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Top Forums UNIX for Advanced & Expert Users problem with netfilter hook function struct skbuff *sock is null.. Post 302369854 by pavan6754 on Monday 9th of November 2009 08:53:44 PM
Old 11-09-2009
problem with netfilter hook function struct skbuff *sock is null..

iam trying to built a i have used netfilter for it.
in function main_hook sock_buff is returning null and in my log file continuously "sock buff null" is printed plse help to solve this problem..
(using print_string iam printing strings on current terminal (terminal we ping))

#include <linux/ip.h>  
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/netdevice.h>
#include <linux/init.h>
#include <linux/sched.h>	
#include <linux/tty.h>		
#include <linux/version.h> 
#include <linux/in.h>
# define NF_IP_PRE_ROUTING 0  
static struct nf_hook_ops netfilter_ops_in; 
struct sk_buff *sock_buff;
struct iphdr *ipptr;
static void print_string(char *str)
	struct tty_struct *my_tty;
        my_tty = current->signal->tty;

	if (my_tty != NULL) {

		((my_tty->driver)->ops->write) (my_tty,str,strlen(str));	

		((my_tty->driver)->ops->write) (my_tty, "\015\012", 2);

unsigned int main_hook(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("sock buff null\n"); return NF_ACCEPT; }
    printk(KERN_ALERT "sockbuff is not zero\n");
    ipptr=(struct iphdr *)skb_network_header(sock_buff);                   
            printk(KERN_ALERT "ipptr is ZERO\n"); 
            return NF_ACCEPT; 
    if(ipptr->saddr == in_aton(""))
         print_string("packet dropped(\n");             
         return NF_DROP; 
    return NF_ACCEPT;
int init_module()
        netfilter_ops_in.hook                   =       (nf_hookfn *)main_hook;                     =       PF_INET;
        netfilter_ops_in.hooknum                =       NF_IP_PRE_ROUTING;
        netfilter_ops_in.priority               =       NF_IP_PRI_FIRST;
        printk("firewall Setuped\n");
return 0;
void cleanup()
nf_unregister_hook(&netfilter_ops_in); /*unregister NF_IP_PRE_ROUTING hook*/


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WIMAX_MSG_ALLOC(9)						 Linux Networking						WIMAX_MSG_ALLOC(9)

wimax_msg_alloc - Create a new skb for sending a message to userspace SYNOPSIS
struct sk_buff * wimax_msg_alloc(struct wimax_dev * wimax_dev, const char * pipe_name, const void * msg, size_t size, gfp_t gfp_flags); ARGUMENTS
wimax_dev WiMAX device descriptor pipe_name "named pipe" the message will be sent to msg pointer to the message data to send size size of the message to send (in bytes), including the header. gfp_flags flags for memory allocation. RETURNS
0 if ok, negative errno code on error DESCRIPTION
Allocates an skb that will contain the message to send to user space over the messaging pipe and initializes it, copying the payload. Once this call is done, you can deliver it with wimax_msg_send. IMPORTANT
Don't use skb_push/skb_pull/skb_reserve on the skb, as wimax_msg_send depends on skb->data being placed at the beginning of the user message. Unlike other WiMAX stack calls, this call can be used way early, even before wimax_dev_add is called, as long as the wimax_dev->net_dev pointer is set to point to a proper net_dev. This is so that drivers can use it early in case they need to send stuff around or communicate with user space. COPYRIGHT
Kernel Hackers Manual 2.6. July 2010 WIMAX_MSG_ALLOC(9)

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