动手学习TCP：服务端状态变迁

上一篇文章介绍了TCP状态机，并且通过实验了解了TCP客户端正常的状态变迁过程。

那么，本篇文章就一起看看TCP服务端的正常状态变迁过程

服务端状态变迁

根据上一篇文章中的TCP状态变迁图，可以得到服务器的正常状态变迁流程如下：

CLOSED -> LISTEN -> SYN_RECV -> ESTABLISHED -> CLOSE_WAIT -> LAST_ACK -> CLOSED

具体的将状态跟TCP包关联起来就如下表示：

From State	To State	Recv Packet	Send Packet
CLOSED	LISTEN	-	-
LISTEN	SYN_RCVD	[SYN]	[SYN, ACK]
SYN_RCVD	ESTABLISHED	[ACK]	-
ESTABLISHED	CLOSE_WAIT	[FIN, ACK]	[ACK]
CLOSE_WAIT	LAST_ACK	-	[FIN, ACK]
LAST_ACK	CLOSED	[ACK]	-

服务端状态变迁实验

下面就结合上面分析出来的服务端状态变迁表，利用Pcap.Net来模拟服务端正常的状态变迁过程。

代码实现

跟前面几次正好相反，这次我们将在宿主机运行Pcap.Net实现的服务端，然后在虚拟机运行一个客户端。

对于服务端，主程序中设置了源和目的端的连接信息，这次宿主机中的服务端将监听“3333”端口。

然后，程序中设置了服务端TCP初始状态为"LISTENING"，然后就直接运行监听函数了。

// Open the output device
using (PacketCommunicator communicator = selectedDevice.Open(System.Int32.MaxValue, // name of the device
                                                             PacketDeviceOpenAttributes.Promiscuous, // promiscuous mode
                                                             1)) // read timeout
{
    EndPointInfo endPointInfo = new EndPointInfo();
    endPointInfo.SourceMac = "08:00:27:00:C0:D5";
    endPointInfo.DestinationMac = "";
    endPointInfo.SourceIp = "192.168.56.101";
    endPointInfo.DestinationIp = "";
    endPointInfo.SourcePort = 3333;
    endPointInfo.DestinationPort = 0;

    using (BerkeleyPacketFilter filter = communicator.CreateFilter("tcp port " + endPointInfo.SourcePort))
    {
        // Set the filter
        communicator.SetFilter(filter);
    }

    tcpStatus = TCPStatus.LISTENING;
    PacketHandler(communicator, endPointInfo);
}

这次的监听函数"PacketHandler"中的逻辑，跟上一次客户端的例子还是有很大差别的。

首先是期待接收和实际发送的TCP包类型有很大的差别，其次就是状态之间的变迁是完全不同的。但是，代码的逻辑依然是根据上面的服务端状态变迁表。

private static void PacketHandler(PacketCommunicator communicator, EndPointInfo endPointInfo)
{
    Packet packet = null;
    bool running = true;

    do
    {
        PacketCommunicatorReceiveResult result = communicator.ReceivePacket(out packet);

        switch (result)
        {
            case PacketCommunicatorReceiveResult.Timeout:
                // Timeout elapsed
                continue;
            case PacketCommunicatorReceiveResult.Ok:
                bool isRecvedPacket = (packet.Ethernet.IpV4.Destination.ToString() == endPointInfo.SourceIp) ? true : false;

                if (isRecvedPacket)
                {
                    switch (packet.Ethernet.IpV4.Tcp.ControlBits)
                    {
                        case TcpControlBits.Synchronize:
                            if (tcpStatus == TCPStatus.LISTENING)
                            {
                                endPointInfo.DestinationMac = packet.Ethernet.Source.ToString();
                                endPointInfo.DestinationIp = packet.Ethernet.IpV4.Source.ToString();
                                endPointInfo.DestinationPort = packet.Ethernet.IpV4.Tcp.SourcePort;

