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→Investigation 3: What is a runlevel?
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===Investigation 3: What is a runlevel?===
After the kernel boots, it starts a single program, called <code>init</code>. A running instance of a program is called a '''process''' -- the init process always has a process ID (PID) of 1. All other processes on the system are started by init, or they are started by processes started by init.
Most current Linux systems use some variation of the init system originally developed for Unix System V (called "sysvinit") or a newer version called "upstart".
These systems employ the concept of "runlevels" -- groups of software that can be selected, so that the system can be run in various modes. In Fedora systems, these runlevels are used:
{|class="mediawiki" border="1"
!Runlevel
!Description
|-
|0
|Halt (powers off the system)
|-
|1
|Single-user maintenance mode
|-
|3
|Network running, character-mode display only
|-
|5
|Network running, graphical user interface
|-
|6
|Reboot
|}
{{Admon/note|Different runlevel systems|Various Linux distributions may use the runlevel numbers differently. For example, on some Debian/Ubuntu systems, the default (standard) runlevel is 2.}}
* <code>/etc/inittab</code> is the configuration file, which sets the default runlevel.
* <code>/etc/rc.d/init.d</code> is a directory of scripts.
** Note not all distributions use the same runlevel configurations. Debian and Ubuntu based distro's are different.
# To find out the value of the runlevel your GNU/Linux system goes into after boot, enter the command: <code>grep initdefault /etc/inittab</code># You should get a single line containing ":" as the field delimiter. The second field stores the value of the runlevel the init process will use after a reboot. Record the output in your log book.# A list of processes that should be running at a given runlevel can be found in the directory <code>/etc/rc.d/rc'''X'''.d</code> where '''X''' is the runlevel. Do a directory listing of that directory and study what files are in there. Pay attention to the first three characters of each file name. They have special meaning to the system. Record your observation in your log book.# Make a backup of the file /etc/inittab with the command: <code>cp /etc/inittab /etc/inittab.original</code># Edit the file <code>/etc/inittab</code> and change the default runlevel to 3. Save the change and reboot your system.# After the reboot, you should get a "Text Login Screen". Login with your Learn account and type startx at the command prompt. Describe what happens in your log book.# Enter the command: <code>runlevel<code> -- this shows the previous and current runlevel. Record the values in your book.
By default, the Grub boot loader allows anyone with access to the computer at boot time to set the runlevel, or change the boot parameters, which can allow them to influence the init process and which kernel image is loaded. Anyone with access to the boot prompt can therefore bypass security controls and control which software is loaded. For example, rebooting to runlevel 1, known as single user mode, gives the user root priveleges without the need for a password! Obviously, giving a non-administrator this much control can be dangerous, and it is wise to protect the boot loader with a secure password.
