Simple Network Sniffer - Open Source - C# (C-sharp)

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main.cs
TCP packet structure

// Author: Toomas Kaljus
// http://www.digigrupp.com

namespace DG
{
    public class TCPPacket //rfc793
    {
        public ushort SourcePort;
        public ushort DestinationPort;
        public uint SequenceNumber;
        public uint AcknowledgmentNumber;
        public byte DataOffset;
        public byte ControlBits;
        public ushort Window;
        public ushort Checksum;
        public ushort UrgentPointer;
        public byte[] Options;
        public byte[] PacketData;

        public TCPPacket() : base() { }

        public TCPPacket(ref byte[] Packet)
            : base()
        {
            try
            {
                SourcePort = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 0));
                DestinationPort = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 2));
                SequenceNumber = (uint)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 4));
                AcknowledgmentNumber = (uint)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 8));
                DataOffset = (byte)((Packet[12] >> 4) * 4);
                ControlBits = (byte)((Packet[13] & 0x3F));
                Window = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 14));
                Checksum = (ushort)(System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 16)));
                UrgentPointer = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 18));
                Options = new byte[DataOffset - 20];
                System.Buffer.BlockCopy(Packet, 20, Options, 0, Options.Length);
                PacketData = new byte[Packet.Length - DataOffset];
                System.Buffer.BlockCopy(Packet, DataOffset, PacketData, 0, Packet.Length - DataOffset);
            }
            catch { }
        }

        public byte[] GetBytes(ref System.Net.IPAddress SourceAddress, ref System.Net.IPAddress DestinationAddress)
        {
            if (PacketData == null) PacketData = new byte[0];
            if (Options == null) Options = new byte[0];
            int OptionsLength = ((int)((Options.Length + 3) / 4)) * 4;
            DataOffset = (byte)(20 + OptionsLength);
            byte[] Packet = new byte[20 + OptionsLength + PacketData.Length];
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)SourcePort)), 0, Packet, 0, 2);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)DestinationPort)), 0, Packet, 2, 2);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((int)SequenceNumber)), 0, Packet, 4, 4);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((int)AcknowledgmentNumber)), 0, Packet, 8, 4);
            Packet[12] = (byte)(((Packet[12] & 0x0F) | ((DataOffset & 0x0F) << 4)) / 4);
            Packet[13] = (byte)(((Packet[13] & 0xC0) | (ControlBits & 0x3F)));
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)Window)), 0, Packet, 14, 2);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes((short)0), 0, Packet, 16, 2);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)UrgentPointer)), 0, Packet, 18, 2);
            System.Buffer.BlockCopy(Options, 0, Packet, 20, Options.Length);
            if (OptionsLength > Options.Length) System.Buffer.BlockCopy(System.BitConverter.GetBytes((long)0), 0, Packet, 20 + Options.Length, OptionsLength - Options.Length);
            System.Buffer.BlockCopy(PacketData, 0, Packet, DataOffset, PacketData.Length);
            Checksum = GetChecksum(ref Packet, 0, DataOffset - 1, ref SourceAddress, ref DestinationAddress);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes((short)Checksum), 0, Packet, 16, 2);
            return PacketData;
        }

        public ushort GetChecksum(ref byte[] Packet, int start, int end, ref System.Net.IPAddress SourceAddress, ref System.Net.IPAddress DestinationAddress)
        {
            byte[] PseudoPacket;
            PseudoPacket = new byte[12 + Packet.Length];
            System.Buffer.BlockCopy(SourceAddress.GetAddressBytes(), 0, PseudoPacket, 0, 4);
            System.Buffer.BlockCopy(DestinationAddress.GetAddressBytes(), 0, PseudoPacket, 4, 4);
            PseudoPacket[8] = 0;
            PseudoPacket[9] = 6;
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)Packet.Length)), 0, PseudoPacket, 10, 2);
            System.Buffer.BlockCopy(Packet, 0, PseudoPacket, 12, Packet.Length);
            return IPPacket.GetChecksum(ref PseudoPacket, 0, PseudoPacket.Length - 1);
        }
    }
}