以下是我谷歌搜索后得到的结果：
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT #allow loopback access
iptables -A OUTPUT -d 255.255.255.255 -j ACCEPT #make sure you can communicate with any DHCP server
iptables -A INPUT -s 255.255.255.255 -j ACCEPT #make sure you can communicate with any DHCP server
iptables -A INPUT -s 192.168.0.0/16 -d 192.168.0.0/16 -j ACCEPT #make sure that you can communicate within your own network
iptables -A OUTPUT -s 192.168.0.0/16 -d 192.168.0.0/16 -j ACCEPT
iptables -A FORWARD -i eth+ -o tun+ -j ACCEPT
iptables -A FORWARD -i tun+ -o eth+ -j ACCEPT # make sure that eth+ and tun+ can communicate
iptables -t nat -A POSTROUTING -o tun+ -j MASQUERADE # in the POSTROUTING chain of the NAT table, map the tun+ interface outgoing packet IP address, cease examining rules and let the header be modified, so that we don't have to worry about ports or any other issue - please check this rule with care if you have already a NAT table in your chain
这就是我所做的：
答：由于我使用的是 Debian，所以我安装了 iptables-persistent。
B.我sudo /etc/iptables/rules.v4用以下内容替换了其中的默认值：
*filter
-A INPUT -i lo -j ACCEPT
-A OUTPUT -o lo -j ACCEPT #allow loopback access
-A OUTPUT -d 255.255.255.255 -j ACCEPT #make sure you can communicate with any DHCP server
-A INPUT -s 255.255.255.255 -j ACCEPT #make sure you can communicate with any DHCP server
-A INPUT -s 192.168.0.0/16 -d 192.168.0.0/16 -j ACCEPT #make sure that you can communicate within your own network
-A OUTPUT -s 192.168.0.0/16 -d 192.168.0.0/16 -j ACCEPT
-A FORWARD -i eth+ -o tun+ -j ACCEPT
-A FORWARD -i tun+ -o eth+ -j ACCEPT # make sure that eth+ and tun+ can communicate
-t nat -A POSTROUTING -o tun+ -j MASQUERADE # in the POSTROUTING chain of the NAT table, map the tun+ interface outgoing packet IP address, cease examining rules and let the header be modified, so that we don't have to worry about ports or any other issue - please check this rule with care if you have already a NAT table in your chain
-A OUTPUT -o eth+ ! -d 111.222.333.444 -j DROP # if destination for outgoing packet on eth+ is NOT a.b.c.d, drop the packet, so that nothing leaks if VPN disconnects
COMMIT
C. 我将更改保存到 /etc/iptables/rules.v4 并重新启动计算机。请注意，111.222.333.444 是位于 XYZ 国家/地区的 VPN 服务器的 IP 地址示例
D. 重新启动后，我无法连接到 VPN 服务器。 rules.v4 的内容有问题