Netfilter学习之NAT类型动态配置（四）nf_nat_core.c源码解析

Bertha 。 2022-05-28 12:58 642阅读 0赞

在研究了masquerade的用户空间和内核源码之后，我们发现最后进入了`nf_nat_setup_info()`函数，该函数位于nf\_nat\_core.c之中，是Netfilter的NAT表核心函数的实现。

\#\#\#1 nf\_nat\_core.c源码分析

本小节分析了nf\_nat\_core的源码，重点分析当Iptables指定的钩子函数激活后，在Netfilter中是如何识别和起作用的。这里沿用[前文][Link 1]中内部tuple，外部tuple，目标tuple的定义。源码中本身已有很多英文注释。这里用中文加以补充，若英文已经很完备则没有画蛇添足。

/*
     * (C) 1999-2001 Paul `Rusty' Russell
     * (C) 2002-2006 Netfilter Core Team <coreteam@netfilter.org>
     * (C) 2011 Patrick McHardy <kaber@trash.net>
     *
     * This program is free software; you can redistribute it and/or modify
     * it under the terms of the GNU General Public License version 2 as
     * published by the Free Software Foundation.
     */
    
    #include <linux/module.h>
    #include <linux/types.h>
    #include <linux/timer.h>
    #include <linux/skbuff.h>
    #include <linux/gfp.h>
    #include <net/xfrm.h>
    #include <linux/jhash.h>
    #include <linux/rtnetlink.h>
    
    #include <net/netfilter/nf_conntrack.h>
    #include <net/netfilter/nf_conntrack_core.h>
    #include <net/netfilter/nf_nat.h>
    #include <net/netfilter/nf_nat_l3proto.h>
    #include <net/netfilter/nf_nat_l4proto.h>
    #include <net/netfilter/nf_nat_core.h>
    #include <net/netfilter/nf_nat_helper.h>
    #include <net/netfilter/nf_conntrack_helper.h>
    #include <net/netfilter/nf_conntrack_seqadj.h>
    #include <net/netfilter/nf_conntrack_l3proto.h>
    #include <net/netfilter/nf_conntrack_zones.h>
    #include <linux/netfilter/nf_nat.h>
    
    static spinlock_t nf_nat_locks[CONNTRACK_LOCKS];
    
    static DEFINE_MUTEX(nf_nat_proto_mutex);
    static const struct nf_nat_l3proto __rcu *nf_nat_l3protos[NFPROTO_NUMPROTO]
    						__read_mostly;
    static const struct nf_nat_l4proto __rcu **nf_nat_l4protos[NFPROTO_NUMPROTO]
    						__read_mostly;
    
    /*
     * NAT为内部tuple提供了一个基于src ip的哈希表
     * 由此，NAT可以实现对已分配IP的内部tuple直接转换，对目标tuple的访问转换为对应的内部tuple
     */
    static struct hlist_head *nf_nat_bysource __read_mostly;
    static unsigned int nf_nat_htable_size __read_mostly;
    static unsigned int nf_nat_hash_rnd __read_mostly;
    
    /*返回L3层协议类型*/
    inline const struct nf_nat_l3proto *
    __nf_nat_l3proto_find(u8 family)
    {
    	return rcu_dereference(nf_nat_l3protos[family]);
    }
    
    /*返回L4层协议类型*/
    inline const struct nf_nat_l4proto *
    __nf_nat_l4proto_find(u8 family, u8 protonum)
    {
    	return rcu_dereference(nf_nat_l4protos[family][protonum]);
    }
    EXPORT_SYMBOL_GPL(__nf_nat_l4proto_find);
    
    #ifdef CONFIG_XFRM
    static void __nf_nat_decode_session(struct sk_buff *skb, struct flowi *fl)
    {
    	const struct nf_nat_l3proto *l3proto;
    	const struct nf_conn *ct;
    	enum ip_conntrack_info ctinfo;
    	enum ip_conntrack_dir dir;
    	unsigned  long statusbit;
    	u8 family;
    
    	ct = nf_ct_get(skb, &ctinfo);
    	if (ct == NULL)
    		return;
    
    	family = nf_ct_l3num(ct);
    	l3proto = __nf_nat_l3proto_find(family);
    	if (l3proto == NULL)
    		return;
    
    	dir = CTINFO2DIR(ctinfo);
    	if (dir == IP_CT_DIR_ORIGINAL)
    		statusbit = IPS_DST_NAT;
    	else
    		statusbit = IPS_SRC_NAT;
    
