linux-A 主机配置eth0即可：
[root@linux-A ~]# ifconfig eth0|sed -n '2p'
inet addr:192.168.20.3 Bcast:192.168.20.255 Mask:255.255.255.0
[root@linux-A ~]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.20.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 1002 0 0 eth0
0.0.0.0 192.168.20.2 0.0.0.0 UG 0 0 0 eth0
[root@linux-A ~]# /etc/init.d/iptables status
iptables: Firewall is not running.
linux-B网关服务器配置及步骤：
1.先配置好每个网卡的IP地址（linux-B的eth0必须要可以上外网），并置零所有规则和计数器
[root@linux-B ~]# ifconfig eth0|sed -n '2p'
inet addr:10.0.0.4 Bcast:10.0.0.255 Mask:255.255.255.0
[root@linux-B ~]# ifconfig eth1|sed -n '2p' #<===无需配置网关
inet addr:192.168.20.2 Bcast:192.168.20.255 Mask:255.255.255.0
[root@linux-B ~]# route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.20.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
10.0.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 1002 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 1003 0 0 eth1
0.0.0.0 10.0.0.254 0.0.0.0 UG 0 0 0 eth0
[root@linux-B~]# iptables -F
[root@linux-B~]# iptables -X
[root@linux-B~]# iptables -Z
[root@linux-B ~]# iptables -P FORWARD ACCEPT
[root@linux-B ~]# iptables -L -n #<==实验时确保以下为允许状态，然后关闭iptables防火墙测试 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
[root@ B-linux ~]# /etc/init.d/iptables stop #<==先配置好网关，共享上网成功后再看情况开防火墙，表不同，先一个表调整好再调整另一个表
2.linux-B服务器主机开启 ip_forward 转发功能，并加载相应的模块
[root@linux-B ~]# grep "ip_forward" /etc/sysctl.conf
net.ipv4.ip_forward = 1 #<==确保转发功能开启
[root@linux-B ~]# sysctl -p
[root@linux-B~]# modprobe ip_tables
[root@linux-B~]# modprobe iptable_filter
[root@linux-B~]# modprobe iptable_nat
[root@linux-B~]# modprobe ip_conntrack
[root@linux-B~]# modprobe ip_conntrack_ftp
[root@linux-B ~]# modprobe ip_nat_ftp
[root@linux-B ~]# lsmod |egrep ^ip #<===确保模块加载，模块加载最好放在 /etc/rc.local 下
3.配置地址转换实现共享上网（POSTROUTING链）
[root@linux-B ~]# iptables -t nat -A POSTROUTING -s 192.168.20.0/24 -o eth0 -j SNAT --to-source 10.0.0.4
参数说明：
-s 192.168.20.0/24 类似生产环境中的内网网段
-o eth0 类似路由器中，有外网IP网卡的接口
-j SNAT --to-source 10.0.0.4 将内网网段IP地址及端口转换为外网IP地址及端口
或使用以下命令也可以实现地址转换
[root@linux-B ~]# iptables -t nat -A POSTROUTING -s 192.168.20.0/24 -o eth0 -j MASQUERADE #<==将外网eth0接口动态获取的IP地址作为地址转换
4.配置目的地址转换/端口转换（PREROUTING链）
[root@linux-B ~]# iptables -t nat -A PREROUTING -i eth0 -d 10.0.0.4 -p tcp --dport 80 -j DNAT --to-destination 192.168.20.3:8080
5.在linux-A主机上ping测试，或其他方法测试查看转换是否成功
[root@linux-A ~]# traceroute www.baidu.com
traceroute to www.baidu.com (14.215.177.39), 30 hops max, 60 byte packets
1 192.168.20.2 (192.168.20.2) 0.421 ms 0.284 ms 0.362 ms
2 10.0.0.254 (10.0.0.254) 0.643 ms 0.734 ms 0.627 ms [root@linux-A ~]# curl -I www.baidu.com
HTTP/1.1 200 OK
[root@linux-B ~]# iptables -L -n -t nat
Chain PREROUTING (policy ACCEPT)
target prot opt source destination Chain POSTROUTING (policy ACCEPT)
target prot opt source destination
SNAT all -- 192.168.20.0/24 0.0.0.0/0 to:10.0.0.4 Chain OUTPUT (policy ACCEPT)
