Using DNS SRV records with C#

Question

Is there a built in (to the .net framework) class or function to resolve a SRV entry to the corresponding records?

IE: _dmsc._tcp.network.local to an array of information (host, port, weight, priority)

Answer 1

Alternative solution:

P/Invoke to the DnsQuery function and pass DNS_TYPE_SRV.

An example can be found in this excellent post by Ruslan. I reproduce the code below in case the link breaks, but emphasize it is taken directly from his blog.

public class nDnsQuery
{
    public nDnsQuery()
    {
    }

    [DllImport("dnsapi", EntryPoint = "DnsQuery_W", CharSet = CharSet.Unicode, SetLastError = true, ExactSpelling = true)]
    private static extern int DnsQuery([MarshalAs(UnmanagedType.VBByRefStr)]ref string pszName, QueryTypes wType, QueryOptions options, int aipServers, ref IntPtr ppQueryResults, int pReserved);

    [DllImport("dnsapi", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern void DnsRecordListFree(IntPtr pRecordList, int FreeType);


    public static string[] GetSRVRecords(string needle)
    {

        IntPtr ptr1 = IntPtr.Zero;
        IntPtr ptr2 = IntPtr.Zero;
        SRVRecord recSRV;
        if (Environment.OSVersion.Platform != PlatformID.Win32NT)
        {
            throw new NotSupportedException();
        }
        ArrayList list1 = new ArrayList();
        try
        {

            int num1 = nDnsQuery.DnsQuery(ref needle, QueryTypes.DNS_TYPE_SRV, QueryOptions.DNS_QUERY_BYPASS_CACHE, 0, ref ptr1, 0);
            if (num1 != 0)
            {
                if (num1 == 9003)
                {
                    list1.Add("DNS record does not exist");
                }
                else
                {
                    throw new Win32Exception(num1);
                }
            }
            for (ptr2 = ptr1; !ptr2.Equals(IntPtr.Zero); ptr2 = recSRV.pNext)
            {
                recSRV = (SRVRecord)Marshal.PtrToStructure(ptr2, typeof(SRVRecord));
                if (recSRV.wType == (short)QueryTypes.DNS_TYPE_SRV)
                {
                    string text1 = Marshal.PtrToStringAuto(recSRV.pNameTarget);
                    text1 += ":" + recSRV.wPort;
                    list1.Add(text1);
                }
            }
        }
        finally
        {
            nDnsQuery.DnsRecordListFree(ptr1, 0);
        }
        return (string[])list1.ToArray(typeof(string));
    }

    private enum QueryOptions
    {
        DNS_QUERY_ACCEPT_TRUNCATED_RESPONSE = 1,
        DNS_QUERY_BYPASS_CACHE = 8,
        DNS_QUERY_DONT_RESET_TTL_VALUES = 0x100000,
        DNS_QUERY_NO_HOSTS_FILE = 0x40,
        DNS_QUERY_NO_LOCAL_NAME = 0x20,
        DNS_QUERY_NO_NETBT = 0x80,
        DNS_QUERY_NO_RECURSION = 4,
        DNS_QUERY_NO_WIRE_QUERY = 0x10,
        DNS_QUERY_RESERVED = -16777216,
        DNS_QUERY_RETURN_MESSAGE = 0x200,
        DNS_QUERY_STANDARD = 0,
        DNS_QUERY_TREAT_AS_FQDN = 0x1000,
        DNS_QUERY_USE_TCP_ONLY = 2,
        DNS_QUERY_WIRE_ONLY = 0x100
    }

    private enum QueryTypes
    {
        DNS_TYPE_A = 0x0001,
        DNS_TYPE_MX = 0x000f,
        DNS_TYPE_SRV = 0x0021
    }


    [StructLayout(LayoutKind.Sequential)]
    private struct SRVRecord
    {
        public IntPtr pNext;
        public string pName;
        public short wType;
        public short wDataLength;
        public int flags;
        public int dwTtl;
        public int dwReserved;
        public IntPtr pNameTarget;
        public short wPriority;
        public short wWeight;
        public short wPort;
        public short Pad;
    }
}