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netfilter-在内核态操作网络数据包

一.概述                                                  

netfilter是自2.4内核的一个数据包过滤框架。可以过滤数据包网络地址和端口转换(nat和napt技术),以及其他操作数据包的功能。主要工作原理是在内核模块注册回调函数(hook函数)到内核,内核执行到相关点时会触发这个回调函数,然后根据回调函数里的逻辑,对包含网络协议栈的sk_buff结构进行操作!!!iptables的内核模块就是使用的netfilter。netfilter官方网站:http://www.netfilter.org/

二.基本函数接口                                    

1.把hook函数注册到内核需要使用结构体nf_hook_ops来关联,该结构体定义在内核头文件目录的linux/netfilter.h里面,这里以2.6.39内核为例:

 1 struct nf_hook_ops {
 2     struct list_head list;
 3 
 4     /* User fills in from here down. */
 5     nf_hookfn *hook;
 6     struct module *owner;
 7     u_int8_t pf;
 8     unsigned int hooknum;
 9     /* Hooks are ordered in ascending priority. */
10     int priority;
11 };

nf_hookfn就是要填充的hook函数。pf时8位协议族,如ipv4就是PF_INET。hooknum是指定hook函数注册的数据包的路径地点。priority是该hook的优先级,当有多个hook在同一个点注册的时候,会按照优先级来执行hook。

2.nf_hookfn函数声明也是在linux/netfilter.h里面:

1 typedef unsigned int nf_hookfn(unsigned int hooknum,
2                    struct sk_buff *skb,
3                    const struct net_device *in,
4                    const struct net_device *out,
5                    int (*okfn)(struct sk_buff *));

sk_buff是内核维护的网络协议栈结构,里面可以取出各协议栈信息。该函数的返回值

 

1 /* Responses from hook functions. */
2 #define NF_DROP 0
3 #define NF_ACCEPT 1
4 #define NF_STOLEN 2
5 #define NF_QUEUE 3
6 #define NF_REPEAT 4
7 #define NF_STOP 5

NF_ACCEPT:接收数据包,由内核继续正常的报文传送

NF_DROP:丢弃数据包

NF_STOLEN:数据包的操作全部由hook函数处理

NF_QUEUE:将报文入队,通常交由用户程序处理

NF_REPEAT:再次调用该hook函数。

3.hooknum的5个通用注册点也是定义在linux/netfilter.h

1 enum nf_inet_hooks {
2     NF_INET_PRE_ROUTING,
3     NF_INET_LOCAL_IN,
4     NF_INET_FORWARD,
5     NF_INET_LOCAL_OUT,
6     NF_INET_POST_ROUTING,
7     NF_INET_NUMHOOKS
8 };

NF_INET_PRE_ROUTING:系统收到数据后,且没有经过路由
NF_INET_LOCAL_IN:系统收到数据,经过路由后,如果数据的目标地址是本机就经过该点
NF_INET_FORWARD:系统收到数据,经过路由后,如果数据的目标地址是其他地方就经过该点
NF_INET_LOCAL_OUT:系统发送数据时,未经过路由
NF_INET_POST_ROUTING:系统发送数据时,经过了路由阶段,马上就发出去了

对于ipv4的注册点值跟上面一样,只是用的宏定义:

 1 /* IP Hooks */
 2 /* After promisc drops, checksum checks. */
 3 #define NF_IP_PRE_ROUTING    0
 4 /* If the packet is destined for this box. */
 5 #define NF_IP_LOCAL_IN        1
 6 /* If the packet is destined for another interface. */
 7 #define NF_IP_FORWARD        2
 8 /* Packets coming from a local process. */
 9 #define NF_IP_LOCAL_OUT        3
10 /* Packets about to hit the wire. */
11 #define NF_IP_POST_ROUTING    4
12 #define NF_IP_NUMHOOKS        5
13 #endif /* ! __KERNEL__ */

 4.注册和撤销注册函数:

1 /* Function to register/unregister hook points. */
2 int nf_register_hook(struct nf_hook_ops *reg);
3 void nf_unregister_hook(struct nf_hook_ops *reg);