target prot opt source destination
提示：更好的方法是利用脚本进行管理，参考脚本如下（脚本来源：老男孩老师QQ49000448）
#!/bin/bash
#!/bin/bash
#this is a server firewall created by oldboy 17:03 2006-7-26
# e_mail:31333741@qq.com
# qqinfo:49000448
# function: a server firewall
# version:1.1
################################################
# oldboy trainning info.
# QQ 1986787350 70271111
# site:http://www.etiantian.org
# blog:http://oldboy.blog.51cto.com
# oldboy trainning QQ group: 208160987 45039636
################################################
#define variable PATH
IPT=/sbin/iptables
LAN_GW_IP=192.168.0.15
WAN_GW_IP=10.0.0.15
LAN_SERVER=192.168.0.14 echo 1 > /proc/sys/net/ipv4/ip_forward
modprobe ip_tables
modprobe iptable_filter
modprobe iptable_nat
modprobe ip_conntrack
modprobe ip_conntrack_ftp
modprobe ip_nat_ftp
modprobe ipt_state #Remove any existing rules
$IPT -F
$IPT -X #setting default firewall policy
$IPT --policy OUTPUT ACCEPT
$IPT --policy FORWARD ACCEPT
$IPT -P INPUT DROP #setting for loopback interface
$IPT -A INPUT -i lo -j ACCEPT
$IPT -A OUTPUT -o lo -j ACCEPT $IPT -A INPUT -m state --state INVALID -j DROP
$IPT -A OUTPUT -m state --state INVALID -j DROP # Source Address Spoofing and Other Bad Addresses
$IPT -A INPUT -i eth0 -s 172.16.0.0/12 -j DROP
$IPT -A INPUT -i eth0 -s 0.0.0.0/8 -j DROP
$IPT -A INPUT -i eth0 -s 169.254.0.0/16 -j DROP
$IPT -A INPUT -i eth0 -s 192.0.2.0/24 -j DROP # prevent all Stealth Scans and TCP State Flags
$IPT -A INPUT -p tcp --tcp-flags ALL ALL -j DROP
# All of the bits are cleared
$IPT -A INPUT -p tcp --tcp-flags ALL NONE -j DROP
$IPT -A INPUT -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP
#SYN and RST are both set
$IPT -A INPUT -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
# SYN and FIN are both set
$IPT -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
# FIN and RST are both set
$IPT -A INPUT -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
# FIN is the only bit set, without the expected accompanying ACK
$IPT -A INPUT -p tcp --tcp-flags ACK,FIN FIN -j DROP
# PSH is the only bit set, without the expected accompanying ACK
$IPT -A INPUT -p tcp --tcp-flags ACK,PSH PSH -j DROP
# URG is the only bit set, without the expected accompanying ACK
$IPT -A INPUT -p tcp --tcp-flags ACK,URG URG -j DROP #setting access rules
#one,ip access rules,allow all the ips of hudong.com
$IPT -A INPUT -s 202.81.17.0/24 -p all -j ACCEPT
$IPT -A INPUT -s 202.81.18.0/24 -p all -j ACCEPT
$IPT -A INPUT -s 124.43.62.96/27 -p all -j ACCEPT
$IPT -A INPUT -s 192.168.1.0/24 -p all -j ACCEPT
$IPT -A INPUT -s 10.0.0.0/24 -p all -j ACCEPT #second,port access rules