    	l3proto->decode_session(skb, ct, dir, statusbit, fl);
    }
    
    int nf_xfrm_me_harder(struct net *net, struct sk_buff *skb, unsigned int family)
    {
    	struct flowi fl;
    	unsigned int hh_len;
    	struct dst_entry *dst;
    	int err;
    
    	err = xfrm_decode_session(skb, &fl, family);
    	if (err < 0)
    		return err;
    
    	dst = skb_dst(skb);
    	if (dst->xfrm)
    		dst = ((struct xfrm_dst *)dst)->route;
    	dst_hold(dst);
    
    	dst = xfrm_lookup(net, dst, &fl, skb->sk, 0);
    	if (IS_ERR(dst))
    		return PTR_ERR(dst);
    
    	skb_dst_drop(skb);
    	skb_dst_set(skb, dst);
    
    	/* Change in oif may mean change in hh_len. */
    	hh_len = skb_dst(skb)->dev->hard_header_len;
    	if (skb_headroom(skb) < hh_len &&
    	    pskb_expand_head(skb, hh_len - skb_headroom(skb), 0, GFP_ATOMIC))
    		return -ENOMEM;
    	return 0;
    }
    EXPORT_SYMBOL(nf_xfrm_me_harder);
    #endif /* CONFIG_XFRM */
    
    /* We keep an extra hash for each conntrack, for fast searching. */
    static unsigned int
    hash_by_src(const struct net *n, const struct nf_conntrack_tuple *tuple)
    {
    	unsigned int hash;
    
    	get_random_once(&nf_nat_hash_rnd, sizeof(nf_nat_hash_rnd));
    
    	/* Original src, to ensure we map it consistently if poss. */
    	hash = jhash2((u32 *)&tuple->src, sizeof(tuple->src) / sizeof(u32),
    		      tuple->dst.protonum ^ nf_nat_hash_rnd ^ net_hash_mix(n));
    
    	return reciprocal_scale(hash, nf_nat_htable_size);
    }
    
    /* Is this tuple already taken? (not by us) */
    int
    nf_nat_used_tuple(const struct nf_conntrack_tuple *tuple,
    		  const struct nf_conn *ignored_conntrack)
    {
    	/* Conntrack tracking doesn't keep track of outgoing tuples; only
    	 * incoming ones.  NAT means they don't have a fixed mapping,
    	 * so we invert the tuple and look for the incoming reply.
    	 *
    	 * 这里对tuple做了一个颠倒，意义入上文英文所述
    	 * We could keep a separate hash if this proves too slow.
    	 */
    	struct nf_conntrack_tuple reply;
    
    	nf_ct_invert_tuplepr(&reply, tuple);
    	return nf_conntrack_tuple_taken(&reply, ignored_conntrack);
    }
    EXPORT_SYMBOL(nf_nat_used_tuple);
    
    /* If we source map this tuple so reply looks like reply_tuple, will
     * that meet the constraints of range.
     * 检测range是否合法
     */
    static int in_range(const struct nf_nat_l3proto *l3proto,
    		    const struct nf_nat_l4proto *l4proto,
    		    const struct nf_conntrack_tuple *tuple,
    		    const struct nf_nat_range *range)
    {
    	/* If we are supposed to map IPs, then we must be in the
    	 * range specified, otherwise let this drag us onto a new src IP.
    	 */
    	if (range->flags & NF_NAT_RANGE_MAP_IPS &&
    	    !l3proto->in_range(tuple, range))
    		return 0;
    
    	if (!(range->flags & NF_NAT_RANGE_PROTO_SPECIFIED) ||
    	    l4proto->in_range(tuple, NF_NAT_MANIP_SRC,
    			      &range->min_proto, &range->max_proto))
    		return 1;
    
    	return 0;
    }
    
    static inline int
    same_src(const struct nf_conn *ct,
    	 const struct nf_conntrack_tuple *tuple)
    {
    	const struct nf_conntrack_tuple *t;
    
    	t = &ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple;
    	return (t->dst.protonum == tuple->dst.protonum &&
    		nf_inet_addr_cmp(&t->src.u3, &tuple->src.u3) &&
    		t->src.u.all == tuple->src.u.all);
    }
    
