/* input : IP address of a interface on the same network
 * output : MAC address of this interface if found if arp_cache  */

#include <stdlib.h>
#include <stdio.h>
#include <net/if_arp.h>
#include <sys/ioctl.h>
#include <netinet/in.h>
#include <arpa/inet.h>

struct sockaddr_in sin;
struct arpreq myarp;
int ss;
unsigned char *ptr;  
int main (int argc, char *argv[])
{
    if (argc < 2) {
        printf("Forma de uso: %s <Direccion IP>\n",argv[0]);
        exit(0);
        }
    sin.sin_family = AF_INET;
    if (inet_aton (argv[1], &sin.sin_addr)==0) {
        printf("%s: IP address '%s' no es correcta\n",argv[0],argv[1]);
        exit(0);
        }    
    memcpy (&myarp.arp_pa, &sin, sizeof myarp.arp_pa);         
    strcpy (myarp.arp_dev, "eth0");  
    if ((ss = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
        printf("%s: no es posible crear socket\n",argv[0]);
        exit(0);
        }  
    if (ioctl (ss, SIOCGARP, &myarp) == -1) {
        printf("%s: no es posible encontrar entrada para '%s' en el cache de direcciones\n",argv[0],argv[1]);
        exit(0);
        }  
    ptr = &myarp.arp_ha.sa_data[0];
    printf("%s: MAC address for '%s' is : ",argv[0],argv[1]);
    printf("%x:%x:%x:%x:%x:%x\n",*ptr, *(ptr+1),*(ptr+2),*(ptr+3),*(ptr+4),*(ptr+5));  
    return 1;
}