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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>
<variablelist class='network-directives'>
<varlistentry>
<term><varname>DNS=</varname></term>
<listitem><para>A space-separated list of IPv4 and IPv6
addresses to be used as system DNS servers. DNS requests are
sent to one of the listed DNS servers in parallel to any
per-interface DNS servers acquired from
<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
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 be used as the fallback DNS servers. Any
per-interface 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 search domains. 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></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-interface LLMNR settings. LLMNR will be
enabled on an interface only if the per-interface and the
global setting is on.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>DNSSEC=</varname></term>
<listitem><para>Takes a boolean argument or
<literal>downgrade-ok</literal>. If true all DNS lookups are
DNSSEC-validated locally. If a response for a lookup request
is detected invalid this is returned as lookup failure 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>downgrade-ok</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. However, trust anchors may change
in 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>downgrade-ok</literal> the resolver will
automatically turn of 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 kown 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>missing-ok</literal>.</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>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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