Adding upstream version 255.4.upstream/255.4

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 505 insertions, 0 deletions

diff --git a/man/systemd-resolved.service.xml b/man/systemd-resolved.service.xml
new file mode 100644
index 0000000..85c857d
--- /dev/null
+++ b/man/systemd-resolved.service.xml
@@ -0,0 +1,505 @@
+<?xml version='1.0'?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="systemd-resolved.service" conditional='ENABLE_RESOLVE'
+          xmlns:xi="http://www.w3.org/2001/XInclude">
+
+  <refentryinfo>
+    <title>systemd-resolved.service</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd-resolved.service</refentrytitle>
+    <manvolnum>8</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd-resolved.service</refname>
+    <refname>systemd-resolved</refname>
+    <refpurpose>Network Name Resolution manager</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <para><filename>systemd-resolved.service</filename></para>
+    <para><filename>/usr/lib/systemd/systemd-resolved</filename></para>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><command>systemd-resolved</command> is a system service that provides network name resolution to
+    local applications. It implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR
+    and MulticastDNS resolver and responder. Local applications may submit network name resolution requests
+    via three interfaces:</para>
+
+    <itemizedlist>
+      <listitem><para>The native, fully-featured API <command>systemd-resolved</command> exposes on the bus,
+      see
+      <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      and
+      <citerefentry><refentrytitle>org.freedesktop.LogControl1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      for details. Usage of this API is generally recommended to clients as it is asynchronous and fully
+      featured (for example, properly returns DNSSEC validation status and interface scope for addresses as
+      necessary for supporting link-local networking).</para></listitem>
+
+      <listitem><para>The glibc
+      <citerefentry project='man-pages'><refentrytitle>getaddrinfo</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+      API as defined by <ulink url="https://tools.ietf.org/html/rfc3493">RFC3493</ulink> and its related
+      resolver functions, including
+      <citerefentry project='man-pages'><refentrytitle>gethostbyname</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+      This API is widely supported, including beyond the Linux platform. In its current form it does not
+      expose DNSSEC validation status information however, and is synchronous only. This API is backed by the
+      glibc Name Service Switch
+      (<citerefentry project='man-pages'><refentrytitle>nss</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
+      Usage of the glibc NSS module
+      <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> is
+      required in order to allow glibc's NSS resolver functions to resolve hostnames via
+      <command>systemd-resolved</command>.</para></listitem>
+
+      <listitem><para>Additionally, <command>systemd-resolved</command> provides a local DNS stub listener on
+      the IP addresses 127.0.0.53 and 127.0.0.54 on the local loopback interface. Programs issuing DNS
+      requests directly, bypassing any local API may be directed to this stub, in order to connect them to
+      <command>systemd-resolved</command>. Note however that it is strongly recommended that local programs
+      use the glibc NSS or bus APIs instead (as described above), as various network resolution concepts
+      (such as link-local addressing, or LLMNR Unicode domains) cannot be mapped to the unicast DNS
+      protocol.</para>
+
+      <para id="proxy-stub">The DNS stub resolver on 127.0.0.53 provides the full feature set of the local
+      resolver, which includes offering LLMNR/MulticastDNS resolution. The DNS stub resolver on 127.0.0.54
+      provides a more limited resolver, that operates in "proxy" mode only, i.e. it will pass most DNS
+      messages relatively unmodified to the current upstream DNS servers and back, but not try to process the
+      messages locally, and hence does not validate DNSSEC, or offer up LLMNR/MulticastDNS. (It will
+      translate to DNS-over-TLS communication if needed however.)</para></listitem>
+    </itemizedlist>
+
+    <para>The DNS servers contacted are determined from the global settings in
+    <filename>/etc/systemd/resolved.conf</filename>, the per-link static settings in
+    <filename>/etc/systemd/network/*.network</filename> files (in case
+    <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    is used), the per-link dynamic settings received over DHCP, information provided via
+    <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>, and any
+    DNS server information made available by other system services. See
+    <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
+    <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
+    details about systemd's own configuration files for DNS servers. To improve compatibility,
+    <filename>/etc/resolv.conf</filename> is read in order to discover configured system DNS servers, but
+    only if it is not a symlink to <filename>/run/systemd/resolve/stub-resolv.conf</filename>,
+    <filename>/usr/lib/systemd/resolv.conf</filename> or
+    <filename>/run/systemd/resolve/resolv.conf</filename> (see below).</para>
+
+  </refsect1>
+
+  <refsect1>
+    <title>Synthetic Records</title>
+
+    <para><command>systemd-resolved</command> synthesizes DNS resource records (RRs) for the following
+    cases:</para>
+
+    <itemizedlist>
+      <listitem><para>The local, configured hostname is resolved to all locally configured IP addresses
+      ordered by their scope, or — if none are configured — the IPv4 address 127.0.0.2 (which is on the local
+      loopback interface) and the IPv6 address ::1 (which is the local host).</para></listitem>
+
+      <listitem><para>The hostnames <literal>localhost</literal> and <literal>localhost.localdomain</literal>
+      as well as any hostname ending in <literal>.localhost</literal> or
+      <literal>.localhost.localdomain</literal> are resolved to the IP addresses 127.0.0.1 and ::1.
