浙江大学城市学院实验报告

课程名称                         计算机网络应用                     

实验项目名称               实验七  应用层网络编程（一）             

实验成绩          指导老师（签名）                日期    20##-06-03 

一. 实验目的和要求

1.        通过实现使用Java应用层客户端和服务器来获得关于使用Java Socket网络编程的经验（SMTP、POP3）。

二. 实验内容、原理及实验结果与分析

1.     SMTP编程（参考电子讲义“网络编程参考资料-应用层.pdf”及教材“第2章 Socket编程”）

阅读 “网络编程参考资料-应用层.pdf”中 8.3.1部分，实现“SMTP客户机实现”的源代码（SMTPClientDemo.java），并在机器上编译运行通过。（注：可输入城院SMTP邮件服务器smtp.email.zucc.edu.cn或其他邮件服务器作为SMTP服务器）

【程序源代码】

SMTPClientDemo.java

import java.io.*;

import java.net.*;

import java.util.*;

// Chapter 8, Listing 1

public class SMTPClientDemo {

  protected int port = 25;

  protected String hostname = "localhost";

  protected String from = "";

  protected String to = "";

  protected String subject = "";

  protected String body = "";

  protected Socket socket;

  protected BufferedReader br;

  protected PrintWriter pw;

  // Constructs a new instance of the SMTP Client

  public SMTPClientDemo() throws Exception {

    try {

      getInput();

      sendEmail();

    } catch (Exception e) {

      System.out.println ("Error sending message - " + e);

    }

  }

  public static void main(String[] args) throws Exception {

     // Start the SMTP client, so it can send messages

     SMTPClientDemo client = new SMTPClientDemo();

  }

  // Check the SMTP response code for an error message

  protected int readResponseCode() throws Exception {

    String line = br.readLine();

    System.out.println("< "+line);

    line = line.substring(0,line.indexOf(" "));

    return Integer.parseInt(line);

  }

  // Write a protocol message both to the network socket and to the screen

  protected void writeMsg(String msg) throws Exception {

    pw.println(msg);

    pw.flush();

    System.out.println("> "+msg);

  }

  // Close all readers, streams and sockets

  protected void closeConnection() throws Exception {

    pw.flush();

    pw.close();

    br.close();

    socket.close();

  }

  // Send the QUIT protocol message, and terminate connection

  protected void sendQuit() throws Exception {

    System.out.println("Sending QUIT");

    writeMsg("QUIT");

    readResponseCode();

    System.out.println("Closing Connection");

    closeConnection();

  }

  // Send an email message via SMTP, adhering to the protocol known as RFC 2821

  protected void sendEmail() throws Exception {

    System.out.println("Sending message now: Debug below");

    System.out.println("---------------------------------" +

                       "-----------------------------");

    System.out.println("Opening Socket");

    socket = new Socket(this.hostname,this.port);

    System.out.println("Creating Reader & Writer");

    br = new BufferedReader(new InputStreamReader(socket.getInputStream()));

    pw = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()));

    System.out.println("Reading first line");

    int code = readResponseCode();

    if(code != 220) {

      socket.close();

      throw new Exception("Invalid SMTP Server");

    }

    System.out.println("Sending helo command");

    writeMsg("HELO "+InetAddress.getLocalHost().getHostName());

    code = readResponseCode();

    if(code != 250) {

      sendQuit();

      throw new Exception("Invalid SMTP Server");

    }

    System.out.println("Sending mail from command");

    writeMsg("MAIL FROM:<"+this.from+">");

    code = readResponseCode();

    if(code != 250) {

      sendQuit();

      throw new Exception("Invalid from address");

    }

    System.out.println("Sending rcpt to command");

    writeMsg("RCPT TO:<"+this.to+">");

    code = readResponseCode();

    if(code != 250) {

      sendQuit();

      throw new Exception("Invalid to address");

