如何构造UDP数据包但不发送它

Question

这是一个奇怪的请求。

我有一个字节数组，需要通过串口发送给另一个使用C#的设备。然而，我需要首先将字节数组包装在udp数据包中，但同样，它将通过串口发送，而不是通过udp。是否可以将udp数据包构造为字节数组，然后通过串行端口发送？

我以前使用udp发送和接收过消息，但从来没有在构造udp数据包但不通过udp发送的地方。

Answer 1

Answer 2

我将接受Yochai的回答，因为该链接(以及该站点中的其他页面)提供了构建udp数据包和ip报头的代码。对于其他试图实现它的人，以下是代码：

如何调用它：

var udpPacketBytes = UDPPacket.Construct(IPAddress.Parse("1.1.1.1"), 1000, IPAddress.Parse("2.2.2.2"), 6100, payloadBytes);

UDPPacket类：

public static class UDPPacket
    {
        public static byte[] Construct(IPAddress sourceAddress, ushort sourcePort, IPAddress destinationAddress, ushort destinationPort, byte[] payload)
        {
            var bindAddress = IPAddress.Any;

            // Make sure parameters are consistent
            //if ((sourceAddress.AddressFamily != destinationAddress.AddressFamily) || (sourceAddress.AddressFamily != bindAddress.AddressFamily))
            //{
            //    throw new Exception("Source and destination address families don't match!");
            //}

            // Start building the headers
            byte[] builtPacket;
            UdpHeader udpPacket = new UdpHeader();
            ArrayList headerList = new ArrayList();
            //Socket rawSocket = null;
            //SocketOptionLevel socketLevel = SocketOptionLevel.IP;

            // Fill out the UDP header first
            Console.WriteLine("Filling out the UDP header...");
            udpPacket.SourcePort = sourcePort;
            udpPacket.DestinationPort = destinationPort;
            udpPacket.Length = (ushort)(UdpHeader.UdpHeaderLength + payload.Length);
            udpPacket.Checksum = 0;

            if (sourceAddress.AddressFamily == AddressFamily.InterNetwork)
            {
                Ipv4Header ipv4Packet = new Ipv4Header();

                // Build the IPv4 header
                Console.WriteLine("Building the IPv4 header...");
                ipv4Packet.Version = 4;
                ipv4Packet.Protocol = (byte)ProtocolType.Udp;
                ipv4Packet.Ttl = 2;
                ipv4Packet.Offset = 0;
                ipv4Packet.Length = (byte)Ipv4Header.Ipv4HeaderLength;
                ipv4Packet.TotalLength = (ushort)System.Convert.ToUInt16(Ipv4Header.Ipv4HeaderLength + UdpHeader.UdpHeaderLength + payload.Length);
                ipv4Packet.SourceAddress = sourceAddress;
                ipv4Packet.DestinationAddress = destinationAddress;

                // Set the IPv4 header in the UDP header since it is required to calculate the
                //    pseudo header checksum
                Console.WriteLine("Setting the IPv4 header for pseudo header checksum...");
                udpPacket.ipv4PacketHeader = ipv4Packet;

                // Add IPv4 header to list of headers -- headers should be added in th order
                //    they appear in the packet (i.e. IP first then UDP)
                Console.WriteLine("Adding the IPv4 header to the list of header, encapsulating packet...");
                headerList.Add(ipv4Packet);
                //socketLevel = SocketOptionLevel.IP;
            }
            else if (sourceAddress.AddressFamily == AddressFamily.InterNetworkV6)
            {
                Ipv6Header ipv6Packet = new Ipv6Header();

                // Build the IPv6 header
                Console.WriteLine("Building the IPv6 header...");
                ipv6Packet.Version = 6;
                ipv6Packet.TrafficClass = 1;
                ipv6Packet.Flow = 2;
                ipv6Packet.HopLimit = 2;
                ipv6Packet.NextHeader = (byte)ProtocolType.Udp;
                ipv6Packet.PayloadLength = (ushort)(UdpHeader.UdpHeaderLength + payload.Length);
                ipv6Packet.SourceAddress = sourceAddress;
                ipv6Packet.DestinationAddress = destinationAddress;

                // Set the IPv6 header in the UDP header since it is required to calculate the
                //    pseudo header checksum
                Console.WriteLine("Setting the IPv6 header for pseudo header checksum...");
                udpPacket.ipv6PacketHeader = ipv6Packet;

