check default router interface on macOS.

def_route_iface.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/sysctl.h>
#include <sys/socket.h>
#include <net/route.h>
#include <netinet/in.h>
#include <net/if.h>

int main(int argc,char *argv[]){
    int mib[6];
    size_t needed;
    char *buf, *next, *lim;
    struct rt_msghdr2 *rtm;
    char ifname[IFNAMSIZ + 1];

    mib[0] = CTL_NET;
    mib[1] = PF_ROUTE;
    mib[2] = 0;
    mib[3] = 0;
    mib[4] = NET_RT_DUMP2;
    mib[5] = 0;

    if( sysctl(mib, 6, NULL, &needed, NULL,0) < 0){
        perror("sysctl");
        exit(1);
    }

    if((buf = malloc(needed)) == 0 ){
        perror("malloc");
        exit(1);
    }

    if( sysctl(mib, 6, buf, &needed, NULL,0) < 0){
        perror("sysctl");
        exit(1);
    }

    lim  = buf + needed;
    for (next = buf; next < lim; next += rtm->rtm_msglen) {
        struct sockaddr *sa;
        rtm = (struct rt_msghdr2 *)next;

        sa = (struct sockaddr *)(rtm + 1);
        if(sa->sa_family == AF_INET) {
            struct sockaddr_in *sin  = (struct sockaddr_in *)sa;
            off_t offset = sizeof(struct sockaddr)*RTA_NETMASK;
            struct sockaddr_in *mask = (struct sockaddr_in *)sa + offset;

            /* address: 0.0.0.0/0 */
            if( rtm->rtm_addrs & RTA_DST &&
                sin->sin_addr.s_addr == INADDR_ANY &&
                rtm->rtm_addrs & RTA_NETMASK &&
                ((ntohl(mask->sin_addr.s_addr) == 0L))) {

                if_indextoname(rtm->rtm_index, ifname);
                printf("%s\n",ifname);
            }
        }
    }
}