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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">
+<!--
+ 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>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>