Exercise 15. System boot: runlevels, /etc/init.d, rcconf, update-rc.d
Firstly I will give an outline of typical system boot process:
YOU
Press power switch (or start a virtual machine)
Now computer has the control
This control is passed to BIOS
BIOS
Performs hardware-specific tasks
Makes POST, Power-On Self-Test, which tests your hardware
Detects installed hardware, such as hard disks, memory type and amount, ...
Initializes hardware by writing initial values into their memory
Finds a boot device, which is usually a hard disk
Read and executes MBR (Main Boot Record) located at the beginning of this disk
Control is now passed to MBR
MBR
MBR finds and executes GRUB (Grand Unified Bootloader)
Control is now passed to GRUB
GRUB
Finds available filesystems
Finds and reads its configuration file, to find out:
Where system is located
What system to boot
What other actions perform
Executes Linux Kernel, main part of Linux OS
Control is now passed to Linux Kernel
Linux Kernel
Finds and loads initrd, which is initial ram disk
initrd contains necessary drivers which allow to access and mount real filesystems
Mounts the root file system, which is specified in GRUB config file
Executes /sbin/init, a special program which starts all the others
Control is now passed to init
init
Looks at the /etc/inittab to determine desired runlevel
Loads all programs appropriate for this runlevel
All programs from /etc/rc.d/rc2.d/ are loaded, because 2 is default Debian runlevel
SSH and TTY are started so you are able to connect to your computer
Booting up is now finished
YOU
Connect to your computer using SSH
SSH daemon executes bash shell for you
You can now type stuff
You are in control once again
For now we are interested only in “init” and “runlevel” stages, so I will give you a general overview of how the system starts up and automatically starts some programs. First, a little terminology:
- Daemon — a program which runs in background all the time. This means that it does not care if you are logged into the system, and usually you do not need to start it manually, because daemond are started automatically when computer boots up.
- Runlevel — a mode of system operation. Basically, this is just numbers which are fed to init program, which knows which daemons are associated with each number, and startd and stops those daemons as needed.
In Debian there are the following runlevels:
| ID | Description |
|---|---|
| S | This executed then system is powered on. |
| 0 | Halt, this defines which actions are executed when system is shutting down. |
| 1 | Single-User mode, this is a special mode for troubleshooting. In this mode most daemons are not automatically started. |
| 2-5 | Full Multi-User, configured daemon start in this mode. |
| 6 | Reboot. Like halt, but instead of powering down system reboots. |
But how does init know? Well, where are special directories for this:
user1@vm1:/etc$ find /etc -type d -name 'rc*' 2>/dev/null | sort /etc/rc0.d /etc/rc1.d /etc/rc2.d /etc/rc3.d /etc/rc4.d /etc/rc5.d /etc/rc6.d /etc/rcS.d
As you might guess, each number and S corresponds to a run level from this table. Let us list one of those directories, which start all needed daemons on normal boot:
user1@vm1:/etc$ ls -al /etc/rc2.d | awk '{printf "%-15.15s %-3.3s %s\n",$9,$10,$11}' . .. README S14portmap -> ../init.d/portmap S15nfs-common -> ../init.d/nfs-common S17rsyslog -> ../init.d/rsyslog S17sudo -> ../init.d/sudo S18acpid -> ../init.d/acpid S18atd -> ../init.d/atd S18cron -> ../init.d/cron S18exim4 -> ../init.d/exim4 S18ssh -> ../init.d/ssh S20bootlogs -> ../init.d/bootlogs S21rc.local -> ../init.d/rc.local S21rmnologin -> ../init.d/rmnologin S21stop-bootlog -> ../init.d/stop-bootlogd
As you can see, files in this directory are just symbolic links to actual startup scripts. Let us look at one of this link more closely: S18ssh → ../init.d/ssh. Here is what we are able to say about this file:
- It is a link to the file ./init.d/ssh
- It starts with S which means start. Each script used in Debian start-up system has at least 2 parameters, start and stop. Now we can say that when our system switches to runlevel 2 this script is executed with action start.
- It has a number 18. Scripts in rc directories are executed in alphabetical order, so now we understand that before starting ssh, system starts portmap, nfs-common, rsyslog, and sudo. rsyslog is a system loggin daemon, and ssh in particular want to log who and when accessed the system, so it needs rsyslog to be running before starting up.
