当使用命令 sudo iptables -L -n 查看防火墙配置时，如果出现如下的三个空表：
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
含义如下：
Empty tables mean you have no rules. Having no rules means the table "policy" controls what happens to each packet traversing that table. The policy ACCEPT on each table means that all packets are allowed through each table. Thus, you have no firewall active.
By default, the firewall on Ubuntu (which can't be removed, because its part of the kernel) is unconfigured, and has default allow on everything. Unless you've added ufw or iptables rules, the firewall (which cannot be disabled because its part of the kernel) will not filter/block anything (DefaultALLOW rule).
