本节我们通过代码来实现TCP协议连接时的三次握手过程。首先我们需要再次重温一下TCP数据包的相关结构：
屏幕快照 2019-07-27 下午4.43.06.png
我们们将依照上面结构所示来构建数据包，相关代码如下：
public class TCPProtocolLayer implements IProtocol {
private static int HEADER_LENGTH = 20;
private int sequence_number = 2;
private int acknowledgement_number = 0;
private static int PSEUDO_HEADER_LENGTH = 12;
public static byte TCP_PROTOCOL_NUMBER = 6;
private static int POSITION_FOR_DATA_OFFSET = 12;
private static int POSITION_FOR_CHECKSUM = 16;
private static byte MAXIMUN_SEGMENT_SIZE_OPTION_LENGTH = 4;
private static byte MAXIMUN_SEGMENT_OPTION_KIND = 2;
private static byte WINDOW_SCALE_OPTION_KIND = 3;
private static byte WINDOW_SCALE_OPTION_LENGTH = 3;
private static byte WINDOW_SCALE_SHIFT_BYTES = 6;
private static byte TCP_URG_BIT = (1 << 5);
private static byte TCP_ACK_BIT = (1 << 4);
private static byte TCP_PSH_BIT = (1 << 3);
private static byte TCP_RST_BIT = (1 << 2);
private static byte TCP_SYN_BIT = (1 << 1);
private static byte TCP_FIN_BIT = (1);
@Override
public byte[] createHeader(HashMap<String, Object> headerInfo) {
short data_length = 0;
byte[] data = null;
if (headerInfo.get("data") != null) {
data = (byte[])headerInfo.get("data");
}
byte[] header_buf = new byte[HEADER_LENGTH];
ByteBuffer byteBuffer = ByteBuffer.wrap(header_buf);
if (headerInfo.get("src_port") == null) {
return null;
}
short srcPort = (short)headerInfo.get("src_port");
byteBuffer.putShort(srcPort);
if (headerInfo.get("dest_port") == null) {
return null;
}
short destPort = (short)headerInfo.get("dest_port");
byteBuffer.putShort(destPort);
//设置初始序列号
if (headerInfo.get("seq_num") != null) {
sequence_number = (int)headerInfo.get("seq_num");
}
if (headerInfo.get("ack_num") != null) {
acknowledgement_number = (int)headerInfo.get("ack_num");
}
byteBuffer.putInt(sequence_number);
byteBuffer.putInt(acknowledgement_number);
short control_bits = 0;
//设置控制位
if (headerInfo.get("URG") != null) {
control_bits |= (1 << 5);
}
if (headerInfo.get("ACK") != null) {
control_bits |= (1 << 4);
}
if (headerInfo.get("PSH") != null) {
control_bits |= (1 << 3);
}
if (headerInfo.get("RST") != null) {
control_bits |= (1 << 2);
}
if (headerInfo.get("SYN") != null) {
control_bits |= (1 << 1);
}
if (headerInfo.get("FIN") != null) {
control_bits |= (1);
}
byteBuffer.putShort(control_bits);
System.out.println(Integer.toBinaryString(control_bits));
char window = 65535;
byteBuffer.putChar(window);
short check_sum = 0;
byteBuffer.putShort(check_sum);
short urgent_pointer = 0;
byteBuffer.putShort(urgent_pointer);
byte[] maximun_segment_option = new byte[MAXIMUN_SEGMENT_SIZE_OPTION_LENGTH];
ByteBuffer maximun_segment_buffer = ByteBuffer.wrap(maximun_segment_option);
maximun_segment_buffer.put(MAXIMUN_SEGMENT_OPTION_KIND);
maximun_segment_buffer.put(MAXIMUN_SEGMENT_SIZE_OPTION_LENGTH);
short segment_size = 1460;
maximun_segment_buffer.putShort(segment_size);
byte[] window_scale_option = new byte[WINDOW_SCALE_OPTION_LENGTH];
ByteBuffer window_scale_buffer = ByteBuffer.wrap(window_scale_option);
window_scale_buffer.put(WINDOW_SCALE_OPTION_KIND);
window_scale_buffer.put(WINDOW_SCALE_OPTION_LENGTH);
window_scale_buffer.put(WINDOW_SCALE_SHIFT_BYTES);
byte[] option_end = new byte[1];
option_end[0] = 0;
int total_length = data_length + header_buf.length + maximun_segment_option.length + window_scale_option.length + option_end.length;
