man/systemd-resolved.service.xml

   1 <?xml version='1.0'?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7 -->
   8
   9 <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'>
  10
  11   <refentryinfo>
  12     <title>systemd-resolved.service</title>
  13     <productname>systemd</productname>
  14   </refentryinfo>
  15
  16   <refmeta>
  17     <refentrytitle>systemd-resolved.service</refentrytitle>
  18     <manvolnum>8</manvolnum>
  19   </refmeta>
  20
  21   <refnamediv>
  22     <refname>systemd-resolved.service</refname>
  23     <refname>systemd-resolved</refname>
  24     <refpurpose>Network Name Resolution manager</refpurpose>
  25   </refnamediv>
  26
  27   <refsynopsisdiv>
  28     <para><filename>systemd-resolved.service</filename></para>
  29     <para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
  30   </refsynopsisdiv>
  31
  32   <refsect1>
  33     <title>Description</title>
  34
  35     <para><command>systemd-resolved</command> is a system service that provides network name resolution to local
  36     applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR and MulticastDNS
  37     resolver and responder. Local applications may submit network name resolution requests via three interfaces:</para>
  38
  39     <itemizedlist>
  40       <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus. See the
  41       <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved">API Documentation</ulink> for
  42       details. Usage of this API is generally recommended to clients as it is asynchronous and fully featured (for
  43       example, properly returns DNSSEC validation status and interface scope for addresses as necessary for supporting
  44       link-local networking).</para></listitem>
  45
  46       <listitem><para>The glibc
  47       <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> API as defined
  48       by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related resolver functions,
  49       including <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
  50       API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC
  51       validation status information however, and is synchronous only. This API is backed by the glibc Name Service
  52       Switch (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Usage of the
  53       glibc NSS module <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  54       is required in order to allow glibc's NSS resolver functions to resolve host names via
  55       <command>systemd-resolved</command>.</para></listitem>
  56
  57       <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on IP
  58       address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local
  59       API may be directed to this stub, in order to connect them to <command>systemd-resolved</command>. Note however
  60       that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above),
  61       as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped
  62       to the unicast DNS protocol.</para></listitem>
  63     </itemizedlist>
  64
  65     <para>The DNS servers contacted are determined from the global settings in
  66     <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in
  67     <filename>/etc/systemd/network/*.network</filename> files (in case
  68     <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
  69     used), the per-link dynamic settings received over DHCP, user request made via
  70     <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, and any DNS server
  71     information made available by other system services. See
  72     <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
  73     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details
  74     about systemd's own configuration files for DNS servers. To improve compatibility,
  75     <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but only if it is
  76     not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>,
  77     <filename>/usr/lib/systemd/resolv.conf</filename> or <filename>/run/systemd/resolve/resolv.conf</filename> (see
  78     below).</para>
  79
  80   </refsect1>
  81
  82   <refsect1>
  83     <title>Synthetic Records</title>
  84
  85     <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para>
  86
  87     <itemizedlist>
  88       <listitem><para>The local, configured hostname is resolved to
  89       all locally configured IP addresses ordered by their scope, or
  90       — if none are configured — the IPv4 address 127.0.0.2 (which
  91       is on the local loopback) and the IPv6 address ::1 (which is the
  92       local host).</para></listitem>
  93
  94       <listitem><para>The hostnames <literal>localhost</literal> and
  95       <literal>localhost.localdomain</literal> (as well as any hostname
  96       ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
  97       are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
  98
  99       <listitem><para>The hostname <literal>_gateway</literal> is
 100       resolved to all current default routing gateway addresses,
 101       ordered by their metric. This assigns a stable hostname to the
 102       current gateway, useful for referencing it independently of the
 103       current network configuration state.</para></listitem>
 104
 105       <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved
 106       to their configured addresses and back, but they will not affect lookups for
 107       non-address types (like MX).</para></listitem>
 108     </itemizedlist>
 109   </refsect1>
 110
 111   <refsect1>
 112     <title>Protocols and Routing</title>
 113
 114     <para>Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces according to the
 115     following rules:</para>
 116
 117     <itemizedlist>
 118       <listitem><para>Lookups for the special hostname <literal>localhost</literal> are never routed to the network. (A
 119       few other, special domains are handled the same way.)</para></listitem>
 120
 121       <listitem><para>Single-label names are routed to all local interfaces capable of IP multicasting, using the LLMNR
 122       protocol. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are only
 123       sent via LLMNR on IPv6. Lookups for the locally configured host name and the <literal>_gateway</literal> host
 124       name are never routed to LLMNR.</para></listitem>
 125
 126       <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are routed to all local
 127       interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4 address lookups are
 128       sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem>
 129
 130       <listitem><para>Other multi-label names are routed to all local interfaces that have a DNS server configured,
 131       plus the globally configured DNS server if there is one. Address lookups from the link-local address range are
 132       never routed to DNS. Note that by default lookups for domains with the <literal>.local</literal> suffix are not
 133       routed to DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server
 134       and interface. This means that on networks where the <literal>.local</literal> domain is defined in a
 135       site-specific DNS server, explicit search or routing domains need to be configured to make lookups within this
 136       DNS domain work. Note that today it's generally recommended to avoid defining <literal>.local</literal> in a DNS
 137       server, as <ulink url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
 138       MulticastDNS use.</para></listitem>
 139     </itemizedlist>
 140
 141     <para>If lookups are routed to multiple interfaces, the first
 142     successful response is returned (thus effectively merging the
 143     lookup zones on all matching interfaces). If the lookup failed on
 144     all interfaces, the last failing response is returned.</para>
 145
 146     <para>Routing of lookups may be influenced by configuring per-interface domain names and other settings. See
 147     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
 148     <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for details. The
 149     following query routing logic applies for unicast DNS traffic:</para>
 150
 151     <itemizedlist>
