man/resolved.conf.xml

   1 <?xml version='1.0'?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
   5
   6 <refentry id="resolved.conf" conditional='ENABLE_RESOLVE'
   7     xmlns:xi="http://www.w3.org/2001/XInclude">
   8   <refentryinfo>
   9     <title>resolved.conf</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>resolved.conf</refentrytitle>
  15     <manvolnum>5</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>resolved.conf</refname>
  20     <refname>resolved.conf.d</refname>
  21     <refpurpose>Network Name Resolution configuration files</refpurpose>
  22   </refnamediv>
  23
  24   <refsynopsisdiv>
  25     <para><filename>/etc/systemd/resolved.conf</filename></para>
  26     <para><filename>/etc/systemd/resolved.conf.d/*.conf</filename></para>
  27     <para><filename>/run/systemd/resolved.conf.d/*.conf</filename></para>
  28     <para><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></para>
  29   </refsynopsisdiv>
  30
  31   <refsect1>
  32     <title>Description</title>
  33
  34     <para>These configuration files control local DNS and LLMNR
  35     name resolution.</para>
  36
  37   </refsect1>
  38
  39   <xi:include href="standard-conf.xml" xpointer="main-conf" />
  40
  41   <refsect1>
  42     <title>Options</title>
  43
  44     <para>The following options are available in the [Resolve] section:</para>
  45
  46     <variablelist class='network-directives'>
  47
  48       <varlistentry>
  49         <term><varname>DNS=</varname></term>
  50         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. DNS requests
  51         are sent to one of the listed DNS servers in parallel to suitable per-link DNS servers acquired from
  52         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
  53         set at runtime by external applications.  For compatibility reasons, if this setting is not specified, the DNS
  54         servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
  55         are configured in it. This setting defaults to the empty list.</para></listitem>
  56       </varlistentry>
  57
  58       <varlistentry>
  59         <term><varname>FallbackDNS=</varname></term>
  60         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Any
  61         per-link DNS servers obtained from
  62         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  63         take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
  64         <filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
  65         known. If this option is not given, a compiled-in list of DNS servers is used instead.</para></listitem>
  66       </varlistentry>
  67
  68       <varlistentry>
  69         <term><varname>Domains=</varname></term>
  70         <listitem><para>A space-separated list of domains optionally prefixed with <literal>~</literal>,
  71         used for two distinct purposes described below. Defaults to the empty list.</para>
  72
  73         <para>Any domains <emphasis>not</emphasis> prefixed with <literal>~</literal> are used as search
  74         suffixes when resolving single-label hostnames (domain names which contain no dot), in order to
  75         qualify them into fully-qualified domain names (FQDNs). These "search domains" are strictly processed
  76         in the order they are specified in, until the name with the suffix appended is found. For
  77         compatibility reasons, if this setting is not specified, the search domains listed in
  78         <filename>/etc/resolv.conf</filename> with the <varname>search</varname> keyword are used instead, if
  79         that file exists and any domains are configured in it.</para>
  80
  81         <para>The domains prefixed with <literal>~</literal> are called "routing domains". All domains listed
  82         here (both search domains and routing domains after removing the <literal>~</literal> prefix) define
  83         a search path that preferably directs DNS queries to this interface. This search path has an effect
  84         only when suitable per-link DNS servers are known. Such servers may be defined through the
  85         <varname>DNS=</varname> setting (see above) and dynamically at run time, for example from DHCP
  86         leases. If no per-link DNS servers are known, routing domains have no effect.</para>
  87
  88         <para>Use the construct <literal>~.</literal> (which is composed from <literal>~</literal> to
  89         indicate a routing domain and <literal>.</literal> to indicate the DNS root domain that is the
  90         implied suffix of all DNS domains) to use the DNS servers defined for this link preferably for all
  91         domains.</para></listitem>
  92       </varlistentry>
  93
  94       <varlistentry>
  95         <term><varname>LLMNR=</varname></term>
  96         <listitem><para>Takes a boolean argument or
  97         <literal>resolve</literal>. Controls Link-Local Multicast Name
  98         Resolution support (<ulink
  99         url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
 100         the local host. If true, enables full LLMNR responder and
 101         resolver support. If false, disables both. If set to
 102         <literal>resolve</literal>, only resolution support is enabled,
 103         but responding is disabled. Note that
 104         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 105         also maintains per-link LLMNR settings. LLMNR will be
 106         enabled on a link only if the per-link and the
 107         global setting is on.</para></listitem>
 108       </varlistentry>
 109
 110       <varlistentry>
 111         <term><varname>MulticastDNS=</varname></term>
 112         <listitem><para>Takes a boolean argument or
 113         <literal>resolve</literal>. Controls Multicast DNS support (<ulink
 114         url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
