man/systemd-resolved.service.xml

<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  This file is part of systemd.

  Copyright 2014 Tom Gundersen

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->

<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
    manages network name resolution. It implements a caching DNS stub
    resolver and an LLMNR resolver and responder. It also generates
    <filename>/run/systemd/resolve/resolv.conf</filename> for
    compatibility which may be symlinked from
    <filename>/etc/resolv.conf</filename>. The glibc NSS module
    <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    is necessary to allow libc'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 hostname <literal>localhost</literal> is
      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>
    </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.</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 addres 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, but only through a symlink from
    <filename>/etc/resolv.conf</filename>.</para>
  </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>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>