    /* Only called for SRC manip */
    /*
     * 函数功能：用于确定ip地址、端口等是否在合适的范围,
     * 并且定义了一个nf_conn_nat结构的变量nat，结构体中的成员bysource是一个hash链表(根据源IP来查找)，
     * 把即将进行NAT操作的连接都放到这个hash表里。
     */
    static int
    find_appropriate_src(struct net *net,
    		     const struct nf_conntrack_zone *zone,
    		     const struct nf_nat_l3proto *l3proto,
    		     const struct nf_nat_l4proto *l4proto,
    		     const struct nf_conntrack_tuple *tuple,
    		     struct nf_conntrack_tuple *result,
    		     const struct nf_nat_range *range)
    {
    	unsigned int h = hash_by_src(net, tuple);
    	const struct nf_conn *ct;
    
    	hlist_for_each_entry_rcu(ct, &nf_nat_bysource[h], nat_bysource) {
    		if (same_src(ct, tuple) &&
    		    net_eq(net, nf_ct_net(ct)) &&
    		    nf_ct_zone_equal(ct, zone, IP_CT_DIR_ORIGINAL)) {
    			/* Copy source part from reply tuple. */
    			nf_ct_invert_tuplepr(result,
    				       &ct->tuplehash[IP_CT_DIR_REPLY].tuple);
    			result->dst = tuple->dst;
    
    			if (in_range(l3proto, l4proto, result, range))
    				return 1;
    		}
    	}
    	return 0;
    }
    
    /* For [FUTURE] fragmentation handling, we want the least-used
     * src-ip/dst-ip/proto triple.  Fairness doesn't come into it.  Thus
     * if the range specifies 1.2.3.4 ports 10000-10005 and 1.2.3.5 ports
     * 1-65535, we don't do pro-rata allocation based on ports; we choose
     * the ip with the lowest src-ip/dst-ip/proto usage.
     */
    static void
    find_best_ips_proto(const struct nf_conntrack_zone *zone,
    		    struct nf_conntrack_tuple *tuple,
    		    const struct nf_nat_range *range,
    		    const struct nf_conn *ct,
    		    enum nf_nat_manip_type maniptype)
    {
    	union nf_inet_addr *var_ipp;
    	unsigned int i, max;
    	/* Host order */
    	u32 minip, maxip, j, dist;
    	bool full_range;
    
    	/* No IP mapping?  Do nothing. */
    	if (!(range->flags & NF_NAT_RANGE_MAP_IPS))
    		return;
    
    	if (maniptype == NF_NAT_MANIP_SRC)
    		var_ipp = &tuple->src.u3;
    	else
    		var_ipp = &tuple->dst.u3;
    
    	/* Fast path: only one choice. */
    	if (nf_inet_addr_cmp(&range->min_addr, &range->max_addr)) {
    		*var_ipp = range->min_addr;
    		return;
    	}
    
    	if (nf_ct_l3num(ct) == NFPROTO_IPV4)
    		max = sizeof(var_ipp->ip) / sizeof(u32) - 1;
    	else
    		max = sizeof(var_ipp->ip6) / sizeof(u32) - 1;
    
    	/* Hashing source and destination IPs gives a fairly even
    	 * spread in practice (if there are a small number of IPs
    	 * involved, there usually aren't that many connections
    	 * anyway).  The consistency means that servers see the same
    	 * client coming from the same IP (some Internet Banking sites
    	 * like this), even across reboots.
    	 */
    	j = jhash2((u32 *)&tuple->src.u3, sizeof(tuple->src.u3) / sizeof(u32),
    		   range->flags & NF_NAT_RANGE_PERSISTENT ?
    			0 : (__force u32)tuple->dst.u3.all[max] ^ zone->id);
    
    	full_range = false;
    	for (i = 0; i <= max; i++) {
    		/* If first bytes of the address are at the maximum, use the
    		 * distance. Otherwise use the full range.
    		 */
    		if (!full_range) {
    			minip = ntohl((__force __be32)range->min_addr.all[i]);
    			maxip = ntohl((__force __be32)range->max_addr.all[i]);
    			dist  = maxip - minip + 1;
    		} else {
    			minip = 0;
    			dist  = ~0;
    		}
    
    		var_ipp->all[i] = (__force __u32)
    			htonl(minip + reciprocal_scale(j, dist));
    		if (var_ipp->all[i] != range->max_addr.all[i])
    			full_range = true;
    
    		if (!(range->flags & NF_NAT_RANGE_PERSISTENT))
    			j ^= (__force u32)tuple->dst.u3.all[i];
    	}
    }
    
