man/systemd-resolved.service.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'
   7           xmlns:xi="http://www.w3.org/2001/XInclude">
   8
   9   <refentryinfo>
  10     <title>systemd-resolved.service</title>
  11     <productname>systemd</productname>
  12   </refentryinfo>
  13
  14   <refmeta>
  15     <refentrytitle>systemd-resolved.service</refentrytitle>
  16     <manvolnum>8</manvolnum>
  17   </refmeta>
  18
  19   <refnamediv>
  20     <refname>systemd-resolved.service</refname>
  21     <refname>systemd-resolved</refname>
  22     <refpurpose>Network Name Resolution manager</refpurpose>
  23   </refnamediv>
  24
  25   <refsynopsisdiv>
  26     <para><filename>systemd-resolved.service</filename></para>
  27     <para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
  28   </refsynopsisdiv>
  29
  30   <refsect1>
  31     <title>Description</title>
  32
  33     <para><command>systemd-resolved</command> is a system service that provides network name resolution to
  34     local applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR
  35     and MulticastDNS resolver and responder. Local applications may submit network name resolution requests
  36     via three interfaces:</para>
  37
  38     <itemizedlist>
  39       <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus,
  40       see
  41       <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  42       and
  43       <citerefentry><refentrytitle>org.freedesktop.LogControl1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  44       for details. Usage of this API is generally recommended to clients as it is asynchronous and fully
  45       featured (for example, properly returns DNSSEC validation status and interface scope for addresses as
  46       necessary for supporting link-local networking).</para></listitem>
  47
  48       <listitem><para>The glibc
  49       <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  50       API as defined by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related
  51       resolver functions, including
  52       <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
  53       This API is widely supported, including beyond the Linux platform. In its current form it does not
  54       expose DNSSEC validation status information however, and is synchronous only. This API is backed by the
  55       glibc Name Service Switch
  56       (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
  57       Usage of the glibc NSS module
  58       <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
  59       required in order to allow glibc's NSS resolver functions to resolve hostnames via
  60       <command>systemd-resolved</command>.</para></listitem>
  61
  62       <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on
  63       the IP addresses 127.0.0.53 and 127.0.0.54 on the local loopback interface. Programs issuing DNS
  64       requests directly, bypassing any local API may be directed to this stub, in order to connect them to
  65       <command>systemd-resolved</command>. Note however that it is strongly recommended that local programs
  66       use the glibc NSS or bus APIs instead (as described above), as various network resolution concepts
  67       (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped to the unicast DNS
  68       protocol.</para>
  69
  70       <para id="proxy-stub">The DNS stub resolver on 127.0.0.53 provides the full feature set of the local
  71       resolver, which includes offering LLMNR/MulticastDNS resolution. The DNS stub resolver on 127.0.0.54
  72       provides a more limited resolver, that operates in "proxy" mode only, i.e. it will pass most DNS
  73       messages relatively unmodified to the current upstream DNS servers and back, but not try to process the
  74       messages locally, and hence does not validate DNSSEC, or offer up LLMNR/MulticastDNS. (It will
  75       translate to DNS-over-TLS communication if needed however.)</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>
  82     is used), the per-link dynamic settings received over DHCP, information provided via
  83     <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, and any
  84     DNS server information made available by other system services. See
  85     <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
  86     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
  87     details about systemd's own configuration files for DNS servers. To improve compatibility,
  88     <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but
  89     only if it is not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>,
  90     <filename>/usr/lib/systemd/resolv.conf</filename> or
  91     <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para>
  92
  93   </refsect1>
  94
  95   <refsect1>
  96     <title>Synthetic Records</title>
  97
  98     <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following
  99     cases:</para>
 100
 101     <itemizedlist>
 102       <listitem><para>The local, configured hostname is resolved to all locally configured IP addresses
 103       ordered by their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the local
 104       loopback interface) and the IPv6 address ::1 (which is the local host).</para></listitem>
 105
 106       <listitem><para>The hostnames <literal>localhost</literal> and <literal>localhost.localdomain</literal>
 107       as well as any hostname ending in <literal>.localhost</literal> or
 108       <literal>.localhost.localdomain</literal> are resolved to the IP addresses 127.0.0.1 and ::1.
