Enable Wireless networks in Debian Initramfs

December 7, 2017 by Marc Fargas

Initramfs is a very tiny environment in which your Linux system boots in order to do a lot of initialisation magic before loading your system. The most common use case is to mount the root filesystem, like when it’s encrypted and you need to type a passphrase to mount it. Or if fsck needs to run on an unclean root filesystem. Basically anything that goes before mounting the root filesystem and, after that, before launching INIT (you can read more in the Debian Wiki).

There are scenarios in which you need the network available at this very preliminary state of the system boot. Doing that over wired ethernet is easy (as long as you don’t need authentication on the wire) with the ip= kernel parameter (see here the documentation of the ip= parameter). Wireless connectivity is a whole different issue, WPA needs a helper program (wpa_supplicant), network configuration and, most likely, firmware files.

So, lets find out how can we get a wireless connection up and running on initramfs to do something useful there (in a later post: unattended boot with an encrypted rootfs).

A few warnings:

  • I assume you know how to move around on the console, really.
  • If you screw up you might end up on an emergency shell or event with an unbootable system.
  • This solution kills wpa_supplicant at the last stage of init so the underlying system can take over the wireless connection (ie: NetworkManager) so, be sure that whatever you attempt to achieve can live with that network interruption.
  • Make sure you have a keyboard and display (or a serial console). Do not attempt to do this on a headless system.

This has been tested on:

  • Debian Stretch
  • Intel Corporation Wireless 7260 AC
  • WPA2 (AP is a Time Capsule)

initramfs-tools

The Debian system utilises what is known as initramfs-tools to build the initramfs for the installed kernel images. A command comes with it: update-initramfs which updates/creates suchs images. This tools is highly extendable (in fact, lots of packages extend it) by the end-user in the folder /etc/initramfs-tools/ where hooks and scripts can be placed in order to customize the image build and/or the boot processes. A very good place to start would be man initramfs-tools for our purpose today: the SCRIPTS section.

What we need initramfs-tools to do for us

  • Add our wireless modules and firmware to the initramfs
  • Copy our dependencies to the image (wpa_supplicant, wpa_cli, wpa_supplicant.conf and its libraries)
  • Start wpa_supplicant to connect to the network
  • Setup networking (ip, etc)
  • Hold the boot process until the network is up or a timeout occurs
  • Kill wpa_supplicant before booting the underlying system to not conflict with whatever is there expecting full control of the network interface (aka: NetworkManager)

That sounds easy.

Custom hook

We will use a hook to do the first two steps, note that for the modules part you can just type the module name in /etc/initramfs-tools/modules and it should work. We do it in the hook just to keep everything together.

This goes into /etc/initramfs-tools/hooks/enable-wireless, make sure to put the right modules on the manual_add_modules line.

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

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

# CHANGE HERE for your correct modules.
manual_add_modules iwlwifi iwlmvm
copy_exec /sbin/wpa_supplicant
copy_exec /sbin/wpa_cli
copy_file config /etc/initramfs-tools/wpa_supplicant.conf /etc/wpa_supplicant.conf

Pretty straighforward, the upper half is boilerplate as required by initramfs-tools, see the manpage for more. The rest is quite readable: add the modules (it will also add the firmwares), copy wpa* stuff, copy the configuration.

Now, wpa_supplicant.conf is unique to you, as always, man wpa_supplicant.conf is your friend, and here is an example:

# Sample /etc/initramfs-tools/wpa_supplicant.conf
# note that this is independent of the system /etc/wpa_supplicant.conf (if any)
# only add the network you need at boot time. **And keep the ctrl_interface** !!
ctrl_interface=/tmp/wpa_supplicant

network={
    ssid="MyNetwork"
    scan_ssid=1
    psk="network passphrase"
    key_mgmt=WPA-PSK
}

Connection script (init-premount)

Now, we need the system to startup the supplicant, connect and go on. This can’t be done at the init-top stage because not even the kernel modules are available by then, to init-premount looks fine. Problem? Whatever the reason you are reading this, most likely it also happens in init-premount (mandos-client, cryptsetup, …) and initramfs-tools comes with this warning on the manpage:

No guarantees are made as to the order in which the different scripts are executed unless the prereqs are setup in the script.

So… dirty hack is to assume alphabetical order of execution and put “a_” in front of the script. It works, for now.

This goes into `/etc/initramfs-tools/scripts/init-premount/a_enable_wireless, you need to change the INTERFACE= and, maybe, the AUTH_LIMIT one (the timeout):

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

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

. /scripts/functions

AUTH_LIMIT=30
INTERFACE="wlp5s0"
alias WPACLI="/sbin/wpa_cli -p/tmp/wpa_supplicant -i$INTERFACE "

log_begin_msg "Starting WLAN connection"
/sbin/wpa_supplicant  -i$INTERFACE -c/etc/wpa_supplicant.conf -P/run/initram-wpa_supplicant.pid -B -f /tmp/wpa_supplicant.log

# Wait for AUTH_LIMIT seconds, then check the status
limit=${AUTH_LIMIT}

echo -n "Waiting for connection (max ${AUTH_LIMIT} seconds)"
while [ $limit -ge 0 -a `WPACLI status | grep wpa_state` != "wpa_state=COMPLETED" ]
do
    sleep 1
    echo -n "."
    limit=`expr $limit - 1`
done
echo ""

if [ `WPACLI status | grep wpa_state` != "wpa_state=COMPLETED" ]; then
  ONLINE=0
  log_failure_msg "WLAN offline after timeout"
  panic
else
  ONLINE=1
  log_success_msg "WLAN online"
fi

configure_networking

Kill when done

Last, but not least, `/etc/initramfs-tools/scripts/local-bottom/kill_wireless should contain:

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

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

echo "Killing wpa_supplicant so the system takes over later."
kill `cat /run/initram-wpa_supplicant.pid`

Final touches

You may have noticed we use the provided configure_networking function, it relies on you passing the proper ip=kernel parameter, so better supply it, for GRUB just setup the GRUB_CMDLINE_LINUX in /etc/default/grub like: GRUB_CMDLINE_LINUX="ip=:::::wlp5s0:on panic=10" (see here the documentation of the ip= parameter). The panic=10 makes the system reboot if something goes wrong (like network failure), when testing you might prefer break=premount or some other options, see the initramfs-tools manpage5 or the Debian wiki InitramfsDebug page

Make the scripts executable, and, finally, rebuild initramfs and update-grub:

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

update-initramfs -k all -u
update-grub

And… reboot. I suggest you boot with break=bottomso you can check things work as expected (i.e with ip link and ip addr).

© 2017 | Follow on Twitter | Hucore theme & Hugo