Now you will learn how to list enabled services (daemons), and enable and disable services (daemons).
Do this
1: sudo aptitude install rcconf 2: ls -al /etc/rc2.d 3: sudo rcconf --list 4: sudo update-rc.d exim4 disable 5: ls -al /etc/rc2.d 6: sudo rcconf --list 7: sudo update-rc.d exim4 enable 8: ls -al /etc/rc2.d 9: sudo rcconf --list
What you should see
user1@vm1:/var/log$ sudo aptitude install rcconf The following NEW packages will be installed: rcconf 0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded. Need to get 0 B/23.9 kB of archives. After unpacking 135 kB will be used. Selecting previously deselected package rcconf. (Reading database ... 24239 files and directories currently installed.) Unpacking rcconf (from .../archives/rcconf_2.5_all.deb) ... Processing triggers for man-db ... Setting up rcconf (2.5) ... user1@vm1:/etc$ ls -al /etc/rc2.d total 12 drwxr-xr-x 2 root root 4096 Jun 27 11:42 . drwxr-xr-x 68 root root 4096 Jun 25 18:43 .. -rw-r--r-- 1 root root 677 Mar 27 05:50 README lrwxrwxrwx 1 root root 17 Jun 4 11:53 S14portmap -> ../init.d/portmap lrwxrwxrwx 1 root root 20 Jun 4 11:53 S15nfs-common -> ../init.d/nfs-common lrwxrwxrwx 1 root root 17 Jun 4 11:53 S17rsyslog -> ../init.d/rsyslog lrwxrwxrwx 1 root root 14 Jun 15 19:02 S17sudo -> ../init.d/sudo lrwxrwxrwx 1 root root 15 Jun 4 11:53 S18acpid -> ../init.d/acpid lrwxrwxrwx 1 root root 13 Jun 4 11:53 S18atd -> ../init.d/atd lrwxrwxrwx 1 root root 14 Jun 4 11:53 S18cron -> ../init.d/cron lrwxrwxrwx 1 root root 15 Jun 27 11:42 S18exim4 -> ../init.d/exim4 lrwxrwxrwx 1 root root 13 Jun 4 11:53 S18ssh -> ../init.d/ssh lrwxrwxrwx 1 root root 18 Jun 4 11:53 S20bootlogs -> ../init.d/bootlogs lrwxrwxrwx 1 root root 18 Jun 4 11:53 S21rc.local -> ../init.d/rc.local lrwxrwxrwx 1 root root 19 Jun 4 11:53 S21rmnologin -> ../init.d/rmnologin lrwxrwxrwx 1 root root 23 Jun 4 11:53 S21stop-bootlogd -> ../init.d/stop-bootlogd user1@vm1:/etc$ sudo rcconf --list rsyslog on ssh on bootlogs on portmap on sudo on nfs-common on udev on console-setup on kbd on exim4 on keyboard-setup on acpid on cron on atd on procps on module-init-tools on user1@vm1:/etc$ sudo update-rc.d exim4 disable update-rc.d: using dependency based boot sequencing insserv: warning: current start runlevel(s) (empty) of script `exim4' overwrites defaults (2 3 4 5). insserv: warning: current stop runlevel(s) (0 1 2 3 4 5 6) of script `exim4' overwrites defaults (0 1 6). user1@vm1:/etc$ ls -al /etc/rc2.d total 12 drwxr-xr-x 2 root root 4096 Jun 27 11:43 . drwxr-xr-x 68 root root 4096 Jun 25 18:43 .. lrwxrwxrwx 1 root root 15 Jun 27 11:43 K01exim4 -> ../init.d/exim4 -rw-r--r-- 1 root root 677 Mar 27 05:50 README lrwxrwxrwx 1 root root 17 Jun 4 11:53 S14portmap -> ../init.d/portmap lrwxrwxrwx 1 root root 20 Jun 4 11:53 S15nfs-common -> ../init.d/nfs-common lrwxrwxrwx 1 root root 17 Jun 4 11:53 S17rsyslog -> ../init.d/rsyslog lrwxrwxrwx 1 root root 14 Jun 15 19:02 S17sudo -> ../init.d/sudo lrwxrwxrwx 1 root root 15 Jun 4 11:53 S18acpid -> ../init.d/acpid lrwxrwxrwx 1 root root 13 Jun 4 11:53 S18atd -> ../init.d/atd lrwxrwxrwx 1 root root 14 Jun 4 11:53 S18cron -> ../init.d/cron lrwxrwxrwx 1 root root 13 Jun 