Setup NAT server on Linux

1. change the value of ip_forward from 0 to 1

CentOS

echo "1" > /proc/sys/net/ipv4/ip_forward


Ubuntu

sudo vi /etc/sysctl.conf
--
..
net.ipv4.ip_forward=1
..

sudo sysctl -p # load sysctl settings

2. load related modules

/sbin/modprobe ip_nat_ftp
/sbin/modprobe ip_conntrack
/sbin/modprobe ip_conntrack_ftp
/sbin/modprobe ip_conntrack_irc

3. 192.168.1.1 ~ 192.168.1.254 route through ppp0

/sbin/iptables -t nat -A POSTROUTING -o ppp0 -s 192.168.1.0/24 -j MASQUERADE

4. if you want the above things done every time you boot the machine, you can add these into /etc/rc.local