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

Here is a Class I use to look up MX records only.

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

    namespace Mx.Dns
    {
        public class Query
        {
            //Build a DNS query buffer according to RFC 1035 4.1.1 e 4.1.2
            private readonly int id;
        private readonly int flags;
        private readonly int QDcount;
        private readonly int ANcount;
        private readonly int NScount;
        private readonly int ARcount;
        private readonly string Qname;
        private readonly int Qtype;
        private readonly  int Qclass;
        public byte[] buf;
        
        public Query(int ID, string query, int qtype)
        {
            //init vectors with given + default values
            id = ID;
            flags = 256;
            QDcount = 1;
            ANcount = 0;
            NScount = 0;
            ARcount = 0;
            Qname = query;
            Qtype = qtype;
            Qclass = 1; //Internet = IN = 1

            //build a buffer with formatted query data

            //header information (16 bit padding
            buf = new byte[12 + Qname.Length + 2 + 4];
            buf[0] = (byte)(id / 256);
            buf[1] = (byte)(id - (buf[0] * 256));
            buf[2] = (byte)(flags / 256);
            buf[3] = (byte)(flags - (buf[2] * 256));
            buf[4] = (byte)(QDcount / 256);
            buf[5] = (byte)(QDcount - (buf[4] * 256));
            buf[6] = (byte)(ANcount / 256);
            buf[7] = (byte)(ANcount - (buf[6] * 256));
            buf[8] = (byte)(NScount / 256);
            buf[9] = (byte)(NScount - (buf[8] * 256));
            buf[10] = (byte)(ARcount / 256);
            buf[11] = (byte)(ARcount - (buf[10] * 256));
            //QNAME (RFC 1035 4.1.2)
            //no padding
            string[] s = Qname.Split('.');
            int index = 12;
            foreach (string str in s) {
                buf[index] = (byte)str.Length;
                index++;
                byte[] buf1 = Encoding.ASCII.GetBytes(str);
                buf1.CopyTo(buf, index);
                index += buf1.Length;
            }
            //add root domain label (chr(0))
            buf[index] = 0;
            
            //add Qtype and Qclass (16 bit values)
            index = buf.Length - 4;
            buf[index] = (byte)(Qtype / 256);
            buf[index + 1] = (byte)(Qtype - (buf[index] * 256));
            buf[index + 2] = (byte)(Qclass / 256);
            buf[index + 3] = (byte)(Qclass - (buf[index + 2] * 256));
        }
    }
    public class C_DNSquery
    {
        public StringCollection result = new StringCollection();
        public int Error = 0;
        public string ErrorTxt = "undefined text";
        public bool Done = false;
        public UdpClient udpClient;
        private string DNS;
        private string Query;
        private int Qtype;
        public bool IS_BLACKLIST_QUERY = false;
        public C_DNSquery(string IPorDNSname, string query, int type)
        {
            DNS = IPorDNSname;
            Query = query;
            Qtype = type;
        }
        public void doTheJob()
        {
            //check if provided DNS contains an IP address or a name
            IPAddress ipDNS;
            IPHostEntry he;
            try {
                //try to parse an IPaddress
                ipDNS = IPAddress.Parse(DNS);
            } catch (FormatException ) {
//              Console.WriteLine(e);
                //format error, probably is a FQname, try to resolve it
                try {
                    //try to resolve the hostname
                    he = Dns.GetHostEntry(DNS);
                } catch {
                    //Error, invalid server name or address
                    Error = 98;
                    ErrorTxt = "Invalid server name:" + DNS;
                    Done = true;
                    return;
                }
                //OK, get the first server address
                ipDNS = he.AddressList[0];
            }

            //Query the DNS server
            //our current thread ID is used to match the reply with this process
            
            Query myQuery = new Query(System.Threading.Thread.CurrentThread.ManagedThreadId, Query, Qtype);
            //data buffer for query return value
            Byte[] recBuf;
            