//总长度必须是4的倍数，不足的话以0补全
if (total_length % 4 != 0) {
total_length = (total_length / 4 + 1) * 4;
}
byte[] tcp_buffer = new byte[total_length];
ByteBuffer buffer = ByteBuffer.wrap(tcp_buffer);
buffer.put(header_buf);
buffer.put(maximun_segment_option);
buffer.put(window_scale_option);
buffer.put(option_end);
short data_offset = buffer.getShort(POSITION_FOR_DATA_OFFSET);
data_offset |= (((total_length / 4) & 0x0F) << 12);
System.out.println(Integer.toBinaryString(data_offset));
buffer.putShort(POSITION_FOR_DATA_OFFSET, data_offset);
check_sum = (short)compute_checksum(headerInfo, buffer);
buffer.putShort(POSITION_FOR_CHECKSUM, check_sum);
return buffer.array();
}
private long compute_checksum(HashMap<String, Object> headerInfo, ByteBuffer tcp_buffer) {
byte[] pseudo_header = new byte[PSEUDO_HEADER_LENGTH];
ByteBuffer pseudo_header_buf = ByteBuffer.wrap(pseudo_header);
byte[] src_addr = (byte[])headerInfo.get("src_ip");
byte[] dst_addr = (byte[])headerInfo.get("dest_ip");
pseudo_header_buf.put(src_addr);
pseudo_header_buf.put(dst_addr);
byte reserved = 0;
pseudo_header_buf.put(reserved);
pseudo_header_buf.put(TCP_PROTOCOL_NUMBER);
short tcp_length = (short)tcp_buffer.array().length;
//将伪包头和tcp包头内容合在一起计算校验值
byte[] total_buffer = new byte[PSEUDO_HEADER_LENGTH + tcp_buffer.array().length];
ByteBuffer total_buf = ByteBuffer.wrap(total_buffer);
total_buf.put(pseudo_header);
total_buf.put(tcp_buffer.array());
return Utility.checksum(total_buffer, total_buffer.length);
}
@Override
public HashMap<String, Object> handlePacket(Packet packet) {
ByteBuffer buffer= ByteBuffer.wrap(packet.header);
HashMap<String, Object> headerInfo = new HashMap<String, Object>();
short src_port = buffer.getShort();
headerInfo.put("src_port", src_port);
short dst_port = buffer.getShort();
headerInfo.put("dest_port", dst_port);
int seq_num = buffer.getInt();
headerInfo.put("seq_num", seq_num);
int ack_num = buffer.getInt();
headerInfo.put("ack_num", ack_num);
short control_bits = buffer.getShort();
if ((control_bits & TCP_ACK_BIT) != 0) {
headerInfo.put("ACK", 1);
}
if ((control_bits & TCP_SYN_BIT) != 0) {
headerInfo.put("SYN", 1);
}
if ((control_bits & TCP_FIN_BIT) != 0) {
headerInfo.put("FIN", 1);
}
short win_size = buffer.getShort();
headerInfo.put("window", win_size);
//越过校验值
buffer.getShort();
short urg_pointer = buffer.getShort();
headerInfo.put("urg_ptr", urg_pointer);
return headerInfo;
}
}
上面代码实现了协议层TCP的封包与解包，在函数createHeader中，我们按照上图结构填写相关包头的字段，在函数handlePacket中，我们根据包头的字段获取相应信息。
在ProtocolManager中转层，我们实现下面代码：
private void handleTCPPacket(Packet packet, HashMap<String, Object> infoFromUpLayer) {
IProtocol tcpProtocol = new TCPProtocolLayer();
HashMap<String, Object> headerInfo = tcpProtocol.handlePacket(packet);
short dstPort = (short)headerInfo.get("dest_port");
//根据端口获得应该接收UDP数据包的程序
IApplication app = ApplicationManager.getInstance().getApplicationByPort(dstPort);
if (app != null) {
app.handleData(headerInfo);
}
}
一旦程序通过JPCap收到TCP包后，它会让上面实现的TCPProtocolLayer去解析数据包内的各个字段，然后检测数据包对应的端口是否在应用层有对应的接收对象，如果有的话，它就将解析信息转交给应用层的接收对象，接下来我们看应用层的相关实现：
public class TCPThreeHandShakes extends Application{
private byte[] dest_ip;
private short dest_port;
private int ack_num = 0;
private int seq_num = 0;
public TCPThreeHandShakes(byte[] server_ip, short server_port) {
this.dest_ip = server_ip;
this.dest_port = server_port;
//指定一个固定端口，以便抓包调试
