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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1+ -->
<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 <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 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 for it's IP. If the DNS server does not support
DNS-over-TLS all DNS requests will fail. 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 as the resolver is not capable of authenticating
the server, it is vulnerable for "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.</para>
If <literal>no-negative</literal>, only positive answers are cached.
<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), the DNS stub resolver 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>
</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>
|