            //use UDP protocol to connect
            udpClient = new UdpClient();
            do {
                try {
                    //connect to given nameserver, port 53 (DNS)
                    udpClient.Connect(DNS, 53);
                    //send query
                    udpClient.Send(myQuery.buf, myQuery.buf.Length);
                    //IPEndPoint object allow us to read datagrams..
                    //..selecting only packet coming from our nameserver and port
                    IPEndPoint RemoteIpEndPoint = new IPEndPoint(ipDNS, 53);
                    //Blocks until a message returns on this socket from a remote host.
                    recBuf = udpClient.Receive(ref RemoteIpEndPoint);
                    udpClient.Close();
                } catch (Exception e) {
                    //connection error, probably a wrong server address
                    udpClient.Close();
                    Error = 99;
                    ErrorTxt = e.Message + "(server:" + DNS + ")";
                    Done = true;
                    return;
                }
                //repeat until we get the reply with our threadID
            } while (System.Threading.Thread.CurrentThread.ManagedThreadId != ((recBuf[0] * 256) + recBuf[1]));

            //Check the DNS reply
            //check if bit QR (Query response) is set
            if (recBuf[2] < 128) {
                //response byte not set (probably a malformed packet)
                Error = 2;
                ErrorTxt = "Query response bit not set";
                Done = true;
                return;
            }
            //check if RCODE field is 0
            if ((recBuf[3] & 15) > 0) {
                //DNS server error, invalid reply
                switch (recBuf[3] & 15) {
                    case 1:
                        Error = 31;
                        ErrorTxt = "Format error. The nameserver was unable to interpret the query";
                        break;
                    case 2:
                        Error = 32;
                        ErrorTxt = "Server failure. The nameserver was unable to process the query.";
                        break;
                    case 3:
                        Error = 33;
                        ErrorTxt = "Name error. Check provided domain name!!";
                        break;
                    case 4:
                        Error = 34;
                        ErrorTxt = "Not implemented. The name server does not support the requested query";
                        break;
                    case 5:
                        Error = 35;
                        ErrorTxt = "Refused. The name server refuses to reply for policy reasons";
                        break;
                    default:
                        Error = 36;
                        ErrorTxt = "Unknown. The name server error code was: " + Convert.ToString((recBuf[3] & 15));
                        break;
                }
                Done = true;
                return;
            }
            //OK, now we should have valid header fields
            int QDcnt, ANcnt, NScnt, ARcnt;
            int index;
            QDcnt = (recBuf[4] * 256) + recBuf[5];
            ANcnt = (recBuf[6] * 256) + recBuf[7];
            NScnt = (recBuf[8] * 256) + recBuf[9];
            ARcnt = (recBuf[10] * 256) + recBuf[11];
            index = 12;
            //sometimes there are no erros but blank reply... ANcnt == 0...
            if (ANcnt == 0) { // if blackhole list query, means no spammer !!//if ((ANcnt == 0) & (IS_BLACKLIST_QUERY == false))
                //error blank reply, return an empty array
                Error = 4;
                ErrorTxt = "Empty string array";
                Done = true;
                return;
            }

            //Decode received information
            string s1;
            // START TEST
            s1 = Encoding.ASCII.GetString(recBuf, 0, recBuf.Length);
            // END TEST
            
            if (QDcnt > 0) {
                //we are not really interested to this string, just parse and skip
                s1 = "";
                index = parseString(recBuf, index, out s1);
                index += 4; //skip root domain, Qtype and QClass values... unuseful in this contest
            }
            if (IS_BLACKLIST_QUERY) {
                // get the answers, normally one !
                // int the four last bytes there is the ip address
                Error = 0;
                int Last_Position = recBuf.Length - 1;
                result.Add(recBuf[Last_Position - 3].ToString() + "." + recBuf[Last_Position - 2].ToString() + "." + recBuf[Last_Position - 1].ToString() + "." + recBuf[Last_Position].ToString());
                Done = true;
                return;
            }
            int count = 0;
            //get all answers
            while (count < ANcnt) {
                s1 = "";
                index = parseString(recBuf, index, out s1);
                //Qtype
                int QType = (recBuf[index] * 256) + recBuf[index + 1];
                index += 2;
                s1 += "," + QType.ToString();
                //QClass
                int QClass = (recBuf[index] * 256) + recBuf[index + 1];
                index += 2;
                s1 += "," + QClass.ToString();
                //TTL (Time to live)
                uint TTL = (recBuf[index] * 16777216u) + (recBuf[index + 1] * 65536u) + (recBuf[index + 2] * 256u) + recBuf[index + 3];
                index += 4;
                s1 += "," + TTL.ToString();
                int blocklen = (recBuf[index] * 256) + recBuf[index + 1];
                index += 2;
                if (QType == 15) {
                    int MXprio = (recBuf[index] * 256) + recBuf[index + 1];
                    index += 2;
                    s1 += "," + MXprio.ToString();
                }
                string s2;
                index = parseString(recBuf, index, out s2);
                s1 += "," + s2;
                result.Add(s1);
                count++;
            }
            Error = 0;
            Done = true;
        }
        private int parseString(byte[] buf, int i, out string s)
        {
            int len;
            s = "";
            bool end = false;
            while (!end) {
                if (buf[i] == 192) {
                    //next byte is a pointer to the string, get it..
                    i++;
                    s += getString(buf, buf[i]);
                    i++;
                    end = true;
                } else {
                    //next byte is the string length
                    len = buf[i];
                    i++;
                    //get the string
                    s += Encoding.ASCII.GetString(buf, i, len);
                    i += len;
                    //check for the null terminator
                    if (buf[i] != 0) {
                        //not null, add a point to the name
                        s += ".";
                    } else {
                        //null char..the string is complete, exit
                        end = true;
                        i++;
                    }
                }
            }
            return i;
        }
        private string getString(byte[] buf, int i)
        {
            string s = "";
            int len;
            bool end = false;
            while (!end) {
                len = buf[i];
                i++;
                s += Encoding.ASCII.GetString(buf, i, len);
                i += len;
                if (buf[i] == 192) {
                    i++;
                    s += "." + getString(buf, buf[i]);
                    return s;
                }
                if (buf[i] != 0) {
                    s += ".";
                } else {
                    end = true;
                }
            }
            return s;
        }
    }
}

