如何在内核空间中附加数据包?

我试图在内核空间的数据包上附加一些数据.我有一个echo客户端和服务器.我输入命令行,如:./client“message”,服务器只是回复它.服务器使用./server运行.

现在,客户端和服务器位于两台不同的计算机上(可能是VM).我正在编写一个在客户机上运行的内核模块.它的工作是在数据包离开机器时在“消息”之后附加“12345”.我将在下面介绍代码.

/*
 * 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");

我想从内核空间发送客户端机器时将“消息”变为“message12345”.这样服务器将获得“message12345”并回显它,客户端将只读取“message12345”但是我遇到了skb_put()和skb_add_data()函数的问题.我不明白我犯了什么错误.如果有人可以帮我解决这些问题,我将非常感激.提前致谢.为方便起见,我也给了Makefile.这适用于分发内核,而不适用于构建的内核.

#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

现在我完全相信
    skb-> end – skb-> tail
是如此之小,我将不得不在内核空间中创建新的数据包.我用过
    alloc_skb()
    skb_reserve()
    skb_header_pointer()
以及用于创建新skb的其他有用的skb函数,但我想到的是如何在数据包流路径中路由新创建的数据包.如何使用
    ip_route_me_harder()
我查看了xtables-addons包中的建议,但是他们使用的函数与linux内核中的函数不同.任何建议都受到欢迎.

最佳答案
大约一年前的内核2.6.26我这样做了:

// Do we need extra space?
if(len - skb_tailroom(skb) > 0){

  // Expand skb tail until we have enough room for the extra data
  if (pskb_expand_head(skb, 0, extra_data_len - skb_tailroom(skb), GFP_ATOMIC)) {
    // allocation failed. Do whatever you need to do
  }

  // Allocation succeeded

  // Reserve space in skb and return the starting point
  your_favourite_structure* ptr = (your_favourite_structure*) 
                                  skb_push(skb, sizeof(*ptr)); 

  // Now either set each field of your structure or memcpy into it.
  // Remember you can use a char*

}

别忘了:

>重新计算UDP校验和,因为您更改了传输数据中的数据.
>更改ip标头中的字段tot_len(总长度),因为您已将数据添加到数据包.
>重新计算IP标头校验和,因为您更改了tot_len字段.

额外说明:
这只是一件简单的事情.我在你的代码中看到你将tmp分配为一个200字节的数组并使用它来存储你的消息数据.如果你发送一个更大的数据包,你将很难调试这个,因为内存崩溃是因为内存崩溃太痛苦了.

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