    }

    System.out.println("Sending data command");

    writeMsg("DATA");

    code = readResponseCode();

    if(code != 354) {

      sendQuit();

      throw new Exception("Data entry not accepted");

    }

    System.out.println("Sending message");

    writeMsg("Subject: "+this.subject);

    writeMsg("To: "+this.to);

    writeMsg("From: "+this.from);

    writeMsg("");

    writeMsg(body);

    code = readResponseCode();

    sendQuit();

    if(code != 250)

      throw new Exception("Message may not have been sent correctly");

    else

      System.out.println("Message sent");

  }

  // Obtain input from the user

  protected void getInput() throws Exception {

    // Read input from user console

    String data=null;

    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    // Request hostname for SMTP server

    System.out.print("Please enter SMTP server hostname: ");

    data = br.readLine();

    if (data == null || data.equals("")) hostname="localhost";

    else hostname=data;

    // Request the sender's email address

    System.out.print("Please enter FROM email address: ");

    data = br.readLine();

    from = data;

    // Request the recipient's email address

    System.out.print("Please enter TO email address :");

    data = br.readLine();

    if(!(data == null || data.equals("")))

      to=data;

    System.out.print("Please enter subject: ");

    data = br.readLine();

    subject=data;

    System.out.println("Please enter plain-text message ('.' character" +

                       "on a blank line signals end of message):");

    StringBuffer buffer = new StringBuffer();

    // Read until user enters a . on a blank line

    String line = br.readLine();

    while(line != null) {

      // Check for a '.', and only a '.', on a line

      if(line.equalsIgnoreCase(".")) {

        break;

      }

      buffer.append(line);

      buffer.append("\n");

      line = br.readLine();

    }

    buffer.append(".\n");

    body = buffer.toString();

  }

}

【实验结果与分析】

2.     POP3编程（参考电子讲义“网络编程参考资料-应用层.pdf”及教材“第2章 Socket编程”）

阅读 “网络编程参考资料-应用层.pdf”中 8.3.2部分，实现“POP3客户实现”的源代码（Pop3ClientDemo.java），并在机器上编译运行通过。（注：可输入城院POP3邮件服务器pop3.email.zucc.edu.cn或其他邮件服务器作为POP3服务器）

【程序源代码】

Pop3ClientDemo.java

import java.io.*;

import java.net.*;

import java.util.*;

public class Pop3ClientDemo {

  protected int port = 110;

  protected String hostname = "localhost";

  protected String username = "";

  protected String password = "";

  protected Socket socket;

  protected BufferedReader br;

  protected PrintWriter pw;

  // Constructs a new instance of the POP3 client

  public Pop3ClientDemo() throws Exception {

    try {

      // Get user input

      getInput();

      // Get mail messages

      displayEmails();

    } catch(Exception e) {

      System.err.println ("Error occured - details follow");

      e.printStackTrace();

      System.out.println(e.getMessage());

    }

  }

  // Returns TRUE if POP response indicates success, FALSE if failure

  protected boolean responseIsOk() throws Exception {

    String line = br.readLine();

    System.out.println("< "+line);

    // 和 SMTP 不同的地方，POP3 的回覆不再是一個 number 而是

    // +OK 來代表要求成功。失敗則以 -ERR 來代表。

    return line.toUpperCase().startsWith("+OK");

  }

  // Reads a line from the POP server, and displays it to screen

  protected String readLine(boolean debug) throws Exception {

    String line = br.readLine();

    // Append a < character to indicate this is a server protocol response

    if (debug)

      System.out.println("< "+line);

    else

      System.out.println(line);

    return line;

  }

  // Writes a line to the POP server, and displays it to the screen

  protected void writeMsg(String msg) throws Exception {

    pw.println(msg);

    pw.flush();

    System.out.println("> "+msg);

  }

  // Close all writers, streams and sockets

  protected void closeConnection() throws Exception {

    pw.flush();

    pw.close();

    br.close();

    socket.close();

