using ioctl(2) to get the original hwaddr of a network interface

get_original_hwaddr.c

// gcc -o get_orig_hwaddr get_orig_hwaddr.c
#include <stdio.h>
#include <errno.h>
#include <string.h>

#include <net/if.h>
#include <sys/ioctl.h>
#include <sys/socket.h>

char *get_addr (int fd, struct ifreq *ifrq) {
    char *hwaddr;

    if ((ioctl(fd, SIOCGIFHWADDR, ifrq)) == -1){
        perror("[-] error");
        return NULL;
    }
    hwaddr = ifrq->ifr_ifru.ifru_hwaddr.sa_data;
    return (hwaddr);
}

int main(int argc, char *argv[]) {
    int nlsock;

    if (argc > 2 || ( argc == 2 && (strcmp(argv[1],"-h")) == 0 ) ) {
        fprintf(stderr, "usage: %s [-h] <ifdev>\n", argv[0]);
        return 1;
    }

    nlsock = socket(AF_UNIX, SOCK_RAW, 0);

    if( argc == 2 ){
        struct ifreq ifr;
        strncpy(ifr.ifr_name, argv[1], IF_NAMESIZE-1);

        char *mac = get_addr(nlsock, &ifr);
        if (mac == NULL)
            return errno;

        for(int i=0; i<6;i++){
            printf("%02x", mac[i]);
            if ( i < 5 )
                printf(":");
        }
        printf("\n");
    } else {
        struct if_nameindex *ifindex = if_nameindex();
        while(ifindex->if_index > 0){
            struct ifreq ifr;
            strncpy(ifr.ifr_name, ifindex->if_name, IF_NAMESIZE-1);

            char *mac = get_addr(nlsock, &ifr);
            if (mac == NULL)
                return errno;
            for(int i=0; i<6;i++){
                printf("%02x", mac[i]);
                if ( i < 5 )
                    printf(":");
            }
            printf(" (%s)\n", ifindex->if_name);
            ifindex++;
        }
    }

    shutdown(nlsock, SHUT_RD);
    return 0;
}