Netfilter的使用和实现

本文主要内容：Netfilter的原理和实现浅析，以及示例模块。

内核版本：2.6.37

Author：zhangskd @ csdn blog

概述

Netfilter为多种网络协议（IPv4、IPv6、ARP等）各提供了一套钩子函数。

在IPv4中定义了5个钩子函数，这些钩子函数在数据包流经协议栈的5个关键点被调用。

这就像有5个钓鱼台，在每个钓鱼台放了一个鱼钩(钩子函数)，把经过的数据包钓上来，

然后根据自定义的规则，来决定数据包的命运：

可以原封不动的放回IPv4协议，继续向上层递交；可以进行修改，再放回IPv4协议；也可以直接丢弃。

Netfilter主要采用连接跟踪(Connection Tracking)、包过滤(Packet Filtering)、地址转换(NAT)、包处理

(Packet Mangling)四种技术。

(1) IP层的5个钓鱼台

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
};

支持的协议类型：

enum {
    NFPROTO_UNSPEC = 0,
    NFPROTO_IPV4 = 2,
    NFPROTO_ARP = 3,
    NFPROTO_BRIDGE = 7,
    NFPROTO_IPV6 = 10,
    NFPROTO_DECNET = 12,
    NFPROTO_NUMPROTO,
};

(2) 钩子函数

typedef unsigned int nf_hookfn(unsigned int hooknum,
                               struct sk_buff *skb,
                               const struct net_device *in,
                               const struct net_device *out,
                               int (*okfn) (struct sk_buff *));

/* 处理函数返回值 */
#define NF_DROP 0 /* drop the packet, don't continue traversal */
#define NF_ACCEPT 1 /* continue traversal as normal */
#define NF_STOLEN 2 /* I've taken over the packet, don't continue traversal */
#define NF_QUEUE 3 /* queue the packet (usually for userspace handling) */
#define NF_REPEAT 4 /* call this hook again */
#define NF_STOP 5
#define NF_MAX_VERDICT NF_STOP

(3) Netfilter实体

在使用Netfilter时，需要定义一个nf_hook_ops实例。

struct nf_hook_ops {
    struct list_head list;
    /* User fills in from here down. */
    nf_hookfn *hook; /* 要注册的钩子函数 */
    struct module *owner;
    u_int8_t pf; /* 协议类型 */
    unsigned int hooknum; /* 哪个钓鱼台 */
    /* Hooks are ordered in asending priority. */
    int priority; /* 数值越小，优先级越高 */
};
typedef __u8 u_int8_t;

(4) 注册与注销

/* Functions to register/unregister hook points. */
int nf_register_hook(struct nf_hook_ops *reg);
void nf_unregister_hook(struct nf_hook_ops *reg);

实现

Netfilter定义了一个全局链表：

struct list_head nf_hooks[NFPROTO_NUMPROTO][NF_MAX_HOOKS];
EXPORT_SYMBOL(nf_hooks);
static DEFINE_MUTEX(nf_hook_mutex);

(1) 注册函数

注册函数会把nf_hook_ops放入nf_hooks相应的位置中。

int nf_register_hook(struct nf_hook_ops *reg)
{
    struct nf_hook_ops *elem;
    int err;

    err = mutex_lock_interruptible(&nf_hook_mutex);
    if (err < 0)
        return err;

    list_for_each_entry(elem, &nf_hooks[reg->pf][reg->hooknum], list) {
        if (reg->priority < elem->priority)
            break;
    }

    list_add_rcu(&reg->list, elem->list.prev); /* 把netfilter实例添加到队列中 */
    mutex_unlock(&nf_hook_mutex);
    return 0;
}

(2) 注销函数

void nf_unregister_hook(struct nf_hook_ops *reg)
{
    mutex_lock(&nf_hook_mutex);
    list_del_rcu(&reg->list); /* 把netfilter实例从队列中删除 */
    mutex_unlock(&nf_hook_mutex);
    synchronize_net();
}

(3) 内核接口

内核的Netfilter钩子函数调用：

NF_HOOK

|--> NF_HOOK_THRESH

|--> nf_hook_thresh

|--> nf_hook_slow

|--> nf_iterate

static inline int NF_HOOK(uint8_t pf, unsigned int hook, struct sk_buff *skb,
    struct net_device *in, struct net_device *out, int (*okfn)(struct sk_buff *))
{
    /* INT_MIN表示要调用钓鱼台的所有钩子函数 */
    return NF_HOOK_THRESH(pf, hook, skb, in, out, okfn, INT_MIN);
}

static inline int NF_HOOK_THRESH(uint8_t pf, unsigned int hook, struct sk_buff *skb,
    struct net_device *in, struct net_device *out, int (*okfn)(struct sk_buff *), int thresh)
{
    int ret = nf_hook_thresh(pf, hook, skb, in, out, okfn, thresh);
    if (ret == 1)
        ret = okfn(skb); /* 如果skb没被处理掉，调用此函数 */

