man: update explanation about the format to specify DNS servers

[thirdparty/systemd.git] / man / resolved.conf.xml

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<refentry id="resolved.conf" conditional='ENABLE_RESOLVE'
    xmlns:xi="http://www.w3.org/2001/XInclude">
  <refentryinfo>
    <title>resolved.conf</title>
    <productname>systemd</productname>
  </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 [Resolve] 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. Each address can
        optionally take a port number separated with <literal>:</literal>, a network interface name or index separated with
        <literal>%</literal>, and a Server Name Indication (SNI) separated with <literal>#</literal>. When IPv6 address is
        specified with a port number, then the address must be in the square brackets. That is, the acceptable full formats
        are <literal>111.222.333.444:9953%ifname#example.com</literal> for IPv4 and
        <literal>[1111:2222::3333]:9953%ifname#example.com</literal> for IPv6. 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. Please see
        <varname>DNS=</varname> for acceptable format of adddresses. 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 optionally prefixed with <literal>~</literal>,
        used for two distinct purposes described below. Defaults to the empty list.</para>

        <para>Any domains <emphasis>not</emphasis> prefixed with <literal>~</literal> are used as search
        suffixes when resolving single-label hostnames (domain names which contain no dot), in order to
        qualify them into fully-qualified domain names (FQDNs). These "search domains" are strictly processed
        in the order they are specified in, 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> with the <varname>search</varname> keyword are used instead, if
        that file exists and any domains are configured in it.</para>

        <para>The domains prefixed with <literal>~</literal> are called "routing domains". All domains listed
        here (both search domains and routing domains after removing the <literal>~</literal> prefix) define
        a search path that preferably directs DNS queries to this interface. This search path has an effect
        only when suitable per-link DNS servers are known. Such servers may be defined through the
        <varname>DNS=</varname> setting (see above) and dynamically at run time, for example from DHCP
        leases. If no per-link DNS servers are known, routing domains have no effect.</para>

        <para>Use the construct <literal>~.</literal> (which is composed from <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 DNS servers defined for this link 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 4795</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>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-link Multicast DNS settings. Multicast DNS 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 <literal>allow-downgrade</literal></para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>DNSOverTLS=</varname></term>
        <listitem>
        <para>Takes a boolean argument or <literal>opportunistic</literal>. If
        true all connections to the server will be encrypted. Note that this
        mode requires a DNS server that supports DNS-over-TLS and has a valid
        certificate. If the hostname was specified in <varname>DNS=</varname>
        by using the format format <literal>address#server_name</literal> it
        is used to validate its certificate and also to enable Server Name
        Indication (SNI) when opening a TLS connection. Otherwise
        the certificate is checked against the server's IP.
        If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>

        <para>When set to <literal>opportunistic</literal>
        DNS request are attempted to send encrypted with DNS-over-TLS.
        If the DNS server does not support TLS, DNS-over-TLS is disabled.
        Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
        attacks, where an attacker might be able to trigger a downgrade
        to non-encrypted mode by synthesizing a response that suggests
        DNS-over-TLS was not supported. If set to false, DNS lookups
        are send over UDP.</para>

        <para>Note that DNS-over-TLS requires additional data to be
        send for setting up an encrypted connection, and thus results
        in a small DNS look-up time penalty.</para>

        <para>Note that in <literal>opportunistic</literal> mode the
        resolver is not capable of authenticating the server, so it is
        vulnerable to "man-in-the-middle" attacks.</para>

        <para>In addition to this global DNSOverTLS setting
        <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
        also maintains per-link DNSOverTLS settings. For system DNS
        servers (see above), only the global DNSOverTLS 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>Defaults to off.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>Cache=</varname></term>
        <listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If
        <literal>yes</literal> (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. If <literal>no-negative</literal>, only positive answers are cached.</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>, 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 default), 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>

      <varlistentry>
        <term><varname>ReadEtcHosts=</varname></term>
        <listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default),
        <command>systemd-resolved</command> will read <filename>/etc/hosts</filename>, and try to resolve
        hosts or address by using the entries in the file before sending query to DNS servers.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term><varname>ResolveUnicastSingleLabel=</varname></term>
        <listitem><para>Takes a boolean argument. When false (the default),
        <command>systemd-resolved</command> will not resolve A and AAAA queries for single-label names over
        classic DNS. Note that such names may still be resolved if search domains are specified (see
        <varname>Domains=</varname> above), or using other mechanisms, in particular via LLMNR or from
        <filename>/etc/hosts</filename>. When true, queries for single-label names will be forwarded to
        global DNS servers even if no search domains are defined.
        </para>

        <para>This option is provided for compatibility with configurations where <emphasis>public DNS
        servers are not used</emphasis>. Forwarding single-label names to servers not under your control is
        not standard-conformant, see <ulink
        url="https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/">IAB
        Statement</ulink>, and may create a privacy and security risk.</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>