SYSCTL.D(5)							     sysctl.d							       SYSCTL.D(5)

NAME

       sysctl.d - Configure kernel parameters at boot

SYNOPSIS

       /etc/sysctl.d/*.conf

       /run/sysctl.d/*.conf

       /usr/lib/sysctl.d/*.conf

DESCRIPTION

       At boot, systemd-sysctl.service(8) reads configuration files from the above directories to configure sysctl(8) kernel parameters.

CONFIGURATION FORMAT

       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 either "/" or "."  may be used as separators within sysctl variable names. If the first separator is a slash, remaining slashes
       and dots are left intact. If the first separator is a dot, dots and slashes are interchanged.  "kernel.domainname=foo" and
       "kernel/domainname=foo" are equivalent and will cause "foo" to be written to /proc/sys/kernel/domainname. Either
       "net.ipv4.conf.enp3s0/200.forwarding" or "net/ipv4/conf/enp3s0.200/forwarding" may be used to refer to
       /proc/sys/net/ipv4/conf/enp3s0.200/forwarding.

       The settings configured with sysctl.d files will be applied early on boot. The network interface-specific options will also be applied
       individually for each network interface as it shows up in the system. (More specifically, net.ipv4.conf.*, net.ipv6.conf.*,
       net.ipv4.neigh.*  and net.ipv6.neigh.*).

       Many sysctl parameters only become available when certain kernel modules are loaded. Modules are usually loaded on demand, e.g. when
       certain hardware is plugged in or network brought up. This means that systemd-sysctl.service(8) which runs during early boot will not
       configure such parameters if they become available after it has run. To set such parameters, it is recommended to add an udev(7) rule to
       set those parameters when they become available. Alternatively, a slightly simpler and less efficient option is to add the module to
       modules-load.d(5), causing it to be loaded statically before sysctl settings are applied (see example below).

CONFIGURATION DIRECTORIES AND PRECEDENCE

       Configuration files are read from directories in /etc/, /run/, and /lib/, in order of precedence. Each configuration file in these
       configuration directories shall be named in the style of filename.conf. Files in /etc/ override files with the same name in /run/ and
       /lib/. Files in /run/ override files with the same name in /lib/.

       Packages should install their configuration files in /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 lexicographic
       order, regardless of which of the directories they reside in. If multiple files specify the same option, the entry in the file with the
       lexicographically latest name will take precedence. It is recommended to prefix all filenames with a two-digit number and a dash, to
       simplify the ordering of the files.

       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 the configuration directory in /etc/, with the same filename as the vendor configuration file. If the vendor configuration file is
       included in the initrd image, the image has to be regenerated.

EXAMPLES

       Example 1. Set kernel YP domain name

       /etc/sysctl.d/domain-name.conf:

	   kernel.domainname=example.com

       Example 2. Apply settings available only when a certain module is loaded (method one)

       /etc/udev/rules.d/99-bridge.rules:

	   ACTION=="add", SUBSYSTEM=="module", KERNEL=="br_netfilter", 
		 RUN+="/lib/systemd/systemd-sysctl --prefix=/net/bridge"

       /etc/sysctl.d/bridge.conf:

	   net.bridge.bridge-nf-call-ip6tables = 0
	   net.bridge.bridge-nf-call-iptables = 0
	   net.bridge.bridge-nf-call-arptables = 0

       This method applies settings when the module is loaded. Please note that, unless the br_netfilter module is loaded, bridged packets will
       not be filtered by Netfilter (starting with kernel 3.18), so simply not loading the module is sufficient to avoid filtering.

       Example 3. Apply settings available only when a certain module is loaded (method two)

       /etc/modules-load.d/bridge.conf:

	   br_netfilter

       /etc/sysctl.d/bridge.conf:

	   net.bridge.bridge-nf-call-ip6tables = 0
	   net.bridge.bridge-nf-call-iptables = 0
	   net.bridge.bridge-nf-call-arptables = 0

       This method forces the module to be always loaded. Please note that, unless the br_netfilter module is loaded, bridged packets will not be
       filtered with Netfilter (starting with kernel 3.18), so simply not loading the module is sufficient to avoid filtering.

SEE ALSO

       systemd(1), systemd-sysctl.service(8), systemd-delta(1), sysctl(8), sysctl.conf(5), modprobe(8)

systemd 237															       SYSCTL.D(5)