特别说明：此文章完全转载于https://www.cnblogs.com/EasonJim/p/6851007.html
1.查看系统是否安装防火墙
root@localhost:/usr# which iptables
/sbin/iptables
root@localhost:/usr# whereis iptables
iptables: /sbin/iptables /etc/iptables /usr/share/iptables /usr/share/man/man8/iptables.8.gz
如果是这样的信息，那么表明iptables就是安装了的。
如果没有安装，那么使用sudo apt-get install iptables 安装。
2.查看防火墙的配置信息
配置好了的，是这个样子。
root@localhost:/usr# sudo iptables -L
Chain INPUT (policy DROP)
target prot opt source destination
ACCEPT all -- anywhere anywhere
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt: 22
ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:http
ACCEPT icmp -- anywhere anywhere limit: avg 100/sec burst 100
ACCEPT icmp -- anywhere anywhere limit: avg 1/sec burst 10
syn-flood tcp -- anywhere anywhere tcp flags:FIN,SYN,RST,ACK/SYN
REJECT all -- anywhere anywhere reject-with icmp-host-prohibited
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
Chain syn-flood (1 references)
target prot opt source destination
RETURN tcp -- anywhere anywhere limit: avg 3/sec burst 6
REJECT all -- anywhere anywhere reject-with icmp-port-unreachable
3.新建规则文件
mkdir /etc/iptables #先新建目录，本身无此目录
vim /etc/iptables/rules.v4
/etc/iptables/rules.v4 中的内容是
*filter
:INPUT DROP [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:syn-flood - [0:0]
-A INPUT -i lo -j ACCEPT
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 3306 -j ACCEPT
-A INPUT -p icmp -m limit --limit 100/sec --limit-burst 100 -j ACCEPT
-A INPUT -p icmp -m limit --limit 1/s --limit-burst 10 -j ACCEPT
-A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j syn-flood
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A syn-flood -p tcp -m limit --limit 3/sec --limit-burst 6 -j RETURN
-A syn-flood -j REJECT --reject-with icmp-port-unreachable
COMMIT
4.使防火墙生效
iptables-restore < /etc/iptables/rules.v4
5.创建文件，添加以下内容，使防火墙开机启动
vim /etc/network/if-pre-up.d/iptables
#!/bin/bash
iptables-restore < /etc/iptables/rules.v4
6.添加执行权限
chmod +x /etc/network/if-pre-up.d/iptables
7.查看规则是否生效
iptables -L -n
Ubuntu中没有直接停止关闭iptables的命令，像service iptables stop这类命令，是centos才有的。关闭的话，可以暂时开放所有端口作为替代方案
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT
还可以参考https://blog.csdn.net/langsim/article/details/42644451