Introduction

Sysctl is an interface that allows you to examine and dynamically modify parameters of BSD and Linux operating systems. The implementation is very different between these two systems.

In Linux, the sysctl interface mechanism is also exported as part of procfs in the sys directory. This difference means that checking the value of certain parameters requires opening a file in the virtual file system, reading and interpreting its content, and then closing it. The sysctl system call exists in Linux, but is not encapsulated by a glibc function and its use is discouraged.

Usage

First, you should know that there are two main paths: /proc and /sys:

  • /proc: concerns memory, CPU, network, etc.
  • /sys: concerns devices, disks, etc.

Now let’s get to the point. To read a sysctl parameter, we’ll use the cat command:

  cat /proc/sys/net/ipv4/ip_forward

/proc

/sys corresponds to what we call sysctls.

For persistence, after “/proc/sys/”, you simply need to replace the “/” with “.” to know the name of the parameter:

  /proc/sys/net/ipv4/ip_forward

corresponds to

  net.ipv4.ip_forward

Applying values on the fly

Method 1

Here the default value is 0. If I want to enable IP forwarding on the fly, here’s how to do it:

  echo 1 > /proc/sys/net/ipv4/ip_forward

This will activate the new value but in a non-persistent way (value reset after reboot).

Method 2

To apply on the fly:

  sysctl -w net.ipv4.ip_forward=1

Applying values persistently

Simply add this to the /etc/sysctl.conf file:

  net.ipv4.ip_forward = 1

If you haven’t yet activated your new parameter, you can activate everything in the /etc/sysctl.conf file like this:

  sysctl -p

/sys

For /sys, it’s unfortunately less straightforward than for /proc since only the /etc/rc.local file exists for persistence.

Documentation

Debian

To get the kernel documentation, install this:

  aptitude linux-doc-*

The documentation is located in /usr/share/doc/linux-doc*/Documentation

RedHat

To get the kernel documentation, install this:

  aptitude kernel-doc

The documentation is located in /usr/share/doc/kernel-doc*/Documentation

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Last updated 12 Feb 2012, 14:26 +0200. history