How to add package data from kernel space?

I am trying to add some package data from kernel space. I have an echo client and server. I enter the command line, for example: ./ client "message", and the server simply drives it away. The server was started from. / server.

Now the client and server are on two different machines (can be virtual machines). I am writing a kernel module that runs on a client machine. His job is to add “12345” after the “message” while the packet exits the machine. I present the code below.

/*
 * This is ibss_obsf_cat.c
 */

#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/kernel.h>
#include <linux/netfilter.h>
#include <linux/skbuff.h>
#include <linux/netdevice.h>
#include <linux/udp.h>
#include <linux/ip.h>

#undef __KERNEL__
#include <linux/netfilter_ipv4.h>
#define __KERNEL__


/*
 * Function prototypes ...
 */

static unsigned int cat_obsf_begin (unsigned int hooknum,
                struct sk_buff *skb,
                const struct net_device *in,
                const struct net_device *out,
                int (*okfn)(struct sk_buff *));

static void hex_dump (char str[], int len)
{

}

/*
 * struct nf_hook_ops instance initialization
 */

static struct nf_hook_ops cat_obsf_ops __read_mostly = {
    .pf = NFPROTO_IPV4,
    .priority = 1,
    .hooknum = NF_IP_POST_ROUTING,
    .hook = cat_obsf_begin,
};

/*
 * Module init and exit functions. 
 * No need to worry about that.
 */

static int __init cat_obsf_init (void)
{
    printk(KERN_ALERT "cat_obsf module started...\n");
    return nf_register_hook(&cat_obsf_ops);
}

static void __exit cat_obsf_exit (void)
{
    nf_unregister_hook(&cat_obsf_ops);
    printk(KERN_ALERT "cat_obsf module stopped...\n");
}

/*
 * Modification of the code begins here. 
 * Here are all the functions and other things.
 */

static unsigned int cat_obsf_begin (unsigned int hooknum,
                struct sk_buff *skb,
                const struct net_device *in,
                const struct net_device *out,
                int (*okfn)(struct sk_buff *))
{ 
    struct iphdr *iph;
    struct udphdr *udph;
    unsigned char *data;
    unsigned char dt[] = "12345";
    unsigned char *tmp;
    unsigned char *ptr;

    int i, j, len;

    if (skb){
        iph = ip_hdr(skb);

        if (iph && iph->protocol && (iph->protocol == IPPROTO_UDP)){
            udph = (struct udphdr *) ((__u32 *)iph + iph->ihl);
            data = (char *)udph + 8;

            if(ntohs(udph->dest) == 6000){
                for (i=0; data[i]; i++);
                len = i;

                //printk(KERN_ALERT "\nData length without skb: %d", len);
                //printk(KERN_ALERT "Data is: %s", data);
                //printk(KERN_ALERT "dt size: %lu", sizeof(dt));
                //printk(KERN_ALERT "skb->len: %d", skb->len);
                tmp = kmalloc(200*sizeof(char), GFP_KERNEL);

                memcpy(tmp, data, len);
                ptr = tmp + len;
                memcpy(ptr, dt, sizeof(dt));

                printk(KERN_ALERT "tmp: %s", tmp);


                printk(KERN_ALERT "skb->tail: %d", skb->tail);
                //skb_put(skb, sizeof(dt));
                printk(KERN_ALERT "skb->end: %d", skb->end);
                printk(KERN_ALERT "skb->tail: %d", skb->tail);
                printk(KERN_ALERT "skb->tail(int): %d", (unsigned int)skb->tail);

                //memset(data, 0, len + sizeof(dt));

                //memcpy(data, tmp, len + sizeof(dt));

                //skb_add_data(skb, tmp, len+sizeof(dt));

                printk(KERN_ALERT "Now data is: %s", data);
                for(i=0; data[i]; i++);
                printk(KERN_ALERT "data length: %d", i);

                kfree(tmp);

            }       
        }   
    }
    return NF_ACCEPT;
}

/*
 * Nothing to be touched hereafter
 */

module_init(cat_obsf_init);
module_exit(cat_obsf_exit);

MODULE_AUTHOR("Rifat");
MODULE_DESCRIPTION("Module for packet mangling");
MODULE_LICENSE("GPL");

, "" "message12345" . "message12345" , "message12345" . skb_put() skb_add_data(). , . - , . . Makefile . , .

#If KERNELRELEASE is defined, we've been invoked from the
#kernel build system and use its language
ifneq ($(KERNELRELEASE),)
    obj-m := ibss_obsf_cat.o

#Otherwise we were called directly from the command
#line; invoke the kernel build system.
else

    KERNELDIR ?= /lib/modules/$(shell uname -r)/build
    PWD := $(shell pwd)

default:
    $(MAKE) -C $(KERNELDIR) M=$(PWD) modules

endif

,   skb- > end - skb- > tail , .   alloc_skb()   skb_reserve()   skb_header_pointer() skb skb, , . |   ip_route_me_harder() xtables-addons , , , , linux. .