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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"