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 interface) 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 (the local hostname,
 123       <literal>localhost</literal> and <literal>localdomain</literal>, local gateway, as listed in the
 124       previous section) and addresses configured in <filename>/etc/hosts</filename> are never routed to the
 125       network and a reply is sent immediately.</para></listitem>
 126
 127       <listitem><para>Single-label names are resolved using LLMNR on all local interfaces where LLMNR is
 128       enabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are
 129       only sent via LLMNR on IPv6. Note that lookups for single-label synthesized names are not routed to
 130       LLMNR, MulticastDNS or unicast DNS.</para></listitem>
 131
 132       <listitem><para>Queries for the address records (A and AAAA) of single-label non-synthesized names are
 133       resolved via unicast DNS using search domains. For any interface which defines search domains, such
 134       look-ups are routed to that interface, suffixed with each of the search domains defined on that
 135       interface in turn. When global search domains are defined, such look-ups are routed to all interfaces,
 136       suffixed by each of the global search domains in turn. Additionally, lookup of single-label names via
 137       unicast DNS may be enabled with the <varname>ResolveUnicastSingleLabel=yes</varname> setting. The
 138       details of which servers are queried and how the final reply is chosen are described below. Note that
 139       this means that address queries for single-label names are never sent out to remote DNS servers by
 140       default, and resoulution is only possible if search domains are defined.</para></listitem>
 141
 142       <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are resolved using
 143       MulticastDNS on all local interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups
 144       are sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem>
 145
 146       <listitem><para>Queries for multi-label names are routed via unicast DNS on local interfaces that have
 147       a DNS server configured, plus the globally configured DNS servers if there are any. Which interfaces
 148       are used is determined by the routing logic based on search and route-only domains, described below.
 149       Note that by default, lookups for domains with the <literal>.local</literal> suffix are not routed to
 150       DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server
 151       and interface. This means that on networks where the <literal>.local</literal> domain is defined in a
 152       site-specific DNS server, explicit search or routing domains need to be configured to make lookups work
 153       within this DNS domain. Note that these days, it's generally recommended to avoid defining
 154       <literal>.local</literal> in a DNS server, as <ulink
 155       url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
 156       MulticastDNS use.</para></listitem>
 157
 158       <listitem><para>Address lookups (reverse lookups) are routed similarly to multi-label names, with the
 159       exception that addresses from the link-local address range are never routed to unicast DNS and are only
 160       resolved using LLMNR and MulticastDNS (when enabled).</para></listitem>
 161     </itemizedlist>
 162
 163     <para>If lookups are routed to multiple interfaces, the first successful response is returned (thus
 164     effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces,
 165     the last failing response is returned.</para>
 166
 167     <para>Routing of lookups is determined by the per-interface routing domains (search and route-only) and
 168     global search domains. See
 169     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
 170     <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for a
 171     description how those settings are set dynamically and the discussion of <varname>Domains=</varname> in
 172     <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a
 173     description of globally configured DNS settings.</para>
 174
 175     <para>The following query routing logic applies for unicast DNS traffic:</para>
 176
 177     <itemizedlist>
 178       <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the
 179       configured routing domains (search or route-only) of any link, or the globally configured DNS settings,
 180       "best matching" routing domain is determined: the matching one with the most labels. The query is then
 181       sent to all DNS servers of any links or the globally configured DNS servers associated with this "best
 182       matching" routing domain. (Note that more than one link might have this same "best matching" routing
 183       domain configured, in which case the query is sent to all of them in parallel).</para>
 184
 185       <para>In case of single-label names, when search domains are defined, the same logic applies, except
 186       that the name is first suffixed by each of the search domains in turn. Note that this search logic
 187       doesn't apply to any names with at least one dot. Also see the discussion about compatibility with
 188       the traditional glibc resolver below.</para></listitem>
 189
 190       <listitem><para>If a query does not match any configured routing domain (either per-link or global), it
 191       is sent to all DNS servers that are configured on links with the <varname>DefaultRoute=</varname>
 192       option set, as well as the globally configured DNS server.</para></listitem>
 193
 194       <listitem><para>If there is no link configured as <varname>DefaultRoute=</varname> and no global DNS
 195       server configured, one of the compiled-in fallback DNS servers is used.</para></listitem>
 196
 197       <listitem><para>Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined.