this.port = (short)11940;
}
public void beginThreeHandShakes() throws Exception {
createAndSendPacket(null, "SYN");
}
private void createAndSendPacket(byte[] data, String flags) throws Exception {
byte[] tcpHeader = createTCPHeader(null, flags);
if (tcpHeader == null) {
throw new Exception("tcp Header create fail");
}
byte[] ipHeader = createIP4Header(tcpHeader.length);
byte[] packet = new byte[tcpHeader.length + ipHeader.length];
ByteBuffer packetBuffer = ByteBuffer.wrap(packet);
packetBuffer.put(ipHeader);
packetBuffer.put(tcpHeader);
sendPacket(packet);
}
private void sendPacket(byte[] packet) {
try {
InetAddress ip = InetAddress.getByName("192.168.2.1");
ProtocolManager.getInstance().sendData(packet, ip.getAddress());
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
private byte[] createTCPHeader(byte[] data, String flags) {
IProtocol tcpProto = ProtocolManager.getInstance().getProtocol("tcp");
if (tcpProto == null) {
return null;
}
HashMap<String, Object> headerInfo = new HashMap<String, Object>();
byte[] src_ip = DataLinkLayer.getInstance().deviceIPAddress();
headerInfo.put("src_ip", src_ip);
headerInfo.put("dest_ip", this.dest_ip);
headerInfo.put("src_port", (short)this.port);
headerInfo.put("dest_port", this.dest_port);
headerInfo.put("seq_num", seq_num);
headerInfo.put("ack_num", ack_num);
String[] flag_units = flags.split(",");
for(int i = 0; i < flag_units.length; i++) {
headerInfo.put(flag_units[i], 1);
}
byte[] tcpHeader = tcpProto.createHeader(headerInfo);
return tcpHeader;
}
protected byte[] createIP4Header(int dataLength) {
IProtocol ip4Proto = ProtocolManager.getInstance().getProtocol("ip");
if (ip4Proto == null || dataLength <= 0) {
return null;
}
//创建IP包头默认情况下只需要发送数据长度,下层协议号，接收方ip地址
HashMap<String, Object> headerInfo = new HashMap<String, Object>();
headerInfo.put("data_length", dataLength);
ByteBuffer destIP = ByteBuffer.wrap(this.dest_ip);
headerInfo.put("destination_ip", destIP.getInt());
byte protocol = TCPProtocolLayer.TCP_PROTOCOL_NUMBER;
headerInfo.put("protocol", protocol);
headerInfo.put("identification", (short)this.port);
byte[] ipHeader = ip4Proto.createHeader(headerInfo);
return ipHeader;
}
@Override
public void handleData(HashMap<String, Object> headerInfo) {
short src_port = (short)headerInfo.get("src_port");
System.out.println("receive TCP packet with port:" + src_port);
boolean ack = false, syn = false;
if (headerInfo.get("ACK") != null) {
System.out.println("it is a ACK packet");
ack = true;
}
if (headerInfo.get("SYN") != null) {
System.out.println("it is a SYN packet");
syn = true;
}
if (ack && syn) {
int seq_num = (int)headerInfo.get("seq_num");
int ack_num = (int)headerInfo.get("ack_num");
System.out.println("tcp handshake from othersize with seq_num" + seq_num + " and ack_num: " + ack_num);
this.seq_num = ack_num + 1;
try {
createAndSendPacket(null, "ACK");
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
应用层对象的主要目标是实现TCP连接的三次握手功能。它首先构造了一个TCP数据包，将SYN控制位打开，然后将数据包发送给目标服务器。然后等待对方回应数据包，一旦本机收到对方回发的ACK数据包后，会将数据包内的相关信息转交给当前应用对象，它解读出对方ACK包中回复的ACK数值后，将该数值加一然后再次构造一个ACK包发送给对方，上面程序运行后通过wireshark抓包可看到如下显示：
屏幕快照 2019-08-14 下午5.35.18.png
由此可见，我们成功的完成了TCP协议连接时的三次握手功能，上图显示中有一个数据包设置了RST标志位，它表示重置连接，这个数据包其实不是我们的应用对象发送，很可能是我们绕过了系统网络层发送数据包，当对方数据包回来时，操作系统的网络层发现接收对象没有在它内部不存在，于是自己构造了一个RST数据包发回给对方。
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