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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-10 20:49:52 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-10 20:49:52 +0000
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+<?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>