Boot Issues¶

This page covers diagnosing and resolving boot problems on Ubuntu Server 24.04 LTS.

Boot Process Overview¶

Normal Boot Sequence¶

┌─────────────────────────────────────────────────────────────┐
│                    1. UEFI/BIOS                              │
│              Hardware initialization                         │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                    2. GRUB Bootloader                        │
│            Load kernel and initramfs                         │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                    3. Kernel + initramfs                     │
│        Load drivers, unlock LUKS, mount root                │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                    4. systemd (PID 1)                        │
│            Start services, reach target                      │
└─────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────┐
│                    5. Login Prompt                           │
│                System ready                                  │
└─────────────────────────────────────────────────────────────┘

Identifying Boot Stage¶

Where Did It Fail?¶

Symptom	Stage	See Section
No POST, no display	Hardware/BIOS	Hardware issue
GRUB menu not shown	GRUB	GRUB Recovery
"GRUB rescue>" prompt	GRUB config	GRUB Recovery
Stuck at LUKS prompt	LUKS unlock	LUKS Issues
Kernel panic	Kernel	Kernel Issues
systemd errors	Init	systemd Issues
Services fail	Services	Service Recovery

Accessing Recovery Mode¶

At boot, hold Shift (BIOS) or Esc (UEFI) to show GRUB
Select Advanced options for Ubuntu
Select Ubuntu with Linux x.x.x (recovery mode)

Option	Use For
resume	Continue normal boot
clean	Free up disk space
dpkg	Repair broken packages
fsck	Check filesystem
grub	Update GRUB
network	Enable networking
root	Drop to root shell
system-summary	Show system info

Drop to Root Shell¶

From recovery menu:

Select root
Press Enter for maintenance shell
Remount filesystem as read-write:

mount -o remount,rw /

GRUB Recovery¶

"GRUB rescue>" Prompt¶

System can't find GRUB configuration:

# List available partitions
grub rescue> ls
# (hd0) (hd0,gpt1) (hd0,gpt2) (hd0,gpt3)

# Find partition with /boot
grub rescue> ls (hd0,gpt2)/boot/
# Should list vmlinuz, initrd.img, grub/

# Set prefix and load normal module
grub rescue> set prefix=(hd0,gpt2)/boot/grub
grub rescue> insmod normal
grub rescue> normal

Reinstall GRUB¶

Boot from live USB, then:

# Mount root filesystem
sudo mount /dev/sda2 /mnt

# Mount EFI (if UEFI)
sudo mount /dev/sda1 /mnt/boot/efi

# Mount virtual filesystems
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys

# Chroot into system
sudo chroot /mnt

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

# Exit and reboot
exit
sudo reboot

Fix GRUB Configuration¶

From recovery shell or chroot:

# Update GRUB config
update-grub

# Verify config
cat /boot/grub/grub.cfg | grep menuentry

GRUB Boot Parameters¶

At GRUB menu, press e to edit, then modify the linux line:

Parameter	Purpose
single	Single-user mode
init=/bin/bash	Boot directly to bash
systemd.unit=rescue.target	Rescue target
systemd.unit=emergency.target	Emergency target
nomodeset	Disable kernel mode setting
quiet splash	Hide boot messages
nosplash	Show boot messages

Press Ctrl+X or F10 to boot with modified parameters.

LUKS Encryption Issues¶

Wrong Passphrase¶

If LUKS passphrase isn't working:

# Boot from live USB
# Find LUKS partition
lsblk

# Test unlock
sudo cryptsetup open /dev/sda3 cryptroot
# Enter passphrase

# If successful, mount and continue boot
sudo mount /dev/mapper/vg--system-lv--root /mnt

Add Recovery Key¶

From live USB with volume unlocked:

# Add additional passphrase
sudo cryptsetup luksAddKey /dev/sda3

# Or add key file
sudo cryptsetup luksAddKey /dev/sda3 /path/to/keyfile

LUKS Header Backup/Restore¶

# Backup (do this on healthy system!)
sudo cryptsetup luksHeaderBackup /dev/sda3 --header-backup-file luks-header.img

# Restore from backup
sudo cryptsetup luksHeaderRestore /dev/sda3 --header-backup-file luks-header.img

Kernel Issues¶

Kernel Panic¶

If system panics on boot:

Boot previous kernel from GRUB
At GRUB menu, select Advanced options
Select older kernel version

