如何使用“getaddrinfo”为所有接口选择默认空闲端口？

Question

但当我给它“0”端口时，它在addrinfo结果中保持“0”，这导致每个绑定给我不同的端口号。

我的问题：有没有一种方法告诉getaddrinfo选择一个在所有接口上都是空闲的自由端口，然后将给定的地址绑定到所有接口？

如果没有，有没有其他方法可以找到自由端口号？（不绑定，失败时停止）

#ifdef WIN32
#include <WS2tcpip.h>
#pragma comment(lib, "ws2_32.lib")
#else
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <unistd.h>
#define closesocket close
#endif
#include <iostream>
#include <vector>  
#include <stdio.h>
#include <string>

int GetAddressFamily()
{
    return AF_UNSPEC;
}

std::string ipaddress(addrinfo* info)
{
    std::string retval;   
    char temp[260];
    socklen_t addrlen = (socklen_t)info->ai_addrlen;
    int res = ::getnameinfo(info->ai_addr, addrlen, temp, 256, NULL, 0, NI_NUMERICHOST);
    if(res){
        std::cout<<gai_strerrorA(res)<<std::endl;
    }else{
        retval = temp;
    }
    return retval;
}

int getport(addrinfo* info)
{
    int retval=0;

    if (info->ai_family == AF_INET) {
        retval = htons(((struct sockaddr_in*)(info->ai_addr))->sin_port);
    }else{
        retval = htons(((struct sockaddr_in6*)(info->ai_addr))->sin6_port);
    }
    return retval;
}

int main()
{
    char *hostName = NULL; //GetHostName();
    int portNum = 0;
#ifdef WIN32
    WSADATA w;
    if(0 != WSAStartup(MAKEWORD(2, 2), &w))
    {
       std::cerr<<" WSAStartup() failed \n";
       return -1;
    } 
#endif
    addrinfo hints,*results,*tmp;
    memset(&hints,0,sizeof(hints));
    hints.ai_family = GetAddressFamily();
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_NUMERICSERV;
    if(hostName){
        hints.ai_flags |= AI_CANONNAME; 
        //AI_CANONNAME - fills ai_cannonname in address.
    }else{
        hints.ai_flags |= AI_PASSIVE;
        //AI_PASSIVE - give ADDR_ANY and IN6ADDR_ANY_INIT when hostName is NULL 
    }
    char portbuff[40];
    sprintf(portbuff,"%u",portNum);

    int res = ::getaddrinfo(hostName, portbuff,&hints, &results); 
    if(res){
        std::cerr<<gai_strerrorA(res)<<std::endl;
    }else{
        std::vector<int> sockets;
        for(tmp = results; tmp ; tmp=tmp->ai_next){
            std::cout<<ipaddress(tmp).c_str()<<" : "<<port(tmp)<<std::endl;
            int s = socket(tmp->ai_family,tmp->ai_socktype,tmp->ai_protocol);
            if(s != -1){
                res = bind(s, tmp->ai_addr, (int)tmp->ai_addrlen);
                if(res != -1){
                    sockaddr_storage addr;
                    socklen_t len =sizeof(addr);
                    int res = getsockname(s, (struct sockaddr *)&addr, &len);
                    std::cout<<"Bound to port: ";
                    if(addr.ss_family == AF_INET){
                        std::cout<<htons(((sockaddr_in*)&addr)->sin_port)<<std::endl;
                    }else{
                        std::cout<<htons(((sockaddr_in6*)&addr)->sin6_port)<<std::endl;
                    }
                    sockets.push_back(s);
                }
            }
        }
        for(int i=0;i<sockets.size();i++){
            closesocket(sockets[i]);
        }
    }
    ::freeaddrinfo(results);
    return 0;
}

void setport(addrinfo* info,int port)
{
   for(addrinfo* tmp = info; tmp ; tmp=tmp->ai_next){
   if (tmp->ai_family == AF_INET) {
     ((struct sockaddr_in*)(tmp->ai_addr))->sin_port = htons(port);
   }else{
     ((struct sockaddr_in6*)(tmp->ai_addr))->sin6_port = htons(port);
   }
}

port = getport(result)
//...after bind:   
if(port == 0) {
   port = printed value after succesful bind
   setport(result, port)
}

Answer 1

我认为您的最佳选择是使用禁用ipv6_v6only的IPv6套接字。则该单个套接字将同时连接IPv6和IPv4连接。IPv4客户端将地址设置为映射地址::ffff:a.b.c.d。

这里没有使用getaddrinfo的理由，因为您没有查找任何内容。类似这样的东西应该可以做到（未经测试）：

int s = socket(AF_INET6, SOCK_STREAM, 0);
if (s < 0)
    throw std::system_error(errno, generic_category());

const int off = 0;
if (setsockopt(s, IPPROTO_IPV6, IPV6_V6ONLY, &off, sizeof(off)) {
    close(s);
    throw std::system_error(errno, generic_category());
}

struct sockaddr_in6 addr{};
socklen_t alen = sizeof(addr);
if (bind(s, static_cast<struct sockaddr*>(&addr), alen)) {
    close(s);
    throw std::system_error(errno, generic_category());
}

getsockname(s, static_cast<struct sockaddr*>(&addr), &alen);
int port = ntohs(addr.sin6_port);

ps.最好总是将ipv6_v6only选项设置为您希望的任何值，因为默认值在不同的操作系统之间有所不同。