Gateway script for RPI using HUAWEI E3372 LTE

# simple gateway script working on Raspberry PI with HUAWEI E3372 LTE
#
#!/bin/sh

PATH=/usr/sbin:/sbin:/bin:/usr/bin
LAN="eth0"
NET="ppp0"

#
# 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 ! $NET -j ACCEPT
iptables -A FORWARD -i $NET -o $LAN -m state --state ESTABLISHED,RELATED -j ACCEPT

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

# Masquerade.
iptables -t nat -A POSTROUTING -o $NET -j MASQUERADE

# Don't forward from the outside to the inside.
iptables -A FORWARD -i $NET -o $NET -j REJECT

# Enable routing.
echo 1 > /proc/sys/net/ipv4/ip_forward