systemd/man/resolved.conf.xml

<?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 re-use
        the previous result as long as that is still valid, and thus does not
        need to do an actual network request.</para>

        <para>However, local caching slightly increases the chance of a
        successful DNS poisoning attack, and might also be a privacy problem in
        some environments: By measuring the time it takes to resolve a
        particular network name, a user can determine whether any other user on
        the same machine recently visited that name. If either of these is a
        concern, you may disable the local caching. Be aware that this comes at
        a performance cost, which is <emphasis>very</emphasis> high with DNSSEC.
        </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>