Using Raspbery Pi 3 as wireless client router

The title may seem confusing, but it covers a specific use case: you have a machine with ethernet port only, and you want to connect it to a wifi network. For whatever reason, you can't, or don't want, use a wireless card with your computer. As for me, I have Amiga 500 with Plipbox, and Rapberry Pi B+ with RISC OS 5, and both can be connected to the Internet - but using a few meters long network cable, lying on the floor, to reach a cable TV router is far from what I'd call a comfortable solution.

The Internet is full of recepies of how to use Raspberry Pi 3 as a wireless access point, but I wanted to use it the other way round. I wanted it to be a wireless client, and at the same time act as a router, so I could connect my ethernet-only hardware to the Internet through it. Sadly, I couldn't find any sensible description, so I finally figured it out myself. It's not an ideal solution, and it uses static addresses instead of DHCP, but it's fairly simple, and it works for me. If you have a better one, feel free to point to it in the comments.

My recipie works with Debian 9 (a.k.a. Raspbian Stretch) with desktop. Take note that it may not work with other Linux versions. I assume that you know how to install it on SD card and how to run it, so I'll go straight to the network setup.

First, you need to configure Raspberry Pi to connect to the wifi network. Turn it on and wait for the system to load. Now click on the wifi icon located on the dock and choose your network. Enter the network password and wait until the connection becomes established. You can check if everything works correctly by pinging google.com or some other address.

Now you need to edit a few system files. They are only writeable by the user "root" so you can either use a console editor, like vim or nano, with the "sudo" command, or run "gksu gedit" from the desktop. I prefer the first method, so I'll use this one.

First, you need to disable dhcp for the ethernet interface. It's because Raspberry Pi is meant to work as a client and so it will try to reconfigure the ethernet port everytime it discovers a network cable has been attached to it. So do:

sudo nano /etc/dhcpcd.conf

and add the following line at the end of the file:

denyinterfaces eth0

Next, you need to enable packet forwarding between the network interfaces. To do this, uncomment the following line in /etc/sysctl.conf:

net.ipv4.ip_forward=1

Finally, you need to configure routing. I use 10.0.0.0/24 for my network, but you can use any non-routable addresses for this. So, add the following lines to /etc/rc.local before "exit 0":

ifconfig eth0 10.0.0.1 netmask 255.255.255.0 up
iptables -t nat -A POSTROUTING -s 10.0.0.0/24 -j MASQUERADE
iptables -A FORWARD -s 10.0.0.0/24 -j ACCEPT

The first line sets up the IP address and netmask for the ethernet port. The second one enables packet translation, which means that the network packets will be able to get from the 10.0.0.0/24 network to the outside world. The last rule allows those packets to pass through the built-in firewall.

Now reboot the Raspberry Pi. After a while it will connect to the wifi network (you can say when it is when the yellow LED on the board stops binking), and it will allow clients to use its ethernet port as a gateway.

I will not describe client configuration in detail, because it differs very much among operating systems. I'll just give you the parameters you need to enter to make the client work:
1) IP address: anything between 10.0.0.2 and 10.0.0.254. If you use more than one device (for example connected through a switch) rememer that each one of them needs its own unique IP address.
2) Netmask: 255.255.255.0
3) Gateway: 10.0.0.1
4) DNS servers: use 8.8.8.8 as primary and 8.8.4.4 as secondary. Alternatively, you can use your home router's IP address (e.g. 192.168.1.1) or your ISP provider's DNS hosts - if you know what they are.