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
  28     <programlisting>key.name.under.proc.sys = some value
  29 key/name/under/proc/sys = some value
  30 key/middle.part.with.dots/foo = 123
  31 key.middle/part/with/dots.foo = 123
  32 -key.that.will.not.fail = value
  33 </programlisting>
  34   </refsynopsisdiv>
  35
  36   <refsect1>
  37     <title>Description</title>
  38
  39     <para>At boot,
  40     <citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  41     reads configuration files from the above directories to configure
  42     <citerefentry project='man-pages'><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  43     kernel parameters.</para>
  44   </refsect1>
  45
  46   <refsect1>
  47     <title>Configuration Format</title>
  48
  49     <para>The configuration files contain a list of variable
  50     assignments, separated by newlines. Empty lines and lines whose
  51     first non-whitespace character is <literal>#</literal> or
  52     <literal>;</literal> are ignored.</para>
  53
  54     <para>Note that either <literal>/</literal> or
  55     <literal>.</literal> may be used as separators within sysctl
  56     variable names. If the first separator is a slash, remaining
  57     slashes and dots are left intact. If the first separator is a dot,
  58     dots and slashes are interchanged.
  59     <literal>kernel.domainname=foo</literal> and
  60     <literal>kernel/domainname=foo</literal> are equivalent and will
  61     cause <literal>foo</literal> to be written to
  62     <filename>/proc/sys/kernel/domainname</filename>. Either
  63     <literal>net.ipv4.conf.enp3s0/200.forwarding</literal> or
  64     <literal>net/ipv4/conf/enp3s0.200/forwarding</literal> may be used
  65     to refer to
  66     <filename>/proc/sys/net/ipv4/conf/enp3s0.200/forwarding</filename>.
  67     </para>
  68
  69     <para>Any access permission errors and attempts to write variables not present on the local system are
  70     logged, but do not cause the service to fail. Debug log level is used, which means that the message will
  71     not show up at all by default. Moreover, if a variable assignment is prefixed with a single
  72     <literal>-</literal> character, any failure to set the variable will be logged at debug level, but will
  73     not cause the service to fail. All other errors when setting variables are logged with higher priority
  74     and cause the service to return failure at the end (other variables are still processed).</para>
  75
  76     <para>The settings configured with <filename>sysctl.d</filename>
  77     files will be applied early on boot. The network
  78     interface-specific options will also be applied individually for
  79     each network interface as it shows up in the system. (More
  80     specifically, <filename>net.ipv4.conf.*</filename>,
  81     <filename>net.ipv6.conf.*</filename>,
  82     <filename>net.ipv4.neigh.*</filename> and
  83     <filename>net.ipv6.neigh.*</filename>).</para>
  84
  85     <para>Many sysctl parameters only become available when certain
  86     kernel modules are loaded. Modules are usually loaded on demand,
  87     e.g. when certain hardware is plugged in or network brought up.
  88     This means that
  89     <citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  90     which runs during early boot will not configure such parameters if
  91     they become available after it has run. To set such parameters, it
  92     is recommended to add an
  93     <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  94     rule to set those parameters when they become available.
  95     Alternatively, a slightly simpler and less efficient option is to
  96     add the module to
  97     <citerefentry><refentrytitle>modules-load.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
  98     causing it to be loaded statically before sysctl settings are
  99     applied (see example below).</para>
 100   </refsect1>
 101
 102   <xi:include href="standard-conf.xml" xpointer="confd" />
 103
 104   <refsect1>
 105     <title>Examples</title>
 106     <example>
 107       <title>Set kernel YP domain name</title>
 108       <para><filename>/etc/sysctl.d/domain-name.conf</filename>:
 109       </para>
 110
 111       <programlisting>kernel.domainname=example.com</programlisting>
 112     </example>
 113
 114     <example>
 115       <title>Apply settings available only when a certain module is loaded (method one)</title>
 116       <para><filename>/etc/udev/rules.d/99-bridge.rules</filename>:
 117       </para>
 118
 119       <programlisting>ACTION=="add", SUBSYSTEM=="module", KERNEL=="br_netfilter", \
 120       RUN+="/usr/lib/systemd/systemd-sysctl --prefix=/net/bridge"
 121 </programlisting>
 122
 123       <para><filename>/etc/sysctl.d/bridge.conf</filename>:
 124       </para>
 125
 126       <programlisting>net.bridge.bridge-nf-call-ip6tables = 0
 127 net.bridge.bridge-nf-call-iptables = 0
 128 net.bridge.bridge-nf-call-arptables = 0
 129 </programlisting>
 130
 131       <para>This method applies settings when the module is
 132       loaded. Please note that, unless the <filename>br_netfilter</filename>
 133       module is loaded, bridged packets will not be filtered by
 134       Netfilter (starting with kernel 3.18), so simply not loading the
 135       module is sufficient to avoid filtering.</para>
 136     </example>
 137
 138     <example>
 139       <title>Apply settings available only when a certain module is loaded (method two)</title>
 140       <para><filename>/etc/modules-load.d/bridge.conf</filename>:
 141       </para>
 142
 143       <programlisting>br_netfilter</programlisting>
 144
 145       <para><filename>/etc/sysctl.d/bridge.conf</filename>:
 146       </para>
 147
 148       <programlisting>net.bridge.bridge-nf-call-ip6tables = 0
 149 net.bridge.bridge-nf-call-iptables = 0
 150 net.bridge.bridge-nf-call-arptables = 0
 151 </programlisting>
 152
 153       <para>This method forces the module to be always loaded. Please
 154       note that, unless the <filename>br_netfilter</filename> module is
 155       loaded, bridged packets will not be filtered with Netfilter
 156       (starting with kernel 3.18), so simply not loading the module is
 157       sufficient to avoid filtering.</para>
 158     </example>
 159   </refsect1>
 160
 161   <refsect1>
 162     <title>See Also</title>
 163     <para>
 164       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 165       <citerefentry><refentrytitle>systemd-sysctl.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 166       <citerefentry><refentrytitle>systemd-delta</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 167       <citerefentry project='man-pages'><refentrytitle>sysctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 168       <citerefentry project='man-pages'><refentrytitle>sysctl.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 169       <citerefentry project='man-pages'><refentrytitle>modprobe</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 170     </para>
 171   </refsect1>
 172
 173 </refentry>