man/systemd-resolved.service.xml

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   6 <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'>
   7
   8   <refentryinfo>
   9     <title>systemd-resolved.service</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>systemd-resolved.service</refentrytitle>
  15     <manvolnum>8</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>systemd-resolved.service</refname>
  20     <refname>systemd-resolved</refname>
  21     <refpurpose>Network Name Resolution manager</refpurpose>
  22   </refnamediv>
  23
  24   <refsynopsisdiv>
  25     <para><filename>systemd-resolved.service</filename></para>
  26     <para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
  27   </refsynopsisdiv>
  28
  29   <refsect1>
  30     <title>Description</title>
  31
  32     <para><command>systemd-resolved</command> is a system service that provides network name resolution to
  33     local applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR
  34     and MulticastDNS resolver and responder. Local applications may submit network name resolution requests
  35     via three interfaces:</para>
  36
  37     <itemizedlist>
  38       <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus,
  39       see
  40       <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  41       and
  42       <citerefentry><refentrytitle>org.freedesktop.LogControl1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  43       for details. Usage of this API is generally recommended to clients as it is asynchronous and fully
  44       featured (for example, properly returns DNSSEC validation status and interface scope for addresses as
  45       necessary for supporting link-local networking).</para></listitem>
  46
  47       <listitem><para>The glibc
  48       <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  49       API as defined by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related
  50       resolver functions, including
  51       <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
  52       This API is widely supported, including beyond the Linux platform. In its current form it does not
  53       expose DNSSEC validation status information however, and is synchronous only. This API is backed by the
  54       glibc Name Service Switch
  55       (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
  56       Usage of the glibc NSS module
  57       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
  58       required in order to allow glibc's NSS resolver functions to resolve hostnames via
  59       <command>systemd-resolved</command>.</para></listitem>
  60
  61       <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on
  62       IP address 127.0.0.53 on the local loopback interface. Programs issuing DNS requests directly,
  63       bypassing any local API may be directed to this stub, in order to connect them to
  64       <command>systemd-resolved</command>. Note however that it is strongly recommended that local programs
  65       use the glibc NSS or bus APIs instead (as described above), as various network resolution concepts
  66       (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped to the unicast DNS
  67       protocol.</para></listitem>
  68     </itemizedlist>
  69
  70     <para>The DNS servers contacted are determined from the global settings in
  71     <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in
  72     <filename>/etc/systemd/network/*.network</filename> files (in case
  73     <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  74     is used), the per-link dynamic settings received over DHCP, information provided via
  75     <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, and any
  76     DNS server information made available by other system services. See
  77     <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
  78     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
  79     details about systemd's own configuration files for DNS servers. To improve compatibility,
  80     <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but
  81     only if it is not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>,
  82     <filename>/usr/lib/systemd/resolv.conf</filename> or
  83     <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para>
  84
  85   </refsect1>
  86
  87   <refsect1>
  88     <title>Synthetic Records</title>
  89
  90     <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following
  91     cases:</para>
  92
  93     <itemizedlist>
  94       <listitem><para>The local, configured hostname is resolved to all locally configured IP addresses
  95       ordered by their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the local
  96       loopback) and the IPv6 address ::1 (which is the local host).</para></listitem>
  97
  98       <listitem><para>The hostnames <literal>localhost</literal> and <literal>localhost.localdomain</literal>
  99       (as well as any hostname ending in <literal>.localhost</literal> or
 100       <literal>.localhost.localdomain</literal>) are resolved to the IP addresses 127.0.0.1 and ::1.
 101       </para></listitem>
 102
 103       <listitem><para>The hostname <literal>_gateway</literal> is resolved to all current default routing
 104       gateway addresses, ordered by their metric. This assigns a stable hostname to the current gateway,
 105       useful for referencing it independently of the current network configuration state.</para></listitem>
 106
 107       <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their
 108       configured addresses and back, but they will not affect lookups for non-address types (like MX).
 109       Support for <filename>/etc/hosts</filename> may be disabled with <varname>ReadEtcHosts=no</varname>,
 110       see <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 111       </para></listitem>
 112     </itemizedlist>
 113   </refsect1>
 114
 115   <refsect1>
 116     <title>Protocols and Routing</title>
 117
 118     <para>Lookup requests are routed to the available DNS servers, LLMNR, and MulticastDNS interfaces
 119     according to the following rules:</para>
 120
 121     <itemizedlist>
 122       <listitem><para>Names for which synthetic records are generated (as listed in the previous section) are
 123       never routed to the network and a reply is sent immediately. In particular this means that lookups for
 124       <literal>localhost</literal> are never routed to the network.</para></listitem>
 125
 126       <listitem><para>Single-label names are routed to all local interfaces capable of IP multicasting, where
 127       LLMNR is not disabled, using the LLMNR protocol. Lookups for IPv4 addresses are only sent via LLMNR on
 128       IPv4, and lookups for IPv6 addresses are only sent via LLMNR on IPv6. Lookups for the locally
 129       configured hostname and the <literal>_gateway</literal> hostname are never routed to LLMNR.