4 11:53 S18ssh -> ../init.d/ssh lrwxrwxrwx 1 root root 18 Jun 4 11:53 S20bootlogs -> ../init.d/bootlogs lrwxrwxrwx 1 root root 18 Jun 4 11:53 S21rc.local -> ../init.d/rc.local lrwxrwxrwx 1 root root 19 Jun 4 11:53 S21rmnologin -> ../init.d/rmnologin lrwxrwxrwx 1 root root 23 Jun 4 11:53 S21stop-bootlogd -> ../init.d/stop-bootlogd user1@vm1:/etc$ sudo rcconf --list rsyslog on ssh on bootlogs on portmap on sudo on nfs-common on udev on console-setup on kbd on keyboard-setup on acpid on cron on atd on procps on module-init-tools on exim4 off user1@vm1:/etc$ sudo update-rc.d exim4 enable update-rc.d: using dependency based boot sequencing user1@vm1:/etc$ ls -al /etc/rc2.d total 12 drwxr-xr-x 2 root root 4096 Jun 27 11:43 . drwxr-xr-x 68 root root 4096 Jun 25 18:43 .. -rw-r--r-- 1 root root 677 Mar 27 05:50 README lrwxrwxrwx 1 root root 17 Jun 4 11:53 S14portmap -> ../init.d/portmap lrwxrwxrwx 1 root root 20 Jun 4 11:53 S15nfs-common -> ../init.d/nfs-common lrwxrwxrwx 1 root root 17 Jun 4 11:53 S17rsyslog -> ../init.d/rsyslog lrwxrwxrwx 1 root root 14 Jun 15 19:02 S17sudo -> ../init.d/sudo lrwxrwxrwx 1 root root 15 Jun 4 11:53 S18acpid -> ../init.d/acpid lrwxrwxrwx 1 root root 13 Jun 4 11:53 S18atd -> ../init.d/atd lrwxrwxrwx 1 root root 14 Jun 4 11:53 S18cron -> ../init.d/cron lrwxrwxrwx 1 root root 15 Jun 27 11:43 S18exim4 -> ../init.d/exim4 lrwxrwxrwx 1 root root 13 Jun 4 11:53 S18ssh -> ../init.d/ssh lrwxrwxrwx 1 root root 18 Jun 4 11:53 S20bootlogs -> ../init.d/bootlogs lrwxrwxrwx 1 root root 18 Jun 4 11:53 S21rc.local -> ../init.d/rc.local lrwxrwxrwx 1 root root 19 Jun 4 11:53 S21rmnologin -> ../init.d/rmnologin lrwxrwxrwx 1 root root 23 Jun 4 11:53 S21stop-bootlogd -> ../init.d/stop-bootlogd user1@vm1:/etc$ sudo rcconf --list rsyslog on ssh on bootlogs on portmap on sudo on nfs-common on udev on console-setup on kbd on exim4 on keyboard-setup on acpid on cron on atd on procps on module-init-tools on user1@vm1:/etc$
Explanation
- Installs rcconf package which allows you to easily manage runlevels.
- Prints out directory containing startup scripts for runlevel 2. Mail server exim4 is enabled for now.
- Just prints out services for the same runlevel. Notice that there are several services which are not shown because they are considered system services. rcconf –list –expert will list them all and many more which reside on different runlevels.
- Disables automatic startup of exim4, the mail server.
- Prints out directory containing startup scripts for runlevel 2. exim4 startup script is now renamed from S18exim4 to K01exim4 which means that exim4 is stopped (killed) when entering this runlevel. If exim4 was not running to begin with, nothing happens.
- Prints out services for runlevel 2. Service exim4 is now off.
- Enables automatic startup of exim4.
- Prints out directory containing startup scripts for runlevel 2 once more, exim4 in enabled again.
- Prints out services for runlevel 2. Status of exim4 changed to on as expected.
Extra credit
- Read about Debian boot process here: http://www.debian.org/doc/manuals/debian-reference/ch03.en.html
- Try this: aptitude install sysv-rc-conf, sysv-rc-conf –list. Read man sysv-rc-conf.
