man/systemd-resolved.service.xml

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   8   This file is part of systemd.
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  10   Copyright 2014 Tom Gundersen
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  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 and MulticastDNS
  50     resolver and 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 (in case
  81     <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
  82     used), the per-link dynamic settings received over DHCP, and any DNS server information made available by other
  83     system services. See
  84     <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
  85     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details
  86     about systemd's own configuration files for DNS servers. To improve compatibility,
  87     <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but only if it is
  88     not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename> or
  89     <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para>
  90
  91     <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para>
  92
  93     <itemizedlist>
  94       <listitem><para>The local, configured hostname is resolved to
  95       all locally configured IP addresses ordered by their scope, or
  96       — if none are configured — the IPv4 address 127.0.0.2 (which
  97       is on the local loopback) and the IPv6 address ::1 (which is the
  98       local host).</para></listitem>
  99
 100       <listitem><para>The hostnames <literal>localhost</literal> and
 101       <literal>localhost.localdomain</literal> (as well as any hostname
 102       ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
 103       are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
 104
 105       <listitem><para>The hostname <literal>_gateway</literal> is
 106       resolved to all current default routing gateway addresses,
 107       ordered by their metric. This assigns a stable hostname to the
 108       current gateway, useful for referencing it independently of the
 109       current network configuration state.</para></listitem>
 110
 111       <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved
 112       to their configured addresses and back, but they will not affect lookups for
 113       non-address types (like MX).</para></listitem>
 114     </itemizedlist>
 115
 116     <para>Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces according to the
 117     following rules:</para>
 118
 119     <itemizedlist>
 120       <listitem><para>Lookups for the special hostname <literal>localhost</literal> are never routed to the network. (A
 121       few other, special domains are handled the same way.)</para></listitem>
 122
 123       <listitem><para>Single-label names are routed to all local interfaces capable of IP multicasting, using the LLMNR
 124       protocol. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are only
 125       sent via LLMNR on IPv6. Lookups for the locally configured host name and the <literal>_gateway</literal> host
 126       name are never routed to LLMNR.</para></listitem>
 127
 128       <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are routed to all local
 129       interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4 address lookups are
 130       sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem>
 131
 132       <listitem><para>Other multi-label names are routed to all local interfaces that have a DNS server configured,
 133       plus the globally configured DNS server if there is one. Address lookups from the link-local address range are
 134       never routed to DNS. Note that by default lookups for domains with the <literal>.local</literal> suffix are not
 135       routed to DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server
 136       and interface. This means that on networks where the <literal>.local</literal> domain is defined in a
 137       site-specific DNS server, explicit search or routing domains need to be configured to make lookups within this
 138       DNS domain work. Note that today it's generally recommended to avoid defining <literal>.local</literal> in a DNS
 139       server, as <ulink url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
 140       MulticastDNS use.</para></listitem>
 141     </itemizedlist>
 142
 143     <para>If lookups are routed to multiple interfaces, the first
 144     successful response is returned (thus effectively merging the
 145     lookup zones on all matching interfaces). If the lookup failed on
 146     all interfaces, the last failing response is returned.</para>
 147
 148     <para>Routing of lookups may be influenced by configuring
 149     per-interface domain names. See
 150     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 151     for details. Lookups for a hostname ending in one of the
 152     per-interface domains are exclusively routed to the matching
 153     interfaces.</para>
 154
 155     <para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
 156     Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
 157
 158   </refsect1>
 159
 160   <refsect1>
 161     <title><filename>/etc/resolv.conf</filename></title>
 162
 163     <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see
 164     <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
 165     supported:</para>
 166
 167     <itemizedlist>
 168       <listitem><para><command>systemd-resolved</command> maintains the
 169       <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional Linux
 170       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists the 127.0.0.53
 171       DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by
 172       systemd-resolved. The list of search domains is always kept up-to-date. Note that
 173       <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications, but only
 174       through a symlink from <filename>/etc/resolv.conf</filename>. 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> with correct search domains settings. This mode of operation is
 177       recommended.</para></listitem>
 178
 179       <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
 180       the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
 181       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
 182       <command>systemd-resolved</command>. This file does not contain any search domains.</para></listitem>
 183
 184       <listitem><para><command>systemd-resolved</command> maintains the
 185       <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
 186       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept up-to-date,
 187       containing information about all known DNS servers. Note the file format's limitations: it does not know a
 188       concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that
 189       <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications, but only
 190       through a symlink from <filename>/etc/resolv.conf</filename>. If this mode of operation is used local clients
 191       that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the
 192       known DNS servers.</para> </listitem>
 193
 194       <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which
 195       case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation
 196       <command>systemd-resolved</command> is consumer rather than provider of this configuration
 197       file. </para></listitem>
 198     </itemizedlist>
 199
 200     <para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether
 201     <filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or
 202     lists 127.0.0.53 as DNS server.</para>
 203   </refsect1>
 204
 205   <refsect1>
 206     <title>Signals</title>
 207
 208     <variablelist>
 209       <varlistentry>
 210         <term><constant>SIGUSR1</constant></term>
 211
 212         <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
 213         <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it maintains, as
 214         well as all feature level information it learnt about configured DNS servers into the system
 215         logs.</para></listitem>
 216       </varlistentry>
 217
 218       <varlistentry>
 219         <term><constant>SIGUSR2</constant></term>
 220
 221         <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal
 222         <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally not be
 223         necessary to request this explicitly – except for debugging purposes – as <command>systemd-resolved</command>
 224         flushes the caches automatically anyway any time the host's network configuration changes. Sending this signal
 225         to <command>systemd-resolved</command> is equivalent to the <command>resolvectl --flush-caches</command>
 226         command, however the latter is recommended since it operates in a synchronous way.</para></listitem>
 227       </varlistentry>
 228
 229       <varlistentry>
 230         <term><constant>SIGRTMIN+1</constant></term>
 231
 232         <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
 233         <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
 234         servers. Specifically any information about server feature support is flushed out, and the server feature
 235         probing logic is restarted on the next request, starting with the most fully featured level. Note that it
 236         should normally not be necessary to request this explicitly – except for debugging purposes – as
 237         <command>systemd-resolved</command> automatically forgets learnt information any time the DNS server
 238         configuration changes. Sending this signal to <command>systemd-resolved</command> is equivalent to the
 239         <command>resolvectl --reset-server-features</command> command, however the latter is recommended since it
 240         operates in a synchronous way.</para></listitem>
 241       </varlistentry>
 242     </variablelist>
 243
 244   </refsect1>
 245
 246   <refsect1>
 247     <title>See Also</title>
 248     <para>
 249       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 250       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 251       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 252       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 253       <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 254       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 255       <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 256       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 257       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 258     </para>
 259   </refsect1>
 260
 261 </refentry>