sysctl

Sysctl is a utility and API for examining and dynamically configuring kernel parameters in many Unix-like operating systems. It provides a hierarchical keyspace for tuning subsystems such as networking, memory management, and security, enabling changes at runtime without rebooting when permitted.

The kernel exports the sysctl interface, and user-space tools read and modify parameters through it. In Linux, the parameters appear under /proc/sys (and historically via the sysctl command). The sysctl program supports both reading and writing values, using dotted names such as net.ipv4.ip_forward. Changes can be applied immediately or made persistent by placing entries in /etc/sysctl.conf or files in /etc/sysctl.d/ and reloading with sysctl -p or system-wide with sysctl --system.

Common uses include performance tuning and security hardening, for example enabling IP forwarding, adjusting maximum backlog, or configuring kernel randomization settings. Examples: List all parameters: sysctl -a. Read a value: sysctl net.ipv4.ip_forward. Set a value: sysctl -w net.ipv4.ip_forward=1. Persist changes by editing /etc/sysctl.conf or a file in /etc/sysctl.d/ and reloading.

By design, sysctl is a privileged operation; most changes require root access and some can destabilize the system if configured incorrectly. The interface and syntax are similar across BSD-derived systems and Linux, though exact parameter names and persistence mechanisms may differ.