 109       </para></listitem>
 110
 111       <listitem><para>The hostname <literal>_gateway</literal> is resolved to all current default routing
 112       gateway addresses, ordered by their metric. This assigns a stable hostname to the current gateway,
 113       useful for referencing it independently of the current network configuration state.</para></listitem>
 114
 115       <listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6
 116       addresses that are most likely used for communication with other hosts. This is determined by
 117       requesting a routing decision to the configured default gateways from the kernel and then using the
 118       local IP addresses selected by this decision. This hostname is only available if there is at least one
 119       local default gateway configured. This assigns a stable hostname to the local outbound IP addresses,
 120       useful for referencing them independently of the current network configuration state.</para></listitem>
 121
 122       <listitem><para>The hostname <literal>_localdnsstub</literal> is resolved to the IP address 127.0.0.53,
 123       i.e. the address the local DNS stub (see above) is listening on.</para></listitem>
 124
 125       <listitem><para>The hostname <literal>_localdnsproxy</literal> is resolved to the IP address 127.0.0.54,
 126       i.e. the address the local DNS proxy (see above) is listening on.</para></listitem>
 127
 128       <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their
 129       configured addresses and back, but they will not affect lookups for non-address types (like MX).
 130       Support for <filename>/etc/hosts</filename> may be disabled with <varname>ReadEtcHosts=no</varname>,
 131       see <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 132       </para></listitem>
 133     </itemizedlist>
 134   </refsect1>
 135
 136   <refsect1>
 137     <title>Protocols and Routing</title>
 138
 139     <para>The lookup requests that <filename>systemd-resolved.service</filename> receives are routed to the
 140     available DNS servers, LLMNR, and MulticastDNS interfaces according to the following rules:</para>
 141
 142     <itemizedlist>
 143       <listitem><para>Names for which synthetic records are generated (the local hostname,
 144       <literal>localhost</literal> and <literal>localdomain</literal>, local gateway, as listed in the
 145       previous section) and addresses configured in <filename>/etc/hosts</filename> are never routed to the
 146       network and a reply is sent immediately.</para></listitem>
 147
 148       <listitem><para>Single-label names are resolved using LLMNR on all local interfaces where LLMNR is
 149       enabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are
 150       only sent via LLMNR on IPv6. Note that lookups for single-label synthesized names are not routed to
 151       LLMNR, MulticastDNS or unicast DNS.</para></listitem>
 152
 153       <listitem><para>Queries for the address records (A and AAAA) of single-label non-synthesized names are
 154       resolved via unicast DNS using search domains. For any interface which defines search domains, such
 155       look-ups are routed to the servers defined for that interface, suffixed with each of those search
 156       domains. When global search domains are defined, such look-ups are routed to the global servers. For
 157       each search domain, queries are performed by suffixing the name with each of the search domains in
 158       turn. Additionally, lookup of single-label names via unicast DNS may be enabled with the
 159       <varname>ResolveUnicastSingleLabel=yes</varname> setting. The details of which servers are queried and
 160       how the final reply is chosen are described below. Note that this means that address queries for
 161       single-label names are never sent out to remote DNS servers by default, and resolution is only
 162       possible if search domains are defined.</para></listitem>
 163
 164       <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are resolved using
 165       MulticastDNS on all local interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups
 166       are sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem>
 167
 168       <listitem><para>Queries for multi-label names are routed via unicast DNS on local interfaces that have
 169       a DNS server configured, plus the globally configured DNS servers if there are any. Which interfaces
 170       are used is determined by the routing logic based on search and route-only domains, described below.
