c++ - UDP 示例运行时失败

Question

#include<stdio.h>
#include<winsock2.h>

#pragma comment(lib,"ws2_32.lib") //Winsock Library
#define SERVER "127.0.0.1"
#define BUFLEN 512  //Max length of buffer
#define PORT 8888   //The port on which to listen for incoming data

int main()
{
    SOCKET s;
    struct sockaddr_in server, si_other;
    int slen, recv_len;
    char buf[BUFLEN];
    WSADATA wsa;

    slen = sizeof(si_other);

    //Initialise winsock
    printf("\nInitialising Winsock...");
    if (WSAStartup(MAKEWORD(2, 2), &wsa) != 0)
    {
        printf("Failed. Error Code : %d", WSAGetLastError());
        exit(EXIT_FAILURE);
    }
    printf("Initialised.\n");

    //Create a socket
    if ((s = socket(AF_INET, SOCK_DGRAM, 0)) == INVALID_SOCKET)
    {
        printf("Could not create socket : %d", WSAGetLastError());
    }
    printf("Socket created.\n");

    //Prepare the sockaddr_in structure
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(PORT);
    si_other.sin_addr.S_un.S_addr = inet_addr(SERVER);
    //Bind
    if (bind(s, (struct sockaddr *)&server, sizeof(server)) == SOCKET_ERROR)
    {
        printf("Bind failed with error code : %d", WSAGetLastError());
        exit(EXIT_FAILURE);
    }
    puts("Bind done");

    //keep listening for data
    while (1)
    {
        printf("Waiting for data...");
        fflush(stdout);

        //clear the buffer by filling null, it might have previously received data
        memset(buf, '\0', BUFLEN);

        //try to receive some data, this is a blocking call
        if ((recv_len = recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *) &si_other, &slen)) == SOCKET_ERROR)
        {
            printf("recvfrom() failed with error code : %d", WSAGetLastError());
            exit(EXIT_FAILURE);
        }

        //print details of the client/peer and the data received
        printf("Received packet from %s:%d\n", inet_ntoa(si_other.sin_addr), ntohs(si_other.sin_port));
        printf("Data: %s\n", buf);

        //now reply the client with the same data
        if (sendto(s, buf, recv_len, 0, (struct sockaddr*) &si_other, slen) == SOCKET_ERROR)
        {
            printf("sendto() failed with error code : %d", WSAGetLastError());
            exit(EXIT_FAILURE);
        }
    }

    closesocket(s);
    WSACleanup();

    return 0;
}

我使用这个示例来学习 C++ 中的 UDP，但是当我实现它时。

C:\Users\cc>ncat -vv -u localhost 8888
Ncat: Version 7.12 ( https://nmap.org/ncat )
NCAT DEBUG: Using trusted CA certificates from C:\Program Files (x86)\Nmap\ca-bundle.crt.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #1)
libnsock nsock_connect_udp(): UDP connection requested to ::1:8888 (IOD #1) EID 8
libnsock nsock_trace_handler_callback(): Callback: CONNECT SUCCESS for EID 8 [::1:8888]
Ncat: Connected to ::1:8888.
libnsock nsock_iod_new2(): nsock_iod_new (IOD #2)
libnsock nsock_read(): Read request from IOD #1 [::1:8888] (timeout: -1ms) EID 18
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 26
j
libnsock nsock_trace_handler_callback(): Callback: READ SUCCESS for EID 26 [peer unspecified] (2 bytes): j.
libnsock nsock_write(): Write request for 2 bytes to IOD #1 EID 35 [::1:8888]
libnsock nsock_trace_handler_callback(): Callback: WRITE SUCCESS for EID 35 [::1:8888]
libnsock nsock_readbytes(): Read request for 0 bytes from IOD #2 [peer unspecified] EID 42
libnsock nsock_trace_handler_callback(): Callback: READ ERROR [远程主机强迫关闭了一个现有的连接。  (10054)] for EID 18 [::1:8888]
Ncat: 远程主机强迫关闭了一个现有的连接。 .

出现很多调试信息。然后我输入一个字符'j'，结果和我预期的不一样。当我检查我的网络状态时，我发现端口 8888 连接到 ip 0.0.0.0。我真的希望你能帮助我，谢谢。

 UDP    0.0.0.0:8500           *:*
 UDP    0.0.0.0:8888           *:*
 UDP    0.0.0.0:51754          *:*

Answer 1

当查看 ncat 命令的调试输出时，您会看到它试图“连接”到地址 ::1:8888，这是 IPv6 localhost 地址。但是您的服务器正在等待 localhost 的 IPv4 地址上的数据。

您需要告诉 ncat 使用 IPv4 地址，或者通过添加选项 -4 来强制使用 IPv4:

ncat -4 -vv -u localhost 8888

或者明确告诉它连接到 localhost 的 IPv4 地址:

ncat -vv -u 127.0.0.1 8888