class Program
{
  static void Main(string[] args)
  {
    using (DnsServer server = new DnsServer(IPAddress.Any, 10, 10, ProcessQuery))
    {
      server.Start();
      Console.WriteLine("Press any key to stop server");
      Console.ReadLine();
    }
  }

  static DnsMessageBase ProcessQuery(DnsMessageBase message, IPAddress clientAddress, ProtocolType protocol)
  {
    message.IsQuery = false;
    DnsMessage query = message as DnsMessage;
    // check for valid query
    if ((query != null)
      && (query.Questions.Count == 1)
      && (query.Questions[0].RecordType == RecordType.Txt)
      && (query.Questions[0].Name.Equals("example.com", StringComparison.InvariantCultureIgnoreCase)))
    {
      query.ReturnCode = ReturnCode.NoError;
      query.AnswerRecords.Add(new TxtRecord("example.com", 3600, "Hello world"));
    }
    else
    {
      message.ReturnCode = ReturnCode.ServerFailure;
    }
    return message;
  }
}

class Program
{
  static void Main(string[] args)
  {
    using (DnsServer server = new DnsServer(IPAddress.Any, 10, 10, ProcessQuery))
    {
      server.Start();

      Console.WriteLine("Press any key to stop server");
      Console.ReadLine();
    }
  }

  static DnsMessageBase ProcessQuery(DnsMessageBase message, IPAddress clientAddress, ProtocolType protocol)
  {
    message.IsQuery = false;

    DnsMessage query = message as DnsMessage;

    if ((query != null) && (query.Questions.Count == 1))
    {
      // send query to upstream server
      DnsQuestion question = query.Questions[0];
      DnsMessage answer = DnsClient.Default.Resolve(question.Name, question.RecordType, question.RecordClass);

      // if got an answer, copy it to the message sent to the client
      if (answer != null)
      {
        foreach (DnsRecordBase record in (answer.AnswerRecords))
        {
          query.AnswerRecords.Add(record);
        }
        foreach (DnsRecordBase record in (answer.AdditionalRecords))
        {
          query.AnswerRecords.Add(record);
        }

        query.ReturnCode = ReturnCode.NoError;
        return query;
      }
    }

    // Not a valid query or upstream server did not answer correct
    message.ReturnCode = ReturnCode.ServerFailure;
    return message;
  }
}