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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" [
+<!ENTITY % entities SYSTEM "custom-entities.ent" >
+%entities;
+]>
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="resolved.conf" conditional='ENABLE_RESOLVE'
+ xmlns:xi="http://www.w3.org/2001/XInclude">
+ <refentryinfo>
+ <title>resolved.conf</title>
+ <productname>systemd</productname>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>resolved.conf</refentrytitle>
+ <manvolnum>5</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>resolved.conf</refname>
+ <refname>resolved.conf.d</refname>
+ <refpurpose>Network Name Resolution configuration files</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <para><filename>/etc/systemd/resolved.conf</filename></para>
+ <para><filename>/etc/systemd/resolved.conf.d/*.conf</filename></para>
+ <para><filename>/run/systemd/resolved.conf.d/*.conf</filename></para>
+ <para><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></para>
+ </refsynopsisdiv>
+
+ <refsect1>
+ <title>Description</title>
+
+ <para>These configuration files control local DNS and LLMNR
+ name resolution.</para>
+
+ </refsect1>
+
+ <xi:include href="standard-conf.xml" xpointer="main-conf" />
+
+ <refsect1>
+ <title>Options</title>
+
+ <para>The following options are available in the [Resolve] section:</para>
+
+ <variablelist class='network-directives'>
+
+ <varlistentry>
+ <term><varname>DNS=</varname></term>
+ <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. Each address can
+ optionally take a port number separated with <literal>:</literal>, a network interface name or index separated with
+ <literal>%</literal>, and a Server Name Indication (SNI) separated with <literal>#</literal>. When IPv6 address is
+ specified with a port number, then the address must be in the square brackets. That is, the acceptable full formats
+ are <literal>111.222.333.444:9953%ifname#example.com</literal> for IPv4 and
+ <literal>[1111:2222::3333]:9953%ifname#example.com</literal> for IPv6. DNS requests are sent to one of the listed
+ DNS servers in parallel to suitable per-link DNS servers acquired from
+ <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
+ set at runtime by external applications. For compatibility reasons, if this setting is not specified, the DNS
+ servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
+ are configured in it. This setting defaults to the empty list.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>FallbackDNS=</varname></term>
+ <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Please see
+ <varname>DNS=</varname> for acceptable format of addresses. Any per-link DNS servers obtained from
+ <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
+ <filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
+ known. If this option is not given, a compiled-in list of DNS servers is used instead.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>Domains=</varname></term>
+ <listitem><para>A space-separated list of domains, optionally prefixed with <literal>~</literal>,
+ used for two distinct purposes described below. Defaults to the empty list.</para>
+
+ <para>Any domains <emphasis>not</emphasis> prefixed with <literal>~</literal> are used as search
+ suffixes when resolving single-label hostnames (domain names which contain no dot), in order to
+ qualify them into fully-qualified domain names (FQDNs). These "search domains" are strictly processed
+ in the order they are specified in, until the name with the suffix appended is found. For
+ compatibility reasons, if this setting is not specified, the search domains listed in
+ <filename>/etc/resolv.conf</filename> with the <varname>search</varname> keyword are used instead, if
+ that file exists and any domains are configured in it.</para>
+
+ <para>The domains prefixed with <literal>~</literal> are called "route-only domains". All domains
+ listed here (<emphasis>both search domains and route-only domains</emphasis> after removing the
+ <literal>~</literal> prefix) define a search path that preferably directs DNS queries to this
+ interface. This search path has an effect only when suitable per-link DNS servers are known. Such
+ servers may be defined through the <varname>DNS=</varname> setting (see above) and dynamically at run
+ time, for example from DHCP leases. If no per-link DNS servers are known, route-only domains have no
+ effect.</para>
+
+ <para>Use the construct <literal>~.</literal> (which is composed from <literal>~</literal> to
+ indicate a route-only domain and <literal>.</literal> to indicate the DNS root domain that is the
+ implied suffix of all DNS domains) to use the DNS servers defined for this link preferably for all
+ domains.</para>
+
+ <para>See "Protocols and Routing" in
+ <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ for details of how search and route-only domains are used.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>LLMNR=</varname></term>
+ <listitem><para>Takes a boolean argument or
+ <literal>resolve</literal>. Controls Link-Local Multicast Name
+ Resolution support (<ulink
+ url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
+ the local host. If true, enables full LLMNR responder and
+ resolver support. If false, disables both. If set to
+ <literal>resolve</literal>, only resolution support is enabled,
+ but responding is disabled. Note that
