man/systemd-resolved.service.xml

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   6   SPDX-License-Identifier: LGPL-2.1+
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   8   This file is part of systemd.
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  10   Copyright 2014 Tom Gundersen
  11 -->
  12
  13 <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'>
  14
  15   <refentryinfo>
  16     <title>systemd-resolved.service</title>
  17     <productname>systemd</productname>
  18
  19     <authorgroup>
  20       <author>
  21         <contrib>Developer</contrib>
  22         <firstname>Tom</firstname>
  23         <surname>Gundersen</surname>
  24         <email>teg@jklm.no</email>
  25       </author>
  26     </authorgroup>
  27   </refentryinfo>
  28
  29   <refmeta>
  30     <refentrytitle>systemd-resolved.service</refentrytitle>
  31     <manvolnum>8</manvolnum>
  32   </refmeta>
  33
  34   <refnamediv>
  35     <refname>systemd-resolved.service</refname>
  36     <refname>systemd-resolved</refname>
  37     <refpurpose>Network Name Resolution manager</refpurpose>
  38   </refnamediv>
  39
  40   <refsynopsisdiv>
  41     <para><filename>systemd-resolved.service</filename></para>
  42     <para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
  43   </refsynopsisdiv>
  44
  45   <refsect1>
  46     <title>Description</title>
  47
  48     <para><command>systemd-resolved</command> is a system service that provides network name resolution to local
  49     applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR resolver and
  50     responder. Local applications may submit network name resolution requests via three interfaces:</para>
  51
  52     <itemizedlist>
  53       <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus. See the
  54       <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved">API Documentation</ulink> for
  55       details. Usage of this API is generally recommended to clients as it is asynchronous and fully featured (for
  56       example, properly returns DNSSEC validation status and interface scope for addresses as necessary for supporting
  57       link-local networking).</para></listitem>
  58
  59       <listitem><para>The glibc
  60       <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> API as defined
  61       by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related resolver functions,
  62       including <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
  63       API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC
  64       validation status information however, and is synchronous only. This API is backed by the glibc Name Service
  65       Switch (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Usage of the
  66       glibc NSS module <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  67       is required in order to allow glibc's NSS resolver functions to resolve host names via
  68       <command>systemd-resolved</command>.</para></listitem>
  69
  70       <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on IP
  71       address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local
  72       API may be directed to this stub, in order to connect them to <command>systemd-resolved</command>. Note however
  73       that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above),
  74       as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped
  75       to the unicast DNS protocol.</para></listitem>
  76     </itemizedlist>
  77
  78     <para>The DNS servers contacted are determined from the global settings in
  79     <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in
  80     <filename>/etc/systemd/network/*.network</filename> files, the per-link dynamic settings received over DHCP and any
  81     DNS server information made available by other system services. See
  82     <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
  83     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details
  84     about systemd's own configuration files for DNS servers. To improve compatibility,
  85     <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but only if it is
  86     not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename> or
  87     <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para>
  88
  89     <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para>
  90
  91     <itemizedlist>
  92       <listitem><para>The local, configured hostname is resolved to
  93       all locally configured IP addresses ordered by their scope, or
  94       — if none are configured — the IPv4 address 127.0.0.2 (which
  95       is on the local loopback) and the IPv6 address ::1 (which is the
  96       local host).</para></listitem>
  97
  98       <listitem><para>The hostnames <literal>localhost</literal> and
  99       <literal>localhost.localdomain</literal> (as well as any hostname
 100       ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
 101       are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
 102
 103       <listitem><para>The hostname <literal>_gateway</literal> is
 104       resolved to all current default routing gateway addresses,
 105       ordered by their metric. This assigns a stable hostname to the
 106       current gateway, useful for referencing it independently of the
 107       current network configuration state.</para></listitem>
 108
 109       <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved
 110       to their configured addresses and back, but they will not affect lookups for
 111       non-address types (like MX).</para></listitem>
 112     </itemizedlist>
 113
 114     <para>Lookup requests are routed to the available DNS servers
 115     and LLMNR interfaces according to the following rules:</para>
 116
 117     <itemizedlist>
 118       <listitem><para>Lookups for the special hostname
 119       <literal>localhost</literal> are never routed to the
 120       network. (A few other, special domains are handled the same way.)</para></listitem>
 121
 122       <listitem><para>Single-label names are routed to all local
 123       interfaces capable of IP multicasting, using the LLMNR
 124       protocol. Lookups for IPv4 addresses are only sent via LLMNR on
 125       IPv4, and lookups for IPv6 addresses are only sent via LLMNR on
 126       IPv6. Lookups for the locally configured host name and the
 127       <literal>_gateway</literal> host name are never routed to
 128       LLMNR.</para></listitem>
 129
 130       <listitem><para>Multi-label names are routed to all local
 131       interfaces that have a DNS server configured, plus the globally
