Using netfilter Api within OpenWRT

[dawid <dawidwys@xxxxxxxxx>]

Hi I am writing some netfilter kernel module for my OpenWRT Backfire rev 
28680 build for Dir-300(AR23xx).

I have written some hook_function but unfortunatly any sk_buff i get in 
it is corrupted. There is only trash in it and as a
Could some of u tell me what am i doing wrong? The skb_network_header 
function always returns NULL.

I have included example of my code.

I would be extremely grateful for any sort of help.

Regards

Dawid

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/netfilter.h>
#undef __KERNEL__
#include <linux/netfilter_ipv4.h>
#define __KERNEL__
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <net/ip.h>
#include <net/tcp.h>

struct nf_hook_ops nfho;   //net filter hook option struct
struct sk_buff *sock_buff;
struct tcphdr *tcp_header;          // TCP header struct
struct iphdr *ip_header;            // IP header struct


unsigned int hook_func(unsigned int hooknum,
            struct sk_buff *skb,
            const struct net_device *in,
            const struct net_device *out,
            int (*okfn)(struct sk_buff *))
{
    sock_buff = skb;

    if (!sock_buff)
    {
        printk(KERN_INFO "NULL sock buff header\n");
        return NF_ACCEPT;
    }

    printk(KERN_INFO "IP_PROTO %d\n", sock_buff->protocol);
    ip_header = (struct iphdr *)skb_network_header(sock_buff);

    if (!ip_header)
    {
        printk(KERN_INFO "NULL ip header\n");
        return NF_ACCEPT;
    }

    printk(KERN_INFO "SRC: (%u.%u.%u.%u) --> DST: (%u.%u.%u.%u)\n",NIPQUAD(ip_header->saddr),NIPQUAD(ip_header->daddr));

    if(ip_header->protocol == IPPROTO_TCP){
        printk(KERN_INFO "tcp packet received\n");
    }

    if(ip_header->protocol == IPPROTO_UDP){
        printk(KERN_INFO "udp packet received\n");
    }

    if(ip_header->protocol == IPPROTO_ICMP){
        printk(KERN_INFO "icmp packet received\n");
    }


    printk(KERN_INFO "packet received\n");
    return NF_ACCEPT;
}

static int __init custom_init_module(void)
{
 nfho.hook = hook_func;                       //function to call when conditions below met
 nfho.hooknum = NF_IP_PRE_ROUTING;            //called right after packet recieved, first hook in Netfilter
 nfho.pf = PF_INET;                           //IPV4 packets
 nfho.priority = NF_IP_PRI_FIRST;             //set to highest priority over all other hook functions
 nf_register_hook(&nfho);                     //register hook

 printk(KERN_INFO "init_module() called\n");
 return 0;
}

static void __exit custom_cleanup_module(void)
{
 printk(KERN_INFO "cleanup_module() called\n");
 nf_unregister_hook(&nfho);                     //cleanup – unregister hook
}

module_init(custom_init_module);
module_exit(custom_cleanup_module);