从0到1用java再造tcpip协议栈:使用责任链模式实现ICMP错误数据报解析

package ICMPProtocolLayer;

import java.nio.ByteBuffer;
import java.util.ArrayList;
import java.util.Arrays;

import jpcap.PacketReceiver;
import jpcap.packet.EthernetPacket;
import jpcap.packet.Packet;

public class ICMPProtocolLayer implements PacketReceiver{
    private static byte PROTOCL_ICMP = 1;
    public static int ICMP_DATA_OFFSET = 20 + 14; //越过20字节IP包头
    private static int PROTOCOL_FIELD_IN_IP_HEADER = 14 + 9;

    private ArrayList<IICMPErrorMsgHandler> error_handler_list = new ArrayList<IICMPErrorMsgHandler>();
    private enum ICMP_MSG_TYPE {
        ICMP_UNKNOW_MSG,
        ICMP_ERROR_MSG,
        ICMP_INFO_MSG
    };

    private int icmp_type  = 0;
    private int icmp_code = 0;
    private byte[] packet_header = null;
    private byte[] packet_data = null;

    public ICMPProtocolLayer() {
        //添加错误消息处理对象
        error_handler_list.add(new ICMPUnReachableMsgHandler());
    }

    @Override
    public void receivePacket(Packet packet) {
        if (packet == null) {
            return;
        }

        //确保收到数据包是IP类型
        EthernetPacket etherHeader = (EthernetPacket)packet.datalink;
        if (etherHeader.frametype != EthernetPacket.ETHERTYPE_IP) {
            return;
        }

        //读取IP包头，也就是接在数据链路层后面的20字节，读取协议字段是否表示ICMP，也就是偏移第10个字节的值为1
        if (packet.header[PROTOCOL_FIELD_IN_IP_HEADER] != PROTOCL_ICMP) {
            return;
        }

        packet_header = Arrays.copyOfRange(packet.header, ICMP_DATA_OFFSET, packet.header.length);
        packet_data = packet.data;

        analyzeICMPMessage(packet_header);
    }

    private ICMP_MSG_TYPE  checkType(int type) {
        /*
         * 传递错误消息的ICMP数据报type处于0到127，
         * 传递控制信息的ICMP数据报type处于128到255
         */
        if (type >= 0 && type <= 127) {
            return ICMP_MSG_TYPE.ICMP_ERROR_MSG;
        }

        if (type >= 128 && type <= 255) {
            return ICMP_MSG_TYPE.ICMP_INFO_MSG;
        }

        return ICMP_MSG_TYPE.ICMP_UNKNOW_MSG;
    }

    private void analyzeICMPMessage(byte[] data) {
        ByteBuffer byteBuffer = ByteBuffer.wrap(data);
        icmp_type = byteBuffer.get(0);
        icmp_code = byteBuffer.get(1);

        //检测当前数据包是错误信息还是控制信息,并根据不同情况分别处理
        if (checkType(icmp_type) == ICMP_MSG_TYPE.ICMP_ERROR_MSG) {
            handleICMPErrorMsg(packet_data);
        }
    }

    private void handleICMPErrorMsg(byte[] data) {
        /*
         * ICMP错误数据报的类型很多，一个type和code的组合就能对应一种数据类型，我们不能把对所有数据类型的处理全部
         * 塞入一个函数，那样会造成很多个if..else分支，使得代码复杂，膨胀，极难维护，因此我们使用责任链模式来处理
         */
        for (int i = 0; i < error_handler_list.size(); i++) {
            if (error_handler_list.get(i).handleICMPErrorMsg(icmp_type, icmp_code, data) == true) {
                break;
            }
        }
    }

}

package ICMPProtocolLayer;

public interface IICMPErrorMsgHandler {
    public  boolean handleICMPErrorMsg(int type, int code, byte[] data);
}

package ICMPProtocolLayer;

import java.nio.ByteBuffer;

import jpcap.packet.IPPacket;

public class ICMPUnReachableMsgHandler implements IICMPErrorMsgHandler {
    private static int ICMP_UNREACHABLE_TYPE = 3;
    private static int IP_HEADER_LENGTH = 20;

    enum ICMP_ERROR_MSG_CODE {
      ICMP_NETWORK_UNREACHABLE,
      ICMP_HOST_UNREACHABLE,
      ICMP_PROTOCAL_UNREACHABLE,
      ICMP_PORT_UNREACHABLE,
      ICMP_FRAGMETATION_NEEDED_AND_DF_SET
      //后面需要在继续添加其他类型
    } ;
    @Override
    public boolean handleICMPErrorMsg(int type, int code, byte[] data) {
        if (type != ICMPUnReachableMsgHandler.ICMP_UNREACHABLE_TYPE) {
            return false;    
        }

        ByteBuffer buffer = ByteBuffer.wrap(data);

        switch (ICMP_ERROR_MSG_CODE.values()[code]) {
        case ICMP_PORT_UNREACHABLE:
            //错误数据格式:IP包头和8字节内容
            //获取协议类型
            byte protocol = buffer.get(9);
            if (protocol == IPPacket.IPPROTO_UDP) {
                handleUDPError(buffer);
            }
            break;
        default:
            return false;
        }

        return true;
    }

    private void handleUDPError(ByteBuffer buffer) {
        System.out.println("protocol of error packet is UDP");
        System.out.println("Source IP Address is :");
        int source_ip_offset = 12;
        for (int i = 0; i < 4; i++) {
            int v = buffer.get(source_ip_offset + i) & 0xff;
            System.out.print(v + ".");
        }
        System.out.println("\nDest IP Address is :");
        int dest_ip_offset = 16;
        for (int i = 0; i < 4; i++) {
            int v = buffer.get(dest_ip_offset + i) & 0xff;
            System.out.print(v + ".");
        }

        /*
         * 打印UDP数据包头前8个字节信息，其格式为：
         * source_port(2 byte),
         * dest_port (2byte)
         * length (2byte)
         * check_sum(2byte)
         */
        int source_port = (int) (buffer.getShort(IP_HEADER_LENGTH) & 0xFFFF);
        System.out.println("\nSource Port: " + source_port);
        int source_port_len = 2;
        int dest_port = (int) (buffer.getShort(IP_HEADER_LENGTH + source_port_len) & 0xFFFF);
        System.out.println("dest port: " + dest_port);
    }

}