 132       configured DNS server if there is one. Address lookups from the
 133       link-local address range are never routed to
 134       DNS.</para></listitem>
 135     </itemizedlist>
 136
 137     <para>If lookups are routed to multiple interfaces, the first
 138     successful response is returned (thus effectively merging the
 139     lookup zones on all matching interfaces). If the lookup failed on
 140     all interfaces, the last failing response is returned.</para>
 141
 142     <para>Routing of lookups may be influenced by configuring
 143     per-interface domain names. See
 144     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 145     for details. Lookups for a hostname ending in one of the
 146     per-interface domains are exclusively routed to the matching
 147     interfaces.</para>
 148
 149     <para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
 150     Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
 151
 152   </refsect1>
 153
 154   <refsect1>
 155     <title><filename>/etc/resolv.conf</filename></title>
 156
 157     <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see
 158     <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
 159     supported:</para>
 160
 161     <itemizedlist>
 162       <listitem><para><command>systemd-resolved</command> maintains the
 163       <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional Linux
 164       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists the 127.0.0.53
 165       DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by
 166       systemd-resolved. The list of search domains is always kept up-to-date. Note that
 167       <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications, but only
 168       through a symlink from <filename>/etc/resolv.conf</filename>. This file may be symlinked from
 169       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
 170       <command>systemd-resolved</command> with correct search domains settings. This mode of operation is
 171       recommended.</para></listitem>
 172
 173       <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
 174       the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
 175       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
 176       <command>systemd-resolved</command>. This file does not contain any search domains.</para></listitem>
 177
 178       <listitem><para><command>systemd-resolved</command> maintains the
 179       <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
 180       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept up-to-date,
 181       containing information about all known DNS servers. Note the file format's limitations: it does not know a
 182       concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that
 183       <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications, but only
 184       through a symlink from <filename>/etc/resolv.conf</filename>. If this mode of operation is used local clients
 185       that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the
 186       known DNS servers.</para> </listitem>
 187
 188       <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which
 189       case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation
 190       <command>systemd-resolved</command> is consumer rather than provider of this configuration
 191       file. </para></listitem>
 192     </itemizedlist>
 193
 194     <para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether
 195     <filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or
 196     lists 127.0.0.53 as DNS server.</para>
 197   </refsect1>
 198
 199   <refsect1>
 200     <title>Signals</title>
 201
 202     <variablelist>
 203       <varlistentry>
 204         <term><constant>SIGUSR1</constant></term>
 205
 206         <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
 207         <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it maintains, as
 208         well as all feature level information it learnt about configured DNS servers into the system
 209         logs.</para></listitem>
 210       </varlistentry>
 211
 212       <varlistentry>
 213         <term><constant>SIGUSR2</constant></term>
 214
 215         <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal
 216         <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally not be
 217         necessary to request this explicitly – except for debugging purposes – as <command>systemd-resolved</command>
 218         flushes the caches automatically anyway any time the host's network configuration changes. Sending this signal
 219         to <command>systemd-resolved</command> is equivalent to the <command>systemd-resolve --flush-caches</command>
 220         command, however the latter is recommended since it operates in a synchronous way.</para></listitem>
 221       </varlistentry>
 222
 223       <varlistentry>
 224         <term><constant>SIGRTMIN+1</constant></term>
 225
 226         <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
 227         <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
 228         servers. Specifically any information about server feature support is flushed out, and the server feature
 229         probing logic is restarted on the next request, starting with the most fully featured level. Note that it
 230         should normally not be necessary to request this explicitly – except for debugging purposes – as
 231         <command>systemd-resolved</command> automatically forgets learnt information any time the DNS server
 232         configuration changes. Sending this signal to <command>systemd-resolved</command> is equivalent to the
 233         <command>systemd-resolve --reset-server-features</command> command, however the latter is recommended since it
 234         operates in a synchronous way.</para></listitem>
 235       </varlistentry>
 236     </variablelist>
 237
 238   </refsect1>
 239
 240   <refsect1>
 241     <title>See Also</title>
 242     <para>
 243       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 244       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 245       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 246       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 247       <citerefentry><refentrytitle>systemd-resolve</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 248       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 249       <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 250       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 251       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 252     </para>
 253   </refsect1>
 254
 255 </refentry>