+      </para></listitem>
+
+      <listitem><para>The hostname <literal>_gateway</literal> is resolved to all current default routing
+      gateway addresses, ordered by their metric. This assigns a stable hostname to the current gateway,
+      useful for referencing it independently of the current network configuration state.</para></listitem>
+
+      <listitem><para>The hostname <literal>_outbound</literal> is resolved to the local IPv4 and IPv6
+      addresses that are most likely used for communication with other hosts. This is determined by
+      requesting a routing decision to the configured default gateways from the kernel and then using the
+      local IP addresses selected by this decision. This hostname is only available if there is at least one
+      local default gateway configured. This assigns a stable hostname to the local outbound IP addresses,
+      useful for referencing them independently of the current network configuration state.</para></listitem>
+
+      <listitem><para>The hostname <literal>_localdnsstub</literal> is resolved to the IP address 127.0.0.53,
+      i.e. the address the local DNS stub (see above) is listening on.</para></listitem>
+
+      <listitem><para>The hostname <literal>_localdnsproxy</literal> is resolved to the IP address 127.0.0.54,
+      i.e. the address the local DNS proxy (see above) is listening on.</para></listitem>
+
+      <listitem><para>The mappings defined in <filename>/etc/hosts</filename> are resolved to their
+      configured addresses and back, but they will not affect lookups for non-address types (like MX).
+      Support for <filename>/etc/hosts</filename> may be disabled with <varname>ReadEtcHosts=no</varname>,
+      see <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+      </para></listitem>
+    </itemizedlist>
+  </refsect1>
+
+  <refsect1>
+    <title>Protocols and Routing</title>
+
+    <para>The lookup requests that <filename>systemd-resolved.service</filename> receives are routed to the
+    available DNS servers, LLMNR, and MulticastDNS interfaces according to the following rules:</para>
+
+    <itemizedlist>
+      <listitem><para>Names for which synthetic records are generated (the local hostname,
+      <literal>localhost</literal> and <literal>localdomain</literal>, local gateway, as listed in the
+      previous section) and addresses configured in <filename>/etc/hosts</filename> are never routed to the
+      network and a reply is sent immediately.</para></listitem>
+
+      <listitem><para>Single-label names are resolved using LLMNR on all local interfaces where LLMNR is
+      enabled. Lookups for IPv4 addresses are only sent via LLMNR on IPv4, and lookups for IPv6 addresses are
+      only sent via LLMNR on IPv6. Note that lookups for single-label synthesized names are not routed to
+      LLMNR, MulticastDNS or unicast DNS.</para></listitem>
+
+      <listitem><para>Queries for the address records (A and AAAA) of single-label non-synthesized names are
+      resolved via unicast DNS using search domains. For any interface which defines search domains, such
+      look-ups are routed to the servers defined for that interface, suffixed with each of those search
+      domains. When global search domains are defined, such look-ups are routed to the global servers. For
+      each search domain, queries are performed by suffixing the name with each of the search domains in
+      turn. Additionally, lookup of single-label names via unicast DNS may be enabled with the
+      <varname>ResolveUnicastSingleLabel=yes</varname> setting. The details of which servers are queried and
+      how the final reply is chosen are described below. Note that this means that address queries for
+      single-label names are never sent out to remote DNS servers by default, and resolution is only
+      possible if search domains are defined.</para></listitem>
+
+      <listitem><para>Multi-label names with the domain suffix <literal>.local</literal> are resolved using
+      MulticastDNS on all local interfaces where MulticastDNS is enabled. As with LLMNR, IPv4 address lookups
+      are sent via IPv4 and IPv6 address lookups are sent via IPv6.</para></listitem>
+
+      <listitem><para>Queries for multi-label names are routed via unicast DNS on local interfaces that have
+      a DNS server configured, plus the globally configured DNS servers if there are any. Which interfaces
+      are used is determined by the routing logic based on search and route-only domains, described below.
