man/systemd-resolved.service.xml

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   8   Copyright © 2014 Tom Gundersen
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  11 <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'>
  12
  13   <refentryinfo>
  14     <title>systemd-resolved.service</title>
  15     <productname>systemd</productname>
  16
  17     <authorgroup>
  18       <author>
  19         <contrib>Developer</contrib>
  20         <firstname>Tom</firstname>
  21         <surname>Gundersen</surname>
  22         <email>teg@jklm.no</email>
  23       </author>
  24     </authorgroup>
  25   </refentryinfo>
  26
  27   <refmeta>
  28     <refentrytitle>systemd-resolved.service</refentrytitle>
  29     <manvolnum>8</manvolnum>
  30   </refmeta>
  31
  32   <refnamediv>
  33     <refname>systemd-resolved.service</refname>
  34     <refname>systemd-resolved</refname>
  35     <refpurpose>Network Name Resolution manager</refpurpose>
  36   </refnamediv>
  37
  38   <refsynopsisdiv>
  39     <para><filename>systemd-resolved.service</filename></para>
  40     <para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
  41   </refsynopsisdiv>
  42
  43   <refsect1>
  44     <title>Description</title>
  45
  46     <para><command>systemd-resolved</command> is a system service that provides network name resolution to local
  47     applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR and MulticastDNS
  48     resolver and responder. Local applications may submit network name resolution requests via three interfaces:</para>
  49
  50     <itemizedlist>
  51       <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus. See the
  52       <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved">API Documentation</ulink> for
  53       details. Usage of this API is generally recommended to clients as it is asynchronous and fully featured (for
  54       example, properly returns DNSSEC validation status and interface scope for addresses as necessary for supporting
  55       link-local networking).</para></listitem>
  56
  57       <listitem><para>The glibc
  58       <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry> API as defined
  59       by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related resolver functions,
  60       including <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
  61       API is widely supported, including beyond the Linux platform. In its current form it does not expose DNSSEC
  62       validation status information however, and is synchronous only. This API is backed by the glibc Name Service
  63       Switch (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>). Usage of the
  64       glibc NSS module <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  65       is required in order to allow glibc's NSS resolver functions to resolve host names via
  66       <command>systemd-resolved</command>.</para></listitem>
  67
  68       <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on IP
  69       address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly, bypassing any local
  70       API may be directed to this stub, in order to connect them to <command>systemd-resolved</command>. Note however
  71       that it is strongly recommended that local programs use the glibc NSS or bus APIs instead (as described above),
  72       as various network resolution concepts (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped
  73       to the unicast DNS protocol.</para></listitem>
  74     </itemizedlist>
  75
  76     <para>The DNS servers contacted are determined from the global settings in
  77     <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in
  78     <filename>/etc/systemd/network/*.network</filename> files (in case
  79     <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
  80     used), the per-link dynamic settings received over DHCP, and any DNS server information made available by other
  81     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, 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
 147     per-interface domain names. See
 148     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 149     for details. Lookups for a hostname ending in one of the
 150     per-interface domains are exclusively routed to the matching
 151     interfaces.</para>
 152
 153     <para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
 154     Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
 155
 156   </refsect1>
 157
 158   <refsect1>
 159     <title><filename>/etc/resolv.conf</filename></title>
 160
 161     <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see
 162     <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
 163     supported:</para>
 164
 165     <itemizedlist>
 166       <listitem><para><command>systemd-resolved</command> maintains the
 167       <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional Linux
 168       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists the 127.0.0.53
 169       DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by
 170       systemd-resolved. The list of search domains is always kept up-to-date. Note that
 171       <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications, but only
 172       through a symlink from <filename>/etc/resolv.conf</filename>. This file may be symlinked from
 173       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
 174       <command>systemd-resolved</command> with correct search domains settings. This mode of operation is
 175       recommended.</para></listitem>
 176
 177       <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
 178       the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
 179       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
 180       <command>systemd-resolved</command>. This file does not contain any search domains.</para></listitem>
 181
 182       <listitem><para><command>systemd-resolved</command> maintains the
 183       <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
 184       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept up-to-date,
 185       containing information about all known DNS servers. Note the file format's limitations: it does not know a
 186       concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that
 187       <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications, but only
 188       through a symlink from <filename>/etc/resolv.conf</filename>. If this mode of operation is used local clients
 189       that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the
 190       known DNS servers.</para> </listitem>
 191
 192       <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which
 193       case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation
 194       <command>systemd-resolved</command> is consumer rather than provider of this configuration
 195       file. </para></listitem>
 196     </itemizedlist>
 197
 198     <para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether
 199     <filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or
 200     lists 127.0.0.53 as DNS server.</para>
 201   </refsect1>
 202
 203   <refsect1>
 204     <title>Signals</title>
 205
 206     <variablelist>
 207       <varlistentry>
 208         <term><constant>SIGUSR1</constant></term>
 209
 210         <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
 211         <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it maintains, as
 212         well as all feature level information it learnt about configured DNS servers into the system
 213         logs.</para></listitem>
 214       </varlistentry>
 215
 216       <varlistentry>
 217         <term><constant>SIGUSR2</constant></term>
 218
 219         <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal
 220         <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally not be
 221         necessary to request this explicitly – except for debugging purposes – as <command>systemd-resolved</command>
 222         flushes the caches automatically anyway any time the host's network configuration changes. Sending this signal
 223         to <command>systemd-resolved</command> is equivalent to the <command>resolvectl --flush-caches</command>
 224         command, however the latter is recommended since it operates in a synchronous way.</para></listitem>
 225       </varlistentry>
 226
 227       <varlistentry>
 228         <term><constant>SIGRTMIN+1</constant></term>
 229
 230         <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
 231         <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
 232         servers. Specifically any information about server feature support is flushed out, and the server feature
 233         probing logic is restarted on the next request, starting with the most fully featured level. Note that it
 234         should normally not be necessary to request this explicitly – except for debugging purposes – as
 235         <command>systemd-resolved</command> automatically forgets learnt information any time the DNS server
 236         configuration changes. Sending this signal to <command>systemd-resolved</command> is equivalent to the
 237         <command>resolvectl --reset-server-features</command> command, however the latter is recommended since it
 238         operates in a synchronous way.</para></listitem>
 239       </varlistentry>
 240     </variablelist>
 241
 242   </refsect1>
 243
 244   <refsect1>
 245     <title>See Also</title>
 246     <para>
 247       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 248       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 249       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 250       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 251       <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 252       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 253       <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 254       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 255       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 256     </para>
 257   </refsect1>
 258
 259 </refentry>