双栈网络服务器未按预期处理 IPv4 请求

下面是一个实现双栈网络基础设施的 C++ 服务器的最小工作示例。（这意味着单个套接字可同时处理 ipv4 和 ipv6 连接。）

#include <format>
#include <print>
#include <cstring>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <signal.h>
#include <arpa/inet.h>
#include <unistd.h>

int main(int argc, char* argv[]) {

    const auto SERVER_PORT = 7778;

    const auto server_fd = socket(AF_INET6, SOCK_STREAM, 0);
    int opt = 0;

    setsockopt(server_fd, IPPROTO_IPV6, IPV6_V6ONLY, &opt, sizeof(opt));

    sockaddr_in6 server_address;
    std::memset(&server_address, 0, sizeof(server_address));

    server_address.sin6_family = AF_INET6;
    server_address.sin6_addr = in6addr_any;
    server_address.sin6_port = htons(SERVER_PORT);
    bind(server_fd, reinterpret_cast<sockaddr*>(&server_address), sizeof(server_address));

    listen(server_fd, 10);

    sockaddr_storage peer_address;
    socklen_t peer_address_length = sizeof(peer_address);
    auto peer_fd = accept(server_fd, reinterpret_cast<sockaddr*>(&peer_address), &peer_address_length);

    if(peer_address.ss_family == AF_INET)
    {
        const auto p_peer_address = &peer_address;
        sockaddr_in* ipv4 = (sockaddr_in*)p_peer_address;
        std::println("Client port (IPv4): {}", ntohs(ipv4->sin_port));
    }
    else if(peer_address.ss_family == AF_INET6)
    {
        const auto p_peer_address = &peer_address;
        sockaddr_in6* ipv6 = (sockaddr_in6*)p_peer_address;
        std::println("Client port (IPv6): {}", ntohs(ipv6->sin6_port));
    }
    else
    {
        throw std::runtime_error("unrecognized ss_family");
    }

    close(peer_fd);
    close(server_fd);

    return 0;
}

但是，它并没有按预期工作。无论客户端是通过 ipv4 还是 ipv6 进行连接，逻辑始终遵循语句else if的分支if。第一个if分支从未运行，表示没有与该系列建立任何连接AF_INET。

两个客户端似乎都可以正常工作。当连接启动时，服务器会做出响应 - 无论客户端连接类型如何，都会打印 ipv6 消息。（ipv4 或 ipv6）

如果有什么区别的话，客户端和服务器都在同一台机器上运行，通过 localhost 连接（127.0.0.1）

下面提供了两种客户端类型的示例 C++ 代码。

// Client: ipv6

#include <format>
#include <print>
#include <format>
#include <cstring>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>


int main(int argc, char* argv[]) {

    const auto PORT = 7778;

    const auto socket_fd = socket(AF_INET6, SOCK_STREAM, 0);

    in6_addr server_sin6_address;
    std::memset(&server_sin6_address, 0, sizeof(server_sin6_address));
    inet_pton(AF_INET6, "::1", &server_sin6_address);

    sockaddr_storage server_address;
    std::memset(&server_address, 0, sizeof(server_address));

    server_address.ss_family = AF_INET6;
    sockaddr_storage *p_server_address = &server_address;
    sockaddr_in6 *p_server_address_in6 = reinterpret_cast<sockaddr_in6*>(p_server_address);
    p_server_address_in6->sin6_family = AF_INET6; // why repeat?
    p_server_address_in6->sin6_port = htons(PORT);
    p_server_address_in6->sin6_flowinfo = 0; // not used?
    p_server_address_in6->sin6_addr = server_sin6_address;
    p_server_address_in6->sin6_scope_id = 0; // not used?

    const auto connect_result = connect(socket_fd, reinterpret_cast<sockaddr*>(&server_address), sizeof(server_address));

    const char* const buffer = "hello world ipv6";
    const auto buffer_length = strlen(buffer) + 1;
    send(socket_fd, buffer, buffer_length, 0);

    close(socket_fd);

    return 0;
}

// Client: ipv4

#include <format>
#include <print>
#include <format>
#include <cstring>

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>


int main(int argc, char* argv[]) {

    const auto PORT = 7778;

    const auto socket_fd = socket(AF_INET, SOCK_STREAM, 0);

    in_addr server_sin_address;
    std::memset(&server_sin_address, 0, sizeof(server_sin_address));
    inet_pton(AF_INET, "127.0.0.1", &server_sin_address);

    sockaddr_storage server_address;
    std::memset(&server_address, 0, sizeof(server_address));

    server_address.ss_family = AF_INET;
    sockaddr_storage *p_server_address = &server_address;
    sockaddr_in *p_server_address_in = reinterpret_cast<sockaddr_in*>(p_server_address);
    p_server_address_in->sin_family = AF_INET; // why repeat?
    p_server_address_in->sin_port = htons(PORT);
    p_server_address_in->sin_addr = server_sin_address;
    //p_server_address_in->sin_zero = 0; // not used?

    const auto connect_result = connect(socket_fd, reinterpret_cast<sockaddr*>(&server_address), sizeof(server_address));

    const char* const buffer = "hello world ipv4";
    const auto buffer_length = strlen(buffer) + 1;
    const auto send_result = send(socket_fd, buffer, buffer_length, 0);

    close(socket_fd);
    std::println("Server quit");

    return 0;
}

也许这是 IPv4 客户端实现中的一个错误，但到目前为止我还没有发现任何看起来明显不正确的东西。

到 ipv6 目标的 ipv4 连接是否会以某种方式由操作系统自动升级？这似乎是一种非常奇怪的行为。