man/systemd-resolved.service.xml

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   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, and any DNS server information made available by other
  70     system services. See
  71     <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
  72     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for details
  73     about systemd's own configuration files for DNS servers. To improve compatibility,
  74     <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but only if it is
  75     not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename> or
  76     <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para>
  77
  78     <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following cases:</para>
  79
  80     <itemizedlist>
  81       <listitem><para>The local, configured hostname is resolved to
  82       all locally configured IP addresses ordered by their scope, or
  83       — if none are configured — the IPv4 address 127.0.0.2 (which
  84       is on the local loopback) and the IPv6 address ::1 (which is the
  85       local host).</para></listitem>
  86
  87       <listitem><para>The hostnames <literal>localhost</literal> and
  88       <literal>localhost.localdomain</literal> (as well as any hostname
  89       ending in <literal>.localhost</literal> or <literal>.localhost.localdomain</literal>)
  90       are resolved to the IP addresses 127.0.0.1 and ::1.</para></listitem>
  91
  92       <listitem><para>The hostname <literal>_gateway</literal> is
  93       resolved to all current default routing gateway addresses,
  94       ordered by their metric. This assigns a stable hostname to the
  95       current gateway, useful for referencing it independently of the
  96       current network configuration state.</para></listitem>
  97
  98       <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved
  99       to their configured addresses and back, but they will not affect lookups for
 100       non-address types (like MX).</para></listitem>
 101     </itemizedlist>
 102
 103     <para>Lookup requests are routed to the available DNS servers, LLMNR and MulticastDNS interfaces according to the
 104     following rules:</para>
 105
 106     <itemizedlist>
 107       <listitem><para>Lookups for the special hostname <literal>localhost</literal> are never routed to the network. (A
 108       few other, special domains are handled the same way.)</para></listitem>
 109
 110       <listitem><para>Single-label names are routed to all local interfaces capable of IP multicasting, using the LLMNR
 111       protocol. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are only
 112       sent via LLMNR on IPv6. Lookups for the locally configured host name and the <literal>_gateway</literal> host
 113       name are never routed to LLMNR.</para></listitem>
 114
 115       <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are routed to all local
 116       interfaces capable of IP multicasting, using the MulticastDNS protocol. As with LLMNR IPv4 address lookups are
 117       sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem>
 118
 119       <listitem><para>Other multi-label names are routed to all local interfaces that have a DNS server configured,
 120       plus the globally configured DNS server if there is one. Address lookups from the link-local address range are
 121       never routed to DNS. Note that by default lookups for domains with the <literal>.local</literal> suffix are not
 122       routed to DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server
 123       and interface. This means that on networks where the <literal>.local</literal> domain is defined in a
 124       site-specific DNS server, explicit search or routing domains need to be configured to make lookups within this
 125       DNS domain work. Note that today it's generally recommended to avoid defining <literal>.local</literal> in a DNS
 126       server, as <ulink url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
 127       MulticastDNS use.</para></listitem>
 128     </itemizedlist>
 129
 130     <para>If lookups are routed to multiple interfaces, the first
 131     successful response is returned (thus effectively merging the
 132     lookup zones on all matching interfaces). If the lookup failed on
 133     all interfaces, the last failing response is returned.</para>
 134
 135     <para>Routing of lookups may be influenced by configuring
 136     per-interface domain names. See
 137     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 138     for details. Lookups for a hostname ending in one of the
 139     per-interface domains are exclusively routed to the matching
 140     interfaces.</para>
 141
 142     <para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved"> resolved D-Bus API
 143     Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.</para>
 144
 145   </refsect1>
 146
 147   <refsect1>
 148     <title><filename>/etc/resolv.conf</filename></title>
 149
 150     <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see
 151     <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
 152     supported:</para>
 153
 154     <itemizedlist>
 155       <listitem><para><command>systemd-resolved</command> maintains the
 156       <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional Linux
 157       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists the 127.0.0.53
 158       DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by
 159       systemd-resolved. The list of search domains is always kept up-to-date. Note that
 160       <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications, but only
 161       through a symlink from <filename>/etc/resolv.conf</filename>. This file may be symlinked from
 162       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
 163       <command>systemd-resolved</command> with correct search domains settings. This mode of operation is
 164       recommended.</para></listitem>
 165
 166       <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
 167       the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
 168       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs to
 169       <command>systemd-resolved</command>. This file does not contain any search domains.</para></listitem>
 170
 171       <listitem><para><command>systemd-resolved</command> maintains the
 172       <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
 173       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept up-to-date,
 174       containing information about all known DNS servers. Note the file format's limitations: it does not know a
 175       concept of per-interface DNS servers and hence only contains system-wide DNS server definitions. Note that
 176       <filename>/run/systemd/resolve/resolv.conf</filename> should not be used directly by applications, but only
 177       through a symlink from <filename>/etc/resolv.conf</filename>. If this mode of operation is used local clients
 178       that bypass any local DNS API will also bypass <command>systemd-resolved</command> and will talk directly to the
 179       known DNS servers.</para> </listitem>
 180
 181       <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages, in which
 182       case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode of operation
 183       <command>systemd-resolved</command> is consumer rather than provider of this configuration
 184       file. </para></listitem>
 185     </itemizedlist>
 186
 187     <para>Note that the selected mode of operation for this file is detected fully automatically, depending on whether
 188     <filename>/etc/resolv.conf</filename> is a symlink to <filename>/run/systemd/resolve/resolv.conf</filename> or
 189     lists 127.0.0.53 as DNS server.</para>
 190   </refsect1>
 191
 192   <refsect1>
 193     <title>Signals</title>
 194
 195     <variablelist>
 196       <varlistentry>
 197         <term><constant>SIGUSR1</constant></term>
 198
 199         <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
 200         <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it maintains, as
 201         well as all feature level information it learnt about configured DNS servers into the system
 202         logs.</para></listitem>
 203       </varlistentry>
 204
 205       <varlistentry>
 206         <term><constant>SIGUSR2</constant></term>
 207
 208         <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal
 209         <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally not be
 210         necessary to request this explicitly – except for debugging purposes – as <command>systemd-resolved</command>
 211         flushes the caches automatically anyway any time the host's network configuration changes. Sending this signal
 212         to <command>systemd-resolved</command> is equivalent to the <command>resolvectl --flush-caches</command>
 213         command, however the latter is recommended since it operates in a synchronous way.</para></listitem>
 214       </varlistentry>
 215
 216       <varlistentry>
 217         <term><constant>SIGRTMIN+1</constant></term>
 218
 219         <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
 220         <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
 221         servers. Specifically any information about server feature support is flushed out, and the server feature
 222         probing logic is restarted on the next request, starting with the most fully featured level. Note that it
 223         should normally not be necessary to request this explicitly – except for debugging purposes – as
 224         <command>systemd-resolved</command> automatically forgets learnt information any time the DNS server
 225         configuration changes. Sending this signal to <command>systemd-resolved</command> is equivalent to the
 226         <command>resolvectl --reset-server-features</command> command, however the latter is recommended since it
 227         operates in a synchronous way.</para></listitem>
 228       </varlistentry>
 229     </variablelist>
 230
 231   </refsect1>
 232
 233   <refsect1>
 234     <title>See Also</title>
 235     <para>
 236       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 237       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 238       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 239       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 240       <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 241       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 242       <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 243       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 244       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 245     </para>
 246   </refsect1>
 247
 248 </refentry>