How to get mx records for a dns name with System.Net.DNS?

Question

Is there any built in method in the .NET library that will return all of the MX records for a given domain? I see how you get CNAMES, but not MX records.

Answer 1

I spent all day figuring out how to send/receive dns requests and came up with this. Its a complete generic handler. You just have to set the dns server and pass in 'd' eg. my.website.com?d=itmanx.com

<%@ WebHandler Language="C#" Class="Handler" %>

using System;
using System.Web;
using System.Text;
using System.Net;
using System.Net.Sockets;
using System.Collections.Generic;

public class Handler : IHttpHandler
{
    string dns = "dc1";  //change to your dns
    string qtype = "15"; //A=1  MX=15
    string domain = "";
    int[] resp;

    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "text/plain";

        try
        {
            if (context.Request["t"] != null) qtype = context.Request["t"];
            if (context.Request["d"] != null) domain = context.Request["d"];

            if (string.IsNullOrEmpty(domain)) throw new Exception("Add ?d=<domain name> to url or post data");

            Do(context);
        }
        catch (Exception ex)
        {
            string msg = ex.Message;
            if (msg == "1") msg = "Malformed packet";
            else if (msg == "5") msg = "Refused";
            else if (msg == "131") msg = "No such name";

            context.Response.Write("Error: " + msg);
        }
    }

    public void Do(HttpContext context)
    {
        UdpClient udpc = new UdpClient(dns, 53);

        // SEND REQUEST--------------------
        List<byte> list = new List<byte>();
        list.AddRange(new byte[] { 88, 89, 1, 0, 0, 1, 0, 0, 0, 0, 0, 0 });

        string[] tmp = domain.Split('.');
        foreach (string s in tmp)
        {
            list.Add(Convert.ToByte(s.Length));
            char[] chars = s.ToCharArray();
            foreach (char c in chars)
                list.Add(Convert.ToByte(Convert.ToInt32(c)));
        }
        list.AddRange(new byte[] { 0, 0, Convert.ToByte(qtype), 0, 1 });

        byte[] req = new byte[list.Count];
        for (int i = 0; i < list.Count; i++) { req[i] = list[i]; }

        udpc.Send(req, req.Length);


        // RECEIVE RESPONSE--------------
        IPEndPoint ep = null;
        byte[] recv = udpc.Receive(ref ep);
        udpc.Close();

        resp = new int[recv.Length];
        for (int i = 0; i < resp.Length; i++)
            resp[i] = Convert.ToInt32(recv[i]);

        int status = resp[3];
        if (status != 128) throw new Exception(string.Format("{0}", status));
        int answers = resp[7];
        if (answers == 0) throw new Exception("No results");

        int pos = domain.Length + 18;
        if (qtype == "15") // MX record
        {
            while (answers > 0)
            {
                int preference = resp[pos + 13];
                pos += 14; //offset
                string str = GetMXRecord(pos, out pos);
                context.Response.Write(string.Format("{0}: {1}\n", preference, str));
                answers--;
            }
        }
        else if (qtype == "1") // A record
        {
            while (answers > 0)
            {
                pos += 11; //offset
                string str = GetARecord(ref pos);
                context.Response.Write(string.Format("{0}\n", str));
                answers--;
            }
        }
    }

    //------------------------------------------------------
    private string GetARecord(ref int start)
    {
        StringBuilder sb = new StringBuilder();

        int len = resp[start];
        for (int i = start; i < start + len; i++)
        {
            if (sb.Length > 0) sb.Append(".");
            sb.Append(resp[i + 1]);
        }
        start += len + 1;
        return sb.ToString();
    }
    private string GetMXRecord(int start, out int pos)
    {
        StringBuilder sb = new StringBuilder();
        int len = resp[start];
        while (len > 0)
        {
            if (len != 192)
            {
                if (sb.Length > 0) sb.Append(".");
                for (int i = start; i < start + len; i++)
                    sb.Append(Convert.ToChar(resp[i + 1]));
                start += len + 1;
                len = resp[start];
            }
            if (len == 192)
            {
                int newpos = resp[start + 1];
                if (sb.Length > 0) sb.Append(".");
                sb.Append(GetMXRecord(newpos, out newpos));
                start++;
                break;
            }
        }
        pos = start + 1;
        return sb.ToString();
    }

    //------------------------------------------------------
    public bool IsReusable { get { return false; } }
}