Tuesday, July 26, 2011

using chkconfig

Many system 5 UNIX variants use scripts in the /etc/rcN.d/ directories to control which services should be started in the various runlevels. If, for instance, you wanted the secure shell daemon to run in runlevel 4, you would put a script named something like “S55sshd” in “/etc/rc4.d”. This script would usually accept the “start” “stop” and “restart” arguments, as well as the commands to perform these functions. When the system came up, it would execute “/etc/rc4.d/S55sshd start” when it transitioned into runlevel 4. On the way down, it would execute “/etc/rc4.d/S55sshd stop” as the system passed from runlevel 4 to runlevel 3. If you had made some changes to the sshd configuration file, and wanted to restart the service, you could manually execute “/etc/rc3.d/sshd restart” to kill and then restart the daemon.

Since this model involved having multiple copies of the same script in many different directories, Linux and others have adopted the standard of putting all service control scripts in “/etc/init.d/”, and using symbolic links to these scripts in the various “/etc/rcN.d/” directories. This allowed for the SGI IRIX innovation of the “chkconfig” command, which is command line tool that manages the symbolic links for you.

How to use “chkconfig” in Red Hat Enterprise Linux:

First, all your service control scripts need to be in the “/etc/init.d/” directory. They should reflect the name of the service they control. In our example, the file is named /etc/init.d/sshd”.

Secondly, they have a tag at the head of the script that looks something like this so that “chkconfig” understands that it can controll it:

# Basic support for IRIX style chkconfig
# chkconfig: 2345 55 25
# description: Manages the services you are controlling with the chkconfig command

The first set of numbers “2345″ is are the default runlevels for the service, and “55″ and “25″ represent the name of the “S” and “K” symbolic links, and the order in which the service will be started and stopped in the respective runlevel. You will need to change these last two numbers, making them unique.

Once these requirements are met, using the command is fairly simple. When we go into /etc/rc3.d, we see a file called “S55sshd”.

[root@rhel55x86 rc3.d]# cd /etc/rc3.d/
[root@rhel55x86 rc3.d]# ls -al S55sshd
lrwxrwxrwx 1 root root 14 Oct 14  2010 S55sshd -> ../init.d/sshd

We see this file is a symbolic link to “../init.d/sshd”. Let’s run the “chkconfig” command to turn sshd off.

[root@rhel55x86 init.d]# /sbin/chkconfig sshd off
[root@rhel55x86 init.d]# /sbin/chkconfig --list sshd
sshd 0:off 1:off 2:off 3:off 4:off 5:off 6:off

chkconfig --list sshd confirms that sshd has been disabled in all runlevels, and the symbolic link has been removed from all “/etc/rcN.d/” directories.

Let’s turn sshd back on:

[root@rhel55x86 init.d]# /sbin/chkconfig sshd on
[root@rhel55x86 rc2.d]# /sbin/chkconfig --list sshd
sshd 0:off 1:off 2:on 3:on 4:on 5:on 6:off

chkconfig --list sshd confirms that sshd has now been enabled in runlevels 2, 3, 4 and 5, and we see s symbolic link to “/etc/init.d/sshd” named “S55sshd” in “/etc/rc2.d/”, “/etc/rc3.d/”, “/etc/rc4.d/” and “/etc/rc5.d/”.

Let’s imagine now that we only want sshd to be enabled in runlevel 5. We run the following command to accomplish this:

[root@rhel55x86 rc2.d]# /sbin/chkconfig sshd --level 234 off
[root@calvin rc2.d]# /sbin/chkconfig --list sshd
sshd 0:off 1:off 2:off 3:off 4:off 5:on 6:off

chkconfig --list sshd confirms that sshd has been disabled in all runlevels except 5, and the “S55sshd” has been removed from “/etc/rc2.d/”, “/etc/rc3.d/” and “/etc/rc4.d/”.

There is, of course, more to it, but this should get you well on your way to happily managing your system services with “chkconfig”.

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