281 lines
15 KiB
XML
281 lines
15 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="resolved.conf" conditional='ENABLE_RESOLVE'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>resolved.conf</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>resolved.conf</refentrytitle>
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<manvolnum>5</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>resolved.conf</refname>
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<refname>resolved.conf.d</refname>
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<refpurpose>Network Name Resolution configuration files</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<para><filename>/etc/systemd/resolved.conf</filename></para>
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<para><filename>/etc/systemd/resolved.conf.d/*.conf</filename></para>
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<para><filename>/run/systemd/resolved.conf.d/*.conf</filename></para>
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<para><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>These configuration files control local DNS and LLMNR
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name resolution.</para>
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</refsect1>
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<xi:include href="standard-conf.xml" xpointer="main-conf" />
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<refsect1>
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<title>Options</title>
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<para>The following options are available in the <literal>[Resolve]</literal> section:</para>
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<variablelist class='network-directives'>
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<varlistentry>
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<term><varname>DNS=</varname></term>
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<listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. DNS requests
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are sent to one of the listed DNS servers in parallel to suitable per-link DNS servers acquired from
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
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set at runtime by external applications. For compatibility reasons, if this setting is not specified, the DNS
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servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
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are configured in it. This setting defaults to the empty list.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>FallbackDNS=</varname></term>
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<listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Any
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per-link DNS servers obtained from
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
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<filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
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known. If this option is not given, a compiled-in list of DNS servers is used instead.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>Domains=</varname></term>
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<listitem><para>A space-separated list of domains. These domains are used as search suffixes when resolving
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single-label hostnames (domain names which contain no dot), in order to qualify them into fully-qualified
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domain names (FQDNs). Search domains are strictly processed in the order they are specified, until the name
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with the suffix appended is found. For compatibility reasons, if this setting is not specified, the search
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domains listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any domains
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are configured in it. This setting defaults to the empty list.</para>
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<para>Specified domain names may optionally be prefixed with <literal>~</literal>. In this case they do not
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define a search path, but preferably direct DNS queries for the indicated domains to the DNS servers configured
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with the system <varname>DNS=</varname> setting (see above), in case additional, suitable per-link DNS servers
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are known. If no per-link DNS servers are known using the <literal>~</literal> syntax has no effect. Use the
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construct <literal>~.</literal> (which is composed of <literal>~</literal> to indicate a routing domain and
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<literal>.</literal> to indicate the DNS root domain that is the implied suffix of all DNS domains) to use the
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system DNS server defined with <varname>DNS=</varname> preferably for all domains.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>LLMNR=</varname></term>
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<listitem><para>Takes a boolean argument or
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<literal>resolve</literal>. Controls Link-Local Multicast Name
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Resolution support (<ulink
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url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
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the local host. If true, enables full LLMNR responder and
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resolver support. If false, disables both. If set to
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<literal>resolve</literal>, only resolution support is enabled,
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but responding is disabled. Note that
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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also maintains per-link LLMNR settings. LLMNR will be
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enabled on a link only if the per-link and the
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global setting is on.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>MulticastDNS=</varname></term>
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<listitem><para>Takes a boolean argument or
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<literal>resolve</literal>. Controls Multicast DNS support (<ulink
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url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
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the local host. If true, enables full Multicast DNS responder and
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resolver support. If false, disables both. If set to
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<literal>resolve</literal>, only resolution support is enabled,
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but responding is disabled. Note that
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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also maintains per-link Multicast DNS settings. Multicast DNS will be
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enabled on a link only if the per-link and the
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global setting is on.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>DNSSEC=</varname></term>
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<listitem><para>Takes a boolean argument or
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<literal>allow-downgrade</literal>. If true all DNS lookups are
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DNSSEC-validated locally (excluding LLMNR and Multicast
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DNS). If the response to a lookup request is detected to be invalid
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a lookup failure is returned to applications. Note that
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this mode requires a DNS server that supports DNSSEC. If the
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DNS server does not properly support DNSSEC all validations
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will fail. If set to <literal>allow-downgrade</literal> DNSSEC
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validation is attempted, but if the server does not support
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DNSSEC properly, DNSSEC mode is automatically disabled. Note
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that this mode makes DNSSEC validation vulnerable to
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"downgrade" attacks, where an attacker might be able to
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trigger a downgrade to non-DNSSEC mode by synthesizing a DNS
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response that suggests DNSSEC was not supported. If set to
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false, DNS lookups are not DNSSEC validated.</para>
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<para>Note that DNSSEC validation requires retrieval of
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additional DNS data, and thus results in a small DNS look-up
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time penalty.</para>
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<para>DNSSEC requires knowledge of "trust anchors" to prove
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data integrity. The trust anchor for the Internet root domain
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is built into the resolver, additional trust anchors may be
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defined with
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<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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Trust anchors may change at regular intervals, and old trust
