Distro Choice: Based on Using runlevels

Understanding Runlevels

After spending years on Redhat/CentOS versions of Linux which use runlevel 3 for text mode and runlevel 5 for graphical mode, and then after spending several years with Ubuntu which is moving away from runlevels and using Upstart, I was somewhat surprised with Debian Lenny which uses runlevels 2-5 as the same runlevel.  As a result I decided to review why Debian made this choice and if there are advantages to the runlevel changes.

What runlevels are.
The term runlevel stands for the mode that the operating system will operate in  For example, in RedHat based systems, use runlevel 3 in which the mode of operation is completely text based while runlevel 5 uses the graphical interface.  Two completely different modes of operation will be experienced by moving from runlevel 3 to 5.  This system is part of the Unix System V-style initialization.  Typically there have been 7 runlevels, 0-6, but in reality there are 10 but they are not all used currently.

Runlevel Modes

0     shut down
1     single user – system rescue
2     multi-user with no network
3     multiuser in text mode
4     not used
5     multi-user GUI mode
6     reboot

If you check around with multiple Linux systems you will see there are a number of distributions that have modified the runlevels and how they function including Debian.

How runlevels work
Typically during the boot process, once the kernel turns control over to init, init the default runelevel based on /etc/inittab.  This then sets the default runlevel and  calls the script at /etc/rc.d/rc.sysinit which starts many of the necessary system tasks such as kernel parameters from /etc/sysctl.conf, system clock, keymaps, swap, hostname,  executes root file system checks and then mounts it r/w, activates RAID, starts quotas, mounts file systems in /etc/fstab and then cleans up the PIDs and state locks that were set.  This is where the runlevels begin to make some changes to the operating system.  Each runlevel determines which daemons will run in that specific runlevel.  If you look in /etc/ you will see that there are directories that coincide with the runlevel.

/etc/rc0.d        Run level 0
/etc/rc1.d        Run level 1
/etc/rc2.d        Run level 2
/etc/rc3.d        Run level 3
/etc/rc4.d        Run level 4
/etc/rc5.d        Run level 5
/etc/rc6.d        Run level 6

These directories contain symbolic links to daemons so that when you start your system in a specific runlevel it will start those daemons that are supposed to be available in that runlevel.  Notice in this example that you have links that start with “K” and some with “S”.  This will determine which daemons are turned off in that runlevel “K”, for kill and those daemons which are started, “S”.

lrwxrwxrwx   1 root root   19 Dec 17 08:34 K05saslauthd -> ../init.d/saslauthd
lrwxrwxrwx   1 root root   19 Dec 17 08:35 K10dc_server -> ../init.d/dc_server
lrwxrwxrwx   1 root root   16 Dec 17 08:35 K10psacct -> ../init.d/psacct
lrwxrwxrwx   1 root root   19 Dec 17 08:35 K12dc_client -> ../init.d/dc_client
lrwxrwxrwx   1 root root   13 Dec 17 08:35 K20nfs -> ../init.d/nfs
lrwxrwxrwx   1 root root   14 Dec 17 08:35 K24irda -> ../init.d/irda
lrwxrwxrwx   1 root root   22 Dec 17 08:38 K30spamassassin -> ../init.d/spamassa ssin
lrwxrwxrwx   1 root root   19 Dec 17 08:37 K35vncserver -> ../init.d/vncserver
lrwxrwxrwx   1 root root   17 Dec 17 10:41 K35winbind -> ../init.d/winbind
lrwxrwxrwx   1 root root   14 Dec 17 09:17 K36lisa -> ../init.d/lisa
— cut —
lrwxrwxrwx   1 root root   23 Dec 17 08:35 S00microcode_ctl -> ../init.d/microco de_ctl
lrwxrwxrwx   1 root root   15 Dec 17 08:34 S05kudzu -> ../init.d/kudzu
lrwxrwxrwx   1 root root   18 Dec 17 08:35 S06cpuspeed -> ../init.d/cpuspeed
lrwxrwxrwx   1 root root   18 Dec 17 08:35 S08iptables -> ../init.d/iptables
lrwxrwxrwx   1 root root   14 Dec 17 08:35 S09isdn -> ../init.d/isdn
lrwxrwxrwx   1 root root   16 Dec 17 08:35 S09pcmcia -> ../init.d/pcmcia
lrwxrwxrwx   1 root root   17 Dec 17 08:34 S10network -> ../init.d/network
lrwxrwxrwx   1 root root   16 Dec 17 08:34 S12syslog -> ../init.d/syslog
lrwxrwxrwx   1 root root   20 Dec 17 08:35 S13irqbalance -> ../init.d/irqbalance
lrwxrwxrwx   1 root root   17 Dec 17 08:35 S13portmap -> ../init.d/portmap
lrwxrwxrwx   1 root root   17 Dec 17 08:35 S14nfslock -> ../init.d/nfslock

The significance of runlevels is the determination of which daemons are available when the system boots.  This is important in terms of resources used and in terms of security because many of these daemons will listen on ports for connections from the network when they are activated.

The files in the rcX.d directories are designed for the operating system to read.  Each file is a symbolic link to a script in the /etc/init.d directory and controls the starting, or stopping of a daemon or service.  The links start with the “K” or “S” and then are followed by a two digit number which determines the order in which they start or are turned off.  The string at the end is simply to help you understand what the link is to, human readable.

Debian Lenny
Debian uses the same runlevel settings for runlevels 2-5, in other words they do not make any distinction between a runlevel three typically used as a text mode or a runlevel 5 used as a GUI, graphical interface, mode.  The default is runlevel 2.

Here is a look at the contents of /etc/rc2.d
README        S12dbus         S20acct    S20nfs-common      S20xend     S26network-manager        S89anacron    S99rc.local
S05loadcpufreq    S14avahi-daemon  S20cups    S20openbsd-inetd  S21xendomains  S26network-manager-dispatcher    S89atd          S99rmnologin
S10rsyslog    S16ssh         S20exim4    S20postfix      S24dhcdbd     S30gdm                S89cron       S99stop-bootlogd
S12acpid    S19cpufrequtils  S20kerneloops    S20rsync      S24hal     S30system-tools-backends    S99bootchart

Note yours will look different depending upon what daemons you have installed.

If you wanted to boot your Debian Lenny version into text mode instead of the graphical mode, them you can either change the “S30gdm” to “s30gdm” or run this command:

mv /etc/rc2.d/S30gdm /etc/rc2.d/K70gdm

The system will ignore any lines that start with a lower case letter, to that is why the first example is successful.  The second example moves it from a (S)tart to a (K)ill effectively turning of the graphical mode.  Also note you may be using kdm or xdm instead of gdm.

Summary:
When you are making a choice on a Linux distribution to use, you aspect you may consider is. does the operating system make it easy for me to start in either grpahical mode or text mode.  RedHat/CentOS versions certainly make that an easy option by using the command:

init 3
or
init 5

The init command will allow you to move back and forth between runlevels.

In Debian, you do not have that option.  Debian runlevels are all the same in 2-5 so the only way to boot into text mode to to make a change to the /etc/rc2.d file for gdm as explained above.