Netfilter学习之NAT类型动态配置（七）全锥型NAT内核空间实现

男娘i 2022-05-28 13:43 276阅读 0赞

本文主要实现全锥型NAT的内核空间iptables命令行扩展对应的钩子函数及其功能的实现。实现思路见上文。

### 1.关键部分实现代码 ###

（1）建立ipt\_FULLCONE.c以激活钩子函数，关键在于保持和用户空间libipt的一致性。

static struct xt_target fullcone_tg_reg __read_mostly = {
    	.name		= "FULLCONE",
    	.family		= NFPROTO_IPV4,
    	.target		= fullcone_tg,
    	.targetsize	= sizeof(struct nf_nat_ipv4_multi_range_compat),
    	.table		= "nat",
    	.hooks		= 1 << NF_INET_PRE_ROUTING,
    	.checkentry	= fullcone_tg_check,
    	.destroy	= fullcone_tg_destroy,
    	.me		= THIS_MODULE,
    };

在fullcone\_tg中调用nf\_nat\_fullcone\_ipv4

static unsigned int
    fullcone_tg(struct sk_buff *skb, const struct xt_action_param *par)
    {
    	struct nf_nat_range range;
    	const struct nf_nat_ipv4_multi_range_compat *mr;
    
    	mr = par->targinfo;
    	range.flags = mr->range[0].flags;
    	range.min_proto = mr->range[0].min;
    	range.max_proto = mr->range[0].max;
    
    	return nf_nat_fullcone_ipv4(skb, xt_hooknum(par), &range,
    				      xt_out(par));
    }

（2）在nf\_nat\_fullcone\_ipv4.c中实现具体功能

unsigned int
    nf_nat_fullcone_ipv4(struct sk_buff *skb, unsigned int hooknum,
    		    const struct nf_nat_range *range,
    		    const struct net_device *out)
    {
    	struct nf_conn *ct;
    	struct nf_conn_nat *nat;
    	enum ip_conntrack_info ctinfo;
    	struct nf_nat_range newrange;
    	__be32 newdst;
    
    	//we need to monitor packets at prerouting and put dst to nf_nat_setup_info
    	NF_CT_ASSERT(hooknum == NF_INET_PRE_ROUTING);
    
    	ct = nf_ct_get(skb, &ctinfo);		//get infomationn from sockets
    	nat = nfct_nat(ct);
    
    	NF_CT_ASSERT（ct && (ctinfo == IP_CT_NEW || ctinfo == IP_CT_RELATED || 
    			   ctinfo == IP_CT_RELATED_REPLY)）；
    				
    	/* Source address is 0.0.0.0 - locally generated packet that is
    	 * probably not supposed to be masqueraded.
    	 */
    	if (ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u3.ip == 0)
    		return NF_ACCEPT;
    	
    	newdst = nf_nat_fullcone_match(ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple);
    
    	if (newdst)
    	{
    		//transfer from original range
    		memset(&newrange.min_addr, 0, sizeof(newrange.min_addr));
    		memset(&newrange.max_addr, 0, sizeof(newrange.max_addr));
    		newrange.flags = range->flags | NF_NAT_RANGE_MAP_IPS;
    		newrange.min_addr.ip = newdst;
    		newrange.max_addr.ip = newdst;
    		newrange.min_proto = range->min_proto;
    		newrange.max_proto = range->max_proto;
    		
    		//Hnad modified range to generic setup. Change dst by normal way.
    		return nf_nat_setup_info(ct, &newrange, NF_NAT_MANIP_DST);
    	}
    	else
    		return NF_ACCEPT;			
    }
    EXPORT_SYMBOL_GPL(nf_nat_fullcone_ipv4);

（3）进入nf\_nat\_core.c中，调用nf\_nat\_setup\_info，详细源码介绍可见前文《[Netfilter学习之NAT类型动态配置（四）nf\_nat\_core.c源码解析》][Netfilter_NAT_nf_nat_core.c]

