tapup.c

/*
 * tapup.c
 * Bring up a tap device and discard frames
 * By J. Stuart McMurray
 * Created 20190911
 * Last Modified 20190911
 */

#include <linux/if.h>
#include <linux/if_tun.h>
#include <sys/ioctl.h>

#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>

int
main(void)
{
        struct ifreq ifr;
        char buf[2048];
        int fd;
        int udp;

        /* Device from which to request tap device */
        if (-1 == (fd = open("/dev/net/tun", O_RDWR)))
                err(1, "open");

        /* Request our own tap device */
        memset(&ifr, 0, sizeof(ifr));
        ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
        if (-1 == ioctl(fd, TUNSETIFF, (void *)&ifr))
                err(2, "ioctl");

        /* Bring device up.  Why we need a different socket to pass to
         * ioctl(2) is beyond me. */
        if (-1 == (udp = socket(AF_INET, SOCK_DGRAM, 0)))
                err(3, "socket");
        ifr.ifr_flags = IFF_UP | IFF_BROADCAST | IFF_RUNNING | IFF_MULTICAST;
        if (-1 == ioctl(udp, SIOCSIFFLAGS, &ifr)) /* Doesn't work with fd */
                err(4, "ioctl");
        if (-1 == close(udp))                     /* Doesn't work with fd */
                err(5, "close");

        fprintf(stderr, "Device: %s\n", ifr.ifr_name);

        /* Read and discard frames */
        for (;;)
                if (0 >= read(fd, buf, sizeof(buf)))
                        err(6, "read");

        /* Unpossible */
        return 7;
}