    /* Manipulate the tuple into the range given. For NF_INET_POST_ROUTING,
     * we change the source to map into the range. For NF_INET_PRE_ROUTING
     * and NF_INET_LOCAL_OUT, we change the destination to map into the
     * range. It might not be possible to get a unique tuple, but we try.
     * At worst (or if we race), we will end up with a final duplicate in
     * __ip_conntrack_confirm and drop the packet. 
     *
     * 核心函数，为入、出的tuple分配合适的外部、内部tuple,详情入上文英文所述
     * 首先，调用find_appropriate_src( )确定ip地址、端口等是否在合适的范围
     * 然后定义了一个nf_conn_nat结构的变量nat，结构体中的成员bysource把即将进行NAT操作的连接都放到这个hash表里
     * 随后，由find_best_ips_proto分配“最少使用的”tuple（如果自带的不在合适范围内）
     * 最终，调用nat协议的操作集调用unique_tuple成员函数。
     */
    static void
    get_unique_tuple(struct nf_conntrack_tuple *tuple,
    		 const struct nf_conntrack_tuple *orig_tuple,
    		 const struct nf_nat_range *range,
    		 struct nf_conn *ct,
    		 enum nf_nat_manip_type maniptype)
    {
    	const struct nf_conntrack_zone *zone;
    	const struct nf_nat_l3proto *l3proto;
    	const struct nf_nat_l4proto *l4proto;
    	struct net *net = nf_ct_net(ct);
    
    	zone = nf_ct_zone(ct);
    
    	rcu_read_lock();
    	l3proto = __nf_nat_l3proto_find(orig_tuple->src.l3num);
    	l4proto = __nf_nat_l4proto_find(orig_tuple->src.l3num,
    					orig_tuple->dst.protonum);
    
    	/* 1) If this srcip/proto/src-proto-part is currently mapped,
    	 * and that same mapping gives a unique tuple within the given
    	 * range, use that.
    	 *
    	 * This is only required for source (ie. NAT/masq) mappings.
    	 * So far, we don't do local source mappings, so multiple
    	 * manips not an issue.
    	 *
    	 * 仔细看这里的标记位检查，发现了RANDOM_ALL，这是iptables命令行masquerade --random所对应的标记位
    	 */
    	if (maniptype == NF_NAT_MANIP_SRC &&
    	    !(range->flags & NF_NAT_RANGE_PROTO_RANDOM_ALL)) {
    		/* try the original tuple first */
    		if (in_range(l3proto, l4proto, orig_tuple, range)) {
    			if (!nf_nat_used_tuple(orig_tuple, ct)) {
    				*tuple = *orig_tuple;
    				goto out;
    			}
    		} else if (find_appropriate_src(net, zone, l3proto, l4proto,
    						orig_tuple, tuple, range)) {
    			pr_debug("get_unique_tuple: Found current src map\n");
    			if (!nf_nat_used_tuple(tuple, ct))
    				goto out;
    		}
    	}
    
    	/* 2) Select the least-used IP/proto combination in the given range */
    	*tuple = *orig_tuple;
    	find_best_ips_proto(zone, tuple, range, ct, maniptype);
    
    	/* 3) The per-protocol part of the manip is made to map into
    	 * the range to make a unique tuple.
    	 */
    
    	/* Only bother mapping if it's not already in range and unique */
    	if (!(range->flags & NF_NAT_RANGE_PROTO_RANDOM_ALL)) {
    		if (range->flags & NF_NAT_RANGE_PROTO_SPECIFIED) {
    			if (l4proto->in_range(tuple, maniptype,
    					      &range->min_proto,
    					      &range->max_proto) &&
    			    (range->min_proto.all == range->max_proto.all ||
    			     !nf_nat_used_tuple(tuple, ct)))
    				goto out;
    		} else if (!nf_nat_used_tuple(tuple, ct)) {
    			goto out;
    		}
    	}
    
    	/* Last change: get protocol to try to obtain unique tuple. */
    	l4proto->unique_tuple(l3proto, tuple, range, maniptype, ct);
    out:
    	rcu_read_unlock();
    }
    
    struct nf_conn_nat *nf_ct_nat_ext_add(struct nf_conn *ct)
    {
    	struct nf_conn_nat *nat = nfct_nat(ct);
    	if (nat)
    		return nat;
    
    	if (!nf_ct_is_confirmed(ct))
    		nat = nf_ct_ext_add(ct, NF_CT_EXT_NAT, GFP_ATOMIC);
    
    	return nat;
    }
    EXPORT_SYMBOL_GPL(nf_ct_nat_ext_add);
    
