How to get gateway ip and nameserver ip using ioctl in linux

Question

I have got ip, subnet and broadcast address using ioctl call. but don't know how to get default gateway and nameserver ip. if I pick nameserver from /etc/resolv.conf then is it reliable?

Here is my code:

int main(void)
{
char buf[1024];
struct ifconf ifc;
struct ifreq *ifr;
int sck, nInterfaces;
int i;
unsigned char mac[6];

sck = socket(AF_INET, SOCK_DGRAM, 0);
if(sck < 0)
{
    perror("socket");
    return 1;
}

ifc.ifc_len = sizeof(buf);
ifc.ifc_buf = buf;
if(ioctl(sck, SIOCGIFCONF, &ifc) < 0)
{
    perror("ioctl(SIOCGIFCONF)");
    return 1;
}

ifr = ifc.ifc_req;
nInterfaces = ifc.ifc_len / sizeof(struct ifreq);
for(i = 0; i < nInterfaces; i++)
{
    struct ifreq *item = &ifr[i];
    printf("Interface Name = %s\nIP = %s\n",
           item->ifr_name,
           inet_ntoa(((struct sockaddr_in *)&item->ifr_addr)->sin_addr));

    ioctl(sck, SIOCGIFNETMASK, item);
    printf("SubNet Mask = %s\n", inet_ntoa(((struct sockaddr_in *)&item->ifr_netmask)->sin_addr));

    ioctl(sck, SIOCGIFBRDADDR, item);
    printf("BroadCat Address = %s\n", inet_ntoa(((struct sockaddr_in *)&item->ifr_broadaddr)->sin_addr));

    ioctl(sck, SIOCGIFHWADDR, item);
    memcpy(mac, item->ifr_hwaddr.sa_data, 6);
    printf("MAC:%02X:%02X:%02X:%02X:%02X:%02X\n",mac[0],mac[1],mac[2],mac[3],mac[4],mac[5]); 
}
 return 0;
}

Answer 1

To get the default gateway you can parse /proc/net/route :

# cat /proc/net/route
Iface   Destination     Gateway         Flags   RefCnt  Use     Metric  Mask   MTU      Window  IRTT
eth0    00000000        010110AC        0003    0       0       0       000000000       0       0

If you need nameserver address, parsing /etc/resolv.conf seems like a reliable option to me.

Answer 2

You can use

struct rtentry route;
(...)
ioctl(sck, SIOCADDRT, &route)

More information her:

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