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sysctl

system administrationLinux

The sysctl command is one of the most frequently used commands in Linux/Unix-like operating systems. sysctl Configure kernel parameters at runtime

See Examples View Syntax

Quick Reference

Command Name:

sysctl

Category:

system administration

Platform:

Linux

Basic Usage:

sysctl [options] [arguments]

Common Use Cases

Syntax

sysctl [options] [variable[=value]] [...]

Options

Option	Description
`-a, --all`	Display all variables
`-A`	Alias of -a
`--deprecated`	Include deprecated parameters to --all
`-b, --binary`	Print values without new line
`-e, --ignore`	Ignore unknown variables errors
`-N, --names`	Print variable names only
`-n, --values`	Print values only
`-p, --load[=FILE]`	Read values from file
`-q, --quiet`	Do not display variable changes
`--system`	Read settings from all system configuration files
`-r, --pattern PATTERN`	Only apply settings matching pattern
`-w, --write`	Change sysctl setting
`-o`	Doesn't print variable name when printing values
`-d`	Show help instead of setting a variable
`-h, --help`	Display help and exit
`-V, --version`	Output version information and exit

Examples

How to Use These Examples

The examples below show common ways to use the sysctl command. Try them in your terminal to see the results. You can copy any example by clicking on the code block.

Basic Examples:

Display all kernel parameters

sysctl -a

Display specific kernel parameter

sysctl net.ipv4.ip_forward

Set a kernel parameter temporarily (until reboot)

sysctl -w net.ipv4.ip_forward=1

Load settings from all system configuration files

sysctl --system

Advanced Examples:

Enable IP forwarding for routing

sysctl -w net.ipv4.ip_forward=1

Verify the change

sysctl net.ipv4.ip_forward

Increase maximum open file descriptors

sysctl -w fs.file-max=100000

Check the new value

sysctl fs.file-max

Change swappiness (memory management)

sysctl -w vm.swappiness=10

Protect against TCP SYN flood attacks

sysctl -w net.ipv4.tcp_syncookies=1

Disable ICMP echo requests (ping)

sysctl -w net.ipv4.icmp_echo_ignore_all=1

Display kernel parameters related to IPv6

sysctl -a | grep ipv6

Modify multiple parameters at once

sysctl -w net.ipv4.ip_forward=1 net.ipv6.conf.all.forwarding=1

Load settings from a specific configuration file

sysctl -p /etc/sysctl.d/99-custom.conf

Add a custom parameter to a configuration file

echo "net.ipv4.tcp_fin_timeout=15" >> /etc/sysctl.d/99-network-tuning.conf

sysctl -p /etc/sysctl.d/99-network-tuning.conf

Optimize network performance for high-traffic server

sysctl -w net.core.somaxconn=4096

sysctl -w net.ipv4.tcp_max_syn_backlog=4096 sysctl -w net.core.netdev_max_backlog=4096

Enable kernel protection features

sysctl -w kernel.kptr_restrict=1

sysctl -w kernel.dmesg_restrict=1

Try It Yourself

Practice makes perfect! The best way to learn is by trying these examples on your own system with real files.

Understanding Syntax

Pay attention to the syntax coloring: commands, options, and file paths are highlighted differently.

Notes

The `sysctl` command is a powerful system administration tool in Linux that allows users to view and modify kernel parameters at runtime. These parameters control various aspects of the operating system's behavior, including networking, memory management, file system operations, and security features. The Linux kernel exposes a virtual file system called procfs, typically mounted at `/proc`, which provides an interface to kernel internal data structures. Within this file system, `/proc/sys` contains files representing various kernel parameters that can be viewed and modified. The `sysctl` command provides a convenient interface to read from and write to these files. Kernel parameters are organized hierarchically in a tree-like structure with dot-separated names. For example, `net.ipv4.ip_forward` controls whether the system can forward IP packets (essentially functioning as a router). Changes made using `sysctl -w` are temporary and will be lost after a system reboot. To make permanent changes, parameters should be added to configuration files in the `/etc/sysctl.d/` directory or to the `/etc/sysctl.conf` file. These changes can then be applied without a reboot using `sysctl -p` or `sysctl --system`. Some common kernel parameters frequently modified with `sysctl` include: 1. **Network parameters**: Controls like `net.ipv4.ip_forward` for packet forwarding, `net.ipv4.tcp_syncookies` for protection against SYN flood attacks, and various TCP/IP stack tuning parameters. 2. **Memory management**: Parameters like `vm.swappiness` to control how aggressively the kernel swaps memory to disk, and `vm.overcommit_memory` to control memory allocation policy. 3. **File system settings**: Settings such as `fs.file-max` to control the maximum number of file handles. 4. **Security features**: Parameters like `kernel.randomize_va_space` for address space layout randomization (ASLR) and `kernel.dmesg_restrict` to control access to kernel logs. The `sysctl` command is essential for system tuning and hardening, allowing administrators to optimize system performance for specific workloads and implement security best practices without requiring a system restart. However, it should be used with caution, as improper settings can negatively impact system stability and performance.

Related Commands

These commands are frequently used alongside sysctl or serve similar purposes:

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Use Cases

Learn By Doing

The best way to learn Linux commands is by practicing. Try out these examples in your terminal to build muscle memory and understand how the sysctl command works in different scenarios.

See Examples Try Lab Exercises

$ sysctl

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