  }

  // Send the QUIT command, and close connection

  protected void sendQuit() throws Exception {

    System.out.println("Sending QUIT");

    writeMsg("QUIT");

    readLine(true);

    System.out.println("Closing Connection");

    closeConnection();

  }

  // Display emails in a message

  protected void displayEmails() throws Exception {

    BufferedReader userinput = new BufferedReader( new

                                   InputStreamReader (System.in) );

    System.out.println("Displaying mailbox with protocol commands" +

                       "and responses below");

    System.out.println("-----------------------------------------" +

                       "---------------------");

    // Open a connection to POP3 server

    System.out.println("Opening Socket");

    socket = new Socket(this.hostname, this.port);

    br = new BufferedReader(new InputStreamReader(socket.getInputStream()));

    pw = new PrintWriter(new OutputStreamWriter(socket.getOutputStream()));

    // If response from server is not okay

    if(! responseIsOk()) {

      socket.close();

      throw new Exception("Invalid POP3 Server");

    }

    // Login by sending USER and PASS commands

    System.out.println("Sending username");

    writeMsg("USER "+this.username);

    if(!responseIsOk()) {

      sendQuit();

      throw new Exception("Invalid username");

    }

    System.out.println("Sending password");

    writeMsg("PASS "+this.password);

    if(!responseIsOk()) {

      sendQuit();

      throw new Exception("Invalid password");

    }

    // Get mail count from server ....

    System.out.println("Checking mail");

    writeMsg("STAT");

    // ... and parse for number of messages

    String line = readLine(true);

    StringTokenizer tokens = new StringTokenizer(line," ");

    // +OK

    tokens.nextToken();

    // number of messages

    int messages = Integer.parseInt(tokens.nextToken());

    // size of all messages

    int maxsize = Integer.parseInt(tokens.nextToken());

    if (messages == 0) {

      System.out.println ("There are no messages.");

      sendQuit();

      return;

    }

    System.out.println ("There are " + messages + " messages.");

    System.out.println("Press enter to continue.");

    userinput.readLine();

   

    for(int i = 1; i <= messages ; i++) {

      System.out.println("Retrieving message number "+i);

      writeMsg("RETR "+i);

      System.out.println("--------------------");

      line = readLine(false);

      while(line != null && !line.equals(".")) {

        line = readLine(false);

      }

      System.out.println("--------------------");

      System.out.println("Press enter to continue. To stop, " +

                         "type Q then enter");

      String response = userinput.readLine();

      if (response.toUpperCase().startsWith("Q"))

        break;

    }

    sendQuit();

  }

  public static void main(String[] args) throws Exception {

    Pop3ClientDemo client = new Pop3ClientDemo();

  }

  // Read user input

  protected void getInput() throws Exception {

    String data=null;

    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    System.out.print("Please enter POP3 server hostname:");

    data = br.readLine();

    if(data == null || data.equals("")) hostname="localhost";

    else hostname=data;

    System.out.print("Please enter mailbox username:");

    data = br.readLine();

    if(!(data == null || data.equals("")))

      username=data;

    System.out.print("Please enter mailbox password:");

    data = br.readLine();

    if(!(data == null || data.equals("")))

      password=data;

  }

}

【实验结果与分析】

3.     Ethereal抓包分析

用Ethereal软件截获上面两个程序运行时客户机和服务器之间发送的数据包，并且根据截获的数据包内容进行分析。

【实验结果与分析】

host 10.66.19.27 tcp port 25

SMTP数据包结构

下面的表格的值取自图中选中的数据包：

前32位

第2、3个32位：

下一个16位:1b（1位）

再下一个32位：

host 10.66.19.27 tcp port 110

Pop3数据包结构

下面的表格的值取自图中选中的数据包：

前32位

第2、3个32位：

下一个16位:1b（1位）

再下一个32位：

三. 讨论、心得

记录实验感受、上机过程中遇到的困难及解决办法、遗留的问题、意见和建议等。