sysctl.conf
also: kernel parameters file, sysctl configuration
A configuration file that sets kernel and system parameters at boot time. Changes made here persist across reboots, unlike temporary sysctl command adjustments.
sysctl.conf is located at /etc/sysctl.conf (or in /etc/sysctl.d/ directories) and contains key-value pairs that tune kernel behavior. When the system boots, the sysctl command reads these files and applies the settings.
Common use cases include adjusting network parameters (like TCP buffer sizes), security settings (IP forwarding, ICMP echo), and process limits. For example, net.ipv4.ip_forward = 1 enables IP packet forwarding.
To apply changes immediately without rebooting, run sysctl -p. Modern systems prefer placing custom settings in /etc/sysctl.d/*.conf files rather than editing sysctl.conf directly, making upgrades safer.