ipchains -P forward DENY
ipchains -A forward -s 192.168.0.0/24 -d 0/0 -j MASQ
thats it
you also need to set forward to 1
[david@xxxxxx /]$ cat /proc/sys/net/ipv4/ip_forward
1
[david@xxxxxx /]$
if it says 0 do this
[root@xxxxxx /]# echo 1 > /proc/sys/net/ipv4/ip_forward