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

#define MTU 1500

typedef struct ipv4 {
    u_int16_t tun_flags; //tun specific 16 bits
    u_int16_t tun_proto; //tun specific 16 bits
    u_int8_t version; // 4 bits
    u_int8_t header_length; // 4 bits
    u_int8_t type_of_service; // 8 bits
    u_int16_t total_length; //16 bits
    u_int16_t identification; // 16 bits
    u_int8_t flags; // 3 bits

} ipv4_t;

int create_tun(char *dev_name, int flags){
    struct ifreq ifr;
    int fd, err;
    char *clonedev = "/dev/net/tun";

    if ((fd = open(clonedev, O_RDWR)) < 0)
    {
        return fd;
    }

    memset(&ifr, 0, sizeof(ifr));
    ifr.ifr_ifru.ifru_flags = flags;

    if(*dev_name) {
        strncpy(ifr.ifr_ifrn.ifrn_name, dev_name, IFNAMSIZ);
    }

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

    strcpy(dev_name, ifr.ifr_ifrn.ifrn_name);

    return fd;
}

void print_packet(u_int8_t* buf, int buf_size) {
    int i;

    printf("received %d bytes: ", buf_size);

    for (i = 0; i < buf_size; i++)
    {
        if (i > 0) printf(":");
        printf("%02X", buf[i]);
    }
    printf("\n");
}


// 0100 0110 46
// 1111 0000 
// 0100 0000 40
// 46 - 40 = 6

// 0100 0110 46
// 0000 1111 
// 0000 0110 6
// 46 - 6 = 40


ipv4_t parse_packet(u_int8_t *buf, int buf_size) {
    
    ipv4_t p;
    p.tun_flags = buf[1]<<8 | buf[0];
    p.tun_proto = buf[2]<<8 | buf[3];
    p.version =  buf[4] - (buf[4] & 00001111);
    p.header_length = buf[4] - (buf[4] & 11110000);
    p.type_of_service = buf[5];
    p.total_length = buf[6]<<8 | buf[7];
    p.identification = buf[8]<<8 | buf[9];
    p.flags = buf[10] - (buf[10] & 00011111);
    return p;
}

int main (int argc, char** argv) {
    char tun_name[IFNAMSIZ];
    u_int8_t buf[MTU];
    int ret;
    ipv4_t curr_packet;

    sprintf(tun_name, "tun01");
    int tunfd = create_tun(tun_name, (IFF_TUN));
    if (tunfd < 0) {
        fprintf(stderr, "could not allocate device tun.\n");
        return(EXIT_FAILURE);
    }
    
    for (;;) {
        ret = read(tunfd, buf, MTU);
        curr_packet = parse_packet(buf, ret);

        if (curr_packet.tun_proto == 0x800) {
            printf("proto: %02X\n", curr_packet.tun_proto);
            printf("version: %02X\n", curr_packet.version);
            printf("header length %02X\n", curr_packet.header_length);
            print_packet(buf, ret);
        }
    }
   
    return (0);
}