RED HAT ENTERPRISE LINUX

Booting into Different
Targets Manually

Boot systems into different targets manually

CIS126RH | RHEL System Administration 1
Mesa Community College

Not every administrative task can be performed on a fully running system. Resetting a forgotten root password, repairing a corrupted filesystem, or diagnosing a service that prevents normal boot all require entering a different system state. This module covers how to switch targets while the system is running, how to select a different target at boot time through the GRUB2 menu, and how to use emergency and rescue targets for recovery work. These skills are tested on the RHCSA exam.

Learning Objectives

1. Switch targets on a running system — Use systemctl isolate to move to a different target without rebooting
2. Change the default boot target — Use systemctl set-default to control which target the system enters on every future boot
3. Select a target at the GRUB2 boot menu — Interrupt the boot sequence, edit the kernel command line, and append a target specification to boot into rescue or emergency mode
4. Use rescue and emergency targets for recovery — Access a root shell in a minimal environment to repair the system and then return to normal operation

Switching Targets on a Running System

systemctl isolate transitions the running system to a different target immediately — stopping units that are not part of the new target and starting those that are required by it.

# Check the current active target
$ systemctl get-default
graphical.target

# Drop to text-only multi-user mode right now
$ sudo systemctl isolate multi-user.target
# The graphical display manager stops immediately

# Return to graphical mode
$ sudo systemctl isolate graphical.target

# Enter rescue mode — most services stop, root shell only
$ sudo systemctl isolate rescue.target

# List all active targets
$ systemctl list-units --type=target --state=active

Setting the Default Boot Target

The default target controls which state the system enters on every boot. It is stored as a symlink at /etc/systemd/system/default.target.

# View the current default target
$ systemctl get-default
multi-user.target

# Set the default to graphical (adds display manager on boot)
$ sudo systemctl set-default graphical.target
Created symlink /etc/systemd/system/default.target →
    /usr/lib/systemd/system/graphical.target.

# Set the default to multi-user (text login, no GUI)
$ sudo systemctl set-default multi-user.target

# Inspect the symlink directly
$ ls -l /etc/systemd/system/default.target
lrwxrwxrwx ... default.target -> /usr/lib/systemd/system/multi-user.target

systemd Targets Reference

The GRUB2 Boot Menu

GRUB2 is the bootloader on RHEL 9. Its menu appears briefly during boot and allows you to select a kernel or modify boot parameters before the OS starts.

• Appears for a few seconds on boot — time is set in /etc/default/grub
• Press any key (or hold Shift on UEFI systems) to pause the countdown
• Use the ↑ / ↓ arrow keys to select a kernel entry
• Press e to edit the selected entry's boot parameters
• Press Ctrl+X or F10 to boot with the edited parameters
• Press Esc to discard edits and return to the menu

Editing the Kernel Command Line

Pressing e at the GRUB2 menu opens an editor. Find the line beginning with linux — this is the kernel command line.

# Typical GRUB2 kernel line (line starting with 'linux')
linux ($root)/vmlinuz-5.14.0-362.el9.x86_64 \
    root=/dev/mapper/rhel-root \
    ro \
    crashkernel=auto \
    resume=/dev/mapper/rhel-swap \
    rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap \
    rhgb quiet

# To boot into rescue.target — add at the end of the linux line:
systemd.unit=rescue.target

# To boot into emergency.target — add at the end of the linux line:
systemd.unit=emergency.target

# To boot into multi-user.target — add at the end of the linux line:
systemd.unit=multi-user.target

Booting into rescue.target

rescue.target provides a root shell with most services stopped and the root filesystem mounted read-write. It is the first choice for maintenance that cannot be done on a fully running system.

What rescue.target provides

• Root filesystem mounted read-write
• Basic networking may be available
• A root shell — requires the root password
• Most system services are stopped

How to reach rescue.target

1. Reboot and pause at the GRUB2 menu
2. Press e to edit the default entry
3. Navigate to the end of the line beginning with linux
4. Add systemd.unit=rescue.target
5. Press Ctrl+X to boot
6. Enter the root password when prompted

Booting into emergency.target

emergency.target provides the most minimal possible environment — the root filesystem is mounted read-only and almost nothing else runs. Use it when rescue.target itself fails to boot.

What emergency.target provides

• Root filesystem mounted read-only — must be remounted to make changes
• No networking
• A root shell — requires the root password
• Virtually no services running

Remounting root read-write in emergency mode

# After booting into emergency target — remount root read-write
$ mount -o remount,rw /

# Now you can edit files
$ vim /etc/fstab   # fix a broken fstab entry for example

# Reboot to return to normal operation
$ systemctl reboot

The rd.break Method

rd.break interrupts the boot process inside the initramfs, before the real root filesystem is mounted. This provides a shell without requiring the root password — essential for root password recovery.

How rd.break works

• The initramfs provides a shell before systemd loads the actual OS
• The real root filesystem is mounted at /sysroot as read-only
• You must remount it read-write, chroot into it, and then reset the password
• SELinux labels must be updated — a file must be created to trigger relabelling on next boot

To reach rd.break

1. Pause at GRUB2 menu and press e
2. On the linux line, remove rhgb quiet and add rd.break
3. Press Ctrl+X to boot

Root Password Reset: Step by Step

After reaching the rd.break shell, execute these commands in order.

# Step 1: Remount /sysroot read-write
switch_root:/# mount -o remount,rw /sysroot

# Step 2: chroot into /sysroot to make it the working root
switch_root:/# chroot /sysroot

# Step 3: Reset the root password
sh-5.1# passwd root
Changing password for user root.
New password:
Retype new password:
passwd: all authentication tokens updated successfully.

