用C#来实现NslookUP的功能

using System;
using System.Collections;
using System.Net;
using System.IO;
using System.Text;
using System.Net.Sockets;

namespace Wuyin.Network.Components
{
    public class NsLookUp
    {
        private byte[] data;
        private int position, id, length;
        private string name;
        private ArrayList dnsServers;
        private static int DNS_PORT = 53;

        Encoding ASCII = Encoding.ASCII;

        public NsLookUp(ArrayList dnsServers)
        {
            id = DateTime.Now.Millisecond * 60;
            this.dnsServers = dnsServers;
        }

        public ArrayList getMXRecords(string host)
        {
            ArrayList mxRecords = null;
            for (int i = 0; i < dnsServers.Count; i++)
            {
                try
                {
                    mxRecords = getMXRecords(host, (string)dnsServers[i]);
                    break;
                }
                catch (IOException)
                {
                    continue;
                }
            }
            return mxRecords;
        }

        private int getNewId()
        {
            return ++id;
        }

        public ArrayList getMXRecords(string host, string serverAddress)
        {
            UdpClient dnsClient = new UdpClient(serverAddress, DNS_PORT);
            //preparing the DNS query packet. 
            makeQuery(getNewId(), host);
            //send the data packet 
            dnsClient.Send(data, data.Length);
            IPEndPoint endpoint = null;
            //receive the data packet from DNS server 
            data = dnsClient.Receive(ref endpoint);
            length = data.Length;
            //un pack the byte array & makes an array of MXRecord objects. 
            return makeResponse();
        }

        //for packing the information to the format accepted by server 
        public void makeQuery(int id, String name)
        {
            data = new byte[512];
            for (int i = 0; i < 512; ++i)
            {
                data[i] = 0;
            }
            data[0] = (byte)(id >> 8);
            data[1] = (byte)(id & 0xFF);
            data[2] = (byte)1; data[3] = (byte)0;
            data[4] = (byte)0; data[5] = (byte)1;
            data[6] = (byte)0; data[7] = (byte)0;
            data[8] = (byte)0; data[9] = (byte)0;
            data[10] = (byte)0; data[11] = (byte)0;
            string[] tokens = name.Split(new char[] { '.' });
            string label;
            position = 12;
            for (int j = 0; j < tokens.Length; j++)
            {
                label = tokens[j];
                data[position++] = (byte)(label.Length & 0xFF);
                byte[] b = ASCII.GetBytes(label);
                for (int k = 0; k < b.Length; k++)
                {
                    data[position++] = b[k];
                }
            }
            data[position++] = (byte)0; data[position++] = (byte)0;
            data[position++] = (byte)15; data[position++] = (byte)0;
            data[position++] = (byte)1;
        }
        //for un packing the byte array 
        public ArrayList makeResponse()
        {
            ArrayList mxRecords = new ArrayList();
            MXRecord mxRecord;
            //NOTE: we are ignoring the unnecessary fields. 
            //      and takes only the data required to build 
            //      MX records. 
            int qCount = ((data[4] & 0xFF) << 8) | (data[5] & 0xFF);
            if (qCount < 0)
            {
                throw new IOException("invalid question count");
            }
            int aCount = ((data[6] & 0xFF) << 8) | (data[7] & 0xFF);
            if (aCount < 0)
            {
                throw new IOException("invalid answer count");
            }
            position = 12;
            for (int i = 0; i < qCount; ++i)
            {
                name = "";
                position = proc(position);
                position += 4;
            }
            for (int i = 0; i < aCount; ++i)
            {
                name = "";
                position = proc(position);
                position += 10;
                int pref = (data[position++] << 8) | (data[position++] & 0xFF);
                name = "";
                position = proc(position);
                mxRecord = new MXRecord();
                mxRecord.preference = pref;
                mxRecord.exchange = name;
                mxRecords.Add(mxRecord);
            }
            return mxRecords;
        }

        private int proc(int position)
        {
            int len = (data[position++] & 0xFF);
            if (len == 0)
            {
                return position;
            }
            int offset;
            do
            {
                if ((len & 0xC0) == 0xC0)
                {
                    if (position >= length)
                    {
                        return -1;
                    }
                    offset = ((len & 0x3F) << 8) | (data[position++] & 0xFF);
                    proc(offset);
                    return position;
                }
                else
                {
                    if ((position + len) > length)
                    {
                        return -1;
                    }
                    name += ASCII.GetString(data, position, len);
                    position += len;
                }
                if (position > length)
                {
                    return -1;
                }
                len = data[position++] & 0xFF;
                if (len != 0)
                {
                    name += ".";
                }
            } while (len != 0);
            return position;
        }
    }

    public class MXRecord : IComparable
    {
        public int preference = -1;
        public string exchange = null;
        public override string ToString()
        {
            return "Preference : " + preference + " Exchange : " + exchange;
        }
        int IComparable.CompareTo(object obj)
        {
            if (this.preference < ((MXRecord)obj).preference)
                return -1;
            if (this.preference > ((MXRecord)obj).preference)
                return 1;
            return 0;
        }
    }
}