+ <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ also maintains per-link LLMNR settings. LLMNR will be
+ enabled on a link only if the per-link and the
+ global setting is on.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>MulticastDNS=</varname></term>
+ <listitem><para>Takes a boolean argument or
+ <literal>resolve</literal>. Controls Multicast DNS support (<ulink
+ url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
+ the local host. If true, enables full Multicast DNS responder and
+ resolver support. If false, disables both. If set to
+ <literal>resolve</literal>, only resolution support is enabled,
+ but responding is disabled. Note that
+ <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ also maintains per-link Multicast DNS settings. Multicast DNS will be
+ enabled on a link only if the per-link and the
+ global setting is on.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>DNSSEC=</varname></term>
+ <listitem><para>Takes a boolean argument or
+ <literal>allow-downgrade</literal>. If true all DNS lookups are
+ DNSSEC-validated locally (excluding LLMNR and Multicast
+ DNS). If the response to a lookup request is detected to be invalid
+ a lookup failure is returned to applications. Note that
+ this mode requires a DNS server that supports DNSSEC. If the
+ DNS server does not properly support DNSSEC all validations
+ will fail. If set to <literal>allow-downgrade</literal> DNSSEC
+ validation is attempted, but if the server does not support
+ DNSSEC properly, DNSSEC mode is automatically disabled. Note
+ that this mode makes DNSSEC validation vulnerable to
+ "downgrade" attacks, where an attacker might be able to
+ trigger a downgrade to non-DNSSEC mode by synthesizing a DNS
+ response that suggests DNSSEC was not supported. If set to
+ false, DNS lookups are not DNSSEC validated.</para>
+
+ <para>Note that DNSSEC validation requires retrieval of
+ additional DNS data, and thus results in a small DNS look-up
+ time penalty.</para>
+
+ <para>DNSSEC requires knowledge of "trust anchors" to prove
+ data integrity. The trust anchor for the Internet root domain
+ is built into the resolver, additional trust anchors may be
+ defined with
+ <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+ Trust anchors may change at regular intervals, and old trust
+ anchors may be revoked. In such a case DNSSEC validation is
+ not possible until new trust anchors are configured locally or
+ the resolver software package is updated with the new root
+ trust anchor. In effect, when the built-in trust anchor is
+ revoked and <varname>DNSSEC=</varname> is true, all further
+ lookups will fail, as it cannot be proved anymore whether
+ lookups are correctly signed, or validly unsigned. If
+ <varname>DNSSEC=</varname> is set to
+ <literal>allow-downgrade</literal> the resolver will
+ automatically turn off DNSSEC validation in such a case.</para>
+
+ <para>Client programs looking up DNS data will be informed
+ whether lookups could be verified using DNSSEC, or whether the
+ returned data could not be verified (either because the data
+ was found unsigned in the DNS, or the DNS server did not
+ support DNSSEC or no appropriate trust anchors were known). In
+ the latter case it is assumed that client programs employ a
+ secondary scheme to validate the returned DNS data, should
+ this be required.</para>
+
+ <para>It is recommended to set <varname>DNSSEC=</varname> to
+ true on systems where it is known that the DNS server supports
+ DNSSEC correctly, and where software or trust anchor updates
+ happen regularly. On other systems it is recommended to set
+ <varname>DNSSEC=</varname> to
+ <literal>allow-downgrade</literal>.</para>
+
+ <para>In addition to this global DNSSEC setting
+ <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ also maintains per-link DNSSEC settings. For system DNS
+ servers (see above), only the global DNSSEC setting is in
+ effect. For per-link DNS servers the per-link
+ setting is in effect, unless it is unset in which case the
+ global setting is used instead.</para>
+
+ <para>Site-private DNS zones generally conflict with DNSSEC
+ operation, unless a negative (if the private zone is not
+ signed) or positive (if the private zone is signed) trust
+ anchor is configured for them. If
+ <literal>allow-downgrade</literal> mode is selected, it is
+ attempted to detect site-private DNS zones using top-level
+ domains (TLDs) that are not known by the DNS root server. This
+ logic does not work in all private zone setups.</para>
+
+ <para>Defaults to <literal>&DEFAULT_DNSSEC_MODE;</literal>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>DNSOverTLS=</varname></term>
+ <listitem>
+ <para>Takes a boolean argument or <literal>opportunistic</literal>. If
+ true all connections to the server will be encrypted. Note that this
+ mode requires a DNS server that supports DNS-over-TLS and has a valid
+ certificate. If the hostname was specified in <varname>DNS=</varname>
+ by using the format <literal>address#server_name</literal> it
+ is used to validate its certificate and also to enable Server Name
+ Indication (SNI) when opening a TLS connection. Otherwise
+ the certificate is checked against the server's IP.
+ If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>
+
+ <para>When set to <literal>opportunistic</literal>
+ DNS request are attempted to send encrypted with DNS-over-TLS.
+ If the DNS server does not support TLS, DNS-over-TLS is disabled.
+ Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
+ attacks, where an attacker might be able to trigger a downgrade
+ to non-encrypted mode by synthesizing a response that suggests
+ DNS-over-TLS was not supported. If set to false, DNS lookups
+ are send over UDP.</para>
+
+ <para>Note that DNS-over-TLS requires additional data to be
+ send for setting up an encrypted connection, and thus results
+ in a small DNS look-up time penalty.</para>
+
+ <para>Note that in <literal>opportunistic</literal> mode the
+ resolver is not capable of authenticating the server, so it is
+ vulnerable to "man-in-the-middle" attacks.</para>
+
+ <para>In addition to this global <varname>DNSOverTLS=</varname> setting
+ <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
+ also maintains per-link <varname>DNSOverTLS=</varname> settings. For system DNS servers (see above), only the global
+ <varname>DNSOverTLS=</varname> setting is in effect. For per-link DNS servers the per-link setting is in effect, unless
+ it is unset in which case the global setting is used instead.</para>
+
+ <para>Defaults to <literal>&DEFAULT_DNS_OVER_TLS_MODE;</literal>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>Cache=</varname></term>
+ <listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If
+ <literal>yes</literal> (the default), resolving a domain name which already got queried earlier will
+ return the previous result as long as it is still valid, and thus does not result in a new network
+ request. Be aware that turning off caching comes at a performance penalty, which is particularly high
+ when DNSSEC is used. If <literal>no-negative</literal>, only positive answers are cached.</para>
+
+ <para>Note that caching is turned off by default for host-local DNS servers.
+ See <varname>CacheFromLocalhost=</varname> for details.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>CacheFromLocalhost=</varname></term>
+ <listitem><para>Takes a boolean as argument. If <literal>no</literal> (the default), and response cames from
+ host-local IP address (such as 127.0.0.1 or ::1), the result wouldn't be cached in order to avoid
+ potential duplicate local caching.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>DNSStubListener=</varname></term>
+ <listitem><para>Takes a boolean argument or one of <literal>udp</literal> and
+ <literal>tcp</literal>. If <literal>udp</literal>, a DNS stub resolver will listen for UDP requests
+ on addresses 127.0.0.53 and 127.0.0.54, port 53. If <literal>tcp</literal>, the stub will listen for
+ TCP requests on the same addresses and port. If <literal>yes</literal> (the default), the stub listens
+ for both UDP and TCP requests. If <literal>no</literal>, the stub listener is disabled.</para>
+
+ <xi:include href="systemd-resolved.service.xml" xpointer="proxy-stub" />
+
+ <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
+ in use.</para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>DNSStubListenerExtra=</varname></term>
+ <listitem><para>Takes an IPv4 or IPv6 address to listen on. The address may be optionally
+ prefixed with a protocol name (<literal>udp</literal> or <literal>tcp</literal>) separated with
+ <literal>:</literal>. If the protocol is not specified, the service will listen on both UDP and
+ TCP. It may be also optionally suffixed by a numeric port number with separator
+ <literal>:</literal>. When an IPv6 address is specified with a port number, then the address
+ must be in the square brackets. If the port is not specified, then the service uses port 53.
+ Note that this is independent of the primary DNS stub configured with
+ <varname>DNSStubListener=</varname>, and only configures <emphasis>additional</emphasis>
+ sockets to listen on. This option can be specified multiple times. If an empty string is
+ assigned, then the all previous assignments are cleared. Defaults to unset.</para>
+
+ <para>Examples:
+ <programlisting>DNSStubListenerExtra=192.168.10.10
+DNSStubListenerExtra=2001:db8:0:f102::10
+DNSStubListenerExtra=192.168.10.11:9953
+DNSStubListenerExtra=[2001:db8:0:f102::11]:9953
+DNSStubListenerExtra=tcp:192.168.10.12
+DNSStubListenerExtra=udp:2001:db8:0:f102::12
+DNSStubListenerExtra=tcp:192.168.10.13:9953
+DNSStubListenerExtra=udp:[2001:db8:0:f102::13]:9953</programlisting>
+ </para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>ReadEtcHosts=</varname></term>
+ <listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default),
+ <command>systemd-resolved</command> will read <filename>/etc/hosts</filename>, and try to resolve
+ hosts or address by using the entries in the file before sending query to DNS servers.
+ </para></listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><varname>ResolveUnicastSingleLabel=</varname></term>
+ <listitem><para>Takes a boolean argument. When false (the default),
+ <command>systemd-resolved</command> will not resolve A and AAAA queries for single-label names over
+ classic DNS. Note that such names may still be resolved if search domains are specified (see
+ <varname>Domains=</varname> above), or using other mechanisms, in particular via LLMNR or from
+ <filename>/etc/hosts</filename>. When true, queries for single-label names will be forwarded to
+ global DNS servers even if no search domains are defined.
+ </para>
+
+ <para>This option is provided for compatibility with configurations where <emphasis>public DNS
+ servers are not used</emphasis>. Forwarding single-label names to servers not under your control is
+ not standard-conformant, see <ulink
+ url="https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/">IAB
+ Statement</ulink>, and may create a privacy and security risk.</para></listitem>
+ </varlistentry>
+ </variablelist>
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+ <para>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>
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