参数都是nf_hook_ops结构指针。

三.简单例子                                          

我们用一个简单例子���察vxlan的数据包在各hook点的ip情况,vxlan环境可以参考Docker+OpenvSwitch搭建VxLAN实验环境

本例子是内核模块编程,可以参考初探Linux内核编程,参数传递以及模块间函数调用

  1 /**
  2  * @file netfilter_hook.c
  3  */
  4 
  5 #include <linux/module.h>
  6 #include <linux/kernel.h>
  7 
  8 #include <linux/ip.h>
  9 #include <linux/netfilter_ipv4.h>
 10 
 11 MODULE_LICENSE("Dual BSD/GPL");
 12 MODULE_AUTHOR("yuuyuu");
 13 MODULE_DESCRIPTION("netfilter");
 14 MODULE_VERSION("1.0");
 15 
 16 /* 打印点分制ip地址 */
 17 #define printk_ip(info, be32_addr) \
 18     printk("%s %d.%d.%d.%d\n", \
 19     info, \
 20     ((unsigned char *)&(be32_addr))[0], \
 21     ((unsigned char *)&(be32_addr))[1], \
 22     ((unsigned char *)&(be32_addr))[2], \
 23     ((unsigned char *)&(be32_addr))[3])
 24 
 25 int filter_ip(__be32 addr)
 26 {
 27     unsigned char net_num = ((unsigned char *)&addr)[0];
 28     unsigned char host_num = ((unsigned char *)&addr)[3];
 29     if (net_num == 10 || host_num == 1 || host_num == 2)
 30         return 1;
 31     return 0;
 32 }
 33 
 34 int filter_src_dst_ip(__be32 s_addr, __be32 d_addr)
 35 {
 36     int i = filter_ip(s_addr) && filter_ip(d_addr);
 37     return i;
 38 }
 39 
 40 /* NF_INET_PRE_ROUTING */
 41 unsigned int pre_routing_hook(unsigned int hooknum, struct sk_buff *skb,
 42                                 const struct net_device *in, const struct net_device *out,
 43                                 int (*okfn)(struct sk_buff *))
 44 {
 45     struct iphdr *ip_header;
 46 
 47     ip_header = ip_hdr(skb);
 48     if (filter_src_dst_ip(ip_header->saddr, ip_header->daddr))
 49     {
 50         printk("pre_routing_hook()==================================\n");
 51         printk_ip("src ip:", ip_header->saddr);
 52         printk_ip("dst ip:", ip_header->daddr);
 53     }
 54 
 55     return NF_ACCEPT;
 56 }
 57 
 58 struct nf_hook_ops pre_routing_ops =
 59 {
 60     .hook = pre_routing_hook,
 61     .pf = PF_INET,
 62     .hooknum = NF_INET_PRE_ROUTING,
 63     .priority = NF_IP_PRI_FIRST
 64 };
 65 
 66 /* NF_INET_LOCAL_IN */
 67 unsigned int local_in_hook(unsigned int hooknum, struct sk_buff *skb,
 68                                 const struct net_device *in, const struct net_device *out,
 69                                 int (*okfn)(struct sk_buff *))
 70 {
 71     struct iphdr *ip_header;
 72 
 73     ip_header = ip_hdr(skb);
 74     if (filter_src_dst_ip(ip_header->saddr, ip_header->daddr))
 75     {
 76         printk("local_in_hook()========================================\n");
 77         printk_ip("src ip:", ip_header->saddr);
 78         printk_ip("dst ip:", ip_header->daddr);
 79     }
 80 
 81     return NF_ACCEPT;
 82 }
 83 
 84 struct nf_hook_ops local_in_ops =
 85 {
 86     .hook = local_in_hook,
 87     .pf = PF_INET,
 88     .hooknum = NF_INET_LOCAL_IN,
 89     .priority = NF_IP_PRI_FIRST
 90 };
 91 
 92 /* NF_INET_FORWARD */
 93 unsigned int forward_hook(unsigned int hooknum, struct sk_buff *skb,
 94                                 const struct net_device *in, const struct net_device *out,
 95                                 int (*okfn)(struct sk_buff *))
 96 {
 97     struct iphdr *ip_header;
 98 
 99     ip_header = ip_hdr(skb);
100     if (filter_src_dst_ip(ip_header->saddr, ip_header->daddr))
101     {
102         printk("forward_hook=========================================\n");
