public class Startup
{
    public void Configuration(IAppBuilder appBuilder)
    {
        appBuilder.UseIpFiltering(
            remoteAddress =>
            {
                var bytes = remoteAddress.GetAddressBytes();
                return bytes[0] != 192 && bytes[0] != 172 && bytes[0] != 10 && bytes[0] != 127 && bytes[0] != 0;
            });
    }
}
	namespace Owin.IpFilter
	{
	using System;
	using System.Net;

	public static class Extensions
	{
	public static IAppBuilder UseIpFiltering(this IAppBuilder appBuilder, Func<IPAddress, bool> rejectRequest)
	{
	appBuilder.Use(typeof(IpFilterMiddleware), rejectRequest);
	return appBuilder;
	}
	}
	}
	namespace Owin.IpFilter
	{
	using System;
	using System.Collections.Generic;
	using System.Globalization;
	using System.IO;
	using System.Net;
	using System.Text;
	using System.Threading.Tasks;

	using AppFunc = System.Func<System.Collections.Generic.IDictionary<string, object>, System.Threading.Tasks.Task>;

	public class IpFilterMiddleware
	{
	private const string Html404 = "<!doctype html><html><head><meta charset=\"utf-8\"><title>404 Not Found</title></head><body>The resource cannot be found.</body>" + "</html>";

	private readonly AppFunc nextMiddleware;

	private readonly Func<IPAddress, bool> rejectRequest;

	public IpFilterMiddleware(AppFunc nextMiddleware, Func<IPAddress, bool> rejectRequest)
	{
	if (nextMiddleware == null)
	{
	throw new ArgumentNullException("nextMiddleware");
	}

	this.nextMiddleware = nextMiddleware;
	this.rejectRequest = rejectRequest;
	}

	public Task Invoke(IDictionary<string, object> environment)
	{
	var remoteAddress = IPAddress.Parse((string)environment["server.RemoteIpAddress"]).MapToIPv4();

	if (this.rejectRequest(remoteAddress))
	{
	var responseStream = (Stream)environment["owin.ResponseBody"];
	var responseHeaders =
	(IDictionary<string, string[]>)environment["owin.ResponseHeaders"];

	var responseBytes = Encoding.UTF8.GetBytes(Html404);

	responseHeaders["Content-Type"] = new[] { "text/html" };
	responseHeaders["Content-Length"] = new[] { responseBytes.Length.ToString(CultureInfo.InvariantCulture) };

	environment["owin.ResponseStatusCode"] = 404;

	return responseStream.WriteAsync(responseBytes, 0, responseBytes.Length);
	}

	return this.nextMiddleware(environment);
	}
	}
	}
	public class Startup
	{
	public void Configuration(IAppBuilder appBuilder)
	{
	appBuilder.UseIpFiltering(
	remoteAddress =>
	{
	var bytes = remoteAddress.GetAddressBytes();
	return bytes[0] != 192 && bytes[0] != 172 && bytes[0] != 10 && bytes[0] != 127 && bytes[0] != 0;
	});
	}
	}