我正在尝试从内核空间附加一些数据到数据包上。我有一个回声客户端和服务器。我在命令行中输入如下命令:./client "message",服务器就会将其回显。服务器是通过./server运行的。
现在,客户端和服务器位于两台不同的机器上(可能是虚拟机)。我正在编写一个运行在客户端机器上的内核模块。它的工作是在数据包离开机器时在"message"之后附加"12345"。我在下面展示代码。
/*
* 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");
我希望在从内核空间发送时,将“消息”变为“message12345”,以便服务器可以得到“message12345”并将其回显,客户端只需读取“message12345”。但是我在使用skb_put()和skb_add_data()函数时遇到了麻烦,不知道哪里出错了。如果有人能帮我检查代码,我将非常感激。提前致谢。我还提供了Makefile以方便使用,这是针对分布式内核而不是构建内核的。
#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
现在我非常确定skb->end - skb->tail太小了,以至于我必须在内核空间创建新的数据包。我已经使用了alloc_skb()、skb_reserve()、skb_header_pointer()和其他有用的skb函数来创建一个新的skb,但是我现在遇到的问题是如何在数据包流动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软件包以获取建议,但他们使用的函数与Linux内核中的不同。欢迎任何建议。