同一端口的两个TCP客户端连接不同服务器失败问题咨询

背景 #

TCP套接字由源IP/源端口/目的IP/目的端口四元组唯一标识，因此同一IP下使用相同端口的两个TCP客户端，若连接不同服务器，理论上是可行的。

在MacBook上的测试步骤：

• 两台服务器（A和B）监听不同端口
• 第一个客户端绑定到端口1234并连接服务器A，成功建立连接
• 第二个客户端绑定到同一端口1234并连接服务器B，失败并返回错误码48（Address already in use）

问题 #

理论上只要目的IP或端口不同，同一IP的相同端口可绑定多个TCP套接字，但实际操作系统不允许，仅服务器端处理连接请求时可分配新套接字。请问这个理解是否有误？

测试代码运行方法 #

编译 #

gcc server.c -o server
gcc client.c -o client

运行 #

./server 9000        # 端口9000的服务器A
./server 8000        # 端口8000的服务器B
./client 9000 1234   # 第一个客户端用本地端口1234连接服务器A，成功
./client 8000 1234   # 第二个客户端用本地端口1234连接服务器B，失败

代码 #

server.c #

#include <stdio.h>
#include <netdb.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h> // read(), write(), close()
#define MAX 80
#define SA struct sockaddr

// Function designed for chat between client and server.
void func(int connfd)
{
    char buff[MAX];
    int n;
    // infinite loop for chat
    for (;;) {
        bzero(buff, MAX);

        // read the message from client and copy it in buffer
        read(connfd, buff, sizeof(buff));
        // print buffer which contains the client contents
        printf("From client: %s\t To client : ", buff);
        bzero(buff, MAX);
        n = 0;
        // copy server message in the buffer
        while ((buff[n++] = getchar()) != '\n')
            ;

        // and send that buffer to client
        write(connfd, buff, sizeof(buff));

        // if msg contains "Exit" then server exit and chat ended.
        if (strncmp("exit", buff, 4) == 0) {
            printf("Server Exit...\n");
            break;
        }
    }
}

// Driver function
int main(int argc, char **argv)
{
    int sockfd, connfd, len;
    struct sockaddr_in servaddr, cli;

    // socket create and verification
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd == -1) {
        printf("socket creation failed...\n");
        exit(0);
    }
    else
        printf("Socket successfully created..\n");
    
    bzero(&servaddr, sizeof(servaddr));

    // assign IP, PORT
    int port = atoi(argv[1]);
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    servaddr.sin_port = htons(port);

    // Binding newly created socket to given IP and verification
    if ((bind(sockfd, (SA*)&servaddr, sizeof(servaddr))) != 0) {
        printf("socket bind failed...\n");
        exit(0);
    }
    else
        printf("Socket successfully binded..\n");

    // Now server is ready to listen and verification
    if ((listen(sockfd, 5)) != 0) {
        printf("Listen failed...\n");
        exit(0);
    }
    else
        printf("Server listening..\n");
    len = sizeof(cli);

    // Accept the data packet from client and verification
    connfd = accept(sockfd, (SA*)&cli, &len);
    if (connfd < 0) {
        printf("server accept failed...\n");
        exit(0);
    }
    else
        printf("server accept the client...\n");

    // Function for chatting between client and server
    func(connfd);
    printf("close connection directly");

    // After chatting close the socket
    close(sockfd);
}

client.c #

#include <arpa/inet.h> // inet_addr()
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h> // bzero()
#include <sys/socket.h>
#include <unistd.h> // read(), write(), close()
#include <errno.h>
#define MAX 80
#define SA struct sockaddr

void func(int sockfd)
{
    char buff[MAX];
    int n;
    for (;;) {
        bzero(buff, sizeof(buff));
        printf("Enter the string : ");
        n = 0;
        while ((buff[n++] = getchar()) != '\n')
            ;
        long writeRes = write(sockfd, buff, sizeof(buff));
        printf("wrtie result : %d\n", writeRes);
        bzero(buff, sizeof(buff));

        long readRes = read(sockfd, buff, sizeof(buff));
        printf("read result : %d\n", readRes);

        printf("From Server : %s", buff);
        if ((strncmp(buff, "exit", 4)) == 0) {
            printf("Client Exit...\n");
            break;
        }
    }
}

int main(int argc, char **argv)
{
    int sockfd, connfd;
    struct sockaddr_in servaddr, cliaddr;

    // socket create and verification
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd == -1) {
        printf("socket creation failed...\n");
        exit(0);
    }
    else
        printf("Socket successfully created..\n");
    bzero(&servaddr, sizeof(servaddr));

    ////////////////////////////////////////
    // bind client socket to specific address
    bzero(&cliaddr, sizeof(cliaddr));

    // assign client IP, PORT
    int cliport = atoi(argv[2]);
    cliaddr.sin_family = AF_INET;
    cliaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    cliaddr.sin_port = htons(cliport);

    // Binding client socket to given IP and verification
    if ((bind(sockfd, (SA*)&cliaddr, sizeof(cliaddr))) != 0) {
        printf("client socket bind failed...\n");
        printf("Error code: %d\n", errno);
        exit(0);
    }
    else
        printf("client socket successfully binded..\n");
    ////////////////////////////////////////

    // assign server IP, PORT
    int serverport = atoi(argv[1]);
    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    servaddr.sin_port = htons(serverport);

    // connect the client socket to server socket
    if (connect(sockfd, (SA*)&servaddr, sizeof(servaddr)) != 0) {
        printf("connection with the server failed...\n");
        exit(0);
    }
    else
        printf("connected to the server..\n");

    // function for chat
    func(sockfd);

    // close the socket
    close(sockfd);
}

核心解答 #

你的理论理解是正确的：TCP连接由**四元组（源IP/源端口/目的IP/目的端口）**唯一标识，所以同一本地IP+端口，只要连接的目的地址（IP或端口）不同，就可以存在多个独立连接。

为什么默认bind失败？ #

默认情况下，操作系统的bind系统调用会做严格检查：只要本地IP+端口已经被任何套接字绑定（不管该套接字的连接目的是什么），就会返回EADDRINUSE（错误码48）。这是系统的默认安全限制，防止意外的端口冲突。

如何实现预期的场景？ #

需要在调用bind之前，给客户端套接字设置两个套接字选项：

int opt = 1;
// 允许绑定到已被占用的本地端口（只要四元组不冲突）
setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
// 允许多个套接字绑定同一本地IP+端口（macOS/Linux需要）
setsockopt(sockfd, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt));

修改后的客户端main函数开头部分示例：

// socket create and verification
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd == -1) {
    printf("socket creation failed...\n");
    exit(0);
}
else
    printf("Socket successfully created..\n");

// 设置套接字选项
int opt = 1;
setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
setsockopt(sockfd, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt));

bzero(&servaddr, sizeof(servaddr));

关于服务器端的差异 #

服务器端listen后能接受多个连接，是因为accept会为每个新连接创建全新的套接字，这些新套接字的四元组是（客户端IP/客户端端口/服务器IP/服务器端口），和监听套接字的四元组（0.0.0.0/服务器端口/任意/任意）完全不同，所以不会产生冲突。

内容的提问来源于stack exchange，提问作者obanadingyo