strongswan/ipsec正常搭建后，但无法连接。可能的问题是：需要配置下iptables设置：
sudo iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
sudo iptables -A FORWARD -s 10.31.0.0/24 -j ACCEPT
sudo iptables -A FORWARD -s 10.31.1.0/24 -j ACCEPT
sudo iptables -A FORWARD -s 10.31.2.0/24 -j ACCEPT
sudo iptables -A INPUT -i eth0 -p esp -j ACCEPT
sudo iptables -A INPUT -i eth0 -p udp --dport 500 -j ACCEPT
sudo iptables -A INPUT -i eth0 -p tcp --dport 500 -j ACCEPT
sudo iptables -A INPUT -i eth0 -p udp --dport 4500 -j ACCEPT
sudo iptables -A INPUT -i eth0 -p udp --dport 1701 -j ACCEPT
sudo iptables -A INPUT -i eth0 -p tcp --dport 1723 -j ACCEPT
sudo iptables -A FORWARD -j REJECT
sudo iptables -t nat -A POSTROUTING -s 10.31.0.0/24 -o eth0 -j MASQUERADE
sudo iptables -t nat -A POSTROUTING -s 10.31.1.0/24 -o eth0 -j MASQUERADE
sudo iptables -t nat -A POSTROUTING -s 10.31.2.0/24 -o eth0 -j MASQUERADE
