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Socket Console for SMTP in C#

, 25 May 2004
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This is a byproduct while writing a fully functional mail sender. A good example for learning socket communication.

Sample image

Introduction

This console is used for learning SMTP communication.

  1. How to use packaged socket client class TcpClient.
  2. How to make a simple TextBox look and act like a console box.
  3. How to communicate with SMTP servers.

Classes

  1. There is a class called SocketHelper used to process SMTP commands and receive response from server.
    public class SocketHelper
    {
        private TcpClient client=null;
        private NetworkStream stream=null;
        private StreamReader reader=null;
        private StreamWriter writer=null;
        private string resp="";
        private int state=-1;
    
        public SocketHelper(string name,int port)
        {
            //
            // TODO: Add constructor logic here
            //
            client=new TcpClient(name,port);
            stream=client.GetStream();
            reader=new StreamReader(stream);
            writer=new StreamWriter(stream);
        }
    
        public SocketHelper(TcpClient tc)
        {
            client=tc;
            stream=client.GetStream();
            reader=new StreamReader(stream);
            writer=new StreamWriter(stream);
        }
    
        public void SendData(byte[] bts)
        {
            if(GetResponseState()!=221)
            {
                stream.Write(bts,0,bts.Length);
                stream.Flush();
            }
        }
    
        public void SendCommand(string cmd)
        {
            if(GetResponseState()!=221)
            {
                writer.WriteLine(cmd);
                writer.Flush();
            }
        }
    
        public string RecvResponse()
        {
            if(GetResponseState()!=221)
                resp=reader.ReadLine();
            else
                resp="221 closed!";
    
            return resp;
        }
    
        public int GetResponseState()
        {
            if(resp.Length>=3 && IsNumber(resp[0]) 
              && IsNumber(resp[1]) && IsNumber(resp[2]))
                state=Convert.ToInt32(resp.Substring(0,3));
    
                return state;
        }
    
        private bool IsNumber(char c)
        {
            return c>='0' && c<='9';
        }
    
        public string GetFullResponse()
        {
            System.Text.StringBuilder sb=new System.Text.StringBuilder();
            sb.Append(RecvResponse());
            sb.Append("\r\n");
            while(HaveNextResponse())
            {
                sb.Append(RecvResponse());
                sb.Append("\r\n");
            }
            return sb.ToString();
        }
    
        public bool HaveNextResponse()
        {
            if(GetResponseState()>-1)
            {
                if(resp.Length>=4 && resp[3]!=' ')
                    return true;
                else
                    return false;
            }
            else
                return false;
        }
    }

    It is a simple package for TcpClient. My biggest problem while programming was that NetworkStream has no Length. My solution is to parse server response and then decide whether to get more response (some advise please).

  2. The code makes a TextBox like a console, as follows:
    private void tbConsole_KeyDown(object sender, 
                System.Windows.Forms.KeyEventArgs e)
    {
        if(tbConsole.SelectionStart < 
         tbConsole.TextLength-tbConsole.Lines[tbConsole.Lines.Length-1].Length)
            tbConsole.SelectionStart=tbConsole.TextLength;
    
        if(e.KeyValue==13)    //enter
        {
    
            int pos=tbConsole.Text.LastIndexOf("\n");
            string cmd="";
            string res="\r\n";
            bool getresp=true;
            bool datamode=false;
            byte[] bts=null;
            if(pos>-1)
              cmd=tbConsole.Text.Substring(pos+1,tbConsole.TextLength-pos-1);
            else
              cmd=tbConsole.Text;
    
            if(cmd==string.Empty)
              return;
    
            if(cmd[0]=='$')
            {
                cmd=Convert.ToBase64String(
                System.Text.Encoding.ASCII.GetBytes(cmd.Substring(1)));
                tbConsole.Text+="["+cmd+"]";
            }
            else if(cmd[0]=='.' && cmd.Length>1)
            {
                cmd=cmd.Substring(1);
                getresp=false;
            }
            else if(cmd[0]=='@')
            {
                cmd=cmd.Substring(1);
                if(System.IO.File.Exists(cmd))
                {
                    FileStream file=System.IO.File.OpenRead(cmd);
                    bts=new byte[file.Length];
                    file.Read(bts,0,bts.Length);
                    file.Close();
                    datamode=true;
                    getresp=false;
                }
            }
            if(datamode)
                this.Helper.SendData(bts);
            else
                this.Helper.SendCommand(cmd);
            if(getresp)
                res+=this.Helper.GetFullResponse();
    
            tbConsole.Text+=res;
            tbConsole.SelectionStart=tbConsole.TextLength;
            e.Handled=true;
        }
        else if(e.KeyValue==38 || e.KeyValue==40)    //move up or down
            e.Handled=true;
        else if(e.KeyValue==39)    //move right
        {
            if(tbConsole.SelectionStart < tbConsole.TextLength)
            {
                char c=tbConsole.Text[tbConsole.SelectionStart];
                if(c=='\r' || c=='\n')
                    e.Handled=true;
            }
        }
        else if(e.KeyValue==37 || e.KeyValue==8)    //move left
        {
            if(tbConsole.SelectionStart>0)
            {
                char c=tbConsole.Text[tbConsole.SelectionStart-1];
                if(c=='\r' || c=='\n')
                    e.Handled=true;
            }
        }
    }

    tbConsole.SelectionStart tells us where the current cursor is. In order to limit the actions of the TextBox, I changed the manner of arrow keys, enter key, and backspace. There are several special commands: the style "$string" means convert "string" to base64 string, then send "@filename" means load "filename" from disk, and send to server ".string" means send "string" to server but don't try to get response. That's the main difference from command line tool Telnet.

  3. The common command sequence to communicate with a SMTP server should be as follows:
    HELO dear server
    AUTH LOGIN
    (enter base64 string as username)
    (enter base64 string as password)
    MAIL From:sa@mail.local
    RCPT To:target@mail.local
    DATA
    (send mail data to server)
    .(send over)
    QUIT

    HELO is some level equivalent to EHLO (without detailed command prompt). So in my console, just enter the following commands to send a mail:

    HELO dear server
    AUTH Login
    $myname
    $mypass
    MAIL From:sa@mail.local
    RCPT To:test@mail.local
    DATA
    @c:\mail.txt
    .
    QUIT

    Here is a sample mail:

    From: from@mail.local
    To: to@mail.local
    Subject: hello boy
    
    nice to meet you

    Make some changes, you can turn this console into any kind of TCP client. So that's all.

License

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About the Author

vmlinuxx
Web Developer
China China
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