Linux Kernel Parameters

The Linux kernel acts as a bridge between the hardware and software of a system, managing resources like CPU, memory, and devices. To control and fine-tune the behavior of the Linux kernel, we use kernel parameters. These parameters influence how the system behaves at runtime, and they can be viewed or modified without restarting the system.

Kernel parameters are adjustable settings that control how the kernel behaves. These parameters influence system performance, security, networking, memory management, and hardware interactions. They act like “tuning knobs” to optimize your system for specific needs.

• Kernel parameters are settings that control the behavior of the Linux kernel.
• They are useful for performance tuning, enabling/disabling features, and troubleshooting.
• Most parameters can be modified at runtime using the /proc/sys virtual filesystem.
• Changes made at runtime are temporary unless explicitly made permanent.

The /proc/sys Directory

The /proc/sys directory is a virtual filesystem that that exposes kernel parameters as files. It provides real-time access to kernel parameters.  Each file in this directory represents a tunable kernel parameter. The parameters are organized in a hierarchical directory structure based on their function. For example, networking-related parameters are located in /proc/sys/net, and virtual memory parameters in /proc/sys/vm.

Unlike regular files, the contents of this directory represent live kernel settings. Changes made here take effect immediately but are temporary (lost on reboot). The directory is organized into subfolders like:

• kernel: Core kernel settings
• net: Networking configurations
• vm: Memory management
• fs: File system settings

Viewing Kernel Parameters

You can view the current value of a kernel parameter by reading its corresponding file in the /proc/sys directory.

Use the cat command to view parameter values:

$ cat /proc/sys/net/ipv4/ip_forward

Alternatively, you can use the sysctl command:

$ sysctl net.ipv4.ip_forward

Modifying Parameters Temporarily

To temporarily change a kernel parameter, you can use the echo command to write a value to the respective file.

For example, to change a value temporarily (until reboot):

Using echo:

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

Using sysctl -w:

$ sysctl -w net.ipv4.ip_forward=1

These changes are immediate but will be lost after a reboot.

Making Changes Persistent

To make kernel parameter changes permanent across reboots, add them to the /etc/sysctl.conf file or create a new file under /etc/sysctl.d/.

# Add this line to the file
net.ipv4.ip_forward = 1

Apply changes without rebooting:

$ sysctl -p /etc/sysctl.conf