example of adding ipv6 address via ioctl(SIOCAIFADDR_IN6)

darwin-SIOCAIFADDR_IN6.c

#include <sys/socket.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <net/if.h>
#include <netinet/in.h>
#include <netinet/in_var.h>
#include <arpa/inet.h>
#include <netinet6/nd6.h>
#include <unistd.h>

#include <string.h>
#include <stdio.h>

int main() {
    struct in6_aliasreq ifr6;
    int fd;

    memset(&ifr6, 0, sizeof ifr6);

    strcpy(ifr6.ifra_name, "en0");

    ifr6.ifra_addr.sin6_len = sizeof ifr6.ifra_addr;
    ifr6.ifra_addr.sin6_family = AF_INET6;
    if (inet_pton(AF_INET6, "2001:da8:ecd1::1", &ifr6.ifra_addr.sin6_addr) != 1) {
        perror("inet_pton");
        return -1;
    }

    ifr6.ifra_prefixmask.sin6_len = sizeof ifr6.ifra_prefixmask;
    memset(&ifr6.ifra_prefixmask.sin6_addr, 0xff, 8); /* 8 * 8 = 64 bit */

    /* important!!! */
    ifr6.ifra_lifetime.ia6t_vltime = ifr6.ifra_lifetime.ia6t_pltime = ND6_INFINITE_LIFETIME;

    fd = socket(PF_INET6, SOCK_DGRAM, 0);
    if (fd < 0) {
        perror("socket");
        return -1;
    }

    if (ioctl(fd, SIOCAIFADDR_IN6, &ifr6) < 0) {
        perror("ioctl");
        close(fd);
        return -2;
    }

    close(fd);

    return 0;
}