Something

#!/bin/sh

PATH=/usr/sbin:/sbin:/bin:/usr/bin

#
# delete all existing rules.
#
iptables -F
iptables -t nat -F
iptables -t mangle -F
iptables -X

# Always accept loopback traffic
iptables -A INPUT -i lo -j ACCEPT


# Allow established connections, and those not coming from the outside
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -m state --state NEW -i ! eth1 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT

# Allow outgoing connections from the LAN side.
iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT

# Masquerade.
iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
# Don't forward from the outside to the inside.
iptables -A FORWARD -i eth1 -o eth1 -j REJECT

iptables -A INPUT -p udp --sport 67 --dport 68 -j ACCEPT
iptables -A INPUT -p udp -j REJECT --reject-with icmp-port-unreachable
iptables -A INPUT -p icmp -j REJECT --reject-with icmp-port-unreachable
iptables -A INPUT -p tcp -j REJECT --reject-with tcp-reset
iptables -A INPUT -p tcp --dport 113 -j REJECT
/sbin/iptables -A INPUT -j REJECT

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