getaddrinfo not returning ipv6 details

Question

I am facing issues in getting ipv6 using getaddrinfo():

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h>

int main()
{
        int soc = 00;
        struct addrinfo hints,*results,*res;
        hints.ai_family = AF_UNSPEC;
        hints.ai_socktype = SOCK_STREAM;
        hints.ai_protocol = IPPROTO_TCP;
        hints.ai_flags = AI_PASSIVE;

        int m = getaddrinfo("myhost","2018",&hints,&results);
        if (m<0)
        {
                printf("getaddrinfo error :  %s\n", gai_strerror(m));
                return 1;
        }
        res=results;
        do
        {
                if(res->ai_family == AF_INET6)
                {
                        struct sockaddr_in6* a= (struct sockaddr_in6*)res->ai_addr;
                        char straddr[INET6_ADDRSTRLEN];
                        inet_ntop(AF_INET6, &a->sin6_addr, straddr, sizeof(straddr));
                        printf("IP v6 - %s\n",straddr);
                }
                if(res->ai_family == AF_INET)
                {
                        struct sockaddr_in* b= (struct sockaddr_in*)res->ai_addr;
                        const char* dotted_decimal1 = inet_ntoa(b->sin_addr);
                        printf("IP v4 - %s\n",dotted_decimal1);
                }
        }
        while((res=res->ai_next) !=NULL);

        return 0;
}

Output:

IP v4 - xx.xx.xx.xx

Since I have set hints.ai_family = AF_UNSPEC, it should return me both the details (ipv4 and ipv6). But it is returning details of ipv4 only.

Note: on command ifconfig I can see both ipv4 and ipv6 address.

> $ ifconfig
> eth3      Link encap:Ethernet  HWaddr xx:xx:xx:xx:xx:xx
> inet addr:x.x.x.x  Bcast:x.x.x.x  Mask:255.255.255.128
> inet6 addr: xx::xx:xx:xx:xx/64 Scope:Link

1. Can onyone explain me why it is not picking ipv6?

2. Do I need to make any changes on my host (Linux)?

I have removed the actual ip details and replaced with xxxx.

Answer 1

To retrieve local addresses (since you are using AI_PASSIVE), you can (and should) set the node parameter to NULL instead of specifying an actual name, per the documentation:

If the AI_PASSIVE flag is specified in hints.ai_flags, and node is NULL, then the returned socket addresses will be suitable for bind(2)ing a socket that will accept(2) connections.

Also, inet_ntop() supports AF_INET, so you can get rid of your if statements and use inet_ntop() unconditionally for both address families.

Try this:

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

void* getSinAddr(int af, sockaddr *addr)
{
    switch (af)
    {
        case AF_INET:
            return &(reinterpret_cast<sockaddr_in*>(addr)->sin_addr);

        case AF_INET6:
            return &(reinterpret_cast<sockaddr_in6*>(addr)->sin6_addr);
    }
    return NULL;
}

int main()
{
    addrinfo hints = {}, *results, *res;

    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;
    hints.ai_flags = AI_PASSIVE;

    int m = getaddrinfo(NULL, "2018", &hints, &results);
    if (m < 0)
    {
        std::cout << "getaddrinfo error : " << gai_strerror(m) << std::endl;
        return 1;
    }

    for(res = results; res != NULL; res = res->ai_next)
    {
        char straddr[INET6_ADDRSTRLEN] = {};
        if (inet_ntop(res->ai_family, getSinAddr(res->ai_family, res->ai_addr), straddr, sizeof(straddr)))
            std::cout << "IP - " << straddr << "\n";
        else
            std::cout << "inet_ntop error : " << strerror(errno) << std::endl;
    }

    freeaddrinfo(results);

    return 0;
}

Answer 2

Since I have set hints.ai_family = AF_UNSPEC it should return me both the details (ipv4 and ipv6)

No, it should not. getaddrinfo API does not guarantee to return both IPv4 and IPv6 addresses. It returns IP address type that is associated with given host name.

In order to make sure that your code is working fine you can simply check google.com host over 80 or 443 ports i.e.

 int m = getaddrinfo("google.com","80",&hints,&results);

This check returns both IPv4 and IPv6 addresses in my Linux PC(fyi, I also don't have global IPv6 address).

If you need to retrieve your local IPv6 address, I would recommend to use getifaddrs or read /proc/net/if_inet6 file in Linux, and GetAdaptersAddresses in Windows.
Also please pay attention that your IPv6 address scope is link local, which is auto generated by OS.