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

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

namespace DG
{
    public class IPPacket // RFC791
    {
        public byte Version;
        public byte HeaderLength;
        public byte TypeOfService;
        public ushort TotalLength;
        public ushort Identification;
        public byte Flags;
        public ushort FragmentOffset;
        public byte TimeToLive;
        public byte Protocol;
        public ushort HeaderChecksum;
        public System.Net.IPAddress SourceAddress;
        public System.Net.IPAddress DestinationAddress;
        public byte[] PacketData;

        public ICMPPacket ICMP;
        public TCPPacket TCP;
        public UDPPacket UDP;

        public IPPacket() : base() { }
        
        public IPPacket(ref byte[] Packet) : base()
        {
            try {
                Version = (byte)(Packet[0] >> 4);
                HeaderLength = (byte)((Packet[0] & 0x0F) * 4);
                TypeOfService = Packet[1];
                TotalLength = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 2));
                Identification = (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 4));
                Flags = (byte)((Packet[6] & 0xE0) >> 5);
                FragmentOffset = (ushort)(System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 6)) & 0x1FFF);
                TimeToLive = Packet[8];
                Protocol = Packet[9];
                HeaderChecksum = (ushort)(System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, 10)));
                SourceAddress = new System.Net.IPAddress(System.BitConverter.ToInt32(Packet, 12) & 0x00000000FFFFFFFF);
                DestinationAddress = new System.Net.IPAddress(System.BitConverter.ToInt32(Packet, 16) & 0x00000000FFFFFFFF);
                PacketData = new byte[TotalLength - HeaderLength];
                System.Buffer.BlockCopy(Packet, HeaderLength, PacketData, 0, PacketData.Length);
            } catch { }
            
            switch (Protocol) {
                case 1: ICMP = new ICMPPacket(ref PacketData); break;
                case 6: TCP = new TCPPacket(ref PacketData); break;
                case 17: UDP = new UDPPacket(ref PacketData); break;
            }
        }

        public byte[] GetBytes()
        {
            if (ICMP != null) { Protocol = 1; PacketData = ICMP.GetBytes(); }
            if (TCP != null) { Protocol = 6; PacketData = TCP.GetBytes(ref SourceAddress, ref DestinationAddress); }
            if (UDP != null) { Protocol = 17; PacketData = UDP.GetBytes(ref SourceAddress, ref DestinationAddress); }

            if (PacketData == null) PacketData = new byte[0];
            if (Version == 0) Version = 4;
            if (HeaderLength == 0) HeaderLength = 20;
            TotalLength = (ushort)(HeaderLength + PacketData.Length);
            byte[] Packet = new byte[TotalLength];
            if (TimeToLive == 0) TimeToLive = 128;
            Packet[0] = (byte)(((Version & 0x0F) << 4) | ((HeaderLength / 4) & 0x0F));
            Packet[1] = TypeOfService;
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)TotalLength)), 0, Packet, 2, 2);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)Identification)), 0, Packet, 4, 2);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)((FragmentOffset & 0x1F) | ((Flags & 0x03) << 13)))), 0, Packet, 6, 2);
            Packet[8] = TimeToLive;
            Packet[9] = Protocol;
            System.Buffer.BlockCopy(System.BitConverter.GetBytes((short)0), 0, Packet, 10, 2);
            System.Buffer.BlockCopy(SourceAddress.GetAddressBytes(), 0, Packet, 12, 4);
            System.Buffer.BlockCopy(DestinationAddress.GetAddressBytes(), 0, Packet, 16, 4);
            System.Buffer.BlockCopy(PacketData, 0, Packet, HeaderLength, PacketData.Length);
            HeaderChecksum = GetChecksum(ref Packet, 0, HeaderLength - 1);
            System.Buffer.BlockCopy(System.BitConverter.GetBytes(System.Net.IPAddress.HostToNetworkOrder((short)HeaderChecksum)), 0, Packet, 10, 2);
            return Packet;
        }

        public static ushort GetChecksum(ref byte[] Packet, int start, int end)
        {
            uint CheckSum = 0;
            int i;
            for (i=start; i<end; i+=2) CheckSum += (ushort)System.Net.IPAddress.NetworkToHostOrder(System.BitConverter.ToInt16(Packet, i));
            if (i == end) CheckSum += (ushort)System.Net.IPAddress.NetworkToHostOrder((ushort)Packet[end]);
            while (CheckSum >> 16 != 0) CheckSum = (CheckSum & 0xFFFF) + (CheckSum >> 16);
            return (ushort)~CheckSum;
        }
    }
}