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 hostnames <literal>localhost</literal> and
  91       <literal>localhost.localdomain</literal> (as well as any hostname
  92       ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
  93       are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
  94
  95       <listitem><para>The hostname <literal>gateway</literal> is
  96       resolved to all current default routing gateway addresses,
  97       ordered by their metric. This assigns a stable hostname to the
  98       current gateway, useful for referencing it independently of the
  99       current network configuration state.</para></listitem>
 100
 101       <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their configured
 102       addresses and back.</para></listitem>
 103     </itemizedlist>
 104
 105     <para>Lookup requests are routed to the available DNS servers
 106     and LLMNR interfaces according to the following rules:</para>
 107
 108     <itemizedlist>
 109       <listitem><para>Lookups for the special hostname
 110       <literal>localhost</literal> are never routed to the
 111       network. (A few other, special domains are handled the same way.)</para></listitem>
 112
 113       <listitem><para>Single-label names are routed to all local
 114       interfaces capable of IP multicasting, using the LLMNR
 115       protocol. Lookups for IPv4 addresses are only sent via LLMNR on
 116       IPv4, and lookups for IPv6 addresses are only sent via LLMNR on
 117       IPv6. Lookups for the locally configured host name and the
 118       <literal>gateway</literal> host name are never routed to
 119       LLMNR.</para></listitem>
 120
 121       <listitem><para>Multi-label names are routed to all local
 122       interfaces that have a DNS sever configured, plus the globally
 123       configured DNS server if there is one. Address lookups from the
 124       link-local address range are never routed to
 125       DNS.</para></listitem>
 126     </itemizedlist>
 127
 128     <para>If lookups are routed to multiple interfaces, the first
 129     successful response is returned (thus effectively merging the
 130     lookup zones on all matching interfaces). If the lookup failed on
 131     all interfaces, the last failing response is returned.</para>
 132
 133     <para>Routing of lookups may be influenced by configuring
 134     per-interface domain names. See
 135     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 136     for details. Lookups for a hostname ending in one of the
 137     per-interface domains are exclusively routed to the matching
 138     interfaces.</para>
 139
 140     <para>Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications,
 141     but only through a symlink from <filename>/etc/resolv.conf</filename>.</para>
 142
 143     <para>See the <ulink url="http://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
 144     Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
 145
 146   </refsect1>
 147
 148   <refsect1>
 149     <title>See Also</title>
 150     <para>
 151       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 152       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 153       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 154       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 155       <citerefentry><refentrytitle>systemd-resolve</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 156       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 157       <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 158       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 159       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 160     </para>
 161   </refsect1>
 162
 163 </refentry>