USB Relay Linux Docs Update

docs/usb_relay.md

@@ -26,7 +26,7 @@ type nul > ssh
 ```
 
 
-Now we have some incomprensible configuration steps to set up the internet access relay.
+Now we have some incomprehensible configuration steps to set up the internet access relay.
 
 Edit `config.txt`:
 ```
@@ -63,6 +63,7 @@ blah blah rootwait modules-load=dwc2,g_ether [possibly more blah]
 
 Exit and save changes (CTRL-X, then "y" in nano).
 
+If you are on Linux, you need to first [set a static IP](#set-a-static-ip-on-linux) for both your Pi Zero 1.3 and your host computer. Otherwise, continue with the steps below.
 
 Eject the SD card and insert it into your Pi Zero 1.3. Plug a USB cable into your computer and into the Pi's USB connector that is closer to the center. It will draw power from the USB cable and begin powering up.
 
@@ -85,6 +86,7 @@ Are you sure you want to continue connecting (yes/no/[fingerprint])?
 At the password prompt enter the Pi's default password: `raspberry`
 
 If you now see the Pi's command prompt, you're in!
+
 <img src="img/usb_relay_01.png">
 
 Notice this warning in particular:
@@ -139,6 +141,92 @@ If you see a ping response, the Pi has internet access!
 see: https://www.circuitbasics.com/raspberry-pi-zero-ethernet-gadget/
 
 
+### Linux
+To enable IP forwarding on your Host:
+```
+nano /etc/sysctl.conf
+```
+
+and uncomment the following line:
+```
+net.ipv4.ip_forward = 1
+```
+
+Then run `sudo sysctl -p` to have immediate effect.
+
+You should now be able to SSH into your internet connected Pi Zero 1.3, as described in the ["Get started"](#get-started) section.
+
+
+## Set a static IP on Linux
+_This guide is intended for Debian/Ubuntu based distros._
+
+In order to set up the connection between your computer and the Pi Zero, you will have to set a static IP on both endpoints of the connection.
+
+What you will need:
+
+* `def_iface` is the Iface of the "default" Destination network. On your __Host__ machine, type `route` to find it.
+* `host_ip` is the IP address of your Host on the USB interface. For this guide we will use `192.168.21.20`
+* `rpi_ip` is the IP address of your Pi Zero on the USB interface. Use `192.168.21.21`
+
+_Note: Remember to remove the brackets. For example, `ip_address=<rpi_ip>` should be written as: `ip_address=192.168.21.21`_
+
+#### On the SD card
+```
+cd rootfs/etc
+nano dhcpcd.conf
+```
+At the end of the file, add the following lines.
+```
+interface usb0
+static ip_address=<rpi_ip>/24
+static routers=<host_ip>
+static domain_name_servers=8.8.8.8 8.8.4.4
+```
+
+Eject the SD card and insert it into your Pi Zero 1.3. Plug a USB cable into your computer and into the Pi's USB connector that is closer to the center. It will draw power from the USB cable and begin powering up. 
+
+#### On the Host machine
+
+Type `lsusb` to see if your Pi Zero is properly connected. Look for `Ethernet/RNDIS Gadget`:
+
+![Static IP on the seedsigner Interface](img/usb_relay_linux_01.png)
+
+Set a name for your new Interface:
+```
+nano /etc/systemd/network/10-rename-rpi0.link
+```
+
+Now paste in the following text:
+```
+[Match]
+Property=ID_VENDOR_ID=0525 "ID_MODEL_ID=a4a2"
+[Link]
+Name=seedsigner0
+```
+
+To give your Host a static ip:
+```
+nano /etc/network/interfaces.d/seedsigner
+```
+
+Now paste in the following text:
+```
+auto seedsigner0
+iface seedsigner0 inet static
+address <host_ip>
+netmask 255.255.255.0
+post-up /sbin/iptables -t nat -A POSTROUTING -o <def_iface> -j MASQUERADE
+```
+
+Reboot your computer.
+
+To see if everything is set up correctly, type `ifconfig` and look for the `seedsigner0` interface. It should have the <host_ip> assigned to it. 
+
+![Static IP on the seedsigner Interface](img/usb_relay_linux_02.png)
+
+Continue by configuring your host computer to [share internet access](#linux) with your Pi Zero.
+
+
 ## Complete the setup
 Return to the main [README](../README.md) and complete the setup steps. But remember to come back here and disable internet access over USB on your SeedSigner.