/sbin/sysctl command is used to view, set, and automate kernel settings in the
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.
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 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
/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.