 171       Note that by default, lookups for domains with the <literal>.local</literal> suffix are not routed to
 172       DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server
 173       and interface. This means that on networks where the <literal>.local</literal> domain is defined in a
 174       site-specific DNS server, explicit search or routing domains need to be configured to make lookups work
 175       within this DNS domain. Note that these days, it's generally recommended to avoid defining
 176       <literal>.local</literal> in a DNS server, as <ulink
 177       url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
 178       MulticastDNS use.</para></listitem>
 179
 180       <listitem><para>Address lookups (reverse lookups) are routed similarly to multi-label names, with the
 181       exception that addresses from the link-local address range are never routed to unicast DNS and are only
 182       resolved using LLMNR and MulticastDNS (when enabled).</para></listitem>
 183     </itemizedlist>
 184
 185     <para>If lookups are routed to multiple interfaces, the first successful response is returned (thus
 186     effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces,
 187     the last failing response is returned.</para>
 188
 189     <para>Routing of lookups is determined by the per-interface routing domains (search and route-only) and
 190     global search domains. See
 191     <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
 192     <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for a
 193     description how those settings are set dynamically and the discussion of <varname>Domains=</varname> in
 194     <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a
 195     description of globally configured DNS settings.</para>
 196
 197     <para>The following query routing logic applies for unicast DNS lookups initiated by
 198     <filename>systemd-resolved.service</filename>:</para>
 199
 200     <itemizedlist>
 201       <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the
 202       configured routing domains (search or route-only) of any link, or the globally configured DNS settings,
 203       "best matching" routing domain is determined: the matching one with the most labels. The query is then
 204       sent to all DNS servers of any links or the globally configured DNS servers associated with this "best
 205       matching" routing domain. (Note that more than one link might have this same "best matching" routing
 206       domain configured, in which case the query is sent to all of them in parallel).</para>
 207
 208       <para>In case of single-label names, when search domains are defined, the same logic applies, except
 209       that the name is first suffixed by each of the search domains in turn. Note that this search logic
 210       doesn't apply to any names with at least one dot. Also see the discussion about compatibility with
 211       the traditional glibc resolver below.</para></listitem>
 212
 213       <listitem><para>If a query does not match any configured routing domain (either per-link or global), it
 214       is sent to all DNS servers that are configured on links with the <varname>DefaultRoute=</varname>
 215       option set, as well as the globally configured DNS server.</para></listitem>
 216
 217       <listitem><para>If there is no link configured as <varname>DefaultRoute=</varname> and no global DNS
 218       server configured, one of the compiled-in fallback DNS servers is used.</para></listitem>
 219
 220       <listitem><para>Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined.
 221       </para></listitem>
 222     </itemizedlist>
 223
 224     <para>The <varname>DefaultRoute=</varname> option is a boolean setting configurable with
 225     <command>resolvectl</command> or in <filename>.network</filename> files. If not set, it is implicitly
 226     determined based on the configured DNS domains for a link: if there's a route-only domain other than
 227     <literal>~.</literal>, it defaults to false, otherwise to true.</para>
 228
 229     <para>Effectively this means: in order to support single-label non-synthesized names, define appropriate
 230     search domains. In order to preferably route all DNS queries not explicitly matched by routing domain
 231     configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This will
 232     ensure that other links will not be considered for these queries (unless they too carry such a routing
 233     domain). In order to route all such DNS queries to a specific link only if no other link is preferred,
 234     set the <varname>DefaultRoute=</varname> option for the link to true and do not configure a
 235     <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never
 236     receives any DNS traffic not matching any of its configured routing domains, set the
 237     <varname>DefaultRoute=</varname> option for it to false.</para>
 238
 239     <para>See
 240     <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 241     for information about the D-Bus APIs <filename>systemd-resolved</filename> provides.</para>
 242   </refsect1>
 243
 244   <refsect1>
 245     <title>Compatibility with the traditional glibc stub resolver</title>
 246
 247     <para>This section provides a short summary of differences in the resolver implemented by
 248     <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together
