#include <sys/types.h>
#include <sys/socket.h>
#include <sys/sysctl.h>

#include <net/route.h>
#include <netinet/in.h>

#include <arpa/inet.h>

#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <err.h>


/* alignment constraint for routing socket */
#define ROUNDUP(a)							\
     ((a) > 0 ? (1 + (((a) - 1) | (sizeof(long) - 1))) : sizeof(long))
#define ADVANCE(x, n) (x += ROUNDUP((n)->sa_len))

void get_rtaddrs(int bitmask, struct sockaddr *sa, struct sockaddr *addrs[])
{
     int i;

     for (i = 0; i < RTAX_MAX; i++) {
	  if (bitmask & (1 << i)) {
	       addrs[i] = sa;
	       sa = (struct sockaddr *)(ROUNDUP(sa->sa_len) + (char *)sa);
	  } else
	       addrs[i] = NULL;
     }
}

int is_default_route (struct sockaddr *sa, struct sockaddr *mask)
{
    struct sockaddr_in *sin;

    if (sa->sa_family != AF_INET)
	return 0;

    sin = (struct sockaddr_in *)sa;
    if ((sin->sin_addr.s_addr == INADDR_ANY) &&
	mask &&
	(ntohl(((struct sockaddr_in *)mask)->sin_addr.s_addr) == 0L ||
	mask->sa_len == 0))
	return 1;
    else
	return 0;
}

int main()
{
    int mib[6];
    size_t needed;
    char *buf, *next, *lim;
    struct rt_msghdr2 *rtm;
    struct sockaddr *sa;
    struct sockaddr *rti_info[RTAX_MAX];

    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) {
	err(1, "sysctl: net.route.0.0.dump estimate");
    }

    buf = malloc(needed);
    if (buf == 0) {
	err(2, "malloc");
    }

    if (sysctl(mib, 6, buf, &needed, NULL, 0) < 0) {
	err(1, "sysctl: net.route.0.0.dump");
    }

    lim = buf + needed;
    for (next = buf; next < lim; next += rtm->rtm_msglen) {
	rtm = (struct rt_msghdr2 *)next;
	sa = (struct sockaddr *)(rtm + 1);

	if (sa->sa_family == AF_INET) {
            struct sockaddr_in *sin = (struct sockaddr_in *)sa;
	    struct sockaddr addr, mask;

	    get_rtaddrs(rtm->rtm_addrs, sa, rti_info);
	    bzero(&addr, sizeof(addr));
	    if (rtm->rtm_addrs & RTA_DST)
		bcopy(rti_info[RTAX_DST], &addr, rti_info[RTAX_DST]->sa_len);
	    bzero(&mask, sizeof(mask));
	    if (rtm->rtm_addrs & RTA_NETMASK)
		bcopy(rti_info[RTAX_NETMASK], &mask,
			rti_info[RTAX_NETMASK]->sa_len);

	    if (is_default_route(&addr, &mask)) {
		sin = (struct sockaddr_in *)rti_info[RTAX_GATEWAY];
		printf("%s\n", inet_ntoa(sin->sin_addr));
		break;
	    }
	}
	rtm = (struct rt_msghdr2 *)next;
    }
    free(buf);
    exit(0);
}