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https://github.com/systemd/systemd
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409 lines
23 KiB
XML
409 lines
23 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % entities SYSTEM "custom-entities.ent" >
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%entities;
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]>
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="resolved.conf" conditional='ENABLE_RESOLVE'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>resolved.conf</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>resolved.conf</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>resolved.conf</refname>
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<refname>resolved.conf.d</refname>
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<refpurpose>Network Name Resolution configuration files</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><simplelist>
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<member><filename>/etc/systemd/resolved.conf</filename></member>
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<member><filename>/etc/systemd/resolved.conf.d/*.conf</filename></member>
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<member><filename>/run/systemd/resolved.conf.d/*.conf</filename></member>
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<member><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></member>
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</simplelist></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>These configuration files control local DNS and LLMNR
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name resolution.</para>
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</refsect1>
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<xi:include href="standard-conf.xml" xpointer="main-conf" />
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<refsect1>
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<title>Options</title>
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<para>The following options are available in the [Resolve] section:</para>
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<variablelist class='network-directives'>
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<varlistentry>
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<term><varname>DNS=</varname></term>
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<listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. Each address can
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optionally take a port number separated with <literal>:</literal>, a network interface name or index separated with
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<literal>%</literal>, and a Server Name Indication (SNI) separated with <literal>#</literal>. When IPv6 address is
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specified with a port number, then the address must be in the square brackets. That is, the acceptable full formats
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are <literal>111.222.333.444:9953%ifname#example.com</literal> for IPv4 and
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<literal>[1111:2222::3333]:9953%ifname#example.com</literal> for IPv6. DNS requests are sent to one of the listed
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DNS servers in parallel to suitable per-link DNS servers acquired from
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
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set at runtime by external applications. For compatibility reasons, if this setting is not specified, the DNS
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servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
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are configured in it. This setting defaults to the empty list.</para>
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<xi:include href="version-info.xml" xpointer="v213"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>FallbackDNS=</varname></term>
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<listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Please see
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<varname>DNS=</varname> for acceptable format of addresses. Any per-link DNS servers obtained from
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
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<filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
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known. If this option is not given, a compiled-in list of DNS servers is used instead.</para>
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<xi:include href="version-info.xml" xpointer="v216"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>Domains=</varname></term>
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<listitem><para>A space-separated list of domains, optionally prefixed with <literal>~</literal>,
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used for two distinct purposes described below. Defaults to the empty list.</para>
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<para>Any domains <emphasis>not</emphasis> prefixed with <literal>~</literal> are used as search
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suffixes when resolving single-label hostnames (domain names which contain no dot), in order to
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qualify them into fully-qualified domain names (FQDNs). These "search domains" are strictly processed
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in the order they are specified in, until the name with the suffix appended is found. For
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compatibility reasons, if this setting is not specified, the search domains listed in
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<filename>/etc/resolv.conf</filename> with the <varname>search</varname> keyword are used instead, if
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that file exists and any domains are configured in it.</para>
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<para>The domains prefixed with <literal>~</literal> are called "route-only domains". All domains
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listed here (<emphasis>both search domains and route-only domains</emphasis> after removing the
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<literal>~</literal> prefix) define a search path that preferably directs DNS queries to this
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interface. This search path has an effect only when suitable per-link DNS servers are known. Such
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servers may be defined through the <varname>DNS=</varname> setting (see above) and dynamically at run
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time, for example from DHCP leases. If no per-link DNS servers are known, route-only domains have no
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effect.</para>
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<para>Use the construct <literal>~.</literal> (which is composed from <literal>~</literal> to
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indicate a route-only domain and <literal>.</literal> to indicate the DNS root domain that is the
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implied suffix of all DNS domains) to use the DNS servers defined for this link preferably for all
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domains.</para>
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<para>See "Protocols and Routing" in
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<citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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for details of how search and route-only domains are used.</para>
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<para>Note that configuring the MulticastDNS domain <literal>local</literal> as search or routing
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domain has the effect of routing lookups for this domain to classic unicast DNS. This may be used to
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provide compatibility with legacy installations that use this domain in a unicast DNS context,
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against the IANA assignment of this domain to pure MulticastDNS purposes. Search and routing domains
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are a unicast DNS concept, they <emphasis>cannot</emphasis> be used to resolve single-label lookups
