Netfilter学习之NAT类型动态配置（八）nf_nat_proto_common.c代码解析

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#include <linux/types.h>
#include <linux/random.h>
#include <linux/netfilter.h>
#include <linux/export.h>

#include <net/netfilter/nf_nat.h>
#include <net/netfilter/nf_nat_core.h>
#include <net/netfilter/nf_nat_l3proto.h>
#include <net/netfilter/nf_nat_l4proto.h>

/* 判断端口是否在可用范围内*/
bool nf_nat_l4proto_in_range(const struct nf_conntrack_tuple *tuple,
			     enum nf_nat_manip_type maniptype,
			     const union nf_conntrack_man_proto *min,
			     const union nf_conntrack_man_proto *max)
{
	__be16 port;
	
	/*根据SNAT和DNAT的不同取不同的值*/
	if (maniptype == NF_NAT_MANIP_SRC)
		port = tuple->src.u.all;
	else
		port = tuple->dst.u.all;

	return ntohs(port) >= ntohs(min->all) &&
	       ntohs(port) <= ntohs(max->all);
}
EXPORT_SYMBOL_GPL(nf_nat_l4proto_in_range);


/*对称型的NAT端口变化关键算法*/
void nf_nat_l4proto_unique_tuple(const struct nf_nat_l3proto *l3proto,
				 struct nf_conntrack_tuple *tuple,
				 const struct nf_nat_range *range,
				 enum nf_nat_manip_type maniptype,
				 const struct nf_conn *ct,
				 u16 *rover)
{
	unsigned int range_size, min, i;
	__be16 *portptr;
	u_int16_t off;
	
	/*根据SNAT和DNAT的不同取不同的值*/
	if (maniptype == NF_NAT_MANIP_SRC)
		portptr = &tuple->src.u.all;
	else
		portptr = &tuple->dst.u.all;

	/* If no range specified... */
	if (!(range->flags & NF_NAT_RANGE_PROTO_SPECIFIED)) {
		/* If it's dst rewrite, can't change port */
		if (maniptype == NF_NAT_MANIP_DST)
			return;
		
		/*首先取一个min的值*/
		if (ntohs(*portptr) < 1024) {
			/* Loose convention: >> 512 is credential passing */
			if (ntohs(*portptr) < 512) {
				min = 1;
				range_size = 511 - min + 1;
			} else {
				min = 600;
				range_size = 1023 - min + 1;
			}
		} else {
			min = 1024;
			range_size = 65535 - 1024 + 1;
		}
	} else {
		min = ntohs(range->min_proto.all);
		range_size = ntohs(range->max_proto.all) - min + 1;
	}

	/*设置偏移量off值*/
	if (range->flags & NF_NAT_RANGE_PROTO_RANDOM) {
		off = l3proto->secure_port(tuple, maniptype == NF_NAT_MANIP_SRC
						  ? tuple->dst.u.all
						  : tuple->src.u.all);
	} else if (range->flags & NF_NAT_RANGE_PROTO_RANDOM_FULLY) {
		off = prandom_u32();
	} else {
		off = *rover;
	}

	/*不断增加端口号直到找到可用的端口*/
	for (i = 0; ; ++off) {
		*portptr = htons(min + off % range_size);
		if (++i != range_size && nf_nat_used_tuple(tuple, ct))
			continue;
		if (!(range->flags & NF_NAT_RANGE_PROTO_RANDOM_ALL))
			*rover = off;
		return;
	}
}
EXPORT_SYMBOL_GPL(nf_nat_l4proto_unique_tuple);

#if IS_ENABLED(CONFIG_NF_CT_NETLINK)
int nf_nat_l4proto_nlattr_to_range(struct nlattr *tb[],
				   struct nf_nat_range *range)
{
	if (tb[CTA_PROTONAT_PORT_MIN]) {
		range->min_proto.all = nla_get_be16(tb[CTA_PROTONAT_PORT_MIN]);
		range->max_proto.all = range->min_proto.all;
		range->flags |= NF_NAT_RANGE_PROTO_SPECIFIED;
	}
	if (tb[CTA_PROTONAT_PORT_MAX]) {
		range->max_proto.all = nla_get_be16(tb[CTA_PROTONAT_PORT_MAX]);
		range->flags |= NF_NAT_RANGE_PROTO_SPECIFIED;
	}
	return 0;
}
EXPORT_SYMBOL_GPL(nf_nat_l4proto_nlattr_to_range);
#endif