 152       <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the configured search
 153       or route-only domains of any link (or the globally configured DNS settings), the "best matching"
 154       search/route-only domain is determined: the matching one with the most labels. The query is then sent to all DNS
 155       servers of any links or the globally configured DNS servers associated with this "best matching"
 156       search/route-only domain. (Note that more than one link might have this same "best matching" search/route-only
 157       domain configured, in which case the query is sent to all of them in parallel).</para></listitem>
 158
 159       <listitem><para>If a query does not match any configured search/route-only domain (neither per-link nor global),
 160       it is sent to all DNS servers that are configured on links with the "DNS default route" option set, as well as
 161       the globally configured DNS server.</para></listitem>
 162
 163       <listitem><para>If there is no link configured as "DNS default route" and no global DNS server configured, the
 164       compiled-in fallback DNS server is used.</para></listitem>
 165
 166       <listitem><para>Otherwise the query is failed as no suitable DNS servers could be determined.</para></listitem>
 167     </itemizedlist>
 168
 169     <para>The "DNS default route" option is a boolean setting configureable with <command>resolvectl</command> or in
 170     <filename>.network</filename> files. If not set, it is implicitly determined based on the configured DNS domains
 171     for a link: if there's any route-only domain (not matching <literal>~.</literal>) it defaults to false, otherwise
 172     to true.</para>
 173
 174     <para>Effectively this means: in order to preferably route all DNS queries not explicitly matched by
 175     search/route-only domain configuration to a specific link, configure a <literal>~.</literal> route-only domain on
 176     it. This will ensure that other links will not be considered for the queries (unless they too carry such a
 177     route-only domain). In order to route all such DNS queries to a specific link only in case no other link is
 178     preferable, then set the "DNS default route" option for the link to true, and do not configure a
 179     <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never receives any
 180     DNS traffic not matching any of its configured search/route-only domains, set the "DNS default route" option for it
 181     to false.</para>
 182
 183     <para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
 184     Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
 185   </refsect1>
 186
 187   <refsect1>
 188     <title><filename>/etc/resolv.conf</filename></title>
 189
 190     <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see
 191     <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
 192     supported:</para>
 193
 194     <itemizedlist>
 195       <listitem><para><command>systemd-resolved</command> maintains the
 196       <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional Linux
 197       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists the 127.0.0.53
 198       DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by
 199       systemd-resolved. The list of search domains is always kept up-to-date. Note that
 200       <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications, but only
 201       through a symlink from <filename>/etc/resolv.conf</filename>. This file may be symlinked from
 202       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
 203       <command>systemd-resolved</command> with correct search domains settings. This mode of operation is
 204       recommended.</para></listitem>
 205
 206       <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
 207       the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
 208       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
 209       <command>systemd-resolved</command>. This file does not contain any search domains.</para></listitem>
 210
 211       <listitem><para><command>systemd-resolved</command> maintains the
 212       <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
 213       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept up-to-date,
 214       containing information about all known DNS servers. Note the file format's limitations: it does not know a
 215       concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that
 216       <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications, but only
 217       through a symlink from <filename>/etc/resolv.conf</filename>. If this mode of operation is used local clients
 218       that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the
 219       known DNS servers.</para> </listitem>
 220
 221       <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which
 222       case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation
 223       <command>systemd-resolved</command> is consumer rather than provider of this configuration
 224       file. </para></listitem>
 225     </itemizedlist>
 226
 227     <para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether
 228     <filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or
 229     lists 127.0.0.53 as DNS server.</para>
 230   </refsect1>
 231
 232   <refsect1>
 233     <title>Signals</title>
 234
 235     <variablelist>
 236       <varlistentry>
 237         <term><constant>SIGUSR1</constant></term>
 238
 239         <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
 240         <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it maintains, as
 241         well as all feature level information it learnt about configured DNS servers into the system
 242         logs.</para></listitem>
 243       </varlistentry>
 244
 245       <varlistentry>
 246         <term><constant>SIGUSR2</constant></term>
 247
 248         <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal
 249         <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally not be
 250         necessary to request this explicitly – except for debugging purposes – as <command>systemd-resolved</command>
 251         flushes the caches automatically anyway any time the host's network configuration changes. Sending this signal
 252         to <command>systemd-resolved</command> is equivalent to the <command>resolvectl --flush-caches</command>
 253         command, however the latter is recommended since it operates in a synchronous way.</para></listitem>
 254       </varlistentry>
 255
 256       <varlistentry>
 257         <term><constant>SIGRTMIN+1</constant></term>
 258
 259         <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
 260         <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
 261         servers. Specifically any information about server feature support is flushed out, and the server feature
 262         probing logic is restarted on the next request, starting with the most fully featured level. Note that it
 263         should normally not be necessary to request this explicitly – except for debugging purposes – as
 264         <command>systemd-resolved</command> automatically forgets learnt information any time the DNS server
 265         configuration changes. Sending this signal to <command>systemd-resolved</command> is equivalent to the
 266         <command>resolvectl --reset-server-features</command> command, however the latter is recommended since it
 267         operates in a synchronous way.</para></listitem>
 268       </varlistentry>
 269     </variablelist>
 270
 271   </refsect1>
 272
 273   <refsect1>
 274     <title>See Also</title>
 275     <para>
 276       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 277       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 278       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 279       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 280       <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 281       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 282       <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 283       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 284       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 285     </para>
 286   </refsect1>
 287
 288 </refentry>