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via MulticastDNS.</para>
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<xi:include href="version-info.xml" xpointer="v229"/>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>LLMNR=</varname></term>
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<listitem><para>Takes a boolean argument or
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<literal>resolve</literal>. Controls Link-Local Multicast Name
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Resolution support (<ulink
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url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
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the local host. If true, enables full LLMNR responder and
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resolver support. If false, disables both. If set to
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<literal>resolve</literal>, only resolution support is enabled,
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but responding is disabled. Note that
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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also maintains per-link LLMNR settings. LLMNR will be
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enabled on a link only if the per-link and the
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global setting is on.</para>
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<xi:include href="version-info.xml" xpointer="v216"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>MulticastDNS=</varname></term>
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<listitem><para>Takes a boolean argument or
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<literal>resolve</literal>. Controls Multicast DNS support (<ulink
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url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
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the local host. If true, enables full Multicast DNS responder and
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resolver support. If false, disables both. If set to
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<literal>resolve</literal>, only resolution support is enabled,
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but responding is disabled. Note that
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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also maintains per-link Multicast DNS settings. Multicast DNS will be
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enabled on a link only if the per-link and the
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global setting is on.</para>
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<xi:include href="version-info.xml" xpointer="v234"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>DNSSEC=</varname></term>
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<listitem><para>Takes a boolean argument or <literal>allow-downgrade</literal>.</para>
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<para>If set to true, all DNS lookups are DNSSEC-validated locally (excluding LLMNR and Multicast
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DNS). If the response to a lookup request is detected to be invalid a lookup failure is returned to
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applications. Note that this mode requires a DNS server that supports DNSSEC. If the DNS server does
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not properly support DNSSEC all validations will fail.</para>
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<para>If set to <literal>allow-downgrade</literal>, DNSSEC validation is attempted, but if the server
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does not support DNSSEC properly, DNSSEC mode is automatically disabled. Note that this mode makes
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DNSSEC validation vulnerable to "downgrade" attacks, where an attacker might be able to trigger a
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downgrade to non-DNSSEC mode by synthesizing a DNS response that suggests DNSSEC was not
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supported.</para>
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<para>If set to false, DNS lookups are not DNSSEC validated. In this mode, or when set to
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<literal>allow-downgrade</literal> and the downgrade has happened, the resolver becomes
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security-unaware and all forwarded queries have DNSSEC OK (DO) bit unset.</para>
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<para>Note that DNSSEC validation requires retrieval of additional DNS data, and thus results in a
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small DNS lookup time penalty.</para>
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<para>DNSSEC requires knowledge of "trust anchors" to prove
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data integrity. The trust anchor for the Internet root domain
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is built into the resolver, additional trust anchors may be
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defined with
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<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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Trust anchors may change at regular intervals, and old trust
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anchors may be revoked. In such a case DNSSEC validation is
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not possible until new trust anchors are configured locally or
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the resolver software package is updated with the new root
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trust anchor. In effect, when the built-in trust anchor is
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revoked and <varname>DNSSEC=</varname> is true, all further
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lookups will fail, as it cannot be proved anymore whether
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lookups are correctly signed, or validly unsigned. If
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<varname>DNSSEC=</varname> is set to
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<literal>allow-downgrade</literal> the resolver will
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automatically turn off DNSSEC validation in such a case.</para>
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<para>Client programs looking up DNS data will be informed
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whether lookups could be verified using DNSSEC, or whether the
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returned data could not be verified (either because the data
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was found unsigned in the DNS, or the DNS server did not
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support DNSSEC or no appropriate trust anchors were known). In
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the latter case it is assumed that client programs employ a
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secondary scheme to validate the returned DNS data, should
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this be required.</para>
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<para>It is recommended to set <varname>DNSSEC=</varname> to
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true on systems where it is known that the DNS server supports
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DNSSEC correctly, and where software or trust anchor updates
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happen regularly. On other systems it is recommended to set
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<varname>DNSSEC=</varname> to
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<literal>allow-downgrade</literal>.</para>
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<para>In addition to this global DNSSEC setting
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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also maintains per-link DNSSEC settings. For system DNS
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servers (see above), only the global DNSSEC setting is in
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effect. For per-link DNS servers the per-link
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setting is in effect, unless it is unset in which case the
