试着用java实现DNS(一)——DatagramSocket, DatagramPacket, Message

一般来说，自己编写DNS是没有必要的，目前开源的dns服务软件很多，功能也很强大。但是，有时候又是很有必要的，有着诸多好处。比如说，用于企业内网，简化DNS配置，可以根据企业需求添加新的功能，非常灵活。本文试着用java实现一个最简单的DNS服务。

DNS是基于udp协议的，默认端口为53。

在自己电脑上实现dns服务(作为dns服务器)，首先需要程序监听udp 53端口。在java中，和udp相关的类为DatagramSocket以及DatagramPacket。具体信息可以查看API，或者参考http://www.cnblogs.com/hq-antoine/archive/2012/02/11/2346908.html。

之后，需要另一台电脑作为客户端，设置dns地址为服务器的ip地址。

public class UDPServer {
    private static DatagramSocket socket;
    public UDPServer() {
        //设置socket，监听端口53
        try {
            this.socket = new DatagramSocket(53);
        } catch (SocketException e) {
            e.printStackTrace();
        }
    }

    public void start() {
        System.out.println("Starting。。。。。。\n");
        while (true) {
            try {
                byte[] buffer = new byte[1024];
                DatagramPacket request = new DatagramPacket(buffer, buffer.length);
                socket.receive(request);
                //输出客户端的dns请求数据
                InetAddress sourceIpAddr = request.getAddress();
                int sourcePort = request.getPort();
                System.out.println("\nsourceIpAddr = " + sourceIpAddr.toString() + "\nsourcePort = " + sourcePort);
                System.out.println("data = " + new String(request.getData(), 0 , request.getLength()));
            } catch (SocketException e) {
                System.out.println("SocketException:");
                e.printStackTrace();
            } catch (IOException e) {
                System.out.println("IOException:");
                e.printStackTrace();
            }
        }
    }
}

运行程序报错，错误提示如下：

根据异常提示，打开53端口异常，需要确认操作权限。1024以下端口默认系统保留，只有root用户才能使用。使用root账号运行程序，之后在客户端上使用nslookup www.baidu.com命令测试，在服务器上输出信息如下：

得到客户端的ip地址为10.211.55.253，端口为43065，但是下面这个data是什么玩意？

分析：出现乱码，原因在于request.getData()获取到的数据并非为String类型，因此不能简单粗暴的通过new String(request.getData(), 0, request.getLength())强制为String类型输出。猜测应该是符合dns数据格式的字节流，下面通过抓包软件wireshark进行分析。

发现dns数据包中，包括ID, Flags, Questions, Answer RRs, Authority RRs, Additional RRs以及Queries等字段。如果自己编写程序，通过分析字节流来提取出所需要的信息，是一个比较麻烦的事情。幸好有dnsjava这个开源项目，里面的Message类已经帮我们把这些事情都处理好了。

更改代码如下，

 1 package com.everSeeker;
 2
 3 import org.xbill.DNS.Message;
 4
 5 import java.io.IOException;
 6 import java.net.DatagramPacket;
 7 import java.net.DatagramSocket;
 8 import java.net.InetAddress;
 9 import java.net.SocketException;
10
11 public class UDPServer {
12     private static DatagramSocket socket;
13
14     public UDPServer() {
15         //设置socket，监听端口53
16         try {
17             this.socket = new DatagramSocket(53);
18         } catch (SocketException e) {
19             e.printStackTrace();
20         }
21     }
22
23     public void start() {
24         System.out.println("Starting。。。。。。\n");
25         while (true) {
26             try {
27                 byte[] buffer = new byte[1024];
28                 DatagramPacket request = new DatagramPacket(buffer, buffer.length);
29                 socket.receive(request);
30                 //输出客户端的dns请求数据
31                 InetAddress sourceIpAddr = request.getAddress();
32                 int sourcePort = request.getPort();
33                 System.out.println("\nsourceIpAddr = " + sourceIpAddr.toString() + "\nsourcePort = " + sourcePort);
34 //                System.out.println("data = " + new String(request.getData(), 0 , request.getLength()));
35
36                 Message indata = new Message(request.getData());
37                 System.out.println("indata = " + indata.toString());
38             } catch (SocketException e) {
39                 System.out.println("SocketException:");
40                 e.printStackTrace();
41             } catch (IOException e) {
42                 System.out.println("IOException:");
43                 e.printStackTrace();
44             }
45         }
46     }
47 }

重新测试，输出信息为:

发现，输出信息与我们通过抓包得到的信息一致。其中，Questions记录了需要解析的域名www.baidu.com，type为A。而Answers为空，是因为这是一个域名解析请求信息，下面我们只需要把解析的结果放入Answers这个字段，并返回给客户端，即完成了最简单的dns功能。

分析dnsjava的源码，发现Message类中有一个变量private List [] sections， 长度为4，记录了Questions, Answers, Authority RRs, Additional RRs这4个字段。更改代码如下，

 1 package com.everSeeker;
 2
 3 import org.xbill.DNS.*;
 4
 5 import java.io.IOException;
 6 import java.net.DatagramPacket;
 7 import java.net.DatagramSocket;
 8 import java.net.InetAddress;
 9 import java.net.SocketException;
10
11 public class UDPServer {
12     private static DatagramSocket socket;
13
14     public UDPServer() {
15         //设置socket，监听端口53
16         try {
17             this.socket = new DatagramSocket(53);
18         } catch (SocketException e) {
19             e.printStackTrace();
20         }
21     }
22
23     public void start() {
24         System.out.println("Starting。。。。。。\n");
25         while (true) {
26             try {
27                 byte[] buffer = new byte[1024];
28                 DatagramPacket request = new DatagramPacket(buffer, buffer.length);
29                 socket.receive(request);
30                 //输出客户端的dns请求数据
31                 InetAddress sourceIpAddr = request.getAddress();
32                 int sourcePort = request.getPort();
33                 System.out.println("\nsourceIpAddr = " + sourceIpAddr.toString() + "\nsourcePort = " + sourcePort);
34                 //分析dns数据包格式
35                 Message indata = new Message(request.getData());
36                 System.out.println("\nindata = " + indata.toString());
37                 Record question = indata.getQuestion();
38                 System.out.println("question = " + question);
39                 String domain = indata.getQuestion().getName().toString();
40                 System.out.println("domain = " + domain);
41                 //解析域名
42                 InetAddress answerIpAddr = Address.getByName(domain);
43                 Message outdata = (Message)indata.clone();
44                 //由于接收到的请求为A类型，因此应答也为ARecord。查看Record类的继承，发现还有AAAARecord(ipv6)，CNAMERecord等
45                 Record answer = new ARecord(question.getName(), question.getDClass(), 64, answerIpAddr);
46                 outdata.addRecord(answer, Section.ANSWER);
47                 //发送消息给客户端
48                 byte[] buf = outdata.toWire();
49                 DatagramPacket response = new DatagramPacket(buf, buf.length, sourceIpAddr, sourcePort);
50                 socket.send(response);
51             } catch (SocketException e) {
52                 System.out.println("SocketException:");
53                 e.printStackTrace();
54             } catch (IOException e) {
55                 System.out.println("IOException:");
56                 e.printStackTrace();
57             }
58         }
59     }
60 }

继续测试，客户端nslookup www.baidu.com，输出结果为:

测试成功，这样一个最简单的dns就完成了。

时间： 2024-11-07 20:11:54

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