 198       </para></listitem>
 199     </itemizedlist>
 200
 201     <para>The <varname>DefaultRoute=</varname> option is a boolean setting configurable with
 202     <command>resolvectl</command> or in <filename>.network</filename> files. If not set, it is implicitly
 203     determined based on the configured DNS domains for a link: if there's a route-only domain other than
 204     <literal>~.</literal>, it defaults to false, otherwise to true.</para>
 205
 206     <para>Effectively this means: in order to support single-label non-synthesized names, define appropriate
 207     search domains. In order to preferably route all DNS queries not explicitly matched by routing domain
 208     configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This will
 209     ensure that other links will not be considered for these queries (unless they too carry such a routing
 210     domain). In order to route all such DNS queries to a specific link only if no other link is preferred,
 211     set the <varname>DefaultRoute=</varname> option for the link to true and do not configure a
 212     <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never
 213     receives any DNS traffic not matching any of its configured routing domains, set the
 214     <varname>DefaultRoute=</varname> option for it to false.</para>
 215
 216     <para>See
 217     <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 218     for information about the D-Bus APIs <filename>systemd-resolved</filename> provides.</para>
 219   </refsect1>
 220
 221   <refsect1>
 222     <title>Compatibility with the traditional glibc stub resolver</title>
 223
 224     <para>This section provides a short summary of differences in the stub resolver implemented by
 225     <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together
 226     with <command>systemd-resolved</command> and the traditional stub resolver implemented in
 227     <citerefentry><refentrytitle>nss-dns</refentrytitle><manvolnum>8</manvolnum></citerefentry>.</para>
 228
 229     <itemizedlist>
 230       <listitem><para>Some names are always resolved internally (see Synthetic Records above). Traditionally
 231       they would be resolved by <filename>nss-files</filename>, and only if provided in
 232       <filename>/etc/hosts</filename>.</para></listitem>
 233
 234       <listitem><para>Single-label names are not resolved for A and AAAA records using unicast DNS (unless
 235       overridden with <varname>ResolveUnicastSingleLabel=</varname>, see
 236       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
 237       This is similar to the <option>no-tld-query</option> option being set in
 238       <citerefentry><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 239       </para></listitem>
 240
 241       <listitem><para>Search domains are not used for <emphasis>suffixing</emphasis> of multi-label names.
 242       (Search domains are nevertheless used for lookup <emphasis>routing</emphasis>, for names that were
 243       originally specified as single-label or multi-label.) Any name with at least one dot is always
 244       interpreted as a FQDN. <filename>nss-dns</filename> would resolve names both as relative (using search
 245       domains) and absolute FQDN names. Some names would be resolved as relative first, and after that query
 246       has failed, as absolute, while other names would be resolved in opposite order. The
 247       <varname>ndots</varname> option in <filename>/etc/resolv.conf</filename> was used to control how many
 248       dots the name needs to have to be resolved as relative first. This stub resolver does not implement
 249       this at all: multi-label names are only resolved as FQDNs. (There are currently more than 1500
 250       top-level domain names defined, and new ones are added regularly, often using "attractive" names that
 251       are also likely to be used locally. Not looking up multi-label names in this fashion avoids fragility
 252       in both directions: a valid global name could be obscured by a local name, and resolution of a relative
 253       local name could suddenly break when a new top-level domain is created, or when a new subdomain of a
 254       top-level domain in registered. Resolving any given name as either relative or absolute avoids this
 255       ambiguity.)</para></listitem>
 256
 257       <listitem><para>This resolver has a notion of the special <literal>.local</literal> domain used for
 258       MulticastDNS, and will not route queries with that suffix to unicast DNS servers unless explicitly
 259       configured, see above. Also, reverse lookups for link-local addresses are not sent to unicast DNS
 260       servers.</para></listitem>
 261
 262       <listitem><para>This resolver reads and caches <filename>/etc/hosts</filename> internally. (In other
 263       words, <filename>nss-resolve</filename> replaces <filename>nss-files</filename> in addition to
 264       <filename>nss-dns</filename>). Entries in <filename>/etc/hosts</filename> have highest priority.</para>
 265       </listitem>
 266
 267       <listitem><para>This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast
 268       DNS protocol, and will resolve single-label names using LLMNR (when enabled) and names ending in
 269       <literal>.local</literal> using MulticastDNS (when enabled).</para></listitem>
 270
 271       <listitem><para>Environment variables <varname>$LOCALDOMAIN</varname> and
 272       <varname>$RES_OPTIONS</varname> described in
 273       <citerefentry><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> are not
 274       supported currently.</para></listitem>
 275     </itemizedlist>
 276   </refsect1>
 277
 278   <refsect1>
 279     <title><filename>/etc/resolv.conf</filename></title>
 280
 281     <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see
 282     <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
 283     supported:</para>
 284
 285     <itemizedlist>
 286       <listitem><para><command>systemd-resolved</command> maintains the
 287       <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional
 288       Linux programs. This file may be symlinked from <filename>/etc/resolv.conf</filename>. This file lists
 289       the 127.0.0.53 DNS stub (see above) as the only DNS server. It also contains a list of search domains
 290       that are in use by systemd-resolved. The list of search domains is always kept up-to-date. Note that
 291       <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not be used directly by applications,
 292       but only through a symlink from <filename>/etc/resolv.conf</filename>. This file may be symlinked from
 293       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs
 294       to <command>systemd-resolved</command> with correct search domains settings. This mode of operation is
 295       recommended.</para></listitem>
 296
 297       <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
 298       the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
 299       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs
 300       to <command>systemd-resolved</command>. This file does not contain any search domains.
