Where should permanent changes in system settings be made?

Permanent changes in system settings should be made in the /etc/sysctl directory, specifically within a configuration file that can define kernel parameters. The sysctl command is commonly used to read and modify the kernel parameters at runtime, but changes made using this command will not persist after a reboot unless they are saved in a configuration file located in /etc/sysctl.conf or in an individual file under /etc/sysctl.d/. This ensures that the desired settings are reapplied each time the system starts.

The /proc directory is a virtual filesystem that provides a view into the kernel and its current data; however, settings made here are transient and will not survive a reboot. The /etc/hosts file is used for network hostname resolution and does not pertain to overall system kernel parameters or settings. Similarly, the /sys directory is another virtual filesystem that exposes kernel data structures and is typically not used for making permanent configuration changes. Instead, it is used for interacting with the kernel and hardware at runtime. Therefore, for ensuring that changes to system settings persist across reboots, the /etc/sysctl directory is the appropriate location.