usb_relay.md

Relaying internet access to the Pi Zero 1.3 over USB

Note that this is an optional, alternate way to initialize your SeedSigner. The default method is to work on a separate Raspberry Pi device that has internet access. Be aware that by enabling internet access over USB, you are creating a link to the outside world that this project seeks to avoid.

If you use this setup route, we recommend that you disable internet access over USB when these steps are complete.

Get started

Insert the SD card with your Raspberry Pi OS image into a regular computer. Open a terminal window (macOS: Terminal; Windows: Command Prompt) and navigate to the SD card:

# macOS/Linux:
cd /Volumes/boot

# Windows (alter with correct drive letter):
cd e:

We need to create an empty file called ssh to enable remote terminal access to the Pi via SSH:

# macOS/Linux:
touch ssh

# Windows:
type nul > ssh

Now we have some incomprehensible configuration steps to set up the internet access relay.

Edit config.txt:

# macOS/Linux:
nano config.txt

# Windows:
notepad config.txt

Add dtoverlay=dwc2 to the end of config.txt. Exit and save changes (CTRL-X, then "y" in nano).

Alternatively, add to config.txt via the command line:

# macOS/Linux/Windows:
echo dtoverlay=dwc2 >> config.txt

Next, edit cmdline.txt:

# macOS/Linux:
nano cmdline.txt

# Windows:
notepad cmdline.txt

Add modules-load=dwc2,g_ether directly after rootwait:

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 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.

The Pi will take a minute or so to boot up its OS. After waiting a bit, try to communicate with the Pi over SSH:

# Manual builds:
ssh pi@raspberrypi.local

# Pre-built image:
ssh pi@seedsigner.local

If you see the following prompt, type yes to continue:

The authenticity of host 'raspberrypi.local (192.168.2.3)' can't be established.
ECDSA key fingerprint is SHA256:go4yVgii1GcvyxzhOe03atLn5bl2NhZlOR04tJHBo+k.
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!

Notice this warning in particular:

SSH is enabled and the default password for the 'pi' user has not been changed.
This is a security risk - please login as the 'pi' user and type 'passwd' to set a new password.

If someone savvy got access to your SeedSigner, they could sign into it and potentially upload malicious code. To add an extra layer of protection, change the default 'pi' user's password now by typing passwd:

pi@raspberrypi:~ $ passwd
Changing password for pi.
Current password: 
New password: 
Retype new password: 
passwd: password updated successfully

Configure host computer to share internet access with the Pi

macOS

Open the "Network" system settings. You should see an entry called "RNDIS/Ethernet Gadget":

Click "Advanced" and change "Configure IPv6" to "Link-local only":

Click "OK" and save your changes.

Now go to the "Sharing" system settings. Click on "Internet Sharing" and check the "RNDIS/Ethernet Gadget".

Click the "Internet Sharing" checkbox to activate.

Back at your SSH terminal, test the connection:

ping 8.8.8.8

Hit CTRL-C to abort the ping

If you see a ping response, the Pi has internet access!

Windows

see: https://www.circuitbasics.com/raspberry-pi-zero-ethernet-gadget/

Linux

To enable internet sharing on your Linux host:

Step 1: Enable IP Forwarding

# Check current IP forwarding status
cat /proc/sys/net/ipv4/ip_forward

# Enable IP forwarding temporarily
sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"

# Enable permanently
sudo nano /etc/sysctl.conf

Uncomment or add:

net.ipv4.ip_forward = 1

Save and apply:

sudo sysctl -p

Step 2: Configure Network Sharing

# Assuming eth0 is your internet-connected interface and usb0 is the USB interface
sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
sudo iptables -A FORWARD -i usb0 -o eth0 -j ACCEPT
sudo iptables -A FORWARD -i eth0 -o usb0 -m state --state RELATED,ESTABLISHED -j ACCEPT

# Save the rules
sudo apt install iptables-persistent
sudo netfilter-persistent save
sudo netfilter-persistent reload

Step 3: Test the Connection

ping 8.8.8.8

If you see responses, the Pi has internet access.

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:

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.

Continue by configuring your host computer to share internet access with your Pi Zero.

Complete the setup

Return to the main README and complete the setup steps. But remember to come back here and disable internet access over USB on your SeedSigner.

Disable internet access

Power down the SeedSigner and remove the SD card. Put the SD card back into your computer and use a terminal to navigate to the boot drive (same process as above).

Edit config.txt:

# macOS/Linux:
nano config.txt

# Windows:
notepad config.txt

Delete the dtoverlay=dwc2 line that we added to the end of the file. Exit and save changes (CTRL-X, then "y" in nano).

Edit cmdline.txt:

# macOS/Linux:
nano cmdline.txt

# Windows:
notepad cmdline.txt

Delete modules-load=dwc2,g_ether. Exit and save changes (CTRL-X, then "y" in nano).

Eject the SD card and put it back in the SeedSigner. Connect the SeedSigner to your computer with a USB cable. It should no longer show up as a "RNDIS/Ethernet Gadget".

SSH into the pi and try to ping 8.8.8.8. It should fail with no responses.

For full security, put the SD card back in your computer one last time and delete the ssh file from boot:

# macOS/Linux:
rm ssh

# Windows:
del ssh

You're now good to go with a SeedSigner that is back to being fully air-gapped!