etc_initramfs-tools_scripts_init-bottom_deconfigure-interfaces

#!/bin/sh

# ABOUT THIS SCRIPT:
#
# This script was copied from:
# https://github.com/rhansen/unlock-cryptroot
#
# This script deconfigures network interfaces that were configured by
# the initramfs during early boot. This avoids conflicts with the
# normal networking setup that runs during boot after initramfs is
# done.
#
# This script is only intended to be used in conjunction with a
# dropbear ssh server in the initramfs for remote unlocking of
# encrypted filesystems. In particular, it MUST NOT be used to
# deconfigure interfaces that are used by a network filesystem for a
# critical mount (otherwise the system might fail to boot).

# INSTRUCTIONS:
#
#   1. Change eth0 below to whatever is appropriate for your system.
#   2. Install this file to
#      /etc/initramfs-tools/scripts/init-bottom/deconfigure-interfaces
#   3. Make sure the file is executable
#   4. Update your initramfs: sudo update-initramfs -u

# Space-separated list of interface names
INTERFACES="eth0"

# Let dropbear clean up before deconfiguring the interfaces.
[ "$1" != prereqs ] || { echo dropbear; exit 0; }

for i in ${INTERFACES}; do
    # When dropbear (or anything else that configures networking) is
    # installed in the initramfs, Ubuntu 18.04 (Bionic) and later
    # creates a /run/netplan/<interface>.yaml file during boot to keep
    # the interface configuration stable in case a critical mount uses
    # a networking filesystem. The yaml file forces the use of
    # systemd-networkd for managing the interface, overriding the
    # default configuration. Deleting the file allows the regular
    # network configuation to manage the interface.
    rm -f "/run/netplan/${i}.yaml"

    # Some versions of NetworkManager and ifup will fail to configure
    # an interface if the interface already has an IP address.
    ip -f inet address flush dev "${i}"
done