Note: This was a RHCE 7 exam objective until June 2016. It is now removed from the curriculum.
When you want to improve the performance or the characteristics of your server, you need to set the kernel runtime parameters.
In order to do this, you’ve got three ways:
- through the /proc filesystem,
- with the sysctl command,
- through the /etc/sysctl.conf file.
The /proc Filesystem
To get the value of a kernel runtime parameter (here /proc/sys/net/ipv4/ip_forward used for allowing a host to act as an router), type:
# cat /proc/sys/net/ipv4/ip_forward
To set the value of the same parameter, type:
# echo 1 > /proc/sys/net/ipv4/ip_forward
Note: 1 is used for On and 0 for off.
This change is instantaneously active but doesn’t persist a reboot. You have to write it into the /etc/rc.d/rc.local file to get it re-applied at each boot. See below for a better solution.
The sysctl Command
With the sysctl command, you can get all the available kernel runtime parameters with their current value.
# sysctl -a | grep vm.swappiness vm.swappiness = 30
But you can also set a kernel runtime parameter with the -w option.
# sysctl -w vm.swappiness=20 vm.swappiness = 20
Still like the previous method, this change is instantaneously active but doesn’t persist a reboot. You have to write it into the /etc/rc.d/rc.local file to get it re-applied at each boot. See below for a better solution.
The /etc/sysctl.conf File
To permanently store kernel runtime parameters, you need to write them into the /etc/sysctl.conf file.
For example, edit the /etc/sysctl.conf file and paste the following line:
# allow IPv4 forwarding net.ipv4.ip_forward = 1
Caution: Comments are only allowed on a separate line and not at the end of a line!
Note: It is not a coincidence if the net.ipv4.ip_forward kernel runtime parameter name matches the /proc/sys/net/ipv4/ip_forward path name.
Then, you need to apply the change:
# sysctl -p
Many kernel runtime parameters can be set this way. Here are only a few examples:
# don't respond to a ping net.ipv4.icmp_echo_ignore_all = 1 # don't respond to a ping to the broadcast address net.ipv4.icmp_echo_ignore_broadcasts = 1 # disable IPv6 for all network interfaces net.ipv6.conf.all.disable_ipv6 = 1
Note: As seen before, the sysctl -a command gets all the kernel runtime parameters with their current value. By redirecting the output to a file, this is also a good way to back up your configuration before any change.
Default kernel runtime configuration is located in the /usr/lib/sysctl.d directory and is executed before anything else (see sysctl –system).
Caution: Kernel runtime parameters set in the /etc/sysctl.conf file can be overrided by the application of a tuned profile (see this example).
Suse provides an interesting SLES 11/12 OS Tuning & Optimization Guide.
Fedora documentation‘s got a page about Suggested /etc/sysctl.conf config.
Kernel.org also provides documentation about sysctl configuration.