#include <sys/time.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <errno.h>
#include <fcntl.h>
#include <time.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <linux/if.h>
#include <linux/if_tun.h>

#define PERROR_L(s) \
    fprintf(stderr, "(line %d) %s: %s\n", __LINE__, s, strerror(errno))

static int tun_alloc(const char *dev) {
    struct ifreq ifr;
    int fd, err;

    if ((fd = open("/dev/net/tun", O_RDWR)) < 0) {
        PERROR_L("open");
        return fd;
    }

    memset(&ifr, 0, sizeof(ifr));
    ifr.ifr_flags = IFF_TAP|IFF_NO_PI;
    strncpy(ifr.ifr_name, dev, IFNAMSIZ);

    if ((err = ioctl(fd, TUNSETIFF, (void *)&ifr)) < 0) {
        PERROR_L("ioctl");
        close(fd);
        return err;
    }

    return fd;
}

static int ifup(const char *dev) {
    struct ifreq ifr;
    int fd, err;

    if ((fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        PERROR_L("socket");
        return fd;
    }

    memset(&ifr, 0, sizeof(ifr));
    strncpy(ifr.ifr_name, dev, IFNAMSIZ);

    if ((err = ioctl(fd, SIOCGIFFLAGS, (void *)&ifr)) < 0) {
        PERROR_L("ioctl");
        goto out;
    }

    ifr.ifr_flags |= IFF_UP|IFF_RUNNING;

    if ((err = ioctl(fd, SIOCSIFFLAGS, (void *)&ifr)) < 0) {
        PERROR_L("ioctl");
        goto out;
    }

  out:
    close(fd);
    return err;
}

static void hexdump(const ssize_t size, const unsigned char *buf) {
    int i;
    time_t t = time(NULL);

    printf("\n--- %s", ctime(&t));
    for (i = 0; i < size; i++) {
        if (i % 16 == 0) printf("\n  0x%04x: ", i);
        printf("%02x", buf[i]);
        if (i % 2) printf(" ");
    }
    printf("\n");
}

static void mainloop(const int fd) {
    fd_set fds;
    int ret;
    struct timeval tv;
    ssize_t size;
    unsigned char buf[65535];

    while (1) {
        memset(&tv, 0, sizeof(tv));
        tv.tv_sec = 1;
        tv.tv_usec = 0;

        FD_ZERO(&fds);
        FD_SET(fd, &fds);

        ret = select(fd+1, &fds, NULL, NULL, &tv);
        if (ret < 0) {
            if (errno == EINTR) continue;
            PERROR_L("select");
            return;
        }
        else if (ret == 0)
            continue;

        memset(buf, 0, sizeof(buf));
        size = read(fd, buf, sizeof(buf));
        if (size < 0) {
            if (errno == EINTR) continue;
            PERROR_L("read");
            return;
        }
        else if (size == 0)
            break;

        hexdump(size, buf);
    }
}

int main(int argc, char **argv) {
    int fd, ret;

    if (argc != 2) {
        fprintf(stderr, "Syntax: %s DEVNAME\n", argv[0]);
        return 1;
    }

    if (IFNAMSIZ <= strlen(argv[1])) {
        fprintf(stderr, "DEVNAME is too long\n");
        return 2;
    }

    if ((fd = tun_alloc(argv[1])) < 0)
        return 3;

    if ((ret = ifup(argv[1])) < 0)
        goto out;

    mainloop(fd);

  out:
    close(fd);
    return ret;
}