p2p内网穿透技术-udp打洞
在公司如何访问家里的电脑？实现方法有很多，其它p2p传输文件最快，能否实现？
udp打洞原理
假设有两台分别处于各自的私有网络中的主机：A和B；N1和N2是两个网络的NAT设备，分别拥有IP地址P1和P2；S是一个使用了一个众所周知的、从全球任何地方都能访问得到的IP地址的公共服务器 步骤一：A和B分别和S建立UDP连接；NAT设备N1和N2创建UDP转换状态并分配临时的外部端口号 步骤二：S检查UDP包，看A和B的端口是否是正在被使用的（否则的话N1和N2应该是应用了端口随机分配，这会让路由验证变得更麻烦） 步骤三：如果端口不是随机化的，那么A和B各自选择端口X和Y，并告知S。S会让A发送UDP包到P2:Y，让B发送UDP包到P1:X 步骤四：A和B通过转换好的IP地址和端口直接联系到对方的NAT设备；
udp打洞成功前提条件--非对称NAT
完全圆锥型NAT: 无条件转发
受限圆锥型NAT: 忽略掉其它主机的数据包(要求来源ip一直)
端口受限圆锥型NAT: 更进一步地要求源端口（上例中是 2727）必须跟之前发出的数据包的目的端口一致(要求来源ip和端口)
对称NAT： 丢弃报文，拒绝转发(每次连接端口会变)
对称NAT理论上可以使用端口预测，但是基本上用不了。现在国内几个宽带运行商都是使用对称NAT，所以udp打洞不可行。
记得之前上学的时候p2p共享软件很多，后来都销声匿迹，应该是被管控了。
socat测试udp打洞
下面是socat打洞的例子，试过在公司与家里之间打洞，用不了被现实了
https://blog.lilydjwg.me/2012/8/28/udp-hole-punching-experiment.35350.html
frp测试udp打洞
frp采用xtcp协议，试过同样用不了。stcp可以用，但是不是p2p，要经过中间转发。
检查nat类型?
#python直接用pynat包 pip install pynat
pynat
C:\Users\wyq>pynat Network type: Symmetric NAT #对称nat Internal address: 192.168.50.169:54320 External address: xxx.xxx.xxx.xxx:61195
内网穿透可行方法
| 方法 | 描述 | 实现条件 | 实际是否可行 |
| p2p | udp打洞 | 非对称nat | 不可行。宽带基本都是对称nat。除非在自己公网ip上提供p2p穿透 |
| ddns | 路由器使用有公网ip | 宽带提供了公网ip | 不可行。路由器基本没有公网ip |
| 端口转发 | 将端口映射到公网ip | 需要公网ip | 可行 |
| vpn | 要求有公网ip节点，运行vpn服务端 | 需要公网ip | 可行 |
参考来源
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