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global setting is used instead.</para>
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<para>Site-private DNS zones generally conflict with DNSSEC
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operation, unless a negative (if the private zone is not
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signed) or positive (if the private zone is signed) trust
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anchor is configured for them. If
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<literal>allow-downgrade</literal> mode is selected, it is
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attempted to detect site-private DNS zones using top-level
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domains (TLDs) that are not known by the DNS root server. This
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logic does not work in all private zone setups.</para>
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<para>Defaults to <literal>&DEFAULT_DNSSEC_MODE;</literal>.</para>
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<xi:include href="version-info.xml" xpointer="v229"/>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>DNSOverTLS=</varname></term>
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<listitem>
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<para>Takes a boolean argument or <literal>opportunistic</literal>. If
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true all connections to the server will be encrypted. Note that this
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mode requires a DNS server that supports DNS-over-TLS and has a valid
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certificate. If the hostname was specified in <varname>DNS=</varname>
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by using the format <literal>address#server_name</literal> it
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is used to validate its certificate and also to enable Server Name
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Indication (SNI) when opening a TLS connection. Otherwise
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the certificate is checked against the server's IP.
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If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>
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<para>When set to <literal>opportunistic</literal>
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DNS request are attempted to send encrypted with DNS-over-TLS.
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If the DNS server does not support TLS, DNS-over-TLS is disabled.
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Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
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attacks, where an attacker might be able to trigger a downgrade
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to non-encrypted mode by synthesizing a response that suggests
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DNS-over-TLS was not supported. If set to false, DNS lookups
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are send over UDP.</para>
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<para>Note that DNS-over-TLS requires additional data to be
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send for setting up an encrypted connection, and thus results
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in a small DNS look-up time penalty.</para>
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<para>Note that in <literal>opportunistic</literal> mode the
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resolver is not capable of authenticating the server, so it is
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vulnerable to "man-in-the-middle" attacks.</para>
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<para>In addition to this global <varname>DNSOverTLS=</varname> setting
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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also maintains per-link <varname>DNSOverTLS=</varname> settings. For system DNS servers (see above), only the global
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<varname>DNSOverTLS=</varname> setting is in effect. For per-link DNS servers the per-link setting is in effect, unless
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it is unset in which case the global setting is used instead.</para>
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<para>Defaults to <literal>&DEFAULT_DNS_OVER_TLS_MODE;</literal>.</para>
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<xi:include href="version-info.xml" xpointer="v239"/>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>Cache=</varname></term>
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<listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If
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<literal>yes</literal> (the default), resolving a domain name which already got queried earlier will
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return the previous result as long as it is still valid, and thus does not result in a new network
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request. Be aware that turning off caching comes at a performance penalty, which is particularly high
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when DNSSEC is used. If <literal>no-negative</literal>, only positive answers are cached.</para>
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<para>Note that caching is turned off by default for host-local DNS servers.
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See <varname>CacheFromLocalhost=</varname> for details.</para>
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<xi:include href="version-info.xml" xpointer="v231"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>CacheFromLocalhost=</varname></term>
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<listitem><para>Takes a boolean as argument. If <literal>no</literal> (the default), and response cames from
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host-local IP address (such as 127.0.0.1 or ::1), the result wouldn't be cached in order to avoid
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potential duplicate local caching.</para>
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<xi:include href="version-info.xml" xpointer="v248"/>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>DNSStubListener=</varname></term>
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<listitem><para>Takes a boolean argument or one of <literal>udp</literal> and
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<literal>tcp</literal>. If <literal>udp</literal>, a DNS stub resolver will listen for UDP requests
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on addresses 127.0.0.53 and 127.0.0.54, port 53. If <literal>tcp</literal>, the stub will listen for
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TCP requests on the same addresses and port. If <literal>yes</literal> (the default), the stub listens
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for both UDP and TCP requests. If <literal>no</literal>, the stub listener is disabled.</para>
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<xi:include href="systemd-resolved.service.xml" xpointer="proxy-stub" />
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<para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
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in use.</para>
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<xi:include href="version-info.xml" xpointer="v232"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>DNSStubListenerExtra=</varname></term>
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<listitem><para>Takes an IPv4 or IPv6 address to listen on. The address may be optionally
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prefixed with a protocol name (<literal>udp</literal> or <literal>tcp</literal>) separated with
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<literal>:</literal>. If the protocol is not specified, the service will listen on both UDP and
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TCP. It may be also optionally suffixed by a numeric port number with separator
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<literal>:</literal>. When an IPv6 address is specified with a port number, then the address
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must be in the square brackets. If the port is not specified, then the service uses port 53.
