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<?xml version="1.0"?>
<!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2011 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<refentry id="sysctl.d">
<refentryinfo>
<title>sysctl.d</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>sysctl.d</refentrytitle>
<manvolnum>5</manvolnum>
</refmeta>
<refnamediv>
<refname>sysctl.d</refname>
<refpurpose>Configure kernel parameters at boot</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename>/etc/sysctl.d/*.conf</filename></para>
<para><filename>/run/sysctl.d/*.conf</filename></para>
<para><filename>/usr/lib/sysctl.d/*.conf</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>At boot,
<citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
reads configuration files from the above directories
to configure
<citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>
kernel parameters.</para>
</refsect1>
<refsect1>
<title>Configuration Format</title>
<para>The configuration files contain a list of
variable assignments, separated by newlines. Empty
lines and lines whose first non-whitespace character
is <literal>#</literal> or <literal>;</literal> are
ignored.</para>
<para>Each configuration file shall be named in the
style of <filename><replaceable>program</replaceable>.conf</filename>.
Files in <filename>/etc/</filename> override files
with the same name in <filename>/usr/lib/</filename>
and <filename>/run/</filename>. Files in
<filename>/run/</filename> override files with the same
name in <filename>/usr/lib/</filename>. Packages
should install their configuration files in
<filename>/usr/lib/</filename>. Files in
<filename>/etc/</filename> 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 variable name, the entry in the file with the
lexicographically latest name will be applied. It is
recommended to prefix all filenames with a two-digit
number and a dash, to simplify the ordering of the
files.</para>
<para>Note that either <literal>/</literal> or
<literal>.</literal> 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. <literal>kernel.domainname=foo</literal>
and <literal>kernel/domainname=foo</literal> are
equivalent and will cause <literal>foo</literal> to
be written to
<filename>/proc/sys/kernel/domainname</filename>.
Either
<literal>net.ipv4.conf.enp3s0/200.forwarding</literal>
or
<literal>net/ipv4/conf/enp3s0.200/forwarding</literal>
may be used to refer to
<filename>/proc/sys/net/ipv4/conf/enp3s0.200/forwarding</filename>.
</para>
<para>If the administrator wants to disable a
configuration file supplied by the vendor, the
recommended way is to place a symlink to
<filename>/dev/null</filename> in
<filename>/etc/sysctl.d/</filename> bearing the
same filename.</para>
<para>The settings configured with
<filename>sysctl.d</filename> 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,
<filename>net.ipv4.conf.*</filename>,
<filename>net.ipv6.conf.*</filename>,
<filename>net.ipv4.neigh.*</filename> and <filename>net.ipv6.neigh.*</filename>).</para>
<para>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
<citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> 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 <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry> rule to set those parameters when they become
available. Alternatively, a slightly simpler and
less efficient option is to add the module to
<citerefentry><refentrytitle>modules-load.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>, causing it to be loaded statically
before sysctl settings are applied (see
example below).</para>
</refsect1>
<refsect1>
<title>Examples</title>
<example>
<title>Set kernel YP domain name</title>
<para><filename>/etc/sysctl.d/domain-name.conf</filename>:
</para>
<programlisting>kernel.domainname=example.com</programlisting>
</example>
<example>
<title>Disable packet filter on the bridge</title>
<para><filename>/etc/modules-load.d/bridge.conf</filename>:
</para>
<programlisting>bridge</programlisting>
<para><filename>/etc/sysctl.d/bridge.conf</filename>:
</para>
<programlisting>net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0
</programlisting>
</example>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-delta</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sysctl.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
<citerefentry><refentrytitle>modprobe</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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