Question

I would like to check a network devices status e.g. promiscous mode. Basically like shown with ip a command.

Maybe someone could push me in the right direction?

I want to do this in C for linux so linux specific headers are available.

Answer 1

You need to use the SIOCGIFFLAGS ioctl to retrieve the flags associated with an interface. You can then check if the IFF_PROMISC flag is set:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>     
#include <sys/ioctl.h>  /* ioctl()  */
#include <sys/socket.h> /* socket() */
#include <arpa/inet.h>  
#include <unistd.h>     /* close()  */
#include <linux/if.h>   /* struct ifreq */

int main(int argc, char* argv[])
{
    /* this socket doesn't really matter, we just need a descriptor 
     * to perform the ioctl on */
    int fd = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);

    struct ifreq ethreq;

    memset(&ethreq, 0, sizeof(ethreq));

    /* set the name of the interface we wish to check */
    strncpy(ethreq.ifr_name, "eth0", IFNAMSIZ);
    /* grab flags associated with this interface */
    ioctl(fd, SIOCGIFFLAGS, &ethreq);
    if (ethreq.ifr_flags & IFF_PROMISC) {
        printf("%s is in promiscuous mode\n",
               ethreq.ifr_name);
    } else {
        printf("%s is NOT in promiscuous mode\n",
               ethreq.ifr_name);
    }

    close(fd);

    return 0;
}

If you want to set the interface to promiscuous mode, you will need root privileges, but you can simply set the field in ifr_flags and use the SIOCSIFFLAGS ioctl:

/* ... */
ethreq.ifr_flags |= IFF_PROMISC;
ioctl(fd, SIOCSIFFLAGS, &ethreq);