Systemd/man/resolved.conf.xml

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<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>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-interface Multicast DNS settings. Multicast
        DNS 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 (excluding LLMNR and Multicast
        DNS). 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, additional trust anchors may be
        defined with
        <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
        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 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>downgrade-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>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry>
      </para>
  </refsect1>

</refentry>