To do this we're going to use dnsmasq to set up the RPi to be a DHCP server and set up some custom DNS settings. This will allow the device connected to the RPi through ethernet to get an IP address from the RPi and also to for the RPi to pass DNS queries.
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We'll also configure some iptables settings to make a NAT between the ethernet adapter and the Wifi connection.
First, install dnsmasq:
sudo apt-get install dnsmasq
Set your ethernet adapter to a static IP address
This will serve as a gateway for the device that you want to connect to the RPi ethernet port. Most Wifi routers use what's called a Private Network and set the IP range to something similar to:
192.168.1.1
For the ethernet adapter on your RPI you'll want to set that to an address that won't interfere with the routers ability to assign addresses, so we'll increment the subnet of the PRi to be:
192.168.2.1
Along with that you'll need to set up the the netmask to:
255.255.255.0
As well as the DCHP settings to broadcast what IP address are available:
network 192.168.2.0
broadcast 192.168.2.255
Raspberry Pi 2 Wifi Dongle
Use iptables to configure a NAT setting to share the Wifi connection with the ethernet port
NAT stands for Network Address Translation. This allows a single IP address to server as a router on a network. So in this case the ethernet adapter on the RPi will serve as the router for whatever device you attach to it. The NAT settings will route the ethernet requests through the Wifi connection.
There are several commands to run here:
sudo iptables -F
sudo iptables -t nat -F
sudo iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE
sudo iptables -A FORWARD -i wlan0 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
sudo iptables -A FORWARD -i eth0 -o wlan0 -j ACCEPT
Configure the dnsmasq settings
The first thing to do is to turn on IP forwarding. This is done by putting a single number 1 in the /proc/sys/net/ipv4/ip_forward file:
sudo nano /proc/sys/net/ipv4/ip_forward
Put a 1 on the first line and then exit and save. Hint: you also may have to edit /etc/sysctl.conf and uncomment this line:
net.ipv4.ip_forward=1
Next set up ip routing:
Raspberry Pi 2 Projects
sudo ip route del 0/0 dev eth0 &> /dev/null
a=`route | awk '/${wlan}/'{print $5+1;exit}'`
sudo route add -net default gw 192.168.2.1 netmask 0.0.0.0 dev eth0 metric $a
The last thing to do is edit your /etc/dnsmasq.conf file and include these settings;
interface=eth0
bind-interfaces
server=8.8.8.8
domain-needed
bogus-priv
dhcp-range=192.168.2.2,192.168.2.100,12h
Raspberry Pi 2 Wifi Setup
Then run this command to start your dnsmasq services:
sudo systemctl start dnsmasq
Raspberry Pi Usb Wifi
Now plug a CAT5 network cable into the device you want to include on the network and put the other end of the cable into the ethernet port on the RPi and you should be good to go! When we set up the ethernet interface we made it hot pluggable, so you should see the ethernet interface come up when you plug the device into the RPi.