下面介绍centos7关闭firewall安装iptables，并且开启80端口、3306端口的操作记录：
[root@localhost ~]# cat /etc/redhat-release
CentOS Linux release 7.2.1511 (Core)
1、关闭firewall：
[root@localhost ~]# systemctl stop firewalld.service //停止firewall
[root@localhost ~]# systemctl disable firewalld.service //禁止firewall开机启动
2、安装iptables防火墙
[root@localhost ~]# yum install iptables-services //安装
[root@localhost ~]# vim /etc/sysconfig/iptables //编辑防火墙配置文件
# Firewall configuration written by system-config-firewall
# Manual customization of this file is not recommended.
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A INPUT -m state --state NEW -m tcp -p tcp --dport 3306 -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
COMMIT
[root@localhost ~]# systemctl restart iptables.service //最后重启防火墙使配置生效
[root@localhost ~]# systemctl enable iptables.service //设置防火墙开机启动
防止DoS攻击
iptables -A INPUT -p tcp --dport 80 -m limit --limit 25/minute --limit-burst 100 -j ACCEPT
-m limit: 启用limit扩展，限制速度。
--limit 25/minute: 允许最多每分钟25个连接
--limit-burst 100: 当达到100个连接后，才启用上述25/minute限制
--icmp-type 8 表示 Echo request——回显请求（Ping请求）。下面表示本机ping主机192.168.1.109时候的限速设置：
iptables -I INPUT -d 192.168.1.109 -p icmp --icmp-type 8 -m limit --limit 3/minute --limit-burst 5 -j ACCEPT
用一个例子来演示会更加清晰
