Master Linux Runlevels and Systemd Targets: How to View, Change, and Secure Them

Why Linux Runlevels Matter

Runlevels are distinct operating modes that determine which processes and services start or stop, allowing administrators to control system behavior for different use cases such as command‑line only or full graphical environments.

Standard Runlevels and Their Functions

0 : Power‑off the system.

1 : Single‑user (maintenance) mode, sometimes labeled S.

2 : Multi‑user without networking.

3 : Multi‑user with networking (text console).

4 : Reserved for custom configurations.

5 : Multi‑user with graphical display manager.

6 : Reboot.

Checking the Current Runlevel

Run runlevel in a terminal. The output shows two numbers: the previous runlevel and the current one. For example, N 5 means the system is now in runlevel 5.

Temporarily Changing the Runlevel

Use telinit as root. To switch to runlevel 3 on CentOS: $ telinit 3 After the change, runlevel will display the new current level and the previous one.

Systemd Targets vs. Traditional Runlevels

Modern distributions replace SysV init with systemd. Runlevels are now called "targets" and map as follows:

Runlevel 0 = poweroff.target Runlevel 1 = rescue.target Runlevel 2 = multi-user.target Runlevel 3 = multi-user.target Runlevel 4 = multi-user.target Runlevel 5 = graphical.target Runlevel 6 =

reboot.target

Changing the Default Target at Boot

To set runlevel 3 as the default, create a symbolic link pointing to the desired target:

$ ln -sf /lib/systemd/system/runlevel3.target /etc/systemd/system/default.target

Verify the change with: $ systemctl get-default The command should return multi-user.target, which corresponds to runlevel 3.

Listing Services Enabled for a Specific Target

For the graphical target (runlevel 5), run: $ systemctl list-dependencies graphical.target Replace graphical.target with any other target name to see its services.

Finding Which Runlevel a Service Runs In

Query a service’s WantedBy property. Example for SSH: $ systemctl show -p WantedBy sshd.service The output shows multi-user.target, meaning SSH starts in runlevels 2, 3, and 4.

Enabling a Service for All Runlevels

To make SSH start in the graphical target as well, enable it:

$ systemctl enable sshd.service

Security Benefits of Precise Runlevel Control

Limiting the system to only the services required for a given runlevel reduces attack surface and conserves resources. Administrators can avoid unintentionally exposing services by switching to a lower runlevel when a GUI is unnecessary.

Creating a Custom Runlevel (Target)

Copy an existing target, e.g., graphical.target, to a new file:

$ cp /usr/lib/systemd/system/graphical.target /usr/lib/systemd/system/mynew.target

Create a directory for the new target’s wants: $ mkdir /etc/systemd/system/mynew.target.wants Then symlink or enable additional services in that directory as needed.

Choosing the Right Runlevel for Your Use Case

Runlevel 3 is ideal for servers or users who only need a command‑line interface. Runlevel 5 provides a full desktop environment. Runlevel 1 is suited for maintenance tasks on production servers. Runlevel 4 is rarely used and typically reserved for custom configurations.

Runlevels 0 and 6 are used only to power off or reboot the system, respectively, and are not intended for normal operation.