Remove Problematic Kernel¶

From recovery mode or live USB:

# List installed kernels
dpkg --list | grep linux-image

# Remove specific kernel
sudo apt remove linux-image-x.x.x-generic

# Update GRUB
sudo update-grub

Reinstall Current Kernel¶

sudo apt install --reinstall linux-image-$(uname -r)
sudo update-grub

systemd Issues¶

Emergency Mode¶

System boots to emergency shell:

# Check what failed
systemctl --failed

# Check specific unit
systemctl status failed-unit.service
journalctl -u failed-unit.service

# Try to start graphical target
systemctl isolate graphical.target

# Or multi-user
systemctl isolate multi-user.target

Dependency Failures¶

# Check dependency tree
systemctl list-dependencies multi-user.target

# Check what's blocking
systemctl list-jobs

Skip Failed Services¶

Temporarily disable problematic service:

# Mask service (prevents start)
systemctl mask problematic.service

# Boot should proceed
systemctl isolate multi-user.target

# Later, fix and unmask
systemctl unmask problematic.service

Filesystem Issues¶

fsck on Boot¶

If filesystem check fails:

# Boot to recovery mode or live USB

# Check filesystem (unmount first if needed)
sudo umount /dev/sda2
sudo fsck -y /dev/sda2

# For root filesystem, use recovery mode
# It will run fsck automatically

Fix fstab Errors¶

Bad fstab entry can prevent boot:

# From recovery mode with remounted root
mount -o remount,rw /

# Edit fstab
nano /etc/fstab

# Common issues:
# - Wrong UUID (check with blkid)
# - Missing partition
# - Typo in mount point

# Comment out problematic line with #
# Reboot and fix properly

Get Correct UUIDs¶

# List all UUIDs
sudo blkid

# Update fstab with correct UUID
# UUID=xxxx-xxxx /mount/point ext4 defaults 0 2

Network Prevents Boot¶

Waiting for Network¶

If boot hangs waiting for network:

# Temporarily disable network wait
systemctl mask systemd-networkd-wait-online.service

# Boot, then fix network config
# Then unmask
systemctl unmask systemd-networkd-wait-online.service

Fix Netplan¶

From recovery:

mount -o remount,rw /

# Check netplan config
cat /etc/netplan/*.yaml

# Fix syntax issues
netplan generate
# Shows errors

# Apply working config
netplan apply

Live USB Recovery¶

Boot from Live USB¶

Create Ubuntu live USB
Boot from USB
Select "Try Ubuntu"

Chroot into System¶

# Find your partitions
lsblk

# Mount root
sudo mount /dev/sda2 /mnt

# If LUKS encrypted
sudo cryptsetup open /dev/sda3 cryptroot
sudo mount /dev/mapper/vg--system-lv--root /mnt

# Mount other partitions
sudo mount /dev/sda1 /mnt/boot/efi
sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo mount --bind /sys /mnt/sys

# Chroot
sudo chroot /mnt

# Now run repairs as if on actual system
# Exit when done
exit
sudo reboot

Quick Reference¶

Boot to Different Targets¶

From GRUB, add to linux line:

systemd.unit=rescue.target      # Rescue mode
systemd.unit=emergency.target   # Emergency mode
systemd.unit=multi-user.target  # No GUI
init=/bin/bash                  # Direct bash

Key Commands¶

# Recovery mode
mount -o remount,rw /        # Remount root writable

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

# Kernel
dpkg --list | grep linux-image  # List kernels

# Filesystem
fsck -y /dev/sda2            # Fix filesystem
blkid                        # Show UUIDs

# systemd
systemctl --failed           # Show failures
systemctl mask service       # Prevent start
systemctl isolate target     # Switch target

Prevention¶

Keep Backup Kernels¶

# Don't autoremove all old kernels
# Keep at least one previous version

# Check how many to keep
grep -r "APT::NeverAutoRemove" /etc/apt/

Backup LUKS Header¶

# Critical! Do this on healthy system
sudo cryptsetup luksHeaderBackup /dev/sda3 \
    --header-backup-file /safe/location/luks-header-backup.img

Backup GRUB¶

# Copy current config
sudo cp -r /boot/grub /safe/location/grub-backup

Next Steps¶

If you've resolved boot issues, continue to Network Issues for network troubleshooting.