writev on tuntap fd

commands

> sudo ip tuntap add dev test mode tun user $(whoami)
> sudo ip link set test up
> gcc one_pkt.c -o one_pkt
> ./one_pkt

one_pkt.c

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/uio.h>
#include <net/if.h>
#include <linux/if_tun.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdint.h>

int tun_alloc(const char *dev) {
  struct ifreq ifr;
  int fd, err;
  if ((fd = open("/dev/net/tun", O_RDWR)) < 0) {
    perror("open()");
    return fd;
  }
  memset(&ifr, 0, sizeof(ifr));
  ifr.ifr_flags = IFF_TUN | IFF_NO_PI;
  strncpy(ifr.ifr_name, dev, IFNAMSIZ);
  if ((err = ioctl(fd, TUNSETIFF, &ifr)) < 0) {
    perror("ioctl()");
    close(fd);
    return err;
  }

  return fd;
}

int main() {
  int tun_fd = tun_alloc("test");
  uint8_t buf0[500] =
      {0x45, 0x00, 0x00, 0x54, 0x8d, 0x4a, 0x40, 0x00, 0x40, 0x01, 0x64, 0x05, 0xc0, 0xa8, 0x64, 0x03, 0xc0, 0xa8, 0x64,
       0x05};
  uint8_t buf1[500];
  struct iovec iv[2];
  iv[0].iov_base = buf0;
  iv[0].iov_len = 500;
  iv[1].iov_base = buf1;
  iv[1].iov_len = 500;
  writev(tun_fd, iv, 2);
  return 0;
}