 249     with <command>systemd-resolved</command> and the traditional stub resolver implemented in
 250     <filename>nss-dns</filename>.</para>
 251
 252     <itemizedlist>
 253       <listitem><para>Some names are always resolved internally (see Synthetic Records above). Traditionally
 254       they would be resolved by <filename>nss-files</filename> if provided in
 255       <filename>/etc/hosts</filename>. But note that the details of how a query is constructed are under the
 256       control of the client library. <filename>nss-dns</filename> will first try to resolve names using
 257       search domains and even if those queries are routed to <filename>systemd-resolved</filename>, it will
 258       send them out over the network using the usual rules for multi-label name routing <footnote><para>For
 259       example, if <filename>/etc/resolv.conf</filename> has <programlisting>nameserver 127.0.0.53
 260 search foobar.com barbar.com
 261       </programlisting>and we look up <literal>localhost</literal>, <filename>nss-dns</filename> will send
 262       the following queries to <filename>systemd-resolved</filename> listening on 127.0.0.53:53: first
 263       <literal>localhost.foobar.com</literal>, then <literal>localhost.barbar.com</literal>, and finally
 264       <literal>localhost</literal>. If (hopefully) the first two queries fail,
 265       <filename>systemd-resolved</filename> will synthesize an answer for the third query.</para>
 266
 267       <para>When using <filename>nss-dns</filename> with any search domains, it is thus crucial to always
 268       configure <filename>nss-files</filename> with higher priority and provide mappings for names that
 269       should not be resolved using search domains.</para></footnote>.</para></listitem>
 270
 271       <listitem><para>Single-label names are not resolved for A and AAAA records using unicast DNS (unless
 272       overridden with <varname>ResolveUnicastSingleLabel=</varname>, see
 273       <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
 274       This is similar to the <option>no-tld-query</option> option being set in
 275       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 276       </para></listitem>
 277
 278       <listitem><para>Search domains are not used for <emphasis>suffixing</emphasis> of multi-label names.
 279       (Search domains are nevertheless used for lookup <emphasis>routing</emphasis>, for names that were
 280       originally specified as single-label or multi-label.) Any name with at least one dot is always
 281       interpreted as a FQDN. <filename>nss-dns</filename> would resolve names both as relative (using search
 282       domains) and absolute FQDN names. Some names would be resolved as relative first, and after that query
 283       has failed, as absolute, while other names would be resolved in opposite order. The
 284       <varname>ndots</varname> option in <filename>/etc/resolv.conf</filename> was used to control how many
 285       dots the name needs to have to be resolved as relative first. This stub resolver does not implement
 286       this at all: multi-label names are only resolved as FQDNs.<footnote><para>There are currently more than
 287       1500 top-level domain names defined, and new ones are added regularly, often using "attractive" names
 288       that are also likely to be used locally. Not looking up multi-label names in this fashion avoids
 289       fragility in both directions: a valid global name could be obscured by a local name, and resolution of
 290       a relative local name could suddenly break when a new top-level domain is created, or when a new
 291       subdomain of a top-level domain in registered. Resolving any given name as either relative or absolute
 292       avoids this ambiguity.</para></footnote></para></listitem>
 293
 294       <listitem><para>This resolver has a notion of the special <literal>.local</literal> domain used for
 295       MulticastDNS, and will not route queries with that suffix to unicast DNS servers unless explicitly
 296       configured, see above. Also, reverse lookups for link-local addresses are not sent to unicast DNS
 297       servers.</para></listitem>
 298
 299       <listitem><para>This resolver reads and caches <filename>/etc/hosts</filename> internally. (In other
 300       words, <filename>nss-resolve</filename> replaces <filename>nss-files</filename> in addition to
 301       <filename>nss-dns</filename>). Entries in <filename>/etc/hosts</filename> have highest priority.</para>
 302       </listitem>
 303
 304       <listitem><para>This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast
 305       DNS protocol, and will resolve single-label names using LLMNR (when enabled) and names ending in
 306       <literal>.local</literal> using MulticastDNS (when enabled).</para></listitem>
 307
 308       <listitem><para>Environment variables <varname>$LOCALDOMAIN</varname> and
 309       <varname>$RES_OPTIONS</varname> described in
 310       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 311       are not supported currently.</para></listitem>
 312
 313       <listitem><para>The <filename>nss-dns</filename> resolver maintains little state between subsequent DNS
 314       queries, and for each query always talks to the first listed DNS server from
 315       <filename>/etc/resolv.conf</filename> first, and on failure continues with the next until reaching the
