Linux 基于UDP的服务器无法区分不同的用户 这是一个基于UDP协议与所有客户端通信的C++服务器应用程序。当用户从客户端登录到服务器时，客户端应用程序向服务器注册一个UDP通道，该通道采用固定格式：IP+端口，这意味着如果IP保持不变，则无论登录到客户端的用户注册同一通道

服务器的套接字层维护一个心跳机制，如果在3分钟内没有从通道接收到任何心跳数据包，该机制将删除通道。在客户端关闭之前，一切正常，例如，网络线已断开。请看下面的场景：

    1. User-A logs into server. The Client registers channel (IP:Port) 
       to the server. Because the UDP channel is alive, so the Server 
       sets the User status of User-A as Online. 

    2. Kill the client process, and within 3 minutes(before the channel 
       timeouts in server), let User-B logs into server from the same 
       computer. Because the IP remains unchanged, so actually the client 
       registers a same (IP:PORT) pair to the server as it did when User-A
       logs in.

    3. Since the Server receives packets from (IP:PORT), so it considers
       User-A is still alive, thus setting the user status of User-A as 
       Online which is not right anymore.

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在上述场景中，服务器无法区分从同一台计算机登录的不同用户，这将导致错误的用户状态。有人知道如何解决这个问题吗

除非客户端应用程序显式绑定UDP套接字，否则没有理由假定任何两个用户的源端口号相同。发起通信的客户端通常可以同样有效地使用临时端口。对于您的特定用例，临时端口可能是随机的，也可能不是随机的，下面的代码显示了如何从入站UDP数据访问客户端端口。如果它们不够随机，则最好将会话cookie或用户cookie编码到协议中

#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <string.h>

int 
main() {
        /*- setup UDP socket on 8500 */
        int rc;
        int server_socket;
        struct sockaddr_in server_address;

        server_socket = socket(AF_INET, SOCK_DGRAM, 0);
        if (server_socket < 0) {
                perror("failed to init socket");
                return 1;
        }

        memset(&server_address, 0, sizeof(server_address));
        server_address.sin_family      = AF_INET;
        server_address.sin_port        = htons(8500);
        server_address.sin_addr.s_addr = inet_addr("127.0.0.1");

        rc = bind(server_socket, (struct sockaddr*) &server_address
                , sizeof(server_address));

        if (rc < 0) { 
                perror("failed to bind");
                return 2;
        }

        /* - receive from UDP socket and print out origin port - */
        char buffer[4096];
        struct sockaddr_in client_address;
        int client_address_len = sizeof(client_address);

        rc = recvfrom(server_socket
                    , buffer 
                    , sizeof(buffer)
                    , 0
                    , (struct sockaddr*) &client_address
                    , &client_address_len);

        if (rc < 0)
                return 3;

        fprintf(stderr, "%s %d\n", buffer, ntohs(client_address.sin_port));

        return 0;
}

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您可以实现某种身份验证。将通道的签名从IP:PORT扩展到IP:PORT:USER