# Step 4: Tell SELinux to relabel on next boot (CRITICAL)
sh-5.1# touch /.autorelabel

# Step 5: Exit the chroot and resume boot
sh-5.1# exit
switch_root:/# exit

Why the SELinux Relabel Step Matters

The /.autorelabel file triggers a full filesystem relabelling by SELinux on the next boot. Skipping it is the single most common reason the root password reset appears to succeed but then fails at login.

• When passwd runs inside the initramfs chroot, it modifies /etc/shadow without setting the correct SELinux file context
• On the next boot, SELinux enforces file context rules
• If /etc/shadow has the wrong context, SELinux blocks access to it — login fails even though the password was set correctly
• Creating /.autorelabel tells SELinux to reset all file contexts on the next boot, correcting the shadow file context

Alternative: Restore Only the Shadow File Context

If you want to avoid the slow full relabelling, restore only the SELinux context of /etc/shadow before exiting the chroot.

# Inside the chroot (after passwd) — restore shadow file context
sh-5.1# restorecon -v /etc/shadow
Relabeled /etc/shadow from system_u:object_r:unlabeled_t:s0
to system_u:object_r:shadow_t:s0

# This replaces the /.autorelabel step for this specific repair

# Full procedure with restorecon instead of autorelabel:
# 1. mount -o remount,rw /sysroot
# 2. chroot /sysroot
# 3. passwd root
# 4. restorecon -v /etc/shadow    <-- instead of touch /.autorelabel
# 5. exit
# 6. exit

Returning to Normal Operation

After performing recovery work in rescue, emergency, or rd.break mode, return to the normal running state.

From rescue or emergency target (reached via GRUB2 edit)

# Reboot — system returns to the default target
$ systemctl reboot

# Or exit the rescue shell to continue the normal boot sequence
$ exit
# systemd continues starting the remaining units

From rescue target (reached via systemctl isolate on a running system)

# Return to normal multi-user mode
$ systemctl isolate multi-user.target

# Or return to graphical mode
$ systemctl isolate graphical.target

# Or use systemctl default to activate the configured default target
$ systemctl default

Comparing Entry Methods

Common Mistakes

Knowledge Check

Answer these before moving to the next slide.

1. What is the difference between systemctl set-default multi-user.target and systemctl isolate multi-user.target?
2. You need to enter rescue mode at boot time. What do you add to the kernel command line in the GRUB2 editor?
3. You boot into rd.break mode to reset the root password. List the commands you run in order after reaching the initramfs shell.
4. Why is the touch /.autorelabel step required during a root password reset?
5. After entering emergency mode, you need to edit /etc/fstab. What must you do before running the editor?
6. How do you return to normal operation after performing repairs in rescue mode that was entered by editing the GRUB2 kernel command line?

Knowledge Check — Answers

1. set-default changes the default target for all future boots by updating the /etc/systemd/system/default.target symlink. isolate transitions the currently running system to the specified target immediately — it does not affect future boots.
2. Append systemd.unit=rescue.target to the end of the line beginning with linux in the GRUB2 editor. Press Ctrl+X to boot.
3. 1. mount -o remount,rw /sysroot
   2. chroot /sysroot
   3. passwd root
   4. touch /.autorelabel (or restorecon /etc/shadow)
   5. exit (leave chroot)
   6. exit (leave initramfs shell, resume boot)
4. The passwd command inside the initramfs chroot modifies /etc/shadow without setting the correct SELinux file context. Without /.autorelabel, SELinux enforces the wrong context on /etc/shadow at the next boot, blocking login even though the password itself was changed correctly.
5. Run mount -o remount,rw / to remount the root filesystem read-write. Emergency mode mounts it read-only, so files cannot be edited until the remount.
6. Run systemctl reboot to reboot cleanly. The system will boot using its default target (the GRUB2 edit was one-time only and did not change the default target).

Key Takeaways

1. Two ways to change targets: isolate (now) or set-default (next boot). systemctl isolate TARGET changes the running system immediately. systemctl set-default TARGET changes the default for future boots. Verify with systemctl get-default.
2. Reach non-default targets at boot by editing the GRUB2 kernel line. Press e at the GRUB2 menu, navigate to the linux line, and append systemd.unit=rescue.target or systemd.unit=emergency.target. The change is one-time only.
3. Use rd.break to reset the root password when the root password is unknown. Add rd.break to the GRUB2 kernel line. Then: remount, chroot, passwd, touch /.autorelabel, exit, exit. Two exits are required.
4. The SELinux relabel step is mandatory after a root password reset. touch /.autorelabel (full relabel) or restorecon /etc/shadow (targeted). Skipping this step means the reset password will not work at login.

Graded Lab

• Run systemctl get-default to record the current default target. Use systemctl set-default multi-user.target to change it, then confirm with get-default. Restore the original default when done.
• If your lab system is running graphical.target, use sudo systemctl isolate multi-user.target to switch to text mode. Confirm the switch, then isolate back to graphical mode.
• Reboot the lab system and pause at the GRUB2 menu. Press e to enter the editor and locate the linux line. Add systemd.unit=rescue.target to the end, then boot. Confirm you receive a rescue shell, then exit to resume normal boot.
• Reboot again and use the rd.break method to enter the initramfs shell. Work through the complete root password reset procedure, including the touch /.autorelabel step. Reboot and confirm you can log in with the new root password.

Next: Interrupt the boot process in order to gain access to a system