man/systemd-resolved.service.xml

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   8   Copyright 2014 Tom Gundersen
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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
  60     manages network name resolution. It implements a caching DNS stub
  61     resolver and an LLMNR resolver and responder. It also generates
  62     <filename>/run/systemd/resolve/resolv.conf</filename> for
  63     compatibility which may be symlinked from
  64     <filename>/etc/resolv.conf</filename>. The glibc NSS module
  65     <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  66     is necessary to allow libc's NSS resolver functions to resolve
  67     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> is
  91       resolved to the IP addresses 127.0.0.1 and
  92       ::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     </itemizedlist>
 100
 101     <para>Lookup requests are routed to the available DNS servers
 102     and LLMNR interfaces according to the following rules:</para>
 103
 104     <itemizedlist>
 105       <listitem><para>Lookups for the special hostname
 106       <literal>localhost</literal> are never routed to the
 107       network.</para></listitem>
 108
 109       <listitem><para>Single-label names are routed to all local
 110       interfaces capable of IP multicasting, using the LLMNR
 111       protocol. Lookups for IPv4 addresses are only sent via LLMNR on
 112       IPv4, and lookups for IPv6 addresses are only sent via LLMNR on
 113       IPv6. Lookups for the locally configured host name and the
 114       <literal>gateway</literal> host name are never routed to
 115       LLMNR.</para></listitem>
 116
 117       <listitem><para>Multi-label names are routed to all local
 118       interfaces that have a DNS sever configured, plus the globally
 119       configured DNS server if there is one. Address lookups from the
 120       link-local addres range are never routed to
 121       DNS.</para></listitem>
 122     </itemizedlist>
 123
 124     <para>If lookups are routed to multiple interfaces, the first
 125     successful response is returned (thus effectively merging the
 126     lookup zones on all matching interfaces). If the lookup failed on
 127     all interfaces, the last failing response is returned.</para>
 128
 129     <para>Routing of lookups may be influenced by configuring
 130     per-interface domain names. See
 131     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 132     for details. Lookups for a hostname ending in one of the
 133     per-interface domains are exclusively routed to the matching
 134     interfaces.</para>
 135
 136     <para>Note that
 137     <filename>/run/systemd/resolve/resolv.conf</filename> should not
 138     be used directly, but only through a symlink from
 139     <filename>/etc/resolv.conf</filename>.</para>
 140   </refsect1>
 141
 142   <refsect1>
 143     <title>See Also</title>
 144     <para>
 145       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 146       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 147       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 148       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 149       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 150     </para>
 151   </refsect1>
 152
 153 </refentry>