    /*
     * masquerade最终调用的传入函数
     *
     * 本函数会调用get_unique_tuple( )来获取唯一的tuple五元组
     * 之后函数返回到nf_nat_packet继续执行
     * 最终调用manip_pkt( )来完成ip和端口的转换
     */
    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 (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);
    
    static unsigned int
    __nf_nat_alloc_null_binding(struct nf_conn *ct, enum nf_nat_manip_type manip)
    {
    	/* Force range to this IP; let proto decide mapping for
    	 * per-proto parts (hence not IP_NAT_RANGE_PROTO_SPECIFIED).
    	 * Use reply in case it's already been mangled (eg local packet).
    	 */
    	union nf_inet_addr ip =
    		(manip == NF_NAT_MANIP_SRC ?
    		ct->tuplehash[IP_CT_DIR_REPLY].tuple.dst.u3 :
    		ct->tuplehash[IP_CT_DIR_REPLY].tuple.src.u3);
    	struct nf_nat_range range = {
    		.flags		= NF_NAT_RANGE_MAP_IPS,
    		.min_addr	= ip,
    		.max_addr	= ip,
    	};
    	return nf_nat_setup_info(ct, &range, manip);
    }
    
    unsigned int
    nf_nat_alloc_null_binding(struct nf_conn *ct, unsigned int hooknum)
    {
    	return __nf_nat_alloc_null_binding(ct, HOOK2MANIP(hooknum));
    }
    EXPORT_SYMBOL_GPL(nf_nat_alloc_null_binding);
    
    /* Do packet manipulations according to nf_nat_setup_info.根据nf_nat_setup_info做包处理 */
    unsigned int nf_nat_packet(struct nf_conn *ct,
    			   enum ip_conntrack_info ctinfo,
    			   unsigned int hooknum,
    			   struct sk_buff *skb)
    {
    	const struct nf_nat_l3proto *l3proto;
    	const struct nf_nat_l4proto *l4proto;
    	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
    	unsigned long statusbit;
    	enum nf_nat_manip_type mtype = HOOK2MANIP(hooknum);
    
    	if (mtype == NF_NAT_MANIP_SRC)
    		statusbit = IPS_SRC_NAT;
    	else
    		statusbit = IPS_DST_NAT;
    
    	/* Invert if this is reply dir. */
    	if (dir == IP_CT_DIR_REPLY)
    		statusbit ^= IPS_NAT_MASK;
    
    	/* Non-atomic: these bits don't change. */
    	if (ct->status & statusbit) {
    		struct nf_conntrack_tuple target;
    
    		/* We are aiming to look like inverse of other direction. */
    		nf_ct_invert_tuplepr(&target, &ct->tuplehash[!dir].tuple);
    
    		l3proto = __nf_nat_l3proto_find(target.src.l3num);
    		l4proto = __nf_nat_l4proto_find(target.src.l3num,
    						target.dst.protonum);
    		if (!l3proto->manip_pkt(skb, 0, l4proto, &target, mtype))
    			return NF_DROP;
    	}
    	return NF_ACCEPT;
    }
    EXPORT_SYMBOL_GPL(nf_nat_packet);
    
    struct nf_nat_proto_clean {
    	u8	l3proto;
    	u8	l4proto;
    };
    
    /* kill conntracks with affected NAT section */
    static int nf_nat_proto_remove(struct nf_conn *i, void *data)
    {
    	const struct nf_nat_proto_clean *clean = data;
    
    	if ((clean->l3proto && nf_ct_l3num(i) != clean->l3proto) ||
    	    (clean->l4proto && nf_ct_protonum(i) != clean->l4proto))
    		return 0;
    
    	return i->status & IPS_NAT_MASK ? 1 : 0;
    }
    
    static void __nf_nat_cleanup_conntrack(struct nf_conn *ct)
    {
    	unsigned int h;
    
    	h = hash_by_src(nf_ct_net(ct), &ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple);
    	spin_lock_bh(&nf_nat_locks[h % CONNTRACK_LOCKS]);
    	hlist_del_rcu(&ct->nat_bysource);
    	spin_unlock_bh(&nf_nat_locks[h % CONNTRACK_LOCKS]);
    }
    
    static int nf_nat_proto_clean(struct nf_conn *ct, void *data)
    {
    	if (nf_nat_proto_remove(ct, data))
    		return 1;
    
    	/* This module is being removed and conntrack has nat null binding.
    	 * Remove it from bysource hash, as the table will be freed soon.
    	 *
    	 * Else, when the conntrack is destoyed, nf_nat_cleanup_conntrack()
    	 * will delete entry from already-freed table.
    	 */
    	if (test_and_clear_bit(IPS_SRC_NAT_DONE_BIT, &ct->status))
    		__nf_nat_cleanup_conntrack(ct);
    