+      Note that by default, lookups for domains with the <literal>.local</literal> suffix are not routed to
+      DNS servers, unless the domain is specified explicitly as routing or search domain for the DNS server
+      and interface. This means that on networks where the <literal>.local</literal> domain is defined in a
+      site-specific DNS server, explicit search or routing domains need to be configured to make lookups work
+      within this DNS domain. Note that these days, it's generally recommended to avoid defining
+      <literal>.local</literal> in a DNS server, as <ulink
+      url="https://tools.ietf.org/html/rfc6762">RFC6762</ulink> reserves this domain for exclusive
+      MulticastDNS use.</para></listitem>
+
+      <listitem><para>Address lookups (reverse lookups) are routed similarly to multi-label names, with the
+      exception that addresses from the link-local address range are never routed to unicast DNS and are only
+      resolved using LLMNR and MulticastDNS (when enabled).</para></listitem>
+    </itemizedlist>
+
+    <para>If lookups are routed to multiple interfaces, the first successful response is returned (thus
+    effectively merging the lookup zones on all matching interfaces). If the lookup failed on all interfaces,
+    the last failing response is returned.</para>
+
+    <para>Routing of lookups is determined by the per-interface routing domains (search and route-only) and
+    global search domains. See
+    <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry> and
+    <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for a
+    description how those settings are set dynamically and the discussion of <varname>Domains=</varname> in
+    <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry> for a
+    description of globally configured DNS settings.</para>
+
+    <para>The following query routing logic applies for unicast DNS lookups initiated by
+    <filename>systemd-resolved.service</filename>:</para>
+
+    <itemizedlist>
+      <listitem><para>If a name to look up matches (that is: is equal to or has as suffix) any of the
+      configured routing domains (search or route-only) of any link, or the globally configured DNS settings,
+      "best matching" routing domain is determined: the matching one with the most labels. The query is then
+      sent to all DNS servers of any links or the globally configured DNS servers associated with this "best
+      matching" routing domain. (Note that more than one link might have this same "best matching" routing
+      domain configured, in which case the query is sent to all of them in parallel).</para>
+
+      <para>In case of single-label names, when search domains are defined, the same logic applies, except
+      that the name is first suffixed by each of the search domains in turn. Note that this search logic
+      doesn't apply to any names with at least one dot. Also see the discussion about compatibility with
+      the traditional glibc resolver below.</para></listitem>
+
+      <listitem><para>If a query does not match any configured routing domain (either per-link or global), it
+      is sent to all DNS servers that are configured on links with the <varname>DefaultRoute=</varname>
+      option set, as well as the globally configured DNS server.</para></listitem>
+
+      <listitem><para>If there is no link configured as <varname>DefaultRoute=</varname> and no global DNS
+      server configured, one of the compiled-in fallback DNS servers is used.</para></listitem>
+
+      <listitem><para>Otherwise the unicast DNS query fails, as no suitable DNS servers can be determined.
