PacketDotNet问题与CalculateTCPChecksum，ValidTCPChecksum和ValidChecksum

Question

我试图用TCP协议进行一些原始的套接字编程，但是我遇到了PacketDotNet和TCP校验和的问题。

我在PacketDotNet.TCPPacket中获得了空指针异常。我得到的例外是：

ValidTCPChecksum = 'tcpPacket.ValidTCPChecksum' threw an exception of type 'System.NullReferenceException'

和

System.NullReferenceException: Object reference not set to an instance of an object.
at PacketDotNet.TransportPacket.CalculateChecksum(TransportChecksumOption option)
at PacketDotNet.TcpPacket.CalculateTCPChecksum()
at ProjectServer.MainWindow.packetstuff(String toIp, String fromIp, Byte[] payload) in  c:\\Users\\MyUser\\Documents\\Visual Studio 2012\\Projects\\ProjectServer\\ProjectServer\\MainWindow.xaml.cs:line 131
at ProjectServer.MainWindow.Project_Click(Object sender, RoutedEventArgs e) in c:\\Users\\MyUser\\Documents\\Visual Studio 2012\\Projects\\ProjectServer\\ProjectServer\\MainWindow.xaml.cs:line 68

131号线是tcpPacket.Checksum = (ushort)tcpPacket.CalculateTCPChecksum();

这似乎与HelpLink为null有关，但我不能百分之百肯定。

我曾经尝试过自己做校验和，但是到目前为止我还没有能够实现一个工作的校验和算法。

下面是打包方法，它基本构建我的包。也许它有什么问题。

    public void packetstuff(string toIp, string fromIp, byte[] payload)
    {
        ushort tcpSourcePort = 123;
        ushort tcpDestinationPort = 321;
        var tcpPacket = new TcpPacket(tcpSourcePort, tcpDestinationPort);

        var ipSourceAddress = System.Net.IPAddress.Parse(fromIp);
        var ipDestinationAddress = System.Net.IPAddress.Parse(toIp);
        var ipPacket = new IPv4Packet(ipSourceAddress, ipDestinationAddress);

        var sourceHwAddress = "MY-MA-CA-DD-RE-SS";//?actually a bit unsure what this should be
        var ethernetSourceHwAddress = System.Net.NetworkInformation.PhysicalAddress.Parse(sourceHwAddress);
        var destinationHwAddress = "MY-MA-CA-DD-RE-SS";
        var ethernetDestinationHwAddress = System.Net.NetworkInformation.PhysicalAddress.Parse(destinationHwAddress);

        var ethernetPacket = new EthernetPacket(ethernetSourceHwAddress,
            ethernetDestinationHwAddress,
            EthernetPacketType.None);

        if (tcpPacket != null)
        {
            tcpPacket.Checksum = 0;

            tcpPacket.Checksum = (ushort)tcpPacket.CalculateTCPChecksum();  //This is where the error occurs.
        }

        ipPacket.PayloadPacket = tcpPacket;
        ipPacket.UpdateIPChecksum();

        ethernetPacket.PayloadPacket = ipPacket;

        ethernetPacket.UpdateCalculatedValues();

        packetBytes = ethernetPacket.Bytes;
        Thread producer = new Thread(new ThreadStart(ThreadRun));
        device.Open();
        producer.Start();

    }

Windows7，VS2012

Answer 1

我看你已经放弃了，但我会回答其他人，寻找解决办法。

我在做类似的事情时遇到了同样的问题。我发现在调用CalculateTCPChecksum()之前，必须将所有数据包链接在一起。

因此，要修复问题中提出的示例，这应该是可行的(我还没有测试过这段代码，但它与我编写的代码非常相似)：

public void packetstuff(string toIp, string fromIp, byte[] payload)
{
    ushort tcpSourcePort = 123;
    ushort tcpDestinationPort = 321;
    var tcpPacket = new TcpPacket(tcpSourcePort, tcpDestinationPort);

    var ipSourceAddress = System.Net.IPAddress.Parse(fromIp);
    var ipDestinationAddress = System.Net.IPAddress.Parse(toIp);
    var ipPacket = new IPv4Packet(ipSourceAddress, ipDestinationAddress);

    var sourceHwAddress = "MY-MA-CA-DD-RE-SS";
    var ethernetSourceHwAddress = System.Net.NetworkInformation.PhysicalAddress.Parse(sourceHwAddress);
    var destinationHwAddress = "MY-MA-CA-DD-RE-SS";
    var ethernetDestinationHwAddress = System.Net.NetworkInformation.PhysicalAddress.Parse(destinationHwAddress);

    var ethernetPacket = new EthernetPacket(ethernetSourceHwAddress,
        ethernetDestinationHwAddress,
        EthernetPacketType.None);

    ethernetPacket.PayloadPacket = ipPacket;
    ipPacket.ParentPacket = ethernetPacket;

    if (tcpPacket != null)
    {
        ipPacket.PayloadPacket = tcpPacket;
        tcpPacket.ParentPacket = ip;
        ipPacket.UpdateIPChecksum();

        tcpPacket.Checksum = (ushort)tcpPacket.CalculateTCPChecksum();
    }
    else
        ipPacket.UpdateIPChecksum();

    ethernetPacket.UpdateCalculatedValues();

    packetBytes = ethernetPacket.Bytes;
    Thread producer = new Thread(new ThreadStart(ThreadRun));
    device.Open();
    producer.Start();

}