虚机中访问外网；NAT中的POSTROUTING是怎么搞的？
看下docker中是怎么配置的网络
在虚机中访问外网：设定了qemu，在主机上添加路由：sudo iptables -t nat -I POSTROUTING -s 192.168.1.110 -j SNAT --to-source 192.168.0.108
设置了这句话就可以访问外网了。
设置了两个虚拟机：
tap0 (192.168.129.1) --->
tap1 (192,168.130.1) --->
增加nat的NAT的表项设置： sudo iptables -t nat -I POSTROUTING -s 192.168.128.0/20 -j SNAT --to-source 192.168.0.108
同时去访问我的云主机：121.X.X.X，从两个主机中都能ping得通，这说明在NAT记录了这个地址，记录着
两个典型包：
192.168.129.110 --->云主机 ( 192.168.0.108 ---> 云主机)
192.168.130.110 --->云主机 ( 192.168.0.108 ---> 云主机)
NAT内部是怎么记录的这个转换？是记录咋的？从云主机IP中回来了一个包，目的地址是192.168.0.108，怎么分别分流到 192.168.129.110 和 192.168.130.110 两个 IP地址中。
难道是端口的信息在里面？接受数据包的流程
#0 icmp_rcv (skb=0xffff88007c9efc00) at net/ipv4/icmp.c:973
#1 0xffffffff816d97af in ip_local_deliver_finish (
net=0xffffffff81ed8680 <init_net>, sk=<optimized out>,
skb=0xffff88007c9efc00) at net/ipv4/ip_input.c:216
#2 0xffffffff816d9e45 in NF_HOOK_THRESH (thresh=<optimized out>,
okfn=<optimized out>, out=<optimized out>, in=<optimized out>,
skb=<optimized out>, sk=<optimized out>, net=<optimized out>,
hook=<optimized out>, pf=<optimized out>)
at ./include/linux/netfilter.h:232
#3 NF_HOOK (okfn=<optimized out>, out=<optimized out>,
in=<optimized out>, skb=<optimized out>, sk=<optimized out>,
net=<optimized out>, hook=<optimized out>, pf=<optimized out>)
at ./include/linux/netfilter.h:255
#4 ip_local_deliver (skb=0xffff88007c9efc00)
at net/ipv4/ip_input.c:257
#5 0xffffffff816d9a7b in dst_input (skb=<optimized out>)
at ./include/net/dst.h:507
#6 ip_rcv_finish (net=0xffffffff81ed8680 <init_net>,
sk=<optimized out>, skb=0xffff88007c9efc00)
at net/ipv4/ip_input.c:396
#7 0xffffffff816da11e in NF_HOOK_THRESH (thresh=<optimized out>,
okfn=<optimized out>, out=<optimized out>, in=<optimized out>,
skb=<optimized out>, sk=<optimized out>, net=<optimized out>,
hook=<optimized out>, pf=<optimized out>)
at ./include/linux/netfilter.h:232
#8 NF_HOOK (okfn=<optimized out>, out=<optimized out>,
in=<optimized out>, skb=<optimized out>, sk=<optimized out>,
net=<optimized out>, hook=<optimized out>, pf=<optimized out>)
at ./include/linux/netfilter.h:255
#9 ip_rcv (skb=0xffff88007c9efc00, dev=0xffff88007c530000,
pt=<optimized out>, orig_dev=<optimized out>)
at net/ipv4/ip_input.c:487
#10 0xffffffff81684eea in __netif_receive_skb_core (
skb=0xffff88007c9efc00, pfmemalloc=<optimized out>)
at net/core/dev.c:4211
#11 0xffffffff816878cd in __netif_receive_skb (skb=<optimized out>)
at net/core/dev.c:4249
#12 0xffffffff8168793d in netif_receive_skb_internal (
skb=0xffff88007c9efc00) at net/core/dev.c:4277
#13 0xffffffff81688582 in napi_skb_finish (skb=<optimized out>,
ret=<optimized out>) at net/core/dev.c:4626
---Type <return> to continue, or q <return> to quit---
#14 napi_gro_receive (napi=0xffff88007c530b70, skb=0xffff88007c9efc00)
at net/core/dev.c:4658
#15 0xffffffff81532db1 in e1000_receive_skb (skb=<optimized out>,
vlan=<optimized out>, status=<optimized out>,
adapter=<optimized out>)
at drivers/net/ethernet/intel/e1000/e1000_main.c:4035
#16 e1000_clean_rx_irq (adapter=0xffff88007c5308c0,
rx_ring=<optimized out>, work_done=<optimized out>,
work_to_do=<optimized out>)
at drivers/net/ethernet/intel/e1000/e1000_main.c:4491
#17 0xffffffff81531bb0 in e1000_clean (napi=0xffff88007c530b70,
budget=64) at drivers/net/ethernet/intel/e1000/e1000_main.c:3836
#18 0xffffffff8168968a in napi_poll (repoll=<optimized out>,
n=<optimized out>) at net/core/dev.c:5158
#19 net_rx_action (h=<optimized out>) at net/core/dev.c:5223
#20 0xffffffff8187c0d9 in __do_softirq () at kernel/softirq.c:284
#21 0xffffffff81058f70 in invoke_softirq () at kernel/softirq.c:364
#22 irq_exit () at kernel/softirq.c:405
#23 0xffffffff8187be94 in exiting_irq ()
at ./arch/x86/include/asm/apic.h:659
#24 do_IRQ (regs=0xffffc9000006be08) at arch/x86/kernel/irq.c:251
#25 0xffffffff8187a4bf in common_interrupt ()
at arch/x86/entry/entry_64.S:520
#26 0xffffc9000006be08 in ?? ()
#27 0x0000000000000000 in ?? ()
设置完SNAT后接收icmp包: NAT是
当服务器14.17.88.99回复了一个数据包后(src=14.17.88.99 dst=115.22.112.12)，进入到wan侧接口的PRE_ROUTING链时，
则在调用其nat相关的hook函数后，会调用函数ip_nat_packet获取到 origin tuple 值，然后再根据 origin tuple，计算出反方向的tuple，
即为new_tuple.src = 14.17.88.99 new_tuple.dst = 192.168.1.123,然后就会根据这个新的tuple修改其目的ip地址，
修改后的数据包的目的地址即为192.168.1.123 。然后再查找路由，将数据发送到正常的lan口。这就是nat的De-SNAT
路由地址：
ipt_do_table -->
nf_nat_ipv4_fn
在nf_nat_ipv4_fn函数中，首先上来是：nf_ct_get，ct: conntrack, 其中涉及到的数据结构有：
ip_conntrace_info / nf_conn_nat
下面的链接中有一个，详细解释了当设置ＳＮＡＴ之后，出包和进包的一个流程
当数据到达路由器的wan0口，进入到PRE_ROUTING时，会先建立一个nf_conn结构，和两个nf_conntrack_tuple（origin 与reply）
问题
１）prerouting 在哪里？
２）postrouting的代码在哪里？
nf_conntrack_l3proto_ipv4_init 初始化的啥东西？
链接跟踪正是在相应的函数中注册了相应的函数：nf_conntrack_l3proto_ipv4_init函数，
ipv4_conntrack_in -->
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