netfilter filter表(二)

news2024/10/5 18:22:32

这次继续分析filter表，不同与之前的分析方式，这次通过将内核中的数据打印出来，对比结构关系图来分析。这是本次分析涉及的几个数据结构：

struct xt_table {

struct list_head list;

/* What hooks you will enter on */

unsigned int valid_hooks;

/* Man behind the curtain... */

struct xt_table_info *private;

/* Set this to THIS_MODULE if you are a module, otherwise NULL */

struct module *me;

u_int8_t af; /* address/protocol family */

int priority; /* hook order */

/* called when table is needed in the given netns */

int (*table_init)(struct net *net);

/* A unique name... */

const char name[XT_TABLE_MAXNAMELEN];

};

struct xt_table_info {

/* Size per table */

unsigned int size;

/* Number of entries: FIXME. --RR */

unsigned int number;

/* Initial number of entries. Needed for module usage count */

unsigned int initial_entries;

/* Entry points and underflows */

unsigned int hook_entry[NF_INET_NUMHOOKS]; // hook_entry[NF_INET_LOCAL_IN]

unsigned int underflow[NF_INET_NUMHOOKS];

/*

* Number of user chains. Since tables cannot have loops, at most

* @stacksize jumps (number of user chains) can possibly be made.

*/

unsigned int stacksize;

void ***jumpstack;

unsigned char entries[0] __aligned(8);

};

依然用《netfilter filter表》中的方法，打印内核中的数据，这次hello_open的代码如下：

static int hello_open(struct inode* inode, struct file*filep)
{
	printk("hello_open\n");
	struct task_struct *tsk = current;
	struct net *net;
	struct xt_table *xt_filter;
	struct xt_table_info *filter_info;
	const void* table_base;
	int i = 0;
	int local_in_hook_entry;
	struct ipt_entry* ipt_entry;

	struct nsproxy *nsprx = tsk->nsproxy; //命名空间
	net = nsprx->net_ns;

	xt_filter = net->ipv4.iptable_filter;

	// 打印xt_table信息
	printk("xt_table: af - %d\n", xt_filter->af);
	printk("xt_table: name - %s\n", xt_filter->name);
	printk("xt_table: valid_hooks - %d\n", xt_filter->valid_hooks);
	printk("xt_table: priority - %d\n", xt_filter->priority);

	filter_info = xt_filter->private;
	if (NULL == filter_info)
	{
		printk("filter_info is null\n");
		return 0;
	}

	printk("filter_info: size - %d\n", filter_info->size);
	printk("filter_info: number - %d\n", filter_info->number); // 4?
	printk("filter_info: initial_entries - %d\n", filter_info->initial_entries);
	printk("filter_info: stacksize - %d\n", filter_info->stacksize);

	table_base = filter_info->entries;

	//local_in_hook_entry = filter_info->hook_entry[NF_INET_LOCAL_IN];
	// 打印hook_entry数组
	for (i = 0; i < NF_INET_NUMHOOKS; ++i)
	{
		printk("filter_info: hook_entry[%d] - %d\n", i, filter_info->hook_entry[i]);
	}

	for (i = 0; i < NF_INET_NUMHOOKS; ++i)
	{
		printk("filter_info: underflow[%d] - %d\n", i, filter_info->underflow[i]);
	}

	return 0;
}

根据打印的内核日志，整理了下面的图：

涉及的两个枚举类型:

enum {

NFPROTO_UNSPEC = 0,

NFPROTO_INET = 1,

NFPROTO_IPV4 = 2,

NFPROTO_ARP = 3,

NFPROTO_NETDEV = 5,

NFPROTO_BRIDGE = 7,

NFPROTO_IPV6 = 10,

NFPROTO_DECNET = 12,

NFPROTO_NUMPROTO,

};

enum nf_inet_hooks {

NF_INET_PRE_ROUTING,

NF_INET_LOCAL_IN,

NF_INET_FORWARD,

NF_INET_LOCAL_OUT,

NF_INET_POST_ROUTING,

NF_INET_NUMHOOKS

};

现在开始分析日志，其中以“#”开头或标蓝的为注释信息。

[ 3885.915747] hello_open

# 2对应的是NFPROTO_IPV4，将上面NFPROTO_IPV4的定义
[ 3885.915749] xt_table: af - 2
[ 3885.915749] xt_table: name - filter

#14实际是：

#define FILTER_VALID_HOOKS ((1 << NF_INET_LOCAL_IN) | \

(1 << NF_INET_FORWARD) | \

(1 << NF_INET_LOCAL_OUT))

[ 3885.915750] xt_table: valid_hooks - 14
[ 3885.915751] xt_table: priority - 0

#下面是xt_table_info信息

#xt_table_info后面，紧接着是4120大小的内存空间，存放ipt_standard和ipt_error
[ 3885.915751] filter_info: size - 4120

#一共25个ipt_standard/ipt_error对象
[ 3885.915752] filter_info: number - 25
[ 3885.915752] filter_info: initial_entries - 4
[ 3885.915753] filter_info: stacksize - 5
[ 3885.915753] filter_info: hook_entry[0] - -1
[ 3885.915754] filter_info: hook_entry[1] - 0
[ 3885.915754] filter_info: hook_entry[2] - 456
[ 3885.915755] filter_info: hook_entry[3] - 1720
[ 3885.915755] filter_info: hook_entry[4] - -1
[ 3885.915756] filter_info: underflow[0] - -1
[ 3885.915756] filter_info: underflow[1] - 304
[ 3885.915757] filter_info: underflow[2] - 1568
[ 3885.915757] filter_info: underflow[3] - 1720
[ 3885.915758] filter_info: underflow[4] - -1

xt_table_info的hook_entry和underflow需要单独说一下。内核网络栈有5个钩子挂载点，hook_entry[0]和underflow[0]分别表示第一个挂载点在filter表中配置存放的起始位置和结束位置，以此类推。

filter_info: hook_entry[0] - -1

underflow[0] - -1

表示第一个挂载点，filter表不处理此挂载点的数据，那第一个挂载点是哪个呢？就是NF_INET_PRE_ROUTING，见enum nf_inet_hooks的定义。

hook_entry[1] - 0

underflow[1] - 304

同理，这个表示第2个挂载点(NF_INET_LOCAL_IN)在filter表中配置项的起始位置和结束位置。

可以参考前面的图，加深对上述的理解。

再看下本机filter表INPUT链的配置：

Chain INPUT (policy ACCEPT)
target prot opt source destination
DROP tcp -- 1.2.3.5 0.0.0.0/0
ACCEPT all -- 1.2.3.4 0.0.0.0/0

INPUT链有两条配置，每个配置对应一个ipt_standard对象，每个ipt_standard对象大小为152字节。因此INPUT链配置存放的起始位置是0,结束位置是304。