Simple script to enable ip forwarding for basic routing

start-forwarding.sh

#!/bin/bash

logger "Telling kernel to turn on ipv4 ip_forwarding"
echo 1 > /proc/sys/net/ipv4/ip_forward
logger "Done. Setting up iptables rules to allow FORWARDING"

DOWNSTREAM=wlan0 # wlan0 is client network (running hostapd)
UPSTREAM=eth0 # eth0 is upstream network (internet)

# Allow IP Masquerading (NAT) of packets from clients (downstream) to upstream network (internet)
iptables -t nat -A POSTROUTING -o $UPSTREAM -j MASQUERADE

# Forward packets from downstream clients to the upstream internet
iptables -A FORWARD -i $DOWNSTREAM -o $UPSTREAM -j ACCEPT

# Forward packers from the internet to clients IF THE CONNECTION IS ALREADY OPEN!
iptables -A FORWARD -i $UPSTREAM  -o $DOWNSTREAM -m state --state RELATED,ESTABLISHED -j ACCEPT

logger "Done setting up iptables rules. Forwarding enabled"