E.4. Using the sysctl Command

The /sbin/sysctl command is used to view, set, and automate kernel settings in the /proc/sys/ directory.
For a quick overview of all settings configurable in the /proc/sys/ directory, type the /sbin/sysctl -a command as root. This creates a large, comprehensive list, a small portion of which looks something like the following: 
net.ipv4.route.min_pmtu = 552
kernel.sysrq = 0
kernel.sem = 250     32000     32     128
This is the same information seen if each of the files were viewed individually. The only difference is the file location. For example, the /proc/sys/net/ipv4/route/min_pmtu file is listed as net.ipv4.route.min_pmtu, with the directory slashes replaced by dots and the proc.sys portion assumed.
The sysctl command can be used in place of echo to assign values to writable files in the /proc/sys/ directory. For example, instead of using the command 
echo 1 > /proc/sys/kernel/sysrq
use the equivalent sysctl command as follows: 
sysctl -w kernel.sysrq="1"
kernel.sysrq = 1
While quickly setting single values like this in /proc/sys/ is helpful during testing, this method does not work as well on a production system as special settings within /proc/sys/ are lost when the machine is rebooted. To preserve custom settings, add them to the /etc/sysctl.conf file.
The /etc/sysctl.conf file is installed by the initscripts package to override some kernel default values and therefore only contains a few of the possible parameters. Use the sysctl -a command to list the parameters in the sysctl key format. See the /usr/share/doc/kernel-doc-kernel_version/Documentation/networking/ip-sysctl.txt file for more information on the possible settings.
Each time the system boots, the init program runs the /etc/rc.d/rc.sysinit script. This script contains a command to execute sysctl using /etc/sysctl.conf to determine the values passed to the kernel. Any values added to /etc/sysctl.conf therefore take effect each time the system boots. Note that modules loaded after sysctl has parsed this file might override the settings.