 130       </para></listitem>
 131
 132       <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are routed to all
 133       local interfaces capable of IP multicasting, where MulticastDNS is not disabled, using the MulticastDNS
 134       protocol. As with LLMNR, IPv4 address lookups are sent via IPv4 and IPv6 address lookups are sent via
 135       IPv6.</para></listitem>
 136
 137       <listitem><para>Resolution of address records (A and AAAA) via unicast DNS (i.e. not LLMNR or
 138       MulticastDNS) for non-synthesized single-label names is allowed for non-top-level domains. This means
 139       that such records can be resolved when search domains are defined. For any interface which defines
 140       search domains, such look-ups are routed to that interface, suffixed with each of the search domains
 141       defined on that interface in turn. When global search domains are defined, such look-ups are routed to
 142       all interfaces, suffixed by each of the global search domains in turn. Additionally, lookup of
 143       single-label names via unicast DNS may be enabled with the
 144       <varname>ResolveUnicastSingleLabel=yes</varname> setting. The details of which servers are queried and
 145       how the final reply is chosen are described below. Note that this means that address queries for
 146       single-label names are never sent out to remote DNS servers by default, and if no search domains are
 147       defined, resolution will fail.</para></listitem>
 148
 149       <listitem><para>Other multi-label names are routed to all local interfaces that have a DNS server
 150       configured, plus the globally configured DNS servers if there are any. Note that by default, lookups for
 151       domains with the <literal>.local</literal> suffix are not routed to DNS servers, unless the domain is
 152       specified explicitly as routing or search domain for the DNS server and interface. This means that on
 153       networks where the <literal>.local</literal> domain is defined in a site-specific DNS server, explicit
 154       search or routing domains need to be configured to make lookups within this DNS domain work. Note that
 155       these days, it's generally recommended to avoid defining <literal>.local</literal> in a DNS server, as
 156       <ulink url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
 157       MulticastDNS use.</para></listitem>
 158
 159       <listitem><para>Address lookups are routed similarly to multi-label names, with the exception that
 160       addresses from the link-local address range are never routed to unicast DNS and are only resolved using
 161       LLMNR and MulticastDNS (when enabled).</para></listitem>
 162     </itemizedlist>
 163
 164     <para>If lookups are routed to multiple interfaces, the first successful response is returned (thus
 165     effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces,
 166     the last failing response is returned.</para>
 167
 168     <para>Routing of lookups may be influenced by configuring per-interface domain names and other
 169     settings. See
 170     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
 171     <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 172     details. The following query routing logic applies for unicast DNS traffic:</para>
 173
 174     <itemizedlist>
 175       <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the
 176       configured search or route-only domains of any link (see
 177       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
 178       or the globally configured DNS settings (see the discussion of <varname>Domains=</varname> in
 179       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
 180       "best matching" search/route-only domain is determined: the matching one with the most labels. The
 181       query is then sent to all DNS servers of any links or the globally configured DNS servers associated
 182       with this "best matching" search/route-only domain. (Note that more than one link might have this same
 183       "best matching" search/route-only domain configured, in which case the query is sent to all of them in
 184       parallel).</para>
 185
 186       <para>In case of single-label names, when search domains are defined, the same logic applies, except
 187       that the name is first suffixed by the search domain.</para></listitem>
 188
 189       <listitem><para>If a query does not match any configured search/route-only domain (neither per-link nor
 190       global), it is sent to all DNS servers that are configured on links with the "DNS default route" option
 191       set, as well as the globally configured DNS server.</para></listitem>
 192
 193       <listitem><para>If there is no link configured as "DNS default route" and no global DNS server
 194       configured, the compiled-in fallback DNS server is used.</para></listitem>
 195
 196       <listitem><para>Otherwise the query is failed as no suitable DNS servers could be determined.
