实验七--应用层网络编程(一).doc

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 2014-06-03

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 .*;
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()));

Systemout.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 .*;
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
长度
16位
16位
字段
Source port
Destination port
值
3588
25

第2、3个32位：
长度
32位
32位
字段
Sequence number
Acknowledgement number
值
82
126

下一个16位:1b（1位）
长度
4位
6位
1位
1位
1位
1位
1位
1位
字段
Header length
Reserved
URG
ACK
PSH
RST
SYN
FIN
值
20
0
0
1
1
0
0
0

再下一个32位：
长度
16位
16位
字段
Window size
Checksum
值
65410
0x4336

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
长度
16位
16位
字段
Source port
Destination port
值
3629
110

第2、3个32位：
长度
32位
32位
字段
Sequence number
Acknowledgement number
值
1
66

下一个16位:1b（1位）
长度
4位
6位
1位
1位
1位
1位
1位
1位
字段
Header length
Reserved
URG
ACK
PSH
RST
SYN
FIN
值
20
0
0
1
1
0
0
0

再下一个32位：
长度
16位
16位
字段
Window size
Checksum
值
65470
0x2a01

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。

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