gw_up.sh

#!/bin/bash
IF_IN="eth0"
IF_OUT="wlan0"
SUB="192.168.100"

echo "[+] Creating DHCP server config."
cat <<EOF > /etc/dhcp/dhcp.${IF_IN}.conf
option routers ${SUB}.1;
option domain-name-servers ${SUB}.1;
default-lease-time 14440;
ddns-update-style none;
deny bootp;
shared-network intranet {
    subnet ${SUB}.0 netmask 255.255.255.0 {
        option subnet-mask 255.255.255.0;
        pool { range ${SUB}.2 ${SUB}.5; }
    }
}
EOF

echo "[+] Bringing up interface ${IF_IN}"
ip link set dev ${IF_IN} up
ip addr add ${SUB}.1/24 dev ${IF_IN}
sleep 2

echo "[+] Setting up iptable rules"
modprobe iptable_nat
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -t nat -A POSTROUTING -s ${SUB}.0/24 -j MASQUERADE
iptables -A FORWARD -o ${IF_IN} -i ${IF_OUT} -s ${SUB}.0/24 -m conntrack --ctstate NEW -j ACCEPT
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT

echo "[+] Launching DHCP service"
dhcpd -cf /etc/dhcp/dhcp.${IF_IN}.conf ${IF_IN}
echo "[+] Done"