 115         the local host. If true, enables full Multicast DNS responder and
 116         resolver support. If false, disables both. If set to
 117         <literal>resolve</literal>, only resolution support is enabled,
 118         but responding is disabled. Note that
 119         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 120         also maintains per-link Multicast DNS settings. Multicast DNS will be
 121         enabled on a link only if the per-link and the
 122         global setting is on.</para></listitem>
 123       </varlistentry>
 124
 125       <varlistentry>
 126         <term><varname>DNSSEC=</varname></term>
 127         <listitem><para>Takes a boolean argument or
 128         <literal>allow-downgrade</literal>. If true all DNS lookups are
 129         DNSSEC-validated locally (excluding LLMNR and Multicast
 130         DNS). If the response to a lookup request is detected to be invalid
 131         a lookup failure is returned to applications. Note that
 132         this mode requires a DNS server that supports DNSSEC. If the
 133         DNS server does not properly support DNSSEC all validations
 134         will fail. If set to <literal>allow-downgrade</literal> DNSSEC
 135         validation is attempted, but if the server does not support
 136         DNSSEC properly, DNSSEC mode is automatically disabled. Note
 137         that this mode makes DNSSEC validation vulnerable to
 138         "downgrade" attacks, where an attacker might be able to
 139         trigger a downgrade to non-DNSSEC mode by synthesizing a DNS
 140         response that suggests DNSSEC was not supported. If set to
 141         false, DNS lookups are not DNSSEC validated.</para>
 142
 143         <para>Note that DNSSEC validation requires retrieval of
 144         additional DNS data, and thus results in a small DNS look-up
 145         time penalty.</para>
 146
 147         <para>DNSSEC requires knowledge of "trust anchors" to prove
 148         data integrity. The trust anchor for the Internet root domain
 149         is built into the resolver, additional trust anchors may be
 150         defined with
 151         <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 152         Trust anchors may change at regular intervals, and old trust
 153         anchors may be revoked. In such a case DNSSEC validation is
 154         not possible until new trust anchors are configured locally or
 155         the resolver software package is updated with the new root
 156         trust anchor. In effect, when the built-in trust anchor is
 157         revoked and <varname>DNSSEC=</varname> is true, all further
 158         lookups will fail, as it cannot be proved anymore whether
 159         lookups are correctly signed, or validly unsigned. If
 160         <varname>DNSSEC=</varname> is set to
 161         <literal>allow-downgrade</literal> the resolver will
 162         automatically turn off DNSSEC validation in such a case.</para>
 163
 164         <para>Client programs looking up DNS data will be informed
 165         whether lookups could be verified using DNSSEC, or whether the
 166         returned data could not be verified (either because the data
 167         was found unsigned in the DNS, or the DNS server did not
 168         support DNSSEC or no appropriate trust anchors were known). In
 169         the latter case it is assumed that client programs employ a
 170         secondary scheme to validate the returned DNS data, should
 171         this be required.</para>
 172
 173         <para>It is recommended to set <varname>DNSSEC=</varname> to
 174         true on systems where it is known that the DNS server supports
 175         DNSSEC correctly, and where software or trust anchor updates
 176         happen regularly. On other systems it is recommended to set
 177         <varname>DNSSEC=</varname> to
 178         <literal>allow-downgrade</literal>.</para>
 179
 180         <para>In addition to this global DNSSEC setting
 181         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 182         also maintains per-link DNSSEC settings. For system DNS
 183         servers (see above), only the global DNSSEC setting is in
 184         effect. For per-link DNS servers the per-link
 185         setting is in effect, unless it is unset in which case the
 186         global setting is used instead.</para>
 187
 188         <para>Site-private DNS zones generally conflict with DNSSEC
 189         operation, unless a negative (if the private zone is not
 190         signed) or positive (if the private zone is signed) trust
 191         anchor is configured for them. If
 192         <literal>allow-downgrade</literal> mode is selected, it is
 193         attempted to detect site-private DNS zones using top-level
 194         domains (TLDs) that are not known by the DNS root server. This
 195         logic does not work in all private zone setups.</para>
 196
 197         <para>Defaults to <literal>allow-downgrade</literal></para>
 198         </listitem>
 199       </varlistentry>
 200
 201       <varlistentry>
 202         <term><varname>DNSOverTLS=</varname></term>
 203         <listitem>
 204         <para>Takes a boolean argument or <literal>opportunistic</literal>. If
 205         true all connections to the server will be encrypted. Note that this
 206         mode requires a DNS server that supports DNS-over-TLS and has a valid
 207         certificate. If the hostname was specified in <varname>DNS=</varname>
 208         by using the format format <literal>address#server_name</literal> it
 209         is used to validate its certificate and also to enable Server Name
 210         Indication (SNI) when opening a TLS connection. Otherwise
 211         the certificate is checked against the server's IP.
