分类: 网络与安全
2012-07-26 17:27:28
内容基本上来自两篇文章:
《Netfilter源码分析》—(独孤九贱)
《Linux Netfilter实现机制和扩展技术》——(杨沙洲 国防科技大学计算机学院)
一、 IP报文的接收到hook函数的调用
1.1 ip_input.c ip_rcv()函数
以接收到的报文为例,类似的还有ip_forward(ip_forward.c)和ip_output(ip_output.c)
int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt, struct net_device *orig_dev)
{
struct iphdr *iph; //定义一个ip报文的数据报头
u32 len;
if (skb->pkt_type == PACKET_OTHERHOST)
goto drop; //数据包不是发给我们的
IP_INC_STATS_BH(IPSTATS_MIB_INRECEIVES); //收到数据包统计量加1
if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL)
{
/* 如果数据报是共享的,则复制一个出来,此时复制而出的已经和socket脱离了关系 */
IP_INC_STATS_BH(IPSTATS_MIB_INDISCARDS);
goto out;
}
if (!pskb_may_pull(skb, sizeof(struct iphdr)))
goto inhdr_error; //对数据报的头长度进行检查,
iph = skb->nh.iph; //取得数据报的头部位置
if (iph->ihl < 5 || iph->version != 4) //版本号或者头长度不对,
goto inhdr_error; //头长度是以4字节为单位的,所以5表示的是20字节
if (!pskb_may_pull(skb, iph->ihl*4))
goto inhdr_error;
if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))
goto inhdr_error; //检查报文的检验和字段
len = ntohs(iph->tot_len);
if (skb->len < len || len < (iph->ihl*4))
goto inhdr_error; //整个报文长度不可能比报头长度小
if (pskb_trim_rcsum(skb, len))
{ //对数据报进行裁减,这样可以分片发送过来的数据报不会有重复数据
IP_INC_STATS_BH(IPSTATS_MIB_INDISCARDS);
goto drop;
}
return NF_HOOK(PF_INET, NF_IP_PRE_ROUTING, skb, dev, NULL,
ip_rcv_finish); //通过回调函数调用ip_rcv_finish
inhdr_error:
IP_INC_STATS_BH(IPSTATS_MIB_INHDRERRORS);
drop:
kfree_skb(skb); //丢掉数据报
out:
return NET_RX_DROP;
}
1.2 include/linux/netfilter.h NF_HOOK宏
#ifdef CONFIG_NETFILTER_DEBUG
#define NF_HOOK(pf, hook, skb, indev, outdev, okfn) \
nf_hook_slow((pf), (hook), (skb), (indev), (outdev), (okfn), INT_MIN)
#define NF_HOOK_THRESH nf_hook_slow
#else
#define NF_HOOK(pf, hook, skb, indev, outdev, okfn) \
(list_empty(&nf_hooks[(pf)][(hook)]) \
? (okfn)(skb) \
: nf_hook_slow((pf), (hook), (skb), (indev), (outdev), (okfn), INT_MIN))
#define NF_HOOK_THRESH(pf, hook, skb, indev, outdev, okfn, thresh) \
(list_empty(&nf_hooks[(pf)][(hook)]) \
? (okfn)(skb) \
: nf_hook_slow((pf), (hook), (skb), (indev), (outdev), (okfn), (thresh)))
#endif
/* 如果nf_hooks[PF_INET][NF_IP_FORWARD]所指向的链表为空(即该钩子上没有挂处理函数),则直接调用okfn;否则,则调用net/core/netfilter.c::nf_hook_slow()转入Netfilter的处理。 */
1.3 net/core/netfilter.c nf_kook_slow()函数
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;
rcu_read_lock();
/*取得对应的链表首部*/
elem = &nf_hooks[pf][hook];
next_hook:
/*调用对应的钩子函数*/
verdict = nf_iterate(&nf_hooks[pf][hook], pskb, hook, indev,
outdev, &elem, okfn, hook_thresh);
/*判断返回值,做相应的处理*/
if (verdict == NF_ACCEPT || verdict == NF_STOP) {
ret = 1; /*前面提到过,返回1,则表示装继续调用okfn函数指针*/
goto unlock;
} else if (verdict == NF_DROP) {
kfree_skb(*pskb); /*删除数据包,需要释放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;
}
1.4 net/core/netfilter.c nf_iterate()函数
static unsigned int nf_iterate(struct list_head *head,
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)
{
/*
* The caller must not block between calls to this
* function because of risk of continuing from deleted element.
*/
/* 依次调用指定hook点下的所有nf_hook_ops->(*hook)函数,这些nf_hook_ops里有filter表注册的,有mangle表注册的,等等。
list_for_each_continue_rcu函数是一个for循环的宏,当调用结点中的hook函数后,根据返回值进行相应处理。如果hook函数的返回值是NF_QUEUE,NF_STOLEN,NF_DROP时,函数返回该值;如果返回值是NF_REPEAT时,则跳到前一个结点继续处理;如果是其他值,由下一个结点继续处理。如果整条链表处理完毕,返回值不是上面四个值,则返回NF_ACCEPT。*/
list_for_each_continue_rcu(*i, head) {
struct nf_hook_ops *elem = (struct nf_hook_ops *)*i;
if (hook_thresh > elem->priority)
continue;
switch (elem->hook(hook, skb, indev, outdev, okfn)) {
case NF_QUEUE:
return NF_QUEUE;
case NF_STOLEN:
return NF_STOLEN;
case NF_DROP:
return NF_DROP;
case NF_REPEAT:
*i = (*i)->prev;
break;
}
}
return NF_ACCEPT;
}