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Wireless-USB2.md

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WiFi USB Support

This guide will show you how to set up a WiFi USB dongle for use during initramfs. The steps here are intended for Raspberry Pi OS, slight changes will be necessary for Ubuntu.

Installing driver

The steps below are written for the Alfa AWUS036AXML USB dongle, which uses the Mediatek MT7921AU driver. The driver is part of the Linux kernel starting from 6.1.

Set up initramfs

Create the following files and customize them if necessary:

  • /etc/initramfs-tools/scripts/init-premount/a_enable_wireless

    #!/bin/sh
PREREQ=""
prereqs()
{
    echo "$PREREQ"
}

case $1 in
prereqs)
    prereqs
    exit 0
    ;;
esac

echo "Waiting for wlan device to come up..."
while [ ! -d "/sys/class/net/wlan0" ]; do
    sleep 1
done

echo "Initializing wpa-supplicant..."
/sbin/wpa_supplicant -i wlan0 -c /etc/wpa_supplicant.conf -P /run/initram-wpa_supplicant.pid -B

  • /etc/initramfs-tools/hooks/enable-wireless

    #!/bin/sh
set -e
PREREQ=""
prereqs()
{
    echo "${PREREQ}"
}
case "${1}" in
    prereqs)
        prereqs
        exit 0
        ;;
esac

. /usr/share/initramfs-tools/hook-functions

copy_exec /sbin/wpa_supplicant

# copy WiFi driver
manual_add_modules mt7921u

copy_file config /etc/initramfs-tools/wpa_supplicant.conf /etc/wpa_supplicant.conf

  • /etc/initramfs-tools/scripts/init-bottom/kill_wireless

    #!/bin/sh
PREREQ=""
prereqs()
{
    echo "$PREREQ"
}

case $1 in
prereqs)
    prereqs
    exit 0
    ;;
esac

# allow the decrypted OS to handle WiFi on its own
echo "Stopping wlan device..."
kill $(cat /run/initram-wpa_supplicant.pid)
ip link set wlan0 down

  • /etc/initramfs-tools/wpa_supplicant.conf

    ctrl_interface=/tmp/wpa_supplicant
country=GB

network={
    ssid="Foo"
    scan_ssid=1
    psk="Bar"
    key_mgmt=WPA-PSK
}

Note that this will only set up WiFi in the initramfs. The decrypted OS needs to be configured separately.

Chmod all scripts you just created:

chmod +x /etc/initramfs-tools/scripts/init-premount/a_enable_wireless
chmod +x /etc/initramfs-tools/hooks/enable-wireless
chmod +x /etc/initramfs-tools/scripts/init-bottom/kill_wireless

You may want to disable onboard WiFi from the decrypted OS so it doesn't conflict with your external USB card:

echo "dtoverlay=disable-wifi" >> /boot/config.txt

You're done! Follow the rest of the guide to finish building your initramfs.

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