 212         If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>
 213
 214         <para>When set to <literal>opportunistic</literal>
 215         DNS request are attempted to send encrypted with DNS-over-TLS.
 216         If the DNS server does not support TLS, DNS-over-TLS is disabled.
 217         Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
 218         attacks, where an attacker might be able to trigger a downgrade
 219         to non-encrypted mode by synthesizing a response that suggests
 220         DNS-over-TLS was not supported. If set to false, DNS lookups
 221         are send over UDP.</para>
 222
 223         <para>Note that DNS-over-TLS requires additional data to be
 224         send for setting up an encrypted connection, and thus results
 225         in a small DNS look-up time penalty.</para>
 226
 227         <para>Note that in <literal>opportunistic</literal> mode the
 228         resolver is not capable of authenticating the server, so it is
 229         vulnerable to "man-in-the-middle" attacks.</para>
 230
 231         <para>In addition to this global DNSOverTLS setting
 232         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 233         also maintains per-link DNSOverTLS settings. For system DNS
 234         servers (see above), only the global DNSOverTLS setting is in
 235         effect. For per-link DNS servers the per-link
 236         setting is in effect, unless it is unset in which case the
 237         global setting is used instead.</para>
 238
 239         <para>Defaults to off.</para>
 240         </listitem>
 241       </varlistentry>
 242
 243       <varlistentry>
 244         <term><varname>Cache=</varname></term>
 245         <listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If
 246         <literal>yes</literal> (the default), resolving a domain name which already got queried earlier will
 247         return the previous result as long as it is still valid, and thus does not result in a new network
 248         request. Be aware that turning off caching comes at a performance penalty, which is particularly high
 249         when DNSSEC is used. If <literal>no-negative</literal>, only positive answers are cached.</para>
 250
 251         <para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address
 252         (such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.</para></listitem>
 253       </varlistentry>
 254
 255       <varlistentry>
 256         <term><varname>DNSStubListener=</varname></term>
 257         <listitem><para>Takes a boolean argument or one of <literal>udp</literal> and <literal>tcp</literal>. If
 258         <literal>udp</literal>, a DNS stub resolver will listen for UDP requests on address 127.0.0.53
 259         port 53. If <literal>tcp</literal>, the stub will listen for TCP requests on the same address and port. If
 260         <literal>yes</literal> (the default), the stub listens for both UDP and TCP requests.  If <literal>no</literal>, the stub
 261         listener is disabled.</para>
 262
 263         <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
 264         in use.</para></listitem>
 265       </varlistentry>
 266
 267       <varlistentry>
 268         <term><varname>ReadEtcHosts=</varname></term>
 269         <listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default),
 270         <command>systemd-resolved</command> will read <filename>/etc/hosts</filename>, and try to resolve
 271         hosts or address by using the entries in the file before sending query to DNS servers.
 272         </para></listitem>
 273       </varlistentry>
 274
 275       <varlistentry>
 276         <term><varname>ResolveUnicastSingleLabel=</varname></term>
 277         <listitem><para>Takes a boolean argument. When false (the default),
 278         <command>systemd-resolved</command> will not resolve A and AAAA queries for single-label names over
 279         classic DNS. Note that such names may still be resolved if search domains are specified (see
 280         <varname>Domains=</varname> above), or using other mechanisms, in particular via LLMNR or from
 281         <filename>/etc/hosts</filename>. When true, queries for single-label names will be forwarded to
 282         global DNS servers even if no search domains are defined.
 283         </para>
 284
 285         <para>This option is provided for compatibility with configurations where <emphasis>public DNS
 286         servers are not used</emphasis>. Forwarding single-label names to servers not under your control is
 287         not standard-conformant, see <ulink
 288         url="https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/">IAB
 289         Statement</ulink>, and may create a privacy and security risk.</para></listitem>
 290       </varlistentry>
 291     </variablelist>
 292   </refsect1>
 293
 294   <refsect1>
 295       <title>See Also</title>
 296       <para>
 297       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 298       <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 299       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 300       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 301       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 302       </para>
 303   </refsect1>
 304
 305 </refentry>