+      </para></listitem>
+    </itemizedlist>
+
+    <para>The <varname>DefaultRoute=</varname> option is a boolean setting configurable with
+    <command>resolvectl</command> or in <filename>.network</filename> files. If not set, it is implicitly
+    determined based on the configured DNS domains for a link: if there's a route-only domain other than
+    <literal>~.</literal>, it defaults to false, otherwise to true.</para>
+
+    <para>Effectively this means: in order to support single-label non-synthesized names, define appropriate
+    search domains. In order to preferably route all DNS queries not explicitly matched by routing domain
+    configuration to a specific link, configure a <literal>~.</literal> route-only domain on it. This will
+    ensure that other links will not be considered for these queries (unless they too carry such a routing
+    domain). In order to route all such DNS queries to a specific link only if no other link is preferred,
+    set the <varname>DefaultRoute=</varname> option for the link to true and do not configure a
+    <literal>~.</literal> route-only domain on it. Finally, in order to ensure that a specific link never
+    receives any DNS traffic not matching any of its configured routing domains, set the
+    <varname>DefaultRoute=</varname> option for it to false.</para>
+
+    <para>See
+    <citerefentry><refentrytitle>org.freedesktop.resolve1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+    for information about the D-Bus APIs <filename>systemd-resolved</filename> provides.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Compatibility with the traditional glibc stub resolver</title>
+
+    <para>This section provides a short summary of differences in the resolver implemented by
+    <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry> together
+    with <command>systemd-resolved</command> and the traditional stub resolver implemented in
+    <filename>nss-dns</filename>.</para>
+
+    <itemizedlist>
+      <listitem><para>Some names are always resolved internally (see Synthetic Records above). Traditionally
+      they would be resolved by <filename>nss-files</filename> if provided in
+      <filename>/etc/hosts</filename>. But note that the details of how a query is constructed are under the
+      control of the client library. <filename>nss-dns</filename> will first try to resolve names using
+      search domains and even if those queries are routed to <filename>systemd-resolved</filename>, it will
+      send them out over the network using the usual rules for multi-label name routing <footnote><para>For
+      example, if <filename>/etc/resolv.conf</filename> has <programlisting>nameserver 127.0.0.53
+search foobar.com barbar.com
+      </programlisting>and we look up <literal>localhost</literal>, <filename>nss-dns</filename> will send
+      the following queries to <filename>systemd-resolved</filename> listening on 127.0.0.53:53: first
+      <literal>localhost.foobar.com</literal>, then <literal>localhost.barbar.com</literal>, and finally
+      <literal>localhost</literal>. If (hopefully) the first two queries fail,
+      <filename>systemd-resolved</filename> will synthesize an answer for the third query.</para>
+
+      <para>When using <filename>nss-dns</filename> with any search domains, it is thus crucial to always
+      configure <filename>nss-files</filename> with higher priority and provide mappings for names that
+      should not be resolved using search domains.</para></footnote>.</para></listitem>
+
+      <listitem><para>Single-label names are not resolved for A and AAAA records using unicast DNS (unless
+      overridden with <varname>ResolveUnicastSingleLabel=</varname>, see
+      <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
+      This is similar to the <option>no-tld-query</option> option being set in
+      <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+      </para></listitem>
+
+      <listitem><para>Search domains are not used for <emphasis>suffixing</emphasis> of multi-label names.
+      (Search domains are nevertheless used for lookup <emphasis>routing</emphasis>, for names that were
+      originally specified as single-label or multi-label.) Any name with at least one dot is always
+      interpreted as a FQDN. <filename>nss-dns</filename> would resolve names both as relative (using search
+      domains) and absolute FQDN names. Some names would be resolved as relative first, and after that query
+      has failed, as absolute, while other names would be resolved in opposite order. The
+      <varname>ndots</varname> option in <filename>/etc/resolv.conf</filename> was used to control how many
+      dots the name needs to have to be resolved as relative first. This stub resolver does not implement
+      this at all: multi-label names are only resolved as FQDNs.<footnote><para>There are currently more than
+      1500 top-level domain names defined, and new ones are added regularly, often using "attractive" names
+      that are also likely to be used locally. Not looking up multi-label names in this fashion avoids
+      fragility in both directions: a valid global name could be obscured by a local name, and resolution of
+      a relative local name could suddenly break when a new top-level domain is created, or when a new
+      subdomain of a top-level domain in registered. Resolving any given name as either relative or absolute
+      avoids this ambiguity.</para></footnote></para></listitem>
+
+      <listitem><para>This resolver has a notion of the special <literal>.local</literal> domain used for
+      MulticastDNS, and will not route queries with that suffix to unicast DNS servers unless explicitly
+      configured, see above. Also, reverse lookups for link-local addresses are not sent to unicast DNS
