How to quickly configure your box as a gateway granting devices connected on ethernet access to wifi network

simple_gateway

First ensure your ethernet isn't managed by network-manager by adding the following to `/etc/NetworkManager/NetworkManager.conf`:

```
[keyfile]
unmanaged-devices=mac:00:11:22:33:44
```

Replacing the MAC address with that of your ethernet device. 

Then run:

```
sudo systemctl restart NetworkManager
```

Install dnsmasq which we'll use only as a DHCP server:

```
sudo apt install dnsmasq
```

Stop it and prevent it from starting on boot

```
sudo systemctl stop dnsmasq
sudo systemctl disable dnsmasq
```

Run the following script as root to set a static IP on your ethernet interface, configure iptables as a NAT gateway and start dnsmasq as a DHCP server.
Remember to replace the interface names.

```
#!/bin/bash

# Check if root
if [[ $EUID -ne 0 ]]; then
   echo "This script must be run as root" 
   exit 1
fi

ETH=enp0s25
WLAN=wlp4s0
IP=172.23.0.1/24
DHCPRANGE=172.23.0.50,172.23.0.150

# Set stat IP
ip addr flush dev $ETH
ip addr add dev $ETH $IP
ip link set dev $ETH up

# Configure as NAT gateway
echo "1" > /proc/sys/net/ipv4/ip_forward
iptables -P FORWARD ACCEPT
iptables -t nat -F POSTROUTING
iptables -t nat -A POSTROUTING -o $WLAN -j MASQUERADE

# Start dnsmasq as DHCP server and keep in foreground
dnsmasq --conf-file=/dev/null --bind-dynamic -i $ETH --no-daemon --dhcp-option=6,$IP --dhcp-range=${DHCPRANGE},10m --log-dhcp --log-facility -
```

To disable, hit ctrl-c to stop dnsmasq then do something like this:

```
iptables -t nat -F POSTROUTING
iptables -F FORWARD
echo "0" > /proc/sys/net/ipv4/ip_forward
ip link set dev $ETH down
```