Here is how you use it.

/// 

        /// Get the MX from the domain address.
        /// 
        public static string getMXrecord(string domain)
        {
            domain = domain.Substring(domain.IndexOf('@') + 1);
            string LocalDNS = GetDnsAdress().ToString();
            Console.WriteLine("domain: " + domain);
            
            // resolv the authoritative domain (type=2)
            C_DNSquery DnsQry = new C_DNSquery(LocalDNS, domain, 2);
            Thread t1 = new Thread(new ThreadStart(DnsQry.doTheJob));
            t1.Start();
            int timeout = 20;
            while ((timeout > 0) & (!DnsQry.Done)) {
                Thread.Sleep(100);
                timeout--;
            }
            if (timeout == 0) {
                if (DnsQry.udpClient != null) {
                    DnsQry.udpClient.Close();
                }
                t1.Abort();
                DnsQry.Error = 100;
            }

            string[] ns1;
            string MyNs = "";
            if (DnsQry.Error == 0) {
                ns1 = DnsQry.result[0].Split(',');
                MyNs = ns1[4];
                t1.Abort();
            } else {
                t1.Abort();
                MyNs = LocalDNS;
            }
            
            // Resolve MX (type = 15)
            DnsQry = new C_DNSquery(MyNs, domain, 15);
            Thread t2 = new Thread(new ThreadStart(DnsQry.doTheJob));
            t2.Start();
            timeout = 20;
            string TTL = "";
            string MXName = "";
            Int32 preference = 9910000;
            while ((timeout > 0) & (!DnsQry.Done)) {
                Thread.Sleep(100);
                timeout--;
            }
            if (timeout == 0) {
                if (DnsQry.udpClient != null) {
                    DnsQry.udpClient.Close();
                }
                t2.Abort();
                DnsQry.Error = 100;
            }
            if (DnsQry.Error == 0) {
                
                if (DnsQry.result.Count == 1) {
                    string[] ns2 = DnsQry.result[0].Split(',');
                    MXName = ns2[5];
                    TTL = ns2[3];
                    preference = Int32.Parse(ns2[4]);
                    Console.WriteLine("domaine: {0} MX: {1} time: {2} pref: {3} ttl: {4}", domain.Substring(domain.IndexOf('@') + 1), MXName, 
                        DateTime.Now, preference, TTL);
            
            
                } else {
                    for (int indns = 0; indns <= DnsQry.result.Count - 1; indns++) {
                        string[] ns2 = DnsQry.result[indns].Split(',');
                        if (Int32.Parse(ns2[4]) < preference) {
                            MXName = ns2[5];
                            TTL = ns2[3];
                            preference = Int32.Parse(ns2[4]);
Console.WriteLine("domain: {0} MX: {1} time: {2} pref: {3} ttl: {4}", domain.Substring(domain.IndexOf('@') + 1), MXName, 
                                DateTime.Now, preference, TTL);
            
                            }
                    }
                }
            }
            return MXName;
        }