centos双网卡，eth0通外网，eth1内网，其他内部机器和eth1一个段且设置网关为eth1的ip.
#!/bin/sh
echo “\n\nSETTING UP IPTABLES NAT”
IF_INTERN=”eth1″
IF_EXTERN=”eth0″
IF_INTERN_IP=”$(/sbin/ifconfig $IF_INTERN | grep ‘inet addr’ | awk ‘{print $2}’ | sed -e ‘s/.*://’)”
IF_EXTERN_IP=”$(/sbin/ifconfig $IF_EXTERN | grep ‘inet addr’ | awk ‘{print $2}’ | sed -e ‘s/.*://’)”
echo ” Loading required stateful/NAT kernel modules…”
/sbin/depmod -a
/sbin/modprobe ip_tables
/sbin/modprobe ip_conntrack
/sbin/modprobe ip_conntrack_ftp
/sbin/modprobe ip_conntrack_irc
/sbin/modprobe iptable_nat
/sbin/modprobe ip_nat_ftp
/sbin/modprobe ip_nat_irc
/sbin/modprobe ipt_LOG
echo ” Configurating system parameters…”
echo “1” > /proc/sys/net/ipv4/ip_forward
echo “1” > /proc/sys/net/ipv4/ip_dynaddr
echo ” External interface: $IF_EXTERN ($IF_EXTERN_IP)”
echo ” Internal interface: $IF_INTERN ($IF_INTERN_IP)”
echo ” Loading iptables rule sets…”
/sbin/iptables -P INPUT ACCEPT
/sbin/iptables -F INPUT
/sbin/iptables -P OUTPUT ACCEPT
/sbin/iptables -F OUTPUT
/sbin/iptables -P FORWARD DROP
/sbin/iptables -F FORWARD
/sbin/iptables -t nat -F
/sbin/iptables -A FORWARD -i $IF_EXTERN -o $IF_INTERN -m state –state ESTABLISHED,RELATED -j ACCEPT
/sbin/iptables -A FORWARD -i $IF_INTERN -o $IF_EXTERN -j ACCEPT
/sbin/iptables -t nat -A POSTROUTING -o $IF_EXTERN -j MASQUERADE