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);
}