Windows客户端可否指定ephemeral端口实现无数据传输通信

结论 #

Windows系统下的客户端完全可以手动指定临时（ephemeral）端口，你提供的适配代码已经完成了该功能的验证。

实现逻辑 #

这种不发送业务数据的隐式传信方案，核心是利用TCP连接中客户端源端口可自定义的特性：客户端在调用connect()和服务端建立连接前，先通过bind()函数主动给客户端套接字绑定指定的端口，替代系统自动分配的临时端口，服务端接受连接时读取客户端源端口即可解析出预定义的信息。比如示例中指定客户端端口为7273，对应ASCII码就是HI，按顺序使用端口7279、8732、6582、6932、8979、8563发起连接，就能传递信息HOW ARE YOU?。

核心代码示例 #

服务端读取客户端端口代码 #

// Accept a client socket
struct sockaddr_in client_addr;
socklen_t slen = sizeof(client_addr);
ClientSocket = accept(ListenSocket, (struct sockaddr*)&client_addr, &slen);
printf("SECRET: %u\n", ntohs(client_addr.sin_port));

适配后的Windows客户端代码 #

#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdlib.h>
#include <stdio.h>


// Need to link with Ws2_32.lib, Mswsock.lib, and Advapi32.lib
#pragma comment (lib, "Ws2_32.lib")
#pragma comment (lib, "Mswsock.lib")
#pragma comment (lib, "AdvApi32.lib")

#pragma warning(disable : 4996)


int __cdecl main(int argc, char** argv)
{
    WSADATA wsaData;
    SOCKET ConnectSocket = INVALID_SOCKET;
   
    // Initialize Winsock
    WSAStartup(MAKEWORD(2, 2), &wsaData);

    // Two buffer are for message communication
    char buffer1[256], buffer2[256];
    struct sockaddr_in my_addr, my_addr1;
    int client = socket(AF_INET, SOCK_STREAM, 0);
    if (client < 0)
        printf("Error in client creating\n");
    else
        printf("Client Created\n");

    my_addr.sin_family = AF_INET;
    my_addr.sin_addr.s_addr = INADDR_ANY;
    my_addr.sin_port = htons(27015);

    // This ip address will change according to the machine
    my_addr.sin_addr.s_addr = inet_addr("127.0.0.1");

    // Explicitly assigning port number 7273 by 
    // binding client with that port 
    my_addr1.sin_family = AF_INET;
    my_addr1.sin_addr.s_addr = INADDR_ANY;
    my_addr1.sin_port = htons(7273);

    // This ip address will change according to the machine
    my_addr1.sin_addr.s_addr = inet_addr("127.0.0.1");
    if (bind(client, (struct sockaddr*)&my_addr1, sizeof(struct sockaddr_in)) == 0)
        printf("Binded Correctly\n");
    else
        printf("Unable to bind\n");

    socklen_t addr_size = sizeof my_addr;
    int con = connect(client, (struct sockaddr*)&my_addr, sizeof my_addr);
    if (con == 0)
        printf("Client Connected\n");
    else
        printf("Error in Connection\n");

    return 0;

}

运行效果 #

使用注意事项 #

• 手动指定的端口不能被本地其他进程占用，否则会绑定失败
• 端口号只要在0-65535范围内未被占用即可使用，不受Windows默认临时端口范围（49152-65535）限制
• 如果要连续使用不同端口发起多个连接，每个连接需要新建独立的套接字重新绑定，已绑定过的套接字无法重复绑定其他端口（开启SO_REUSEADDR选项的特殊场景除外）

内容的提问来源于stack exchange，提问作者vengy