systemd/man/systemd-resolved.service.xml

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<refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVED'>

  <refentryinfo>
    <title>systemd-resolved.service</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Tom</firstname>
        <surname>Gundersen</surname>
        <email>teg@jklm.no</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-resolved.service</refentrytitle>
    <manvolnum>8</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-resolved.service</refname>
    <refname>systemd-resolved</refname>
    <refpurpose>Network Name Resolution manager</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <para><filename>systemd-resolved.service</filename></para>
    <para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><command>systemd-resolved</command> is a system service that provides network name resolution to local
    applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR resolver and
    responder. In addition it maintains the <filename>/run/systemd/resolve/resolv.conf</filename> file for
    compatibility with traditional Linux programs. This file may be symlinked from
    <filename>/etc/resolv.conf</filename>.</para>

    <para>The glibc NSS module
    <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is required to
    permit glibc's NSS resolver functions to resolve host names via <command>systemd-resolved</command>.</para>

    <para>The DNS servers contacted are determined from the global
    settings in <filename>/etc/systemd/resolved.conf</filename>, the
    per-link static settings in <filename>/etc/systemd/network/*.network</filename> files,
    and the per-link dynamic settings received over DHCP. See
    <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    and
    <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for details. To improve compatibility,
    <filename>/etc/resolv.conf</filename> is read in order to discover
    configured system DNS servers, but only if it is not a symlink
    to <filename>/run/systemd/resolve/resolv.conf</filename> (see above).</para>

    <para><command>systemd-resolved</command> synthesizes DNS RRs for the following cases:</para>

    <itemizedlist>
      <listitem><para>The local, configured hostname is resolved to
      all locally configured IP addresses ordered by their scope, or
      — if none are configured — the IPv4 address 127.0.0.2 (which
      is on the local loopback) and the IPv6 address ::1 (which is the
      local host).</para></listitem>

      <listitem><para>The hostnames <literal>localhost</literal> and
      <literal>localhost.localdomain</literal> (as well as any hostname
      ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
      are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>

      <listitem><para>The hostname <literal>gateway</literal> is
      resolved to all current default routing gateway addresses,
      ordered by their metric. This assigns a stable hostname to the
      current gateway, useful for referencing it independently of the
      current network configuration state.</para></listitem>

      <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their configured
      addresses and back.</para></listitem>
    </itemizedlist>

    <para>Lookup requests are routed to the available DNS servers
    and LLMNR interfaces according to the following rules:</para>

    <itemizedlist>
      <listitem><para>Lookups for the special hostname
      <literal>localhost</literal> are never routed to the
      network. (A few other, special domains are handled the same way.)</para></listitem>

      <listitem><para>Single-label names are routed to all local
      interfaces capable of IP multicasting, using the LLMNR
      protocol. Lookups for IPv4 addresses are only sent via LLMNR on
      IPv4, and lookups for IPv6 addresses are only sent via LLMNR on
      IPv6. Lookups for the locally configured host name and the
      <literal>gateway</literal> host name are never routed to
      LLMNR.</para></listitem>

      <listitem><para>Multi-label names are routed to all local
      interfaces that have a DNS sever configured, plus the globally
      configured DNS server if there is one. Address lookups from the
      link-local address range are never routed to
      DNS.</para></listitem>
    </itemizedlist>

    <para>If lookups are routed to multiple interfaces, the first
    successful response is returned (thus effectively merging the
    lookup zones on all matching interfaces). If the lookup failed on
    all interfaces, the last failing response is returned.</para>

    <para>Routing of lookups may be influenced by configuring
    per-interface domain names. See
    <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    for details. Lookups for a hostname ending in one of the
    per-interface domains are exclusively routed to the matching
    interfaces.</para>

    <para>Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications,
    but only through a symlink from <filename>/etc/resolv.conf</filename>.</para>

    <para>See the <ulink url="http://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
    Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>

  </refsect1>

  <refsect1>
    <title>Signals</title>

    <variablelist>
      <varlistentry>
        <term><constant>SIGUSR1</constant></term>

        <listitem><para>Upon reception of the SIGUSR1 process signal <command>systemd-resolved</command> will dump the
        contents of all DNS resource record caches it maintains into the system logs.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SIGUSR2</constant></term>

        <listitem><para>Upon reception of the SIGUSR2 process signal <command>systemd-resolved</command> will flush all
        caches it maintains. Note that it should normally not be necessary to request this explicitly – except for
        debugging purposes – as <command>systemd-resolved</command> flushes the caches automatically anyway any time
        the host's network configuration changes.</para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-resolve</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>