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Is there a good reason that sysctl is broken into so many configuration files across multiple directories?

Is there a reason I can't delete all the sysctl.d configuration files and put everything in /etc/sysctl.conf?

My system is running Xubuntu 20.04 and sysctl.d directories:

  • /etc/sysctl.d/
  • /usr/lib/sysctl.d/

But there must be other config files with sysctl keys, because if I run:

# sudo sysctl --system

it outputs all the keys from my custom /etc/sysctl.conf file and all the keys from the various sysctl.d files.

However, if I run:

# sudo sysctl -a

it outputs tons of keys not explicitly set (by me anyway).

I guess I also really need to know how to locate all the config files that load sysctl keys into the kernel.

man sysctl only list a total of 6 possible locations for sysctl config files:

  • /run/sysctl.d/ which doesn't exist.
  • /etc/sysctl.d/ which contains 8 configuration files generated by the OS and 99-sysctl.conf symlink to /etc/sysctl.conf.
  • /usr/local/lib/sysctl.d/ which doesn't exist.
  • /usr/lib/sysctl.d/ which contains 3 files set by OS.
  • /lib/sysctl.d/ which is the same as /usr/lib/sysctl.d/ although it doesn't show a symlink.
  • /etc/sysctl.conf my heavily modified sysctl file.

I have gone through all of the individual config files and they do not account for even half of the sysctl keys loaded into the kernel. I understand that some applications can set sysctl keys, but there must be other configuration files loading all these keys into the kernel.

Other than options for sysctl command, the man page doesn't have any other data.

BeastOfCaerbannog
  • 16,703
emptysocket
  • 53

1 Answers1

3

/run/sysctl.d/ which doesn't exist.

/run is temporary, changes in it are lost at reboot. So drop-in configuration files in /run are used for modifying the current boot, but without persisting those changes past reboot.

/etc/sysctl.d/ which contains 8 configuration files generated by the OS and 99-sysctl.conf symlink to /etc/sysctl.conf.

This is where configuration from packages that are allowed to be modified by the admin will be kept. dpkg will inform you when your edits conflict with package updates.

/usr/local/lib/sysctl.d/ which doesn't exist.

/usr/local is meant for the admin's use, and packages won't do anything there. You are supposed to make the directory should you need it.

/usr/lib/sysctl.d/ which contains 3 files set by OS.

Files in /usr/lib are not meant to be modified by the admin. Package updates will overwrite changes here.

/lib/sysctl.d/ which is the same as /usr/lib/sysctl.d/ although it doesn't show a symlink.

Since the /usr merge, /bin, /lib, etc. are symlinks to the corresponding directories in /usr.

/etc/sysctl.conf my heavily modified sysctl file.

Left around for backwards compatibility, since people (and scripts and other tools) will still expect it.

Modifying configuration transactionally is easiest when you can just add or remove a file with the exact contents you want, instead of using regexes or whatever to modify a line in the middle of a configuration file filled with who-knows-what. That is why directories allowing drop-in configuration files are highly preferable to single configuration files.

muru
  • 207,228