 316       end of the list which is when the query fails. The resolver in
 317       <filename>systemd-resolved.service</filename> however maintains state, and will continuously talk to
 318       the same server for all queries on a particular lookup scope until some form of error is seen at which
 319       point it switches to the next, and then continuously stays with it for all queries on the scope until
 320       the next failure, and so on, eventually returning to the first configured server. This is done to
 321       optimize lookup times, in particular given that the resolver typically must first probe server feature
 322       sets when talking to a server, which is time consuming. This different behaviour implies that listed
 323       DNS servers per lookup scope must be equivalent in the zones they serve, so that sending a query to one
 324       of them will yield the same results as sending it to another configured DNS server.</para></listitem>
 325     </itemizedlist>
 326   </refsect1>
 327
 328   <refsect1>
 329     <title><filename>/etc/resolv.conf</filename></title>
 330
 331     <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see
 332     <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
 333     supported:</para>
 334
 335     <itemizedlist>
 336       <listitem><para><command>systemd-resolved</command> maintains the
 337       <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional
 338       Linux programs. This file lists the 127.0.0.53 DNS stub (see above) as the only DNS server. It also
 339       contains a list of search domains that are in use by systemd-resolved. The list of search domains is
 340       always kept up-to-date. Note that <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not
 341       be used directly by applications, but only through a symlink from
 342       <filename>/etc/resolv.conf</filename>. This file may be symlinked from
 343       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs
 344       to <command>systemd-resolved</command> with correct search domains settings. This mode of operation is
 345       recommended.</para></listitem>
 346
 347       <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
 348       the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
 349       <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs
 350       to <command>systemd-resolved</command>. This file does not contain any search domains.
 351       </para></listitem>
 352
 353       <listitem><para><command>systemd-resolved</command> maintains the
 354       <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
 355       programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept
 356       up-to-date, containing information about all known DNS servers. Note the file format's limitations: it
 357       does not know a concept of per-interface DNS servers and hence only contains system-wide DNS server
 358       definitions. Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used
 359       directly by applications, but only through a symlink from <filename>/etc/resolv.conf</filename>. If
 360       this mode of operation is used local clients that bypass any local DNS API will also bypass
 361       <command>systemd-resolved</command> and will talk directly to the known DNS servers.</para></listitem>
 362
 363       <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages,
 364       in which case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode
 365       of operation <command>systemd-resolved</command> is consumer rather than provider of this configuration
 366       file. </para></listitem>
 367     </itemizedlist>
 368
 369     <para>Note that the selected mode of operation for this file is detected fully automatically, depending
 370     on whether <filename>/etc/resolv.conf</filename> is a symlink to
 371     <filename>/run/systemd/resolve/resolv.conf</filename> or lists 127.0.0.53 as DNS server.</para>
 372   </refsect1>
 373
 374   <refsect1>
 375     <title>Signals</title>
 376
 377     <variablelist>
 378       <varlistentry>
 379         <term><constant>SIGUSR1</constant></term>
 380
 381         <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
 382         <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it
 383         maintains, as well as all feature level information it learnt about configured DNS servers into the
 384         system logs.</para>
 385
 386         <xi:include href="version-info.xml" xpointer="v231"/></listitem>
 387       </varlistentry>
 388
 389       <varlistentry>
 390         <term><constant>SIGUSR2</constant></term>
 391
 392         <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal
 393         <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally
 394         not be necessary to request this explicitly – except for debugging purposes – as
 395         <command>systemd-resolved</command> flushes the caches automatically anyway any time the host's
 396         network configuration changes. Sending this signal to <command>systemd-resolved</command> is
 397         equivalent to the <command>resolvectl flush-caches</command> command, however the latter is
 398         recommended since it operates in a synchronous way.</para>
 399
 400         <xi:include href="version-info.xml" xpointer="v231"/></listitem>
 401       </varlistentry>
 402
 403       <varlistentry>
 404         <term><constant>SIGRTMIN+1</constant></term>
 405
 406         <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
