sysctl
also: kernel parameter tool, /proc/sys interface
A command-line tool that reads and modifies kernel parameters at runtime without requiring a reboot. Settings are stored in /proc/sys and can be persisted in /etc/sysctl.conf.
sysctl allows system administrators to tune kernel behavior on the fly. It provides a unified interface to view and change hundreds of kernel tuning parameters related to networking, memory, security, and I/O.
To view all current kernel parameters, run sysctl -a. Individual parameters use dot notation like net.ipv4.ip_forward. For example, sysctl net.ipv4.ip_forward shows if IP forwarding is enabled, while sysctl -w net.ipv4.ip_forward=1 enables it immediately.
Changes made with sysctl -w are temporary and lost on reboot. To make them permanent, edit /etc/sysctl.conf (or files in /etc/sysctl.d/) and run sysctl -p to reload. This is essential for performance tuning, security hardening, and enabling features like IP forwarding or increasing file descriptor limits.