Masquerading Made Simple HOWTO
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2. Summary: (I like doing summaries first)

Assuming external internet card is eth0, and external IP is 123.12.23.43 and the internal network card is eth1, then: 


$>
 
modprobe ipt_MASQUERADE
 # If this fails, try continuing anyway

$>
 
iptables -F; iptables -t nat -F; iptables -t mangle -F


$>
 
iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 123.12.23.43


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

Or for a dial-up connection: 


$>
 
modprobe ipt_MASQUERADE
 # If this fails, try continuing anyway

$>
 
iptables -F; iptables -t nat -F; iptables -t mangle -F


$>
 
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE


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

Then to secure it: 


$>
 
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT


$>
 
iptables -A INPUT -m state --state NEW -i ! eth0 -j ACCEPT


$>
 
iptables -P INPUT DROP
   #only if the first two are succesful

$>
 
iptables -A FORWARD -i eth0 -o eth0 -j REJECT

Or for a dial-up connection (with eth0 as the internal network card): 


$>
 
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT


$>
 
iptables -A INPUT -m state --state NEW -i ! ppp0 -j ACCEPT


$>
 
iptables -P INPUT DROP
   #only if the first two are succesful

$>
 
iptables -A FORWARD -i ppp0 -o ppp0 -j REJECT

And thats it! To view the rules do " iptables -t nat -L "

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