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<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>