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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>allow-downgrade</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>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 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>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-interface DNSSEC settings. For system DNS

servers (see above), only the global DNSSEC setting is in

effect. For per-interface DNS servers the per-interface

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>

</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>