                                Utils.PacketInfoPrinter(packet);
                                Packet synAck = Utils.BuildTcpResponsePacket(packet, TcpControlBits.Synchronize | TcpControlBits.Acknowledgment);
                                communicator.SendPacket(synAck);
                                tcpStatus = TCPStatus.SYN_RECEIVED;
                            }
                            break;
                        case TcpControlBits.Acknowledgment:
                            if (tcpStatus == TCPStatus.SYN_RECEIVED)
                            {
                                tcpStatus = TCPStatus.ESTABLISHED;
                                Utils.PacketInfoPrinter(packet, tcpStatus);
                            }
                            else if (tcpStatus == TCPStatus.LAST_ACK)
                            {
                                tcpStatus = TCPStatus.CLOSED;
                                Utils.PacketInfoPrinter(packet, tcpStatus);
                                tcpStatus = TCPStatus.LISTENING;
                            }
                            else if (tcpStatus == TCPStatus.FIN_WAIT_1)
                            {
                                tcpStatus = TCPStatus.FIN_WAIT_2;
                                Utils.PacketInfoPrinter(packet);
                            }
                            break;
                        case (TcpControlBits.Fin | TcpControlBits.Acknowledgment):
                            if (tcpStatus == TCPStatus.FIN_WAIT_2)
                            {
                                Utils.PacketInfoPrinter(packet);
                                Packet ack = Utils.BuildTcpResponsePacket(packet, TcpControlBits.Acknowledgment);
                                communicator.SendPacket(ack);
                                tcpStatus = TCPStatus.TIME_WAIT;
                            }
                            else if (tcpStatus == TCPStatus.ESTABLISHED)
                            {
                                Utils.PacketInfoPrinter(packet);
                                Packet ack = Utils.BuildTcpResponsePacket(packet, TcpControlBits.Acknowledgment);
                                communicator.SendPacket(ack);
                                tcpStatus = TCPStatus.CLOSE_WAIT;
                            }
                            break;
                        default:
                            Utils.PacketInfoPrinter(packet);
                            break;
                    }
                }
                else
                {
                    switch (packet.Ethernet.IpV4.Tcp.ControlBits)
                    {
                        case (TcpControlBits.Synchronize | TcpControlBits.Acknowledgment):
                            if (tcpStatus == TCPStatus.SYN_RECEIVED)
                            {
                                Utils.PacketInfoPrinter(packet, tcpStatus);
                            }
                            break;
                        case (TcpControlBits.Fin | TcpControlBits.Acknowledgment):
                            if (tcpStatus == TCPStatus.FIN_WAIT_1)
                            {
                                Utils.PacketInfoPrinter(packet, tcpStatus);
                            }
                            else if (tcpStatus == TCPStatus.LAST_ACK)
                            {
                                Utils.PacketInfoPrinter(packet, tcpStatus);
                            }
                            break;
                        case TcpControlBits.Acknowledgment:
                            if (tcpStatus == TCPStatus.TIME_WAIT)
                            {
                                Utils.PacketInfoPrinter(packet, tcpStatus);
                            }
                            else if (tcpStatus == TCPStatus.CLOSE_WAIT)
                            {
                                
                                Utils.PacketInfoPrinter(packet, tcpStatus);

                                Packet fin = Utils.BuildTcpPacket(endPointInfo, TcpControlBits.Fin | TcpControlBits.Acknowledgment);
                                communicator.SendPacket(fin);
                                tcpStatus = TCPStatus.LAST_ACK;
                            }
                            break;
                        default:
                            Utils.PacketInfoPrinter(packet);
                            break;
                    }
                }
                break;
            default:
                throw new InvalidOperationException("The result " + result + " should never be reached here");
        }
    } while (running);

}

对于客户端，通过Python实现了一个简单的Socket程序来模拟客户端行为：

from socket import *
import time

HOST = "192.168.56.101"
PORT = 3333
BUFSIZ = 1024
ADDR = (HOST, PORT)

client = socket(AF_INET, SOCK_STREAM)
client.connect(ADDR)

time.sleep(5)

client.close()