改动代码如下：

unsigned int
    nf_nat_setup_info(struct nf_conn *ct,
    		  const struct nf_nat_range *range,
    		  enum nf_nat_manip_type maniptype)
    {
    	struct net *net = nf_ct_net(ct);
    	struct nf_conntrack_tuple curr_tuple, new_tuple;
    
    	/* Can't setup nat info for confirmed ct. */
    	if (nf_ct_is_confirmed(ct))
    		return NF_ACCEPT;
    
    	WARN_ON(maniptype != NF_NAT_MANIP_SRC &&
    		maniptype != NF_NAT_MANIP_DST);
    
    	if (WARN_ON(nf_nat_initialized(ct, maniptype)))
    		return NF_DROP;
    
    	/* What we've got will look like inverse of reply. Normally
    	 * this is what is in the conntrack, except for prior
    	 * manipulations (future optimization: if num_manips == 0,
    	 * orig_tp = ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple)
    	 */
    	nf_ct_invert_tuplepr(&curr_tuple,
    			     &ct->tuplehash[IP_CT_DIR_REPLY].tuple);
    
    	get_unique_tuple(&new_tuple, &curr_tuple, range, ct, maniptype);
    
    	if (!nf_ct_tuple_equal(&new_tuple, &curr_tuple)) {
    		struct nf_conntrack_tuple reply;
    
    		/* Alter conntrack table so will recognize replies. */
    		nf_ct_invert_tuplepr(&reply, &new_tuple);
    		nf_conntrack_alter_reply(ct, &reply);
    
    		/* Non-atomic: we own this at the moment. */
    		if (maniptype == NF_NAT_MANIP_SRC)
    			ct->status |= IPS_SRC_NAT;
    		else
    			ct->status |= IPS_DST_NAT;
    
    		if (nfct_help(ct) && !nfct_seqadj(ct))
    			if (!nfct_seqadj_ext_add(ct))
    				return NF_DROP;
    	}
    
    	if (ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src.u3.ip != ct->tuplehash[IP_CT_DIR_REPLY].tuple.dst.u3.ip){
    		listnode = (struct MatchTupleList *)kmalloc(sizeof(struct MatchTupleList), GFP_KERNEL);
    		listnode->tuple.src = ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.src;
    		listnode->tuple.dst = ct->tuplehash[IP_CT_DIR_REPLY].tuple.dst;
    		listnode->specifiedIP = ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple.dst.u3.ip;
    		list_add_tail(&listnode->list, &TupleHead.list);
    	}
    	
    
    	if (maniptype == NF_NAT_MANIP_SRC) {
    		unsigned int srchash;
    		spinlock_t *lock;
    
    		srchash = hash_by_src(net,
    				      &ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple);
    		lock = &nf_nat_locks[srchash % CONNTRACK_LOCKS];
    		spin_lock_bh(lock);
    		hlist_add_head_rcu(&ct->nat_bysource,
    				   &nf_nat_bysource[srchash]);
    		spin_unlock_bh(lock);
    	}
    
    	/* It's done. */
    	if (maniptype == NF_NAT_MANIP_DST)
    		ct->status |= IPS_DST_NAT_DONE;
    	else
    		ct->status |= IPS_SRC_NAT_DONE;
    
    	return NF_ACCEPT;
    }
    EXPORT_SYMBOL(nf_nat_setup_info);

### 2.小结 ###

在本文中，我实现了全锥型NAT内核空间的关键部分代码，详细代码可以参考我的github上，在此基础上可以很容易的实现限制型锥形和可变的对称型等类型。

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![在这里插入图片描述][20201219120057415.jpg_pic_center]

[Netfilter_NAT_nf_nat_core.c]: https://blog.csdn.net/u013354486/article/details/79832124
[20201219120057415.jpg_pic_center]: /images/20220528/b0a4421dbd2c41d9b4292d090d340693.png