I was trying to set certain kernel parameters using "/etc/sysctl.conf" file on Cent OS 7.5. I copied "/etc/sysctl.conf" file into "/etc/sysctl.d/sysctl.conf" and updated certain parameters and reloaded settings using "sysctl --system".
But I see parameters inside "/etc/sysctl.conf" overwrites those present inside (/etc/sysctl.d/sysctl.conf) . (I can also see the same when I execute command i.e settings from /etc/sysctl.d/sysctl.conf gets applied first and then settings from "/etc/sysctl.conf" gets applied which causes issue.)
But according to man page as sysctl --system should have ignored settings inside "/etc/sysctl.conf" as I have created file with same name inside "/etc/sysctl.d/sysctl.conf" which gets read first. ( Reference : http://man7.org/linux/man-pages/man8/sysctl.8.html ).
--system
Load settings from all system configuration files. Files are
read from directories in the following list in given order
from top to bottom. ***Once a file of a given filename is
loaded, any file of the same name in subsequent directories is
ignored.***
/run/sysctl.d/*.conf
/etc/sysctl.d/*.conf
/usr/local/lib/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
/lib/sysctl.d/*.conf
/etc/sysctl.conf ```
The man page does not agree with the source code sysctl.c. According to the source code of the PreloadSystem() function, it processes the *.conf files in the various sysctl.d search directories (skipping those *.conf filenames that have already been seen, as described in the man page). Then it processes the default /etc/sysctl.conf file if it exists without checking whether the sysctl.conf filename has already been seen.
In summary, the settings in /etc/sysctl.conf cannot be overridden by the *.conf files in /etc/sysctl.d/ and other sysctl.d directories, because the settings in /etc/sysctl.conf are always applied last.