 197       </para></listitem>
 198     </itemizedlist>
 199
 200     <para>The "DNS default route" option is a boolean setting configurable with <command>resolvectl</command>
 201     or in <filename>.network</filename> files. If not set, it is implicitly determined based on the
 202     configured DNS domains for a link: if there's any route-only domain (not matching <literal>~.</literal>)
 203     it defaults to false, otherwise to true.</para>
 204
 205     <para>Effectively this means: in order to support single-label non-synthetized names, define appropriate
 206     search domains. In order to preferably route all DNS queries not explicitly matched by search/route-only
 207     domain configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This
 208     will ensure that other links will not be considered for these queries (unless they too carry such a
 209     route-only domain). In order to route all such DNS queries to a specific link only if no other link
 210     is preferable, set the "DNS default route" option for the link to true and do not configure a
 211     <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never
 212     receives any DNS traffic not matching any of its configured search/route-only domains, set the "DNS
 213     default route" option for it to false.</para>
 214
 215     <para>See the <ulink url="https://www.freedesktop.org/wiki/Software/systemd/resolved">resolved D-Bus API
 216     Documentation</ulink> for information about the APIs <filename>systemd-resolved</filename> provides.
 217     </para>
 218   </refsect1>
 219
 220   <refsect1>
 221     <title><filename>/etc/resolv.conf</filename></title>
 222
 223     <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see
 224     <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
 225     supported:</para>
 226
 227     <itemizedlist>
 228       <listitem><para><command>systemd-resolved</command> maintains the
 229       <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional
 230       Linux programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists
 231       the 127.0.0.53 DNS stub (see above) as the only DNS server. It also contains a list of search domains
 232       that are in use by systemd-resolved. The list of search domains is always kept up-to-date. Note that
 233       <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications,
 234       but only through a symlink from <filename>/etc/resolv.conf</filename>. This file may be symlinked from
 235       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs
 236       to <command>systemd-resolved</command> with correct search domains settings. This mode of operation is
 237       recommended.</para></listitem>
 238
 239       <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
 240       the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
 241       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs
 242       to <command>systemd-resolved</command>. This file does not contain any search domains.
 243       </para></listitem>
 244
 245       <listitem><para><command>systemd-resolved</command> maintains the
 246       <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
 247       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept
 248       up-to-date, containing information about all known DNS servers. Note the file format's limitations: it
 249       does not know a concept of per-interface DNS servers and hence only contains system-wide DNS server
 250       definitions. Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used
 251       directly by applications, but only through a symlink from <filename>/etc/resolv.conf</filename>. If
 252       this mode of operation is used local clients that bypass any local DNS API will also bypass
 253       <command>systemd-resolved</command> and will talk directly to the known DNS servers.</para></listitem>
 254
 255       <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages,
 256       in which case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode
 257       of operation <command>systemd-resolved</command> is consumer rather than provider of this configuration
 258       file. </para></listitem>
 259     </itemizedlist>
 260
 261     <para>Note that the selected mode of operation for this file is detected fully automatically, depending
 262     on whether <filename>/etc/resolv.conf</filename> is a symlink to
 263     <filename>/run/systemd/resolve/resolv.conf</filename> or lists 127.0.0.53 as DNS server.</para>
 264   </refsect1>
 265
 266   <refsect1>
 267     <title>Signals</title>
 268
 269     <variablelist>
 270       <varlistentry>
 271         <term><constant>SIGUSR1</constant></term>
 272
 273         <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
 274         <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it
 275         maintains, as well as all feature level information it learnt about configured DNS servers into the
 276         system logs.</para></listitem>
 277       </varlistentry>
 278
 279       <varlistentry>
 280         <term><constant>SIGUSR2</constant></term>
 281
 282         <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal
 283         <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally
 284         not be necessary to request this explicitly – except for debugging purposes – as
 285         <command>systemd-resolved</command> flushes the caches automatically anyway any time the host's
 286         network configuration changes. Sending this signal to <command>systemd-resolved</command> is
 287         equivalent to the <command>resolvectl flush-caches</command> command, however the latter is
 288         recommended since it operates in a synchronous way.</para></listitem>
 289       </varlistentry>
 290
 291       <varlistentry>
 292         <term><constant>SIGRTMIN+1</constant></term>
 293
 294         <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
 295         <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
 296         servers. Specifically any information about server feature support is flushed out, and the server
 297         feature probing logic is restarted on the next request, starting with the most fully featured
 298         level. Note that it should normally not be necessary to request this explicitly – except for
 299         debugging purposes – as <command>systemd-resolved</command> automatically forgets learnt information
 300         any time the DNS server configuration changes. Sending this signal to
 301         <command>systemd-resolved</command> is equivalent to the <command>resolvectl
 302         reset-server-features</command> command, however the latter is recommended since it operates in a
 303         synchronous way.</para></listitem>
 304       </varlistentry>
 305     </variablelist>
 306
 307   </refsect1>
 308
 309   <refsect1>
 310     <title>See Also</title>
 311     <para>
 312       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 313       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 314       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 315       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 316       <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 317       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 318       <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 319       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 320       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 321     </para>
 322   </refsect1>
 323
 324 </refentry>