initramfs Early Userspace

# List initramfs contents
lsinitramfs /boot/initrd.img-$(uname -r) | head -20
# bin/
# conf/
# etc/
# lib/
# run/
# sbin/
# usr/
# var/
# init  <- the first script executed by the kernel

# Extract initramfs
mkdir /tmp/initrd && cd /tmp/initrd
unmkinitramfs /boot/initrd.img-$(uname -r) .

# Check what's inside
ls early/
ls main/
# main/modules/ contains kernel modules
# main/run/ contains runtime scripts

# Dracut is the initramfs generator for Fedora/RHEL/CentOS

# Regenerate initramfs
sudo dracut --force

# Include specific modules
sudo dracut --force --add "crypt lvm" /boot/initramfs-custom.img

# List available modules
dracut --list-modules

# View module contents
ls /usr/lib/dracut/modules.d/
# 00-systemd     01-discover   02-lvm
# 03-crypt       04-network    05-btrfs

# Create minimal initramfs
sudo dracut --force --hostonly

# Test in a VM
sudo dracut --force --test-image

# Regenerate initramfs
sudo update-initramfs -u

# Generate new initramfs
sudo update-initramfs -c -k $(uname -r)

# List installed kernels
ls /boot/initrd.img-*

# View hooks
ls /etc/initramfs-tools/hooks/
# cryptroot  lvm2  plymouth

# View scripts
ls /etc/initramfs-tools/scripts/
# init-bottom  init-top  local-bottom  local-premount

# Update initramfs configuration
cat /etc/initramfs-tools/initramfs.conf
# MODULES=most
# BUSYBOX=auto
# COMPRESS=gzip

# The initramfs /init script discovers the root device:

# 1. Parse kernel command line for root=
cat /proc/cmdline | grep -o "root=[^ ]*"
# root=/dev/mapper/vg0-root

# 2. Load required modules
# LVM, RAID, crypto, filesystem drivers

# 3. Assemble RAID arrays
mdadm --assemble --scan

# 4. Unlock encrypted devices
cryptsetup luksOpen /dev/sda2 encrypted_vol

# 5. Activate LVM volumes
vgchange -ay

# 6. Mount root filesystem
mount /dev/mapper/vg0-root /new_root

# 7. Pivot to real root
exec switch_root /new_root /sbin/init

# Configure cryptsetup for initramfs (Debian/Ubuntu)
# /etc/cryptsetup-initramfs/conf-hook
# DEVICE=
# CIPHER=
# KEY_SIZE=
# HASH=
# USE_LUKS=
# LUKS_OPTIONS=

# Add key to initramfs
sudo cryptsetup luksAddKey /dev/sda2 /etc/keys/root.key

# Configure initramfs to use the key
echo "KEYFILE=/etc/keys/root.key" | sudo tee -a /etc/cryptsetup-initramfs/conf-hook

# Regenerate initramfs
sudo update-initramfs -u

# For Dracut (Fedora/RHEL)
# /etc/dracut.conf.d/crypt.conf
# add_dracutmodules+=" crypt "
# install_items+=" /etc/keys/root.key "

# Boot to emergency shell
# In GRUB, add to kernel line:
# init=/bin/bash
# or
# systemd.unit=emergency.target

# After boot, filesystem is read-only
mount -o remount,rw /

# Check filesystem
fsck -y /dev/sda2

# Fix initramfs
update-initramfs -u

# Reinstall GRUB
grub-install /dev/sda
update-grub

# If root password is lost
passwd root

# Reboot
reboot -f

# If initramfs is missing or corrupt
# Boot from live USB
sudo mount /dev/sda2 /mnt
sudo mount /dev/sda1 /mnt/boot
for i in /dev /proc /sys /run; do
  sudo mount --bind $i /mnt$i
done
sudo chroot /mnt
update-initramfs -u
exit