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anchors may be revoked. In such a case DNSSEC validation is
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not possible until new trust anchors are configured locally or
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the resolver software package is updated with the new root
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trust anchor. In effect, when the built-in trust anchor is
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revoked and <varname>DNSSEC=</varname> is true, all further
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lookups will fail, as it cannot be proved anymore whether
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lookups are correctly signed, or validly unsigned. If
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<varname>DNSSEC=</varname> is set to
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<literal>allow-downgrade</literal> the resolver will
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automatically turn off DNSSEC validation in such a case.</para>
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<para>Client programs looking up DNS data will be informed
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whether lookups could be verified using DNSSEC, or whether the
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returned data could not be verified (either because the data
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was found unsigned in the DNS, or the DNS server did not
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support DNSSEC or no appropriate trust anchors were known). In
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the latter case it is assumed that client programs employ a
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secondary scheme to validate the returned DNS data, should
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this be required.</para>
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<para>It is recommended to set <varname>DNSSEC=</varname> to
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true on systems where it is known that the DNS server supports
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DNSSEC correctly, and where software or trust anchor updates
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happen regularly. On other systems it is recommended to set
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<varname>DNSSEC=</varname> to
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<literal>allow-downgrade</literal>.</para>
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<para>In addition to this global DNSSEC setting
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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also maintains per-link DNSSEC settings. For system DNS
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servers (see above), only the global DNSSEC setting is in
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effect. For per-link DNS servers the per-link
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setting is in effect, unless it is unset in which case the
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global setting is used instead.</para>
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<para>Site-private DNS zones generally conflict with DNSSEC
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operation, unless a negative (if the private zone is not
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signed) or positive (if the private zone is signed) trust
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anchor is configured for them. If
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<literal>allow-downgrade</literal> mode is selected, it is
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attempted to detect site-private DNS zones using top-level
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domains (TLDs) that are not known by the DNS root server. This
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logic does not work in all private zone setups.</para>
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<para>Defaults to <literal>allow-downgrade</literal></para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>DNSOverTLS=</varname></term>
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<listitem>
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<para>Takes a boolean argument or <literal>opportunistic</literal>. If
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true all connections to the server will be encrypted. Note that this
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mode requires a DNS server that supports DNS-over-TLS and has a valid
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certificate. If the hostname was specified in <varname>DNS=</varname>
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by using the format format <literal>address#server_name</literal> it
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is used to validate its certificate and also to enable Server Name
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Indication (SNI) when opening a TLS connection. Otherwise
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the certificate is checked against the server's IP.
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If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>
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<para>When set to <literal>opportunistic</literal>
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DNS request are attempted to send encrypted with DNS-over-TLS.
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If the DNS server does not support TLS, DNS-over-TLS is disabled.
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Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
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attacks, where an attacker might be able to trigger a downgrade
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to non-encrypted mode by synthesizing a response that suggests
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DNS-over-TLS was not supported. If set to false, DNS lookups
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are send over UDP.</para>
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<para>Note that DNS-over-TLS requires additional data to be
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send for setting up an encrypted connection, and thus results
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in a small DNS look-up time penalty.</para>
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<para>Note that in <literal>opportunistic</literal> mode the
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resolver is not capable of authenticating the server, so it is
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vulnerable to "man-in-the-middle" attacks.</para>
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<para>In addition to this global DNSOverTLS setting
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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also maintains per-link DNSOverTLS settings. For system DNS
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servers (see above), only the global DNSOverTLS setting is in
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effect. For per-link DNS servers the per-link
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setting is in effect, unless it is unset in which case the
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global setting is used instead.</para>
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<para>Defaults to off.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>Cache=</varname></term>
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<listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If
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<literal>yes</literal> (the default), resolving a domain name which already got queried earlier will
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return the previous result as long as it is still valid, and thus does not result in a new network
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request. Be aware that turning off caching comes at a performance penalty, which is particularly high
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when DNSSEC is used. If <literal>no-negative</literal>, only positive answers are cached.</para>
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<para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address
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(such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>DNSStubListener=</varname></term>
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<listitem><para>Takes a boolean argument or one of <literal>udp</literal> and <literal>tcp</literal>. If
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<literal>udp</literal>, a DNS stub resolver will listen for UDP requests on address 127.0.0.53
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port 53. If <literal>tcp</literal>, the stub will listen for TCP requests on the same address and port. If
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<literal>yes</literal> (the default), the stub listens for both UDP and TCP requests. If <literal>no</literal>, the stub
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listener is disabled.</para>
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<para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
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in use.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>ReadEtcHosts=</varname></term>
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<listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default), the DNS stub resolver will read
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<filename>/etc/hosts</filename>, and try to resolve hosts or address by using the entries in the file before
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sending query to DNS servers.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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