Setting kernel parameters in the /proc/sys
directory need not be a manual process or one that required echoing
values into a virtual file, hoping they are correct. The
sysctl command can make viewing, setting, and
automating special kernel settings very easy.
To get a quick overview of all settings configurable in the
/proc/sys directory, type the sysctl
-a command as root. This will create a large, comprehensive
list, a small portion of which looks something like this:
net.ipv4.route.min_delay = 2
kernel.sysrq = 0
kernel.sem = 250 32000 32 128 |
This is the same basic information you would see if you viewed each of
the files individually. The only difference is the file location. The
/proc/sys/net/ipv4/route/min_delay is signified by
net.ipv4.route.min_delay, with the
directory slashes replaced by dots and the
proc.sys portion assumed.
As we have seen in the section called /proc/sys, you can manually
assign values to writable files by echoing the value to the file. The
sysctl command is able to do the same thing by typing
the sysctl -w
<file>="<new-value>"
command. For example, to activate the System Request Key, the following
command is required:
[root@bleach /]# sysctl -w kernel.sysrq="1"
kernel.sysrq = 1
[root@bleach /]# |
While quickly setting single values like this in
/proc/sys is helpful during testing, it does not
work as well on a production system, as all
/proc/sys special settings are lost when the
machine is rebooted. To preserve the settings that you like to make
permanently to your kernel, add them to the
/etc/sysctl.conf file.
Every time the system boots, the
/etc/rc.d/rc.sysinit script is executed by
init. This script contains a command to execute
sysctl using /etc/sysctl.conf as
the values to set. Therefore, any values added to
/etc/sysctl.conf will take effect right after the
system boots with no need to reconfigure and rebuild the kernel to
incorporate the change.