+      servers.</para></listitem>
+
+      <listitem><para>This resolver reads and caches <filename>/etc/hosts</filename> internally. (In other
+      words, <filename>nss-resolve</filename> replaces <filename>nss-files</filename> in addition to
+      <filename>nss-dns</filename>). Entries in <filename>/etc/hosts</filename> have highest priority.</para>
+      </listitem>
+
+      <listitem><para>This resolver also implements LLMNR and MulticastDNS in addition to the classic unicast
+      DNS protocol, and will resolve single-label names using LLMNR (when enabled) and names ending in
+      <literal>.local</literal> using MulticastDNS (when enabled).</para></listitem>
+
+      <listitem><para>Environment variables <varname>$LOCALDOMAIN</varname> and
+      <varname>$RES_OPTIONS</varname> described in
+      <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+      are not supported currently.</para></listitem>
+
+      <listitem><para>The <filename>nss-dns</filename> resolver maintains little state between subsequent DNS
+      queries, and for each query always talks to the first listed DNS server from
+      <filename>/etc/resolv.conf</filename> first, and on failure continues with the next until reaching the
+      end of the list which is when the query fails. The resolver in
+      <filename>systemd-resolved.service</filename> however maintains state, and will continuously talk to
+      the same server for all queries on a particular lookup scope until some form of error is seen at which
+      point it switches to the next, and then continuously stays with it for all queries on the scope until
+      the next failure, and so on, eventually returning to the first configured server. This is done to
+      optimize lookup times, in particular given that the resolver typically must first probe server feature
+      sets when talking to a server, which is time consuming. This different behaviour implies that listed
+      DNS servers per lookup scope must be equivalent in the zones they serve, so that sending a query to one
+      of them will yield the same results as sending it to another configured DNS server.</para></listitem>
+    </itemizedlist>
+  </refsect1>
+
+  <refsect1>
+    <title><filename>/etc/resolv.conf</filename></title>
+
+    <para>Four modes of handling <filename>/etc/resolv.conf</filename> (see
+    <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>) are
+    supported:</para>
+
+    <itemizedlist>
+      <listitem><para><command>systemd-resolved</command> maintains the
+      <filename>/run/systemd/resolve/stub-resolv.conf</filename> file for compatibility with traditional
+      Linux programs. This file lists the 127.0.0.53 DNS stub (see above) as the only DNS server. It also
+      contains a list of search domains that are in use by systemd-resolved. The list of search domains is
+      always kept up-to-date. Note that <filename>/run/systemd/resolve/stub-resolv.conf</filename> should not
+      be used directly by applications, but only through a symlink from
+      <filename>/etc/resolv.conf</filename>. This file may be symlinked from
+      <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs
+      to <command>systemd-resolved</command> with correct search domains settings. This mode of operation is
+      recommended.</para></listitem>
+
+      <listitem><para>A static file <filename>/usr/lib/systemd/resolv.conf</filename> is provided that lists
+      the 127.0.0.53 DNS stub (see above) as only DNS server. This file may be symlinked from
+      <filename>/etc/resolv.conf</filename> in order to connect all local clients that bypass local DNS APIs
+      to <command>systemd-resolved</command>. This file does not contain any search domains.
+      </para></listitem>
+
+      <listitem><para><command>systemd-resolved</command> maintains the
+      <filename>/run/systemd/resolve/resolv.conf</filename> file for compatibility with traditional Linux
+      programs. This file may be symlinked from <filename>/etc/resolv.conf</filename> and is always kept
+      up-to-date, containing information about all known DNS servers. Note the file format's limitations: it
+      does not know a concept of per-interface DNS servers and hence only contains system-wide DNS server
+      definitions. Note that <filename>/run/systemd/resolve/resolv.conf</filename> should not be used
+      directly by applications, but only through a symlink from <filename>/etc/resolv.conf</filename>. If
+      this mode of operation is used local clients that bypass any local DNS API will also bypass
+      <command>systemd-resolved</command> and will talk directly to the known DNS servers.</para></listitem>
+
+      <listitem><para>Alternatively, <filename>/etc/resolv.conf</filename> may be managed by other packages,
+      in which case <command>systemd-resolved</command> will read it for DNS configuration data. In this mode
+      of operation <command>systemd-resolved</command> is consumer rather than provider of this configuration
+      file. </para></listitem>
+    </itemizedlist>
+
+    <para>Note that the selected mode of operation for this file is detected fully automatically, depending
+    on whether <filename>/etc/resolv.conf</filename> is a symlink to
+    <filename>/run/systemd/resolve/resolv.conf</filename> or lists 127.0.0.53 as DNS server.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Signals</title>
+
+    <variablelist>
+      <varlistentry>
+        <term><constant>SIGUSR1</constant></term>
+
+        <listitem><para>Upon reception of the <constant>SIGUSR1</constant> process signal
+        <command>systemd-resolved</command> will dump the contents of all DNS resource record caches it
+        maintains, as well as all feature level information it learnt about configured DNS servers into the
