server要加默认路由个client
push "route 10.8.0.1"
push "redirect-gateway def1"
;log-append openvpn.log不记录log
内核要有iptable_nat和tun模块，分别是iptables转发和openvpn要用到的
转发：iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE
sysctl -w net.ipv4.ip_forward=1