 407         <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
 408         servers. Specifically any information about server feature support is flushed out, and the server
 409         feature probing logic is restarted on the next request, starting with the most fully featured
 410         level. Note that it should normally not be necessary to request this explicitly – except for
 411         debugging purposes – as <command>systemd-resolved</command> automatically forgets learnt information
 412         any time the DNS server configuration changes. Sending this signal to
 413         <command>systemd-resolved</command> is equivalent to the <command>resolvectl
 414         reset-server-features</command> command, however the latter is recommended since it operates in a
 415         synchronous way.</para>
 416
 417         <xi:include href="version-info.xml" xpointer="v235"/></listitem>
 418       </varlistentry>
 419     </variablelist>
 420   </refsect1>
 421
 422   <refsect1>
 423     <title>Credentials</title>
 424
 425     <para><command>systemd-resolved</command> supports the service credentials logic as implemented by
 426     <varname>ImportCredential=</varname>/<varname>LoadCredential=</varname>/<varname>SetCredential=</varname>
 427     (see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
 428     details). The following credentials are used when passed in:</para>
 429
 430     <variablelist class='system-credentials'>
 431       <varlistentry>
 432         <term><varname>network.dns</varname></term>
 433         <term><varname>network.search_domains</varname></term>
 434
 435         <listitem><para>May contain a space separated list of DNS server IP addresses and DNS search
 436         domains. This information is only used when no explicit configuration via
 437         <filename>/etc/systemd/resolved.conf</filename>, <filename>/etc/resolv.conf</filename> or the kernel
 438         command line has been provided.</para>
 439
 440         <xi:include href="version-info.xml" xpointer="v253"/></listitem>
 441       </varlistentry>
 442     </variablelist>
 443   </refsect1>
 444
 445   <refsect1>
 446     <title>Kernel Command Line</title>
 447
 448     <para><command>systemd-resolved</command> also honours two kernel command line options:</para>
 449
 450     <variablelist class='kernel-commandline-options'>
 451       <varlistentry>
 452         <term><varname>nameserver=</varname></term>
 453         <term><varname>domain=</varname></term>
 454
 455         <listitem><para>Takes the IP address of a DNS server (in case of <varname>nameserver=</varname>), and
 456         a DNS search domain (in case of <varname>domain=</varname>). May be used multiple times, to define
 457         multiple DNS servers/search domains. If either of these options are specified
 458         <filename>/etc/resolv.conf</filename> will not be read and the <varname>DNS=</varname> and
 459         <varname>Domains=</varname> settings of
 460         <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 461         will be ignored. These two kernel command line options hence override system
 462         configuration.</para>
 463
 464         <xi:include href="version-info.xml" xpointer="v253"/></listitem>
 465       </varlistentry>
 466     </variablelist>
 467   </refsect1>
 468
 469   <refsect1>
 470     <title>IP Ports</title>
 471
 472     <para>The <command>systemd-resolved</command> service listens on the following IP ports:</para>
 473
 474     <itemizedlist>
 475       <listitem><para>Port 53 on IPv4 addresses 127.0.0.53 and 127.0.0.54 (both are on the local loopback
 476       interface <literal>lo</literal>). This is the local DNS stub, as discussed above. Both UDP and TCP are
 477       covered.</para></listitem>
 478
 479       <listitem><para>Port 5353 on all local addresses, both IPv4 and IPv6 (0.0.0.0 and ::0), for
 480       MulticastDNS on UDP. Note that even though the socket is bound to all local interfaces via the selected
 481       "wildcard" IP addresses, the incoming datagrams are filtered by the network interface they are coming
 482       in on, and separate MulticastDNS link-local scopes are maintained for each, taking into consideration
 483       whether MulticastDNS is enabled for the interface or not.</para></listitem>
 484
 485       <listitem><para>Port 5355 on all local addresses, both IPv4 and IP6 (0.0.0.0 and ::0), for LLMNR, on
 486       both TCP and UDP. As with MulticastDNS filtering by incoming network interface is applied.</para></listitem>
 487     </itemizedlist>
 488   </refsect1>
 489
 490   <refsect1>
 491     <title>See Also</title>
 492     <para><simplelist type="inline">
 493       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 494       <member><citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 495       <member><citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 496       <member><citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 497       <member><citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 498       <member><citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 499       <member><citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 500       <member><citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 501       <member><citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 502     </simplelist></para>
 503   </refsect1>
 504
 505 </refentry>