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-<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
-<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
- "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
-
-<!--
- This file is part of systemd.
-
- Copyright 2014 Tom Gundersen
-
- systemd is free software; you can redistribute it and/or modify it
- under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 2.1 of the License, or
- (at your option) any later version.
-
- systemd is distributed in the hope that it will be useful, but
- WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
-
- You should have received a copy of the GNU Lesser General Public License
- along with systemd; If not, see <http://www.gnu.org/licenses/>.
--->
-
-<refentry id="resolved.conf" conditional='ENABLE_RESOLVED'
- xmlns:xi="http://www.w3.org/2001/XInclude">
- <refentryinfo>
- <title>resolved.conf</title>
- <productname>systemd</productname>
-
- <authorgroup>
- <author>
- <contrib>Developer</contrib>
- <firstname>Tom</firstname>
- <surname>Gundersen</surname>
- <email>teg@jklm.no</email>
- </author>
- </authorgroup>
- </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 <literal>[Resolve]</literal> 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. 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. 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. These domains are used as search suffixes when resolving
- single-label host names (domain names which contain no dot), in order to qualify them into fully-qualified
- domain names (FQDNs). Search domains are strictly processed in the order they are specified, 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> are used instead, if that file exists and any domains
- are configured in it. This setting defaults to the empty list.</para>
-
- <para>Specified domain names may optionally be prefixed with <literal>~</literal>. In this case they do not
- define a search path, but preferably direct DNS queries for the indicated domains to the DNS servers configured
- with the system <varname>DNS=</varname> setting (see above), in case additional, suitable per-link DNS servers
- are known. If no per-link DNS servers are known using the <literal>~</literal> syntax has no effect. Use the
- construct <literal>~.</literal> (which is composed of <literal>~</literal> to indicate a routing domain and
- <literal>.</literal> to indicate the DNS root domain that is the implied suffix of all DNS domains) to use the
- system DNS server defined with <varname>DNS=</varname> preferably for all domains.</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 4794</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>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 off.</para>
- </listitem>
- </varlistentry>
-
- <varlistentry>
- <term><varname>Cache=</varname></term>
- <listitem><para>Takes a boolean argument. If "yes" (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.</para>
-
- <para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address
- (such as 127.0.0.1 or ::1), in order to avoid 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> (the default), a DNS stub resolver will listen for UDP requests on address 127.0.0.53
- port 53. If <literal>tcp</literal>, the stub will listen for TCP requests on the same address and port. If
- <literal>yes</literal>, the stub listens for both UDP and TCP requests. If <literal>no</literal>, the stub
- listener is disabled.</para>
-
- <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
- in use.</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>4</manvolnum></citerefentry>
- </para>
- </refsect1>
-
-</refentry>