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Note that this is independent of the primary DNS stub configured with
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<varname>DNSStubListener=</varname>, and only configures <emphasis>additional</emphasis>
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sockets to listen on. This option can be specified multiple times. If an empty string is
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assigned, then the all previous assignments are cleared. Defaults to unset.</para>
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<para>Examples:
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<programlisting>DNSStubListenerExtra=192.168.10.10
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DNSStubListenerExtra=2001:db8:0:f102::10
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DNSStubListenerExtra=192.168.10.11:9953
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DNSStubListenerExtra=[2001:db8:0:f102::11]:9953
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DNSStubListenerExtra=tcp:192.168.10.12
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DNSStubListenerExtra=udp:2001:db8:0:f102::12
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DNSStubListenerExtra=tcp:192.168.10.13:9953
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DNSStubListenerExtra=udp:[2001:db8:0:f102::13]:9953</programlisting>
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</para>
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<xi:include href="version-info.xml" xpointer="v247"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>ReadEtcHosts=</varname></term>
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<listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default),
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<command>systemd-resolved</command> will read <filename>/etc/hosts</filename>, and try to resolve
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hosts or address by using the entries in the file before sending query to DNS servers.
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</para>
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<xi:include href="version-info.xml" xpointer="v240"/></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>ResolveUnicastSingleLabel=</varname></term>
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<listitem><para>Takes a boolean argument. When false (the default),
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<command>systemd-resolved</command> will not resolve A and AAAA queries for single-label names over
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classic DNS. Note that such names may still be resolved if search domains are specified (see
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<varname>Domains=</varname> above), or using other mechanisms, in particular via LLMNR or from
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<filename>/etc/hosts</filename>. When true, queries for single-label names will be forwarded to
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global DNS servers even if no search domains are defined.
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</para>
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<para>This option is provided for compatibility with configurations where <emphasis>public DNS
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servers are not used</emphasis>. Forwarding single-label names to servers not under your control is
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not standard-conformant, see <ulink
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url="https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/">IAB
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Statement</ulink>, and may create a privacy and security risk.</para>
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<xi:include href="version-info.xml" xpointer="v246"/></listitem>
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</varlistentry>
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<varlistentry>
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<term>StaleRetentionSec=<replaceable>SECONDS</replaceable></term>
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<listitem><para>Takes a duration value, which determines the length of time DNS resource records can
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be retained in the cache beyond their Time To Live (TTL). This allows these records to be returned as
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stale records. By default, this value is set to zero, meaning that DNS resource records are not
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stored in the cache after their TTL expires.</para>
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<para>This is useful when a DNS server failure occurs or becomes unreachable. In such cases,
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<citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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continues to use the stale records to answer DNS queries, particularly when no valid response can be
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obtained from the upstream DNS servers. However, this doesn't apply to NXDOMAIN responses, as those
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are still perfectly valid responses. This feature enhances resilience against DNS infrastructure
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failures and outages.</para>
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|
|
|
<para><command>systemd-resolved</command> always attempts to reach the upstream DNS servers first,
|
|
before providing the client application with any stale data. If this feature is enabled, cache will
|
|
not be flushed when changing servers.</para>
|
|
|
|
<xi:include href="version-info.xml" xpointer="v254"/>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>See Also</title>
|
|
<para><simplelist type="inline">
|
|
<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
|
|
<member><citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
|
<member><citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
|
|
</simplelist></para>
|
|
</refsect1>
|
|
|
|
</refentry>
|