Linux之iptables(四、网络防火墙及NAT)

网络防火墙

• iptables/netfilter网络防火墙：
• (1) 充当网关
• (2) 使用filter表的FORWARD链
• 注意的问题：
• (1) 请求-响应报文均会经由FORWARD链，要注意规则的方向性
• (2) 如果要启用conntrack机制，建议将双方向的状态为ESTABLISHED的报文直接放行

NAT

• NAT: network address translation
• PREROUTING，INPUT，OUTPUT，POSTROUTING
• 请求报文：修改源/目标IP，由定义如何修改
• 响应报文：修改源/目标IP，根据跟踪机制自动实现
• SNAT：source NAT POSTROUTING, INPUT
• 让本地网络中的主机通过某一特定地址访问外部网络，实现地址伪装
• 请求报文：修改源IP
• DNAT：destination NAT PREROUTING , OUTPUT
• 把本地网络中的主机上的某服务开放给外部网络访问(发布服务和端口映射)，但隐藏真实IP
• 请求报文：修改目标IP
• PNAT: port nat，端口和IP都进行修改

SNAT

• nat表的target：
• SNAT：固定IP
• --to-source [ipaddr[-ipaddr]][:port[-port]]
• --random
• iptables -t nat -A POSTROUTING -s LocalNET ! -d LocalNet -j SNAT --to-source ExtIP

[root@centos7a ~]#iptables -t nat -A POSTROUTING -s 10.0.1.0/24 ! -d 10.0.1.0/24 -j SNAT --to-source 172.16.32.6-172.16.32.10
[root@centos7a ~]#iptables -nvL -t nat
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 SNAT       all  --  *      *       10.0.1.0/24         !10.0.1.0/24          to:172.20.71.105-172.20.71.110

SNAT

• MASQUERADE：动态IP，如拨号网络
• --to-ports port[-port]
• --random
• iptables -t nat -A POSTROUTING -s LocalNET ! -d LocalNet -j MASQUERADE

[root@centos7a ~]#iptables -t nat -I POSTROUTING -s 10.0.1.0/24 ! -d  10.0.1.0/24 -j MASQUERADE
[root@centos7a ~]#iptables -nvL -t nat
Chain PREROUTING (policy ACCEPT 4 packets, 765 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain INPUT (policy ACCEPT 4 packets, 765 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain OUTPUT (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         

Chain POSTROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 MASQUERADE  all  --  *      *       10.0.1.0/24         !10.0.1.0/24     

DNAT

• --to-destination [ipaddr[-ipaddr]][:port[-port]]
• iptables -t nat -A PREROUTING -d ExtIP -p tcp|udp --dport PORT -j DNAT --to-destination InterSeverIP[:PORT]

[root@centos7a ~]#iptables -t nat -A PREROUTING -s 0/0 -d 172.16.32.6 -p tcp --dport 22 -j DNAT --to-destination 10.0.1.22
[root@centos7a ~]#iptables -nvL -t nat
Chain PREROUTING (policy ACCEPT 1 packets, 78 bytes)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 DNAT       tcp  --  *      *       0.0.0.0/0            172.32.20.6          tcp dpt:22 to:10.0.1.22

[root@centos7a ~]#iptables -t nat -A PREROUTING -s 0/0 -d 172.16.32.6 -p tcp --dport 80 -j DNAT --to-destination 10.0.1.22:80
[root@centos7a ~]#iptables -nvL -t nat
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 DNAT       tcp  --  *      *       0.0.0.0/0            172.18.100.6         tcp dpt:80 to:10.0.1.22:80

PNAT：利用虚拟端口进行数据转发

转发

• REDIRECT：
• NAT表
• 可用于：PREROUTING OUTPUT 自定义链
• 通过改变目标IP和端口，将接受的包转发至不同端口
• --to-ports port[-port]

[root@centos7a ~]#iptables -t nat -A PREROUTING -d 172.16.32.6 -p tcp --dport 80 -j REDIRECT --to-ports 8080
[root@centos7a ~]#iptables -nvL -t nat
Chain PREROUTING (policy ACCEPT 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination         
    0     0 REDIRECT   tcp  --  *      *       0.0.0.0/0            172.16.100.10        tcp dpt:80 redir ports 8080