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)

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