    return ret;
}

/**
 * nf_hook_thresh - call a netfilter hook
 * Returns 1 if the hook has allowed the packet to pass.
 * The function okfn must be invoked by the caller in this case.
 * Any other return value indicates the packet has been consumed by the hook.
 */
static inline int nf_hook_thresh(u_int8_t pf, unsigned int hook, struct sk_buff *skb,
    struct net_device *indev, struct net_device *outdev, int (*okfn)(struct sk_buff *), int thresh)
{
#ifndef CONFIG_NETFILTER_DEBUG
    /* 如果协议pf的hook点上没有已注册的nf_hook_ops实例，直接返回1 */
    if (list_empty(&nf_hooks[pf][hook]))
        return 1;
#endif

    return nf_hook_slow(pf, hook, skb, indev, outdev, okfn, thresh);
}

/* Returns 1 if okfn() needs to be executed by the caller, -EPERM for NF_DROP, 0 otherwise. */
int nf_hook_slow(u_int8_t pf, unsigned int hook, struct sk_buff *skb, struct net_device *indev,
    struct net_device *outdev, int (*okfn)(struct sk_buff *), int hook_thresh)
{
    struct list_head *elem;
    unsigned int verdict;
    int ret = 0;

    /* We may already have this, but read-locks nest anyway */
    rcu_read_lock();

    elem = &nf_hooks[pf][hook];
next_hook:
    verdict = nf_iterate(&nf_hooks[pf][hook], skb, hook, indev, outdev, &elem, okfn, hook_thresh);

    if (verdict == NF_ACCEPT || verdict == NF_STOP) {
        ret = 1;
    } else if (verdict == NF_DROP) {
        kfree_skb(skb);
        ret = -EPERM;
    } else if ((verdict & NF_VERDICT_MASK) == NF_QUEUE) {
        if (! nf_queue(skb, elem, ph, hook, indev, outdev, okfn, verdict >> NF_VERDICT_BITS))
            goto next_hook;
    }

    rcu_read_unlock();

    return ret;
}

unsigned int nf_iterate(struct list_head *head, struct sk_buff *skb, unsigned int hook,
    const struct net_device *indev, const struct net_device *outdev, struct list_head **i,
    int (*okfn)(struct sk_buff *), int hook_thresh)
{
    unsigned int verdict;

    /*
     * The caller must not block between calls to this function because of risk of
     * continuing from deleted element.
     */
    list_for_each_continue_rcu(*i, head) {
        struct nf_hook_ops *elem = (struct nf_hook_ops *) *i;

        /* 优先级>=hook_thresh的都会被执行 */
        if (hook_thresh > elem_priority)
            continue;

        verdict = elem->hook(hook, skb, indev, outdev, okfn); /* 已注册的执行函数 */

        if (verdict != NF_ACCEPT) {
#ifdef CONFIG_NETFILTER_DEBUG
            if (unlikely((verdict & NF_VERDICT_MASK) > NF_MAX_VERDICT)) {
                NFDEBUG("Evil return from %p(%u).\n", elem->hook, hook);
                continue;
            }
#endif

            if (verdict != NF_REPEAT)
                return verdict;
            *i = (*i)->prev;
        }
    }

    return NF_ACCEPT;
}

使用

以下是一个简单的模块，加载到一个HTTP服务器上。

通过在PRE_ROUTING处注册my_hookfn，改变接收数据包的源IP为8.8.8.8（Google DNS server）。

当客户端向服务器发送一个请求时，肯定收不到服务器的响应：）

#include <linux/netfilter.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/netfilter_ipv4.h>
#include <linux/ip.h>
#include <linux/inet.h>

/**
 * Hook function to be called.
 * We modify the packet's src IP.
 */
unsigned int my_hookfn(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;
    iph = ip_hdr(skb);

    /* log the original src IP */
    printk(KERN_INFO"src IP %pI4\n", &iph->saddr);

    /* modify the packet's src IP */
    iph->saddr = in_aton("8.8.8.8");

    return NF_ACCEPT;
}

/* A netfilter instance to use */
static struct nf_hook_ops nfho = {
    .hook = my_hookfn,
    .pf = PF_INET,
    .hooknum = NF_INET_PRE_ROUTING,
    .priority = NF_IP_PRI_FIRST,
    .owner = THIS_MODULE,
};

static int __init sknf_init(void)
{
    if (nf_register_hook(&nfho)) {
        printk(KERN_ERR"nf_register_hook() failed\n");
        return -1;
    }
    return 0;
}

static void __exit sknf_exit(void)
{
    nf_unregister_hook(&nfho);
}

module_init(sknf_init);
module_exit(sknf_exit);
MODULE_AUTHOR("zhangsk");
MODULE_LICENSE("GPL");