man/sysctl.d.xml

   1 <?xml version="1.0"?> <!--*-nxml-*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
   5 <refentry id="sysctl.d"
   6     xmlns:xi="http://www.w3.org/2001/XInclude">
   7
   8   <refentryinfo>
   9     <title>sysctl.d</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>sysctl.d</refentrytitle>
  15     <manvolnum>5</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>sysctl.d</refname>
  20     <refpurpose>Configure kernel parameters at boot</refpurpose>
  21   </refnamediv>
  22
  23   <refsynopsisdiv>
  24     <para><filename>/etc/sysctl.d/*.conf</filename></para>
  25     <para><filename>/run/sysctl.d/*.conf</filename></para>
  26     <para><filename>/usr/lib/sysctl.d/*.conf</filename></para>
  27   </refsynopsisdiv>
  28
  29   <refsect1>
  30     <title>Description</title>
  31
  32     <para>At boot,
  33     <citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  34     reads configuration files from the above directories to configure
  35     <citerefentry project='man-pages'><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  36     kernel parameters.</para>
  37   </refsect1>
  38
  39   <refsect1>
  40     <title>Configuration Format</title>
  41
  42     <para>The configuration files contain a list of variable
  43     assignments, separated by newlines. Empty lines and lines whose
  44     first non-whitespace character is <literal>#</literal> or
  45     <literal>;</literal> are ignored.</para>
  46
  47     <para>Note that either <literal>/</literal> or
  48     <literal>.</literal> may be used as separators within sysctl
  49     variable names. If the first separator is a slash, remaining
  50     slashes and dots are left intact. If the first separator is a dot,
  51     dots and slashes are interchanged.
  52     <literal>kernel.domainname=foo</literal> and
  53     <literal>kernel/domainname=foo</literal> are equivalent and will
  54     cause <literal>foo</literal> to be written to
  55     <filename>/proc/sys/kernel/domainname</filename>. Either
  56     <literal>net.ipv4.conf.enp3s0/200.forwarding</literal> or
  57     <literal>net/ipv4/conf/enp3s0.200/forwarding</literal> may be used
  58     to refer to
  59     <filename>/proc/sys/net/ipv4/conf/enp3s0.200/forwarding</filename>.
  60     </para>
  61
  62     <para>The settings configured with <filename>sysctl.d</filename>
  63     files will be applied early on boot. The network
  64     interface-specific options will also be applied individually for
  65     each network interface as it shows up in the system. (More
  66     specifically, <filename>net.ipv4.conf.*</filename>,
  67     <filename>net.ipv6.conf.*</filename>,
  68     <filename>net.ipv4.neigh.*</filename> and
  69     <filename>net.ipv6.neigh.*</filename>).</para>
  70
  71     <para>Many sysctl parameters only become available when certain
  72     kernel modules are loaded. Modules are usually loaded on demand,
  73     e.g. when certain hardware is plugged in or network brought up.
  74     This means that
  75     <citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  76     which runs during early boot will not configure such parameters if
  77     they become available after it has run. To set such parameters, it
  78     is recommended to add an
  79     <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  80     rule to set those parameters when they become available.
  81     Alternatively, a slightly simpler and less efficient option is to
  82     add the module to
  83     <citerefentry><refentrytitle>modules-load.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
  84     causing it to be loaded statically before sysctl settings are
  85     applied (see example below).</para>
  86   </refsect1>
  87
  88   <xi:include href="standard-conf.xml" xpointer="confd" />
  89
  90   <refsect1>
  91     <title>Examples</title>
  92     <example>
  93       <title>Set kernel YP domain name</title>
  94       <para><filename>/etc/sysctl.d/domain-name.conf</filename>:
  95       </para>
  96
  97       <programlisting>kernel.domainname=example.com</programlisting>
  98     </example>
  99
 100     <example>
 101       <title>Apply settings available only when a certain module is loaded (method one)</title>
 102       <para><filename>/etc/udev/rules.d/99-bridge.rules</filename>:
 103       </para>
 104
 105       <programlisting>ACTION=="add", SUBSYSTEM=="module", KERNEL=="br_netfilter", \
 106       RUN+="/usr/lib/systemd/systemd-sysctl --prefix=/net/bridge"
 107 </programlisting>
 108
 109       <para><filename>/etc/sysctl.d/bridge.conf</filename>:
 110       </para>
 111
 112       <programlisting>net.bridge.bridge-nf-call-ip6tables = 0
 113 net.bridge.bridge-nf-call-iptables = 0
 114 net.bridge.bridge-nf-call-arptables = 0
 115 </programlisting>
 116
 117       <para>This method applies settings when the module is
 118       loaded. Please note that, unless the <filename>br_netfilter</filename>
 119       module is loaded, bridged packets will not be filtered by
 120       Netfilter (starting with kernel 3.18), so simply not loading the
 121       module is sufficient to avoid filtering.</para>
 122     </example>
 123
 124     <example>
 125       <title>Apply settings available only when a certain module is loaded (method two)</title>
 126       <para><filename>/etc/modules-load.d/bridge.conf</filename>:
 127       </para>
 128
 129       <programlisting>br_netfilter</programlisting>
 130
 131       <para><filename>/etc/sysctl.d/bridge.conf</filename>:
 132       </para>
 133
 134       <programlisting>net.bridge.bridge-nf-call-ip6tables = 0
 135 net.bridge.bridge-nf-call-iptables = 0
 136 net.bridge.bridge-nf-call-arptables = 0
 137 </programlisting>
 138
 139       <para>This method forces the module to be always loaded. Please
 140       note that, unless the <filename>br_netfilter</filename> module is
 141       loaded, bridged packets will not be filtered with Netfilter
 142       (starting with kernel 3.18), so simply not loading the module is
 143       sufficient to avoid filtering.</para>
 144     </example>
 145   </refsect1>
 146
 147   <refsect1>
 148     <title>See Also</title>
 149     <para>
 150       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 151       <citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 152       <citerefentry><refentrytitle>systemd-delta</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 153       <citerefentry project='man-pages'><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 154       <citerefentry project='man-pages'><refentrytitle>sysctl.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 155       <citerefentry project='man-pages'><refentrytitle>modprobe</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 156     </para>
 157   </refsect1>
 158
 159 </refentry>