                // Add the IPv6 header to the list of headers - headers should be added in the order
                //    they appear in the packet (i.e. IP first then UDP)
                Console.WriteLine("Adding the IPv6 header to the list of header, encapsulating packet...");
                headerList.Add(ipv6Packet);
                //socketLevel = SocketOptionLevel.IPv6;
            }

            // Add the UDP header to list of headers after the IP header has been added
            Console.WriteLine("Adding the UDP header to the list of header, after IP header...");
            headerList.Add(udpPacket);

            // Convert the header classes into the binary on-the-wire representation
            Console.WriteLine("Converting the header classes into the binary...");
            builtPacket = udpPacket.BuildPacket(headerList, payload);

            /*
            // Create the raw socket for this packet
            Console.WriteLine("Creating the raw socket using Socket()...");
            rawSocket = new Socket(sourceAddress.AddressFamily, SocketType.Raw, ProtocolType.Udp);

            // Bind the socket to the interface specified
            Console.WriteLine("Binding the socket to the specified interface using Bind()...");
            rawSocket.Bind(new IPEndPoint(bindAddress, 0));

            // Set the HeaderIncluded option since we include the IP header
            Console.WriteLine("Setting the HeaderIncluded option for IP header...");
            rawSocket.SetSocketOption(socketLevel, SocketOptionName.HeaderIncluded, 1);

            try
            {
                // Send the packet!
                Console.WriteLine("Sending the packet...");
                int rc = rawSocket.SendTo(builtPacket, new IPEndPoint(destinationAddress, destinationPort));
                Console.WriteLine("send {0} bytes to {1}", rc, destinationAddress.ToString());
            }
            catch (SocketException err)
            {
                Console.WriteLine("Socket error occurred: {0}", err.Message);
                // http://msdn.microsoft.com/en-us/library/ms740668.aspx
            }
            finally
            {
                // Close the socket
                Console.WriteLine("Closing the socket...");
                rawSocket.Close();
            }
            */

            return builtPacket;
        }
    }

协议类：(太长，无法在此处发布)

Protocol Class Code

Answer 3

您应该通过创建一个包含标准UDP数据包中包含的所有数据的UDP类来构造UDP数据包。

数据为follows格式

源端口SP：当尝试或正在进行连接时，它指定本地计算机正在等待哪个端口来侦听来自目标计算机的响应。

目的端口DP：当用户想要连接到远程机器上的服务时，应用层程序指定初始连接应该使用哪个端口。如果不是初始连接的一部分，则指定在将数据包发送到其目的地时将用于远程计算机的端口号。

Length Len：这允许接收站知道多少传入比特被认为是有效分组的一部分。长度是UDP数据包的一部分有多少字节的计算，包括报头中的字节。由于UDP在报头中总是有4个字段，并且每个字段都有16位，并且数据/有效载荷的长度可变，因此我们知道长度将是8+(有效载荷中的字节数)。

UDP Checksum UDPCS：这是一种校验和，它覆盖了数据包的报头和数据部分，允许接收主机验证传入的UDP数据包的完整性。UDP数据包在校验和字段中加载一个预定义的数字，然后在计算校验和时，校验和将覆盖先前的值。当数据包到达目的地时，目标机器的操作系统查看4个报头字段(从第16位到第31位产生的字节)，并将它们从数据包中取出，然后重新计算数据包的校验和，而校验和字段中没有任何内容。然后，OS将计算出的校验和与数据包中传输的校验和进行比较。如果校验和相同，则数据没有问题，并允许其继续传递，但如果存在差异，则会丢弃UDP数据包和数据，并且接收机器不会尝试获取新的副本，发送机器也不会尝试发送相同的数据包。数据包将永远丢失。UDP不可靠！有关可靠的传输层TCP/IP协议簇协议，请参阅TCP数据包。

UDP (Variable bits)：如您所料，这是数据包的有效载荷或数据部分。有效负载可以是任意数量的协议(通常是应用层)。一些最常用的UDP协议包括NFS、DNS以及多个音频和视频流协议。如果在UDP分组中出现错误并且希望修复错误，则留给应用层来查找错误并请求其应用层的“块”或“块”数据。

创建一个包含所有这些数据并适当填充它的类，重载ToString以允许您随后转换为字节数组。

希望这能有所帮助。