    	/* don't delete conntrack.  Although that would make things a lot
    	 * simpler, we'd end up flushing all conntracks on nat rmmod.
    	 */
    	return 0;
    }
    
    static void nf_nat_l4proto_clean(u8 l3proto, u8 l4proto)
    {
    	struct nf_nat_proto_clean clean = {
    		.l3proto = l3proto,
    		.l4proto = l4proto,
    	};
    
    	nf_ct_iterate_destroy(nf_nat_proto_remove, &clean);
    }
    
    static void nf_nat_l3proto_clean(u8 l3proto)
    {
    	struct nf_nat_proto_clean clean = {
    		.l3proto = l3proto,
    	};
    
    	nf_ct_iterate_destroy(nf_nat_proto_remove, &clean);
    }
    
    /* Protocol registration. */
    int nf_nat_l4proto_register(u8 l3proto, const struct nf_nat_l4proto *l4proto)
    {
    	const struct nf_nat_l4proto **l4protos;
    	unsigned int i;
    	int ret = 0;
    
    	mutex_lock(&nf_nat_proto_mutex);
    	if (nf_nat_l4protos[l3proto] == NULL) {
    		l4protos = kmalloc(IPPROTO_MAX * sizeof(struct nf_nat_l4proto *),
    				   GFP_KERNEL);
    		if (l4protos == NULL) {
    			ret = -ENOMEM;
    			goto out;
    		}
    
    		for (i = 0; i < IPPROTO_MAX; i++)
    			RCU_INIT_POINTER(l4protos[i], &nf_nat_l4proto_unknown);
    
    		/* Before making proto_array visible to lockless readers,
    		 * we must make sure its content is committed to memory.
    		 */
    		smp_wmb();
    
    		nf_nat_l4protos[l3proto] = l4protos;
    	}
    
    	if (rcu_dereference_protected(
    			nf_nat_l4protos[l3proto][l4proto->l4proto],
    			lockdep_is_held(&nf_nat_proto_mutex)
    			) != &nf_nat_l4proto_unknown) {
    		ret = -EBUSY;
    		goto out;
    	}
    	RCU_INIT_POINTER(nf_nat_l4protos[l3proto][l4proto->l4proto], l4proto);
     out:
    	mutex_unlock(&nf_nat_proto_mutex);
    	return ret;
    }
    EXPORT_SYMBOL_GPL(nf_nat_l4proto_register);
    
    /* No one stores the protocol anywhere; simply delete it. */
    void nf_nat_l4proto_unregister(u8 l3proto, const struct nf_nat_l4proto *l4proto)
    {
    	mutex_lock(&nf_nat_proto_mutex);
    	RCU_INIT_POINTER(nf_nat_l4protos[l3proto][l4proto->l4proto],
    			 &nf_nat_l4proto_unknown);
    	mutex_unlock(&nf_nat_proto_mutex);
    	synchronize_rcu();
    
    	nf_nat_l4proto_clean(l3proto, l4proto->l4proto);
    }
    EXPORT_SYMBOL_GPL(nf_nat_l4proto_unregister);
    
    int nf_nat_l3proto_register(const struct nf_nat_l3proto *l3proto)
    {
    	int err;
    
    	err = nf_ct_l3proto_try_module_get(l3proto->l3proto);
    	if (err < 0)
    		return err;
    
    	mutex_lock(&nf_nat_proto_mutex);
    	RCU_INIT_POINTER(nf_nat_l4protos[l3proto->l3proto][IPPROTO_TCP],
    			 &nf_nat_l4proto_tcp);
    	RCU_INIT_POINTER(nf_nat_l4protos[l3proto->l3proto][IPPROTO_UDP],
    			 &nf_nat_l4proto_udp);
    #ifdef CONFIG_NF_NAT_PROTO_DCCP
    	RCU_INIT_POINTER(nf_nat_l4protos[l3proto->l3proto][IPPROTO_DCCP],
    			 &nf_nat_l4proto_dccp);
    #endif
    #ifdef CONFIG_NF_NAT_PROTO_SCTP
    	RCU_INIT_POINTER(nf_nat_l4protos[l3proto->l3proto][IPPROTO_SCTP],
    			 &nf_nat_l4proto_sctp);
    #endif
    #ifdef CONFIG_NF_NAT_PROTO_UDPLITE
    	RCU_INIT_POINTER(nf_nat_l4protos[l3proto->l3proto][IPPROTO_UDPLITE],
    			 &nf_nat_l4proto_udplite);
    #endif
    	mutex_unlock(&nf_nat_proto_mutex);
    
