systemdsysctl

Systemdsysctl, also known as systemd-sysctl, is a systemd component that loads and applies kernel parameter settings, known as sysctl tunables, to the running Linux kernel. It runs as the systemd-sysctl service and is intended to initialize sysctl values at boot and reapply them as needed. Configuration is provided through .conf files located in multiple directories, typically /etc/sysctl.d and /run/sysctl.d, with some distributions also including /usr/lib/sysctl.d. Each file contains lines in the form key=value, where key is a sysctl parameter name such as net.ipv4.ip_forward and value is the desired setting. Comments start with the hash character and blank lines are ignored. The files are processed in lexical order, allowing later files to override earlier ones, and the final values are written to /proc/sys by the kernel.

On boot, systemd starts systemd-sysctl early in the initialization sequence to apply the configured values. Administrators can manually reapply settings using systemctl restart systemd-sysctl or systemctl try-restart; the runtime values can also be changed with the sysctl command (sysctl -w key=value). To make changes persistent across reboots, place them in a conf file under one of the sysctl.d directories. Some tunables may not be changeable at runtime or may be restricted by kernel configuration; such values require a reboot or cannot be changed at all.

See also: systemd, sysctl, Linux kernel parameters.