 301       </para></listitem>
 302
 303       <listitem><para><command>systemd-resolved</command> maintains the
 304       <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
 305       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept
 306       up-to-date, containing information about all known DNS servers. Note the file format's limitations: it
 307       does not know a concept of per-interface DNS servers and hence only contains system-wide DNS server
 308       definitions. Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used
 309       directly by applications, but only through a symlink from <filename>/etc/resolv.conf</filename>. If
 310       this mode of operation is used local clients that bypass any local DNS API will also bypass
 311       <command>systemd-resolved</command> and will talk directly to the known DNS servers.</para></listitem>
 312
 313       <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages,
 314       in which case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode
 315       of operation <command>systemd-resolved</command> is consumer rather than provider of this configuration
 316       file. </para></listitem>
 317     </itemizedlist>
 318
 319     <para>Note that the selected mode of operation for this file is detected fully automatically, depending
 320     on whether <filename>/etc/resolv.conf</filename> is a symlink to
 321     <filename>/run/systemd/resolve/resolv.conf</filename> or lists 127.0.0.53 as DNS server.</para>
 322   </refsect1>
 323
 324   <refsect1>
 325     <title>Signals</title>
 326
 327     <variablelist>
 328       <varlistentry>
 329         <term><constant>SIGUSR1</constant></term>
 330
 331         <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
 332         <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it
 333         maintains, as well as all feature level information it learnt about configured DNS servers into the
 334         system logs.</para></listitem>
 335       </varlistentry>
 336
 337       <varlistentry>
 338         <term><constant>SIGUSR2</constant></term>
 339
 340         <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal
 341         <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally
 342         not be necessary to request this explicitly – except for debugging purposes – as
 343         <command>systemd-resolved</command> flushes the caches automatically anyway any time the host's
 344         network configuration changes. Sending this signal to <command>systemd-resolved</command> is
 345         equivalent to the <command>resolvectl flush-caches</command> command, however the latter is
 346         recommended since it operates in a synchronous way.</para></listitem>
 347       </varlistentry>
 348
 349       <varlistentry>
 350         <term><constant>SIGRTMIN+1</constant></term>
 351
 352         <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
 353         <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
 354         servers. Specifically any information about server feature support is flushed out, and the server
 355         feature probing logic is restarted on the next request, starting with the most fully featured
 356         level. Note that it should normally not be necessary to request this explicitly – except for
 357         debugging purposes – as <command>systemd-resolved</command> automatically forgets learnt information
 358         any time the DNS server configuration changes. Sending this signal to
 359         <command>systemd-resolved</command> is equivalent to the <command>resolvectl
 360         reset-server-features</command> command, however the latter is recommended since it operates in a
 361         synchronous way.</para></listitem>
 362       </varlistentry>
 363     </variablelist>
 364   </refsect1>
 365
 366   <refsect1>
 367     <title>See Also</title>
 368     <para>
 369       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 370       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 371       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 372       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 373       <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 374       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 375       <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 376       <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 377       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 378     </para>
 379   </refsect1>
 380
 381 </refentry>