    	RCU_INIT_POINTER(nf_nat_l3protos[l3proto->l3proto], l3proto);
    	return 0;
    }
    EXPORT_SYMBOL_GPL(nf_nat_l3proto_register);
    
    void nf_nat_l3proto_unregister(const struct nf_nat_l3proto *l3proto)
    {
    	mutex_lock(&nf_nat_proto_mutex);
    	RCU_INIT_POINTER(nf_nat_l3protos[l3proto->l3proto], NULL);
    	mutex_unlock(&nf_nat_proto_mutex);
    	synchronize_rcu();
    
    	nf_nat_l3proto_clean(l3proto->l3proto);
    	nf_ct_l3proto_module_put(l3proto->l3proto);
    }
    EXPORT_SYMBOL_GPL(nf_nat_l3proto_unregister);
    
    /* No one using conntrack by the time this called. */
    static void nf_nat_cleanup_conntrack(struct nf_conn *ct)
    {
    	if (ct->status & IPS_SRC_NAT_DONE)
    		__nf_nat_cleanup_conntrack(ct);
    }
    
    static struct nf_ct_ext_type nat_extend __read_mostly = {
    	.len		= sizeof(struct nf_conn_nat),
    	.align		= __alignof__(struct nf_conn_nat),
    	.destroy	= nf_nat_cleanup_conntrack,
    	.id		= NF_CT_EXT_NAT,
    };
    
    #if IS_ENABLED(CONFIG_NF_CT_NETLINK)
    
    #include <linux/netfilter/nfnetlink.h>
    #include <linux/netfilter/nfnetlink_conntrack.h>
    
    static const struct nla_policy protonat_nla_policy[CTA_PROTONAT_MAX+1] = {
    	[CTA_PROTONAT_PORT_MIN]	= { .type = NLA_U16 },
    	[CTA_PROTONAT_PORT_MAX]	= { .type = NLA_U16 },
    };
    
    static int nfnetlink_parse_nat_proto(struct nlattr *attr,
    				     const struct nf_conn *ct,
    				     struct nf_nat_range *range)
    {
    	struct nlattr *tb[CTA_PROTONAT_MAX+1];
    	const struct nf_nat_l4proto *l4proto;
    	int err;
    
    	err = nla_parse_nested(tb, CTA_PROTONAT_MAX, attr,
    			       protonat_nla_policy, NULL);
    	if (err < 0)
    		return err;
    
    	l4proto = __nf_nat_l4proto_find(nf_ct_l3num(ct), nf_ct_protonum(ct));
    	if (l4proto->nlattr_to_range)
    		err = l4proto->nlattr_to_range(tb, range);
    
    	return err;
    }
    
    static const struct nla_policy nat_nla_policy[CTA_NAT_MAX+1] = {
    	[CTA_NAT_V4_MINIP]	= { .type = NLA_U32 },
    	[CTA_NAT_V4_MAXIP]	= { .type = NLA_U32 },
    	[CTA_NAT_V6_MINIP]	= { .len = sizeof(struct in6_addr) },
    	[CTA_NAT_V6_MAXIP]	= { .len = sizeof(struct in6_addr) },
    	[CTA_NAT_PROTO]		= { .type = NLA_NESTED },
    };
    
    static int
    nfnetlink_parse_nat(const struct nlattr *nat,
    		    const struct nf_conn *ct, struct nf_nat_range *range,
    		    const struct nf_nat_l3proto *l3proto)
    {
    	struct nlattr *tb[CTA_NAT_MAX+1];
    	int err;
    
    	memset(range, 0, sizeof(*range));
    
    	err = nla_parse_nested(tb, CTA_NAT_MAX, nat, nat_nla_policy, NULL);
    	if (err < 0)
    		return err;
    
    	err = l3proto->nlattr_to_range(tb, range);
    	if (err < 0)
    		return err;
    
    	if (!tb[CTA_NAT_PROTO])
    		return 0;
    
    	return nfnetlink_parse_nat_proto(tb[CTA_NAT_PROTO], ct, range);
    }
    
    /* This function is called under rcu_read_lock() */
    static int
    nfnetlink_parse_nat_setup(struct nf_conn *ct,
    			  enum nf_nat_manip_type manip,
    			  const struct nlattr *attr)
    {
    	struct nf_nat_range range;
    	const struct nf_nat_l3proto *l3proto;
    	int err;
    
    	/* Should not happen, restricted to creating new conntracks
    	 * via ctnetlink.
    	 */
    	if (WARN_ON_ONCE(nf_nat_initialized(ct, manip)))
    		return -EEXIST;
    
