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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

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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 Basic

# 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 Advanced

# 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.

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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.

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$ sysctl

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