sysctl.d — Configure kernel parameters at boot
/etc/sysctl.d/*.conf
/run/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
At boot, systemd-sysctl.service(8) reads configuration files from the above directories to configure sysctl(8) kernel parameters.
The configuration files contain a list of variable assignments, separated by newlines. Empty lines and lines whose first non-whitespace character is # or ; are ignored.
Note that both / and . are accepted as label
                separators within sysctl variable
                names. kernel.domainname=foo and
                kernel/domainname=foo hence are
                entirely equivalent.
Each configuration file shall be named in the
                style of program.conf/etc/ override files
                with the same name in /usr/lib/
                and /run/.  Files in
                /run/ override files with the same
                name in /usr/lib/. Packages
                should install their configuration files in
                /usr/lib/. Files in
                /etc/ are reserved for the local
                administrator, who may use this logic to override the
                configuration files installed by vendor packages. All
                configuration files are sorted by their filename in
                alphabetical order, regardless in which of the
                directories they reside, to guarantee that a specific
                configuration file takes precedence over another file
                with an alphabetically later name, if both files
                contain the same variable setting.
If the administrator wants to disable a
                configuration file supplied by the vendor the
                recommended way is to place a symlink to
                /dev/null in
                /etc/sysctl.d/ bearing the
                same file name.
Example 1. /etc/sysctl.d/domain-name.conf example:
# Set kernel YP domain name kernel.domainname=example.com