http://www.skynet.org.cn/viewthread.php?tid=7&extra=page%3D2
作者:kendo
出處:SecPower[www.skynet.org.cn]
版權所有,轉載請註明作者與出處。
1、nf_hook_slow函數
Hook被註冊後,它就會在那裏守株待兔,等待自動送上門的數據包,那麼內核是如何調用到註冊的Hook的呢?在分析NF_HOOK的時候說過,如果指定協議的指定鉤子類型上註冊了鉤子函數數,會調用nf_hook_slow函數:
* -EPERM for NF_DROP, 0 otherwise. */
int nf_hook_slow(int pf, unsigned int hook, struct sk_buff **pskb,
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();
#ifdef CONFIG_NETFILTER_DEBUG
if (unlikely((*pskb)->nf_debug & (1 << hook))) {
printk("nf_hook: hook %i already set./n", hook);
nf_dump_skb(pf, *pskb);
}
(*pskb)->nf_debug |= (1 << hook);
#endif
/*取得對應的鏈表首部*/
elem = &nf_hooks[pf][hook];
next_hook:
/*調用對應的鉤子函數*/
verdict = nf_iterate(&nf_hooks[pf][hook], pskb, hook, indev,
outdev, &elem, okfn, hook_thresh);
/*判斷返回值,做相應的處理*/
if (verdict == NF_ACCEPT || verdict == NF_STOP) {
ret = 1; /*前面提到過,返回1,則表示裝繼續調用okfn函數指針*/
goto unlock;
} else if (verdict == NF_DROP) {
kfree_skb(*pskb); /*刪除數據包,需要釋放skb*/
ret = -EPERM;
} else if (verdict == NF_QUEUE) {
NFDEBUG("nf_hook: Verdict = QUEUE./n");
if (!nf_queue(*pskb, elem, pf, hook, indev, outdev, okfn))
goto next_hook;
}
unlock:
rcu_read_unlock();
return ret;
}
內核的數據轉發函數,調用宏NF_HOOK時,會告訴它協議簇和Hook類型,nf_hook_slow函數根據這兩個要素,可以很輕易地從nf_hooks 中取得對應的前面註冊好的Hook鏈表的首部:
elem = &nf_hooks[pf][hook];
然後,就調用函數nf_iterate ,遍歷Hook鏈表,調用鏈用上所有的Hook函數
2、nf_iterate 函數
struct sk_buff **skb,
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) { /*遍歷所有註冊的Hook*/
/*取得當前遍歷的Hook*/
struct nf_hook_ops *elem = (struct nf_hook_ops *)*i;
if (hook_thresh > elem->priority)
continue;
/* Optimization: we don't need to hold module
reference here, since function can't sleep. --RR */
/*調用Hook 的函數*/
` verdict = elem->hook(hook, skb, indev, outdev, okfn);
if (verdict != NF_ACCEPT) {
#ifdef CONFIG_NETFILTER_DEBUG
if (unlikely(verdict > 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;
}
我們可以看到,函數在一個鏈表的遍歷過程中,不斷地調用對應的Hook函數:
verdict = elem->hook(hook, skb, indev, outdev, okfn);
以執行我們註冊好的Hook,對數據包進行我們想要的處理。函數的返回值verdict決定了封包的命令,接受或丟棄,Netfilter共有6 個值:
#define NF_DROP 0 丟棄該數據包
#define NF_ACCEPT 1 保留該數據包
#define NF_STOLEN 2 記掉該數據包
#define NF_QUEUE 3 將該數據包插入到用戶空間
#define NF_REPEAT 4 再次調用該Hook函數
#define NF_STOP 5 停止檢測,不再進行下一個Hook函數
逐 個分析每一個Hook函數,就可以瞭解整個Netfilter的運行機理,這是我後面的重點內容。現在需要回答的問題是,如果同時註冊了N個Hook,它 們對封包的處理,有的是ACCEPT,有的是DROP,那麼如果前一個DROP了它,後面的Hook函數還會被執行嗎?來看返回值的處理代碼:
{
if (verdict != NF_REPEAT)
return verdict;
*i = (*i)->prev;
}
很明顯,如果是ACCEPT動作,那麼還會繼續調用下一個Hook,否則,當是REPEAT時,再返回前一個Hook,至於其它動作,則直接返回,不再斷續調用下一個Hook,換句話講,就是如果數據包已經被前一個Hook函數丟棄,當然不會再被交給下一個Hook函數。