    	/* Make sure that L3 NAT is there by when we call nf_nat_setup_info to
    	 * attach the null binding, otherwise this may oops.
    	 */
    	l3proto = __nf_nat_l3proto_find(nf_ct_l3num(ct));
    	if (l3proto == NULL)
    		return -EAGAIN;
    
    	/* No NAT information has been passed, allocate the null-binding */
    	if (attr == NULL)
    		return __nf_nat_alloc_null_binding(ct, manip) == NF_DROP ? -ENOMEM : 0;
    
    	err = nfnetlink_parse_nat(attr, ct, &range, l3proto);
    	if (err < 0)
    		return err;
    
    	return nf_nat_setup_info(ct, &range, manip) == NF_DROP ? -ENOMEM : 0;
    }
    #else
    static int
    nfnetlink_parse_nat_setup(struct nf_conn *ct,
    			  enum nf_nat_manip_type manip,
    			  const struct nlattr *attr)
    {
    	return -EOPNOTSUPP;
    }
    #endif
    
    static struct nf_ct_helper_expectfn follow_master_nat = {
    	.name		= "nat-follow-master",
    	.expectfn	= nf_nat_follow_master,
    };
    
    static int __init nf_nat_init(void)
    {
    	int ret, i;
    
    	/* Leave them the same for the moment. */
    	nf_nat_htable_size = nf_conntrack_htable_size;
    	if (nf_nat_htable_size < CONNTRACK_LOCKS)
    		nf_nat_htable_size = CONNTRACK_LOCKS;
    
    	nf_nat_bysource = nf_ct_alloc_hashtable(&nf_nat_htable_size, 0);
    	if (!nf_nat_bysource)
    		return -ENOMEM;
    
    	ret = nf_ct_extend_register(&nat_extend);
    	if (ret < 0) {
    		nf_ct_free_hashtable(nf_nat_bysource, nf_nat_htable_size);
    		printk(KERN_ERR "nf_nat_core: Unable to register extension\n");
    		return ret;
    	}
    
    	for (i = 0; i < CONNTRACK_LOCKS; i++)
    		spin_lock_init(&nf_nat_locks[i]);
    
    	nf_ct_helper_expectfn_register(&follow_master_nat);
    
    	BUG_ON(nfnetlink_parse_nat_setup_hook != NULL);
    	RCU_INIT_POINTER(nfnetlink_parse_nat_setup_hook,
    			   nfnetlink_parse_nat_setup);
    #ifdef CONFIG_XFRM
    	BUG_ON(nf_nat_decode_session_hook != NULL);
    	RCU_INIT_POINTER(nf_nat_decode_session_hook, __nf_nat_decode_session);
    #endif
    	return 0;
    }
    
    static void __exit nf_nat_cleanup(void)
    {
    	struct nf_nat_proto_clean clean = {};
    	unsigned int i;
    
    	nf_ct_iterate_destroy(nf_nat_proto_clean, &clean);
    
    	nf_ct_extend_unregister(&nat_extend);
    	nf_ct_helper_expectfn_unregister(&follow_master_nat);
    	RCU_INIT_POINTER(nfnetlink_parse_nat_setup_hook, NULL);
    #ifdef CONFIG_XFRM
    	RCU_INIT_POINTER(nf_nat_decode_session_hook, NULL);
    #endif
    	synchronize_rcu();
    
    	for (i = 0; i < NFPROTO_NUMPROTO; i++)
    		kfree(nf_nat_l4protos[i]);
    	synchronize_net();
    	nf_ct_free_hashtable(nf_nat_bysource, nf_nat_htable_size);
    }
    
    MODULE_LICENSE("GPL");
    
    module_init(nf_nat_init);
    module_exit(nf_nat_cleanup);

\#\#\#2 masquerade实现流程的分析  
  通过上文对源码的分析，我们这里总结一下masquerade的整个实现流程（很多人没有耐心看源码，但是看源码和文档才是学习、理解最好的手段）。这里我们通过图来表示。  
![这里写图片描述][70]

主机A通过NAT发送给C，外部的C收到数据包之后回复给A。我们这里用A表示A的IP和端口tuple，B、C有着相同的含义。从这里可以看出来源码中为什么要使用inverse tuple了：因为恰好和C回传的时候的tuple一致。由此，可以实现C向A的通信。

本篇结束。

--------------------

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

[Link 1]: https://blog.csdn.net/u013354486/article/details/78996727
[70]: /images/20220528/5d0c15668aa24f2b8a2bc9a9d4a63ec1.png
[20201219120057415.jpg_pic_center]: /images/20220528/07b4679902164553bbf280ac0c89e57e.png