基于 UDP 的服务器无法区分不同的用户

Question

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，该通道的格式是固定的：IP+Port，即如果IP保持不变，那么无论客户端登录的用户注册什么同一个频道。

服务器的套接字层维护一个心跳机制，如果它在 3 分钟内没有收到来自通道的任何心跳数据包，它将删除该通道。一切正常，直到客户端关闭，例如网线被拔掉了。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.

在上述情况下，服务器无法区分从同一台计算机登录的不同用户，从而导致用户状态错误。有谁知道如何解决这个问题？

Answer 1

我认为没有理由假设任何两个用户的原始端口号都是相同的，除非客户端应用程序显式绑定了 UDP 套接字。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，临时端口可能足够随机，也可能不够随机，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，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;
}