man/systemd-resolved.service.xml

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  23
  24 <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVED'>
  25
  26   <refentryinfo>
  27     <title>systemd-resolved.service</title>
  28     <productname>systemd</productname>
  29
  30     <authorgroup>
  31       <author>
  32         <contrib>Developer</contrib>
  33         <firstname>Tom</firstname>
  34         <surname>Gundersen</surname>
  35         <email>teg@jklm.no</email>
  36       </author>
  37     </authorgroup>
  38   </refentryinfo>
  39
  40   <refmeta>
  41     <refentrytitle>systemd-resolved.service</refentrytitle>
  42     <manvolnum>8</manvolnum>
  43   </refmeta>
  44
  45   <refnamediv>
  46     <refname>systemd-resolved.service</refname>
  47     <refname>systemd-resolved</refname>
  48     <refpurpose>Network Name Resolution manager</refpurpose>
  49   </refnamediv>
  50
  51   <refsynopsisdiv>
  52     <para><filename>systemd-resolved.service</filename></para>
  53     <para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
  54   </refsynopsisdiv>
  55
  56   <refsect1>
  57     <title>Description</title>
  58
  59     <para><command>systemd-resolved</command> is a system service that provides network name resolution to local
  60     applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR resolver and
  61     responder. In addition it maintains the <filename>/run/systemd/resolve/resolv.conf</filename> file for
  62     compatibility with traditional Linux programs. This file may be symlinked from
  63     <filename>/etc/resolv.conf</filename>.</para>
  64
  65     <para>The glibc NSS module
  66     <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is required to
  67     permit glibc's NSS resolver functions to resolve host names via <command>systemd-resolved</command>.</para>
  68
  69     <para>The DNS servers contacted are determined from the global
  70     settings in <filename>/etc/systemd/resolved.conf</filename>, the
  71     per-link static settings in <filename>/etc/systemd/network/*.network</filename> files,
  72     and the per-link dynamic settings received over DHCP. See
  73     <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  74     and
  75     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  76     for details. To improve compatibility,
  77     <filename>/etc/resolv.conf</filename> is read in order to discover
  78     configured system DNS servers, but only if it is not a symlink
  79     to <filename>/run/systemd/resolve/resolv.conf</filename> (see above).</para>
  80
  81     <para><command>systemd-resolved</command> synthesizes DNS RRs for the following cases:</para>
  82
  83     <itemizedlist>
  84       <listitem><para>The local, configured hostname is resolved to
  85       all locally configured IP addresses ordered by their scope, or
  86       — if none are configured — the IPv4 address 127.0.0.2 (which
  87       is on the local loopback) and the IPv6 address ::1 (which is the
  88       local host).</para></listitem>
  89
  90       <listitem><para>The hostname <literal>localhost</literal> (as well as any hostname ending in
  91       <literal>.localhost</literal>, <literal>.localdomain</literal> or equal to <literal>localdomain</literal>) is
  92       resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
  93
  94       <listitem><para>The hostname <literal>gateway</literal> is
  95       resolved to all current default routing gateway addresses,
  96       ordered by their metric. This assigns a stable hostname to the
  97       current gateway, useful for referencing it independently of the
  98       current network configuration state.</para></listitem>
  99
 100       <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their configured
 101       addresses and back.</para></listitem>
 102     </itemizedlist>
 103
 104     <para>Lookup requests are routed to the available DNS servers
 105     and LLMNR interfaces according to the following rules:</para>
 106
 107     <itemizedlist>
 108       <listitem><para>Lookups for the special hostname
 109       <literal>localhost</literal> are never routed to the
 110       network. (A few other, special domains are handled the same way.)</para></listitem>
 111
 112       <listitem><para>Single-label names are routed to all local
 113       interfaces capable of IP multicasting, using the LLMNR
 114       protocol. Lookups for IPv4 addresses are only sent via LLMNR on
 115       IPv4, and lookups for IPv6 addresses are only sent via LLMNR on
 116       IPv6. Lookups for the locally configured host name and the
 117       <literal>gateway</literal> host name are never routed to
 118       LLMNR.</para></listitem>
 119
 120       <listitem><para>Multi-label names are routed to all local
 121       interfaces that have a DNS sever configured, plus the globally
 122       configured DNS server if there is one. Address lookups from the
 123       link-local address range are never routed to
 124       DNS.</para></listitem>
 125     </itemizedlist>
 126
 127     <para>If lookups are routed to multiple interfaces, the first
 128     successful response is returned (thus effectively merging the
 129     lookup zones on all matching interfaces). If the lookup failed on
 130     all interfaces, the last failing response is returned.</para>
 131
 132     <para>Routing of lookups may be influenced by configuring
 133     per-interface domain names. See
 134     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 135     for details. Lookups for a hostname ending in one of the
 136     per-interface domains are exclusively routed to the matching
 137     interfaces.</para>
 138
 139     <para>Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications,
 140     but only through a symlink from <filename>/etc/resolv.conf</filename>.</para>
 141
 142     <para>See the <ulink url="http://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
 143     Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
 144
 145   </refsect1>
 146
 147   <refsect1>
 148     <title>See Also</title>
 149     <para>
 150       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 151       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 152       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 153       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 154       <citerefentry><refentrytitle>systemd-resolve</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 155       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 156       <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 157       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 158       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 159     </para>
 160   </refsect1>
 161
 162 </refentry>