103         printk_ip("src ip:", ip_header->saddr);
104         printk_ip("dst ip:", ip_header->daddr);
105     }
106 
107     return NF_ACCEPT;
108 }
109 
110 struct nf_hook_ops forward_ops =
111 {
112     .hook = forward_hook,
113     .pf = PF_INET,
114     .hooknum = NF_INET_FORWARD,
115     .priority = NF_IP_PRI_FIRST
116 };
117 
118 /* NF_INET_LOCAL_OUT */
119 unsigned int local_out_hook(unsigned int hooknum, struct sk_buff *skb,
120                                 const struct net_device *in, const struct net_device *out,
121                                 int (*okfn)(struct sk_buff *))
122 {
123     struct iphdr *ip_header;
124 
125     ip_header = ip_hdr(skb);
126     if (filter_src_dst_ip(ip_header->saddr, ip_header->daddr))
127     {
128         printk("local_out_hook===========================================\n");
129         printk_ip("src ip:", ip_header->saddr);
130         printk_ip("dst ip:", ip_header->daddr);
131     }
132 
133     return NF_ACCEPT;
134 }
135 
136 struct nf_hook_ops local_out_ops =
137 {
138     .hook = local_out_hook,
139     .pf = PF_INET,
140     .hooknum = NF_INET_LOCAL_OUT,
141     .priority = NF_IP_PRI_FIRST
142 };
143 
144 /* NF_INET_POST_ROUTING */
145 unsigned int post_routing_hook(unsigned int hooknum, struct sk_buff *skb,
146                                 const struct net_device *in, const struct net_device *out,
147                                 int (*okfn)(struct sk_buff *))
148 {
149     struct iphdr *ip_header;
150 
151     ip_header = ip_hdr(skb);
152     if (filter_src_dst_ip(ip_header->saddr, ip_header->daddr))
153     {
154         printk("post_routing_hook====================================\n");
155         printk_ip("src ip:", ip_header->saddr);
156         printk_ip("dst ip:", ip_header->daddr);
157     }
158 
159     return NF_ACCEPT;
160 }
161 
162 struct nf_hook_ops post_routing_ops =
163 {
164     .hook = post_routing_hook,
165     .pf = PF_INET,
166     .hooknum = NF_INET_POST_ROUTING,
167     .priority = NF_IP_PRI_FIRST
168 };
169 
170 /* 注册 */
171 static int hook_init(void)
172 {
173     printk("hook_init()======================\n");
174     nf_register_hook(&pre_routing_ops);
175     nf_register_hook(&local_in_ops);
176     nf_register_hook(&forward_ops);
177     nf_register_hook(&local_out_ops);
178     nf_register_hook(&post_routing_ops);
179 
180     return 0;
181 }
182 
183 static void hook_exit(void)
184 {
185     printk("hook_exit()=====================\n");
186     nf_unregister_hook(&pre_routing_ops);
187     nf_unregister_hook(&local_in_ops);
188     nf_unregister_hook(&forward_ops);
189     nf_unregister_hook(&local_out_ops);
190     nf_unregister_hook(&post_routing_ops);
191 }
192 
193 module_init(hook_init);
194 module_exit(hook_exit);

第27,34行简单过滤下本环境的IP,方便查看结果。

Makefile文件

1 KERNEL_DIR ?= /lib/modules/$(shell uname -r)/build
2 PWD := $(shell pwd)
3 
4 obj-m := netfilter_hook.o
5 
6 default:
7     $(MAKE) -C $(KERNEL_DIR) M=$(PWD) modules

四.验证                                                  

vxlan环境:

host1:eth0:192.168.2.1,虚拟网桥10.1.2.10

host2:eth0:192.168.2.2,虚拟网桥10.1.2.11

编译后插入模块,在host1里面ping一次host2的虚拟网桥10.1.2.11

1 ping 10.2.1.11 -c 1

然后参看内核输出信息:

1 sudo dmesg

可以看到vxlan的icmp请求有2个IP在工作,分别经过local_out_hookpost_routing_hook

同理host2的icmp应答进入本机也是2个IP都经过了pre_routing_hooklocal_in_hook

本文永久更新链接地址:http://www.linuxidc.com/Linux/2016-02/128396.htm

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