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<?xml version="1.0"?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1+ -->
<refentry id="sysctl.d"
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sysctl.d</title>
<productname>systemd</productname>
</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 project='man-pages'><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>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 a variable assignment is prefixed with a single <literal>-</literal> character, any attempts to
set it that fail will be ignored (though are logged). Moreover, any access permission errors, and
attempts to write variables not defined on the local system are ignored (and logged) too.</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>
<xi:include href="standard-conf.xml" xpointer="confd" />
<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>Apply settings available only when a certain module is loaded (method one)</title>
<para><filename>/etc/udev/rules.d/99-bridge.rules</filename>:
</para>
<programlisting>ACTION=="add", SUBSYSTEM=="module", KERNEL=="br_netfilter", \
RUN+="/usr/lib/systemd/systemd-sysctl --prefix=/net/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>
<para>This method applies settings when the module is
loaded. Please note that, unless the <filename>br_netfilter</filename>
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.</para>
</example>
<example>
<title>Apply settings available only when a certain module is loaded (method two)</title>
<para><filename>/etc/modules-load.d/bridge.conf</filename>:
</para>
<programlisting>br_netfilter</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>
<para>This method forces the module to be always loaded. Please
note that, unless the <filename>br_netfilter</filename> 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.</para>
</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 project='man-pages'><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>sysctl.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>modprobe</refentrytitle><manvolnum>8</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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