Using a Raspberry Pi as a USB-tethered Router

For several years, when I was living in rural Vermont in a location that had no wired internet service, I used an Android phone tethered to a Linux laptop via USB. This worked well for that laptop, but I also wanted to share that tethered connection to other machines via Ethernet. I used NetworkManager on the laptop to set the ethernet port to “shared” mode, which effectively turned the laptop into a router running a DHCP server. Recently I tried to duplicate this setup using a Raspberry Pi 4 instead of a laptop, and ran into a problem.

The latest version of Raspberry Pi OS is a variant of Debian 13 “Trixie”, which now uses NetworkManager, as did that aforementioned laptop years ago, so I assumed that I could use the same setup on the Pi. I used nmtui to set the ethernet port to “shared” mode, and that worked perfectly, but I could not get the Pi to connect to the tethered Android phone correctly. The Pi saw the phone, and both ifconfig and nmcli showed the phone as device usb0, but the Pi didn’t assign an IP address to usb0.

I first tried forcing the Pi to use DHCP, using this command:

sudo dhcpcd --waitip=4 usb0

That worked, but this obviously was not a permanent solution, because it had to be done whenever I connected the USB cable to the Pi. After some pondering on the NetworkManager man pages, and studying logs with journalctl, I realized that on the Pi, NetworkManager does not manage USB network connections. This meant that I had to add the USB device to NetworkManager explicitly, using this command:

sudo nmcli con add type ethernet con-name usb-network ifname usb0

Now NetWorkManager brought up the connection properly whenever the Android phone was tethered to the Pi.

I also tried this same approach using an Inseego MiFi X PRO 5G device in place of the Android phone. But the MiFi device shows up on the Pi as eth1, not usb0 or usb1, so the following command was necessary to add it to NetworkManager:

sudo nmcli con add type ethernet con-name mifi ifname eth1

Sharing via Ethernet

As mentioned above, the USB tethered connection can be shared via ethernet. It’s simplest to do this using nmtui. In nmtui, select “Edit a connection”, then select the ethernet connection (“netplan-eth0” on my Pi), then “Edit…”. Change “IPv4 Configuration” to “Shared” and hit “OK”.

Sharing via wi-fi

To turn the Pi into a wi-fi hotspot, set it up the first time using these commands:

sudo nmcli radio wifi on
sudo nmcli device wifi hotspot ssid MyCoolSsId password PaSsWoRd

Change the ssid and password to the values of your choice. These commands need to be run only once; NetworkManager should remember these settings even after a power cycle.

To disable the hotspot in the future, use this command:

sudo nmcli radio wifi off

Then to re-enable the hotspot:

sudo nmcli radio wifi on
sudo nmcli connection up Hotspot

Leases

You can see which DHCP leases NetworkManager has given out via ethernet using this command:

sudo cat /var/lib/NetworkManager/dnsmasq-eth0.leases

Similarly, you can see the wi-fi leases using this command:

sudo cat /var/lib/NetworkManager/dnsmasq-wlan0.leases