code/tun/netw.c

#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>
#include <linux/ip.h>
#include <linux/udp.h>
#include <arpa/inet.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");
}

void parse_packet(u_int8_t *buf, int buf_size) {
    
  
    //struct ethhdr * ethernet_header;
    struct iphdr * ip_header;
    struct udphdr * udp_header;
    ip_header = (struct iphdr*)(buf);
    
    if (ip_header->protocol == IPPROTO_UDP)
    {
        printf("UDP packet found\n");
        printf("TTL: %d \n",ip_header->ttl);
        printf("Dest IP address: %s\n", inet_ntoa( *(struct in_addr*)&ip_header->daddr));
        printf("Source IP address: %s\n", inet_ntoa( *(struct in_addr*)&ip_header->saddr));

        udp_header =    (struct udphdr *) (buf + ip_header->ihl*4);
        printf("Source Port: %d\n", ntohs(udp_header->source));
        printf("Dest Port: %d\n", ntohs(udp_header->dest));
        if (ntohs(udp_header->dest) == 53) 
        {
            printf("Found DNS packet\n");
        }
    }

    return;// NULL;
}

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 | IFF_NO_PI));
    if (tunfd < 0) {
        fprintf(stderr, "could not allocate device tun.\n");
        return(EXIT_FAILURE);
    }

    for (;;) {
        ret = read(tunfd, buf, MTU);
        parse_packet(buf, ret);
    }
   
    return (0);
}