#nagios
$IPT -A INPUT -s 192.168.1.0/24 -p tcp --dport 5666 -j ACCEPT
$IPT -A INPUT -s 202.81.17.0/24 -p tcp --dport 5666 -j ACCEPT
$IPT -A INPUT -s 202.81.18.0/24 -p tcp --dport 5666 -j ACCEPT #db
$IPT -A INPUT -s 192.168.1.0/24 -p tcp --dport 3306 -j ACCEPT
$IPT -A INPUT -s 192.168.1.0/24 -p tcp --dport 3307 -j ACCEPT
$IPT -A INPUT -s 192.168.1.0/24 -p tcp --dport 3308 -j ACCEPT
$IPT -A INPUT -s 192.168.1.0/24 -p tcp --dport 1521 -j ACCEPT #ssh difference from other servers here.........................................................>>
$IPT -A INPUT -s 202.81.17.0/24 -p tcp --dport 50718 -j ACCEPT
$IPT -A INPUT -s 202.81.18.0/24 -p tcp --dport 50718 -j ACCEPT
$IPT -A INPUT -s 124.43.62.96/27 -p tcp --dport 50718 -j ACCEPT
$IPT -A INPUT -s 192.168.1.0/24 -p tcp --dport 50718 -j ACCEPT
$IPT -A INPUT -p tcp --dport 22 -j ACCEPT
#ftp
#$IPT -A INPUT -p tcp --dport 21 -j ACCEPT #http
$IPT -A INPUT -p tcp --dport 80 -j ACCEPT
$IPT -A INPUT -s 192.168.1.0/24 -p tcp -m multiport --dport 8080,8081,8082,8888,8010,8020,8030,8150 -j ACCEPT
$IPT -A INPUT -s 202.81.17.0/24 -p tcp -m multiport --dport 8080,8081,8082,8888,8010,8020,8030,8150 -j ACCEPT
$IPT -A INPUT -s 124.43.62.96/27 -p tcp -m multiport --dport 8080,8081,8082,8888,8010,8020,8030,8150 -j ACCEPT #snmp
$IPT -A INPUT -s 192.168.1.0/24 -p UDP --dport 161 -j ACCEPT
$IPT -A INPUT -s 202.81.17.0/24 -p UDP --dport 161 -j ACCEPT
$IPT -A INPUT -s 202.81.18.0/24 -p UDP --dport 161 -j ACCEPT #rsync
$IPT -A INPUT -s 192.168.1.0/24 -p tcp -m tcp --dport 873 -j ACCEPT
$IPT -A INPUT -s 202.81.17.0/24 -p tcp -m tcp --dport 873 -j ACCEPT
$IPT -A INPUT -s 202.81.18.0/24 -p tcp -m tcp --dport 873 -j ACCEPT
$IPT -A INPUT -s 124.43.62.96/27 -p tcp -m tcp --dport 873 -j ACCEPT #nfs 2049,portmap 111
$IPT -A INPUT -s 192.168.1.0/24 -p udp -m multiport --dport 111,892,2049 -j ACCEPT
$IPT -A INPUT -s 192.168.1.0/24 -p tcp -m multiport --dport 111,892,2049 -j ACCEPT #others RELATED
#$IPT -A INPUT -p icmp -m icmp --icmp-type any -j ACCEPT
$IPT -A INPUT -s 124.43.62.96/27 -p icmp -m icmp --icmp-type any -j ACCEPT
$IPT -A INPUT -s 192.168.1.0/24 -p icmp -m icmp --icmp-type any -j ACCEPT $IPT -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT ###############nat start##############################
#nat internet
iptables -t nat -A POSTROUTING -s 192.168.0.0/255.255.255.0 -o eth1 -j SNAT --to-source $LAN_GW_IP #www server nat wan to lan
iptables -t nat -A PREROUTING -d $WAN_GW_IP -p tcp -m tcp --dport 80 -j DNAT --to-destination $$LAN_SERVER:80
iptables -t nat -A POSTROUTING -d $LAN_SERVER -p tcp --dport 80 -j SNAT --to LAN_GW_IP
更多内幕知识及原理深入了解，看下面好友链接