+        system logs.</para>
+
+        <xi:include href="version-info.xml" xpointer="v231"/></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>SIGUSR2</constant></term>
+
+        <listitem><para>Upon reception of the <constant>SIGUSR2</constant> process signal
+        <command>systemd-resolved</command> will flush all caches it maintains. Note that it should normally
+        not be necessary to request this explicitly – except for debugging purposes – as
+        <command>systemd-resolved</command> flushes the caches automatically anyway any time the host's
+        network configuration changes. Sending this signal to <command>systemd-resolved</command> is
+        equivalent to the <command>resolvectl flush-caches</command> command, however the latter is
+        recommended since it operates in a synchronous way.</para>
+
+        <xi:include href="version-info.xml" xpointer="v231"/></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><constant>SIGRTMIN+1</constant></term>
+
+        <listitem><para>Upon reception of the <constant>SIGRTMIN+1</constant> process signal
+        <command>systemd-resolved</command> will forget everything it learnt about the configured DNS
+        servers. Specifically any information about server feature support is flushed out, and the server
+        feature probing logic is restarted on the next request, starting with the most fully featured
+        level. Note that it should normally not be necessary to request this explicitly – except for
+        debugging purposes – as <command>systemd-resolved</command> automatically forgets learnt information
+        any time the DNS server configuration changes. Sending this signal to
+        <command>systemd-resolved</command> is equivalent to the <command>resolvectl
+        reset-server-features</command> command, however the latter is recommended since it operates in a
+        synchronous way.</para>
+
+        <xi:include href="version-info.xml" xpointer="v235"/></listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
+    <title>Credentials</title>
+
+    <para><command>systemd-resolved</command> supports the service credentials logic as implemented by
+    <varname>ImportCredential=</varname>/<varname>LoadCredential=</varname>/<varname>SetCredential=</varname>
+    (see <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
+    details). The following credentials are used when passed in:</para>
+
+    <variablelist class='system-credentials'>
+      <varlistentry>
+        <term><varname>network.dns</varname></term>
+        <term><varname>network.search_domains</varname></term>
+
+        <listitem><para>May contain a space separated list of DNS server IP addresses and DNS search
+        domains. This information is only used when no explicit configuration via
+        <filename>/etc/systemd/resolved.conf</filename>, <filename>/etc/resolv.conf</filename> or the kernel
+        command line has been provided.</para>
+
+        <xi:include href="version-info.xml" xpointer="v253"/></listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
+    <title>Kernel Command Line</title>
+
+    <para><command>systemd-resolved</command> also honours two kernel command line options:</para>
+
+    <variablelist class='kernel-commandline-options'>
+      <varlistentry>
+        <term><varname>nameserver=</varname></term>
+        <term><varname>domain=</varname></term>
+
+        <listitem><para>Takes the IP address of a DNS server (in case of <varname>nameserver=</varname>), and
+        a DNS search domain (in case of <varname>domain=</varname>). May be used multiple times, to define
+        multiple DNS servers/search domains. If either of these options are specified
+        <filename>/etc/resolv.conf</filename> will not be read and the <varname>DNS=</varname> and
+        <varname>Domains=</varname> settings of
+        <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        will be ignored. These two kernel command line options hence override system
+        configuration.</para>
+
+        <xi:include href="version-info.xml" xpointer="v253"/></listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
+    <title>IP Ports</title>
+
+    <para>The <command>systemd-resolved</command> service listens on the following IP ports:</para>
+
+    <itemizedlist>
+      <listitem><para>Port 53 on IPv4 addresses 127.0.0.53 and 127.0.0.54 (both are on the local loopback
+      interface <literal>lo</literal>). This is the local DNS stub, as discussed above. Both UDP and TCP are
+      covered.</para></listitem>
+
+      <listitem><para>Port 5353 on all local addresses, both IPv4 and IPv6 (0.0.0.0 and ::0), for
+      MulticastDNS on UDP. Note that even though the socket is bound to all local interfaces via the selected
+      "wildcard" IP addresses, the incoming datagrams are filtered by the network interface they are coming
+      in on, and separate MulticastDNS link-local scopes are maintained for each, taking into consideration
+      whether MulticastDNS is enabled for the interface or not.</para></listitem>
+
+      <listitem><para>Port 5355 on all local addresses, both IPv4 and IP6 (0.0.0.0 and ::0), for LLMNR, on
+      both TCP and UDP. As with MulticastDNS filtering by incoming network interface is applied.</para></listitem>
+    </itemizedlist>
+  </refsect1>
+
+  <refsect1>
+    <title>See Also</title>
+    <para>
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>resolved.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>nss-resolve</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>resolvectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry project='man-pages'><refentrytitle>hosts</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.network</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+
+</refentry>