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. Each address can
  51         optionally take a port number separated with <literal>:</literal>, a network interface name or index separated with
  52         <literal>%</literal>, and a Server Name Indication (SNI) separated with <literal>#</literal>. When IPv6 address is
  53         specified with a port number, then the address must be in the square brackets. That is, the acceptable full formats
  54         are <literal>111.222.333.444:9953%ifname#example.com</literal> for IPv4 and
  55         <literal>[1111:2222::3333]:9953%ifname#example.com</literal> for IPv6. DNS requests are sent to one of the listed
  56         DNS servers in parallel to suitable per-link DNS servers acquired from
  57         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
  58         set at runtime by external applications.  For compatibility reasons, if this setting is not specified, the DNS
  59         servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
  60         are configured in it. This setting defaults to the empty list.</para></listitem>
  61       </varlistentry>
  62
  63       <varlistentry>
  64         <term><varname>FallbackDNS=</varname></term>
  65         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Please see
  66         <varname>DNS=</varname> for acceptable format of adddresses. Any per-link DNS servers obtained from
  67         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  68         take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
  69         <filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
  70         known. If this option is not given, a compiled-in list of DNS servers is used instead.</para></listitem>
  71       </varlistentry>
  72
  73       <varlistentry>
  74         <term><varname>Domains=</varname></term>
  75         <listitem><para>A space-separated list of domains optionally prefixed with <literal>~</literal>,
  76         used for two distinct purposes described below. Defaults to the empty list.</para>
  77
  78         <para>Any domains <emphasis>not</emphasis> prefixed with <literal>~</literal> are used as search
  79         suffixes when resolving single-label hostnames (domain names which contain no dot), in order to
  80         qualify them into fully-qualified domain names (FQDNs). These "search domains" are strictly processed
  81         in the order they are specified in, until the name with the suffix appended is found. For
  82         compatibility reasons, if this setting is not specified, the search domains listed in
  83         <filename>/etc/resolv.conf</filename> with the <varname>search</varname> keyword are used instead, if
  84         that file exists and any domains are configured in it.</para>
  85
  86         <para>The domains prefixed with <literal>~</literal> are called "routing domains". All domains listed
  87         here (both search domains and routing domains after removing the <literal>~</literal> prefix) define
  88         a search path that preferably directs DNS queries to this interface. This search path has an effect
  89         only when suitable per-link DNS servers are known. Such servers may be defined through the
  90         <varname>DNS=</varname> setting (see above) and dynamically at run time, for example from DHCP
  91         leases. If no per-link DNS servers are known, routing domains have no effect.</para>
  92
  93         <para>Use the construct <literal>~.</literal> (which is composed from <literal>~</literal> to
  94         indicate a routing domain and <literal>.</literal> to indicate the DNS root domain that is the
  95         implied suffix of all DNS domains) to use the DNS servers defined for this link preferably for all
  96         domains.</para></listitem>
  97       </varlistentry>
  98
  99       <varlistentry>
 100         <term><varname>LLMNR=</varname></term>
 101         <listitem><para>Takes a boolean argument or
 102         <literal>resolve</literal>. Controls Link-Local Multicast Name
 103         Resolution support (<ulink
 104         url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
 105         the local host. If true, enables full LLMNR responder and
 106         resolver support. If false, disables both. If set to
 107         <literal>resolve</literal>, only resolution support is enabled,
 108         but responding is disabled. Note that
 109         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 110         also maintains per-link LLMNR settings. LLMNR will be
 111         enabled on a link only if the per-link and the
 112         global setting is on.</para></listitem>
 113       </varlistentry>
 114
 115       <varlistentry>
 116         <term><varname>MulticastDNS=</varname></term>
 117         <listitem><para>Takes a boolean argument or
 118         <literal>resolve</literal>. Controls Multicast DNS support (<ulink
 119         url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
 120         the local host. If true, enables full Multicast DNS responder and
 121         resolver support. If false, disables both. If set to
 122         <literal>resolve</literal>, only resolution support is enabled,
 123         but responding is disabled. Note that
 124         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 125         also maintains per-link Multicast DNS settings. Multicast DNS will be
 126         enabled on a link only if the per-link and the
 127         global setting is on.</para></listitem>
 128       </varlistentry>
 129
 130       <varlistentry>
 131         <term><varname>DNSSEC=</varname></term>
 132         <listitem><para>Takes a boolean argument or
 133         <literal>allow-downgrade</literal>. If true all DNS lookups are
 134         DNSSEC-validated locally (excluding LLMNR and Multicast
 135         DNS). If the response to a lookup request is detected to be invalid
 136         a lookup failure is returned to applications. Note that
 137         this mode requires a DNS server that supports DNSSEC. If the
 138         DNS server does not properly support DNSSEC all validations
 139         will fail. If set to <literal>allow-downgrade</literal> DNSSEC
 140         validation is attempted, but if the server does not support
 141         DNSSEC properly, DNSSEC mode is automatically disabled. Note
 142         that this mode makes DNSSEC validation vulnerable to
 143         "downgrade" attacks, where an attacker might be able to
 144         trigger a downgrade to non-DNSSEC mode by synthesizing a DNS
 145         response that suggests DNSSEC was not supported. If set to
 146         false, DNS lookups are not DNSSEC validated.</para>
 147
 148         <para>Note that DNSSEC validation requires retrieval of
 149         additional DNS data, and thus results in a small DNS look-up
 150         time penalty.</para>
 151
 152         <para>DNSSEC requires knowledge of "trust anchors" to prove
 153         data integrity. The trust anchor for the Internet root domain
 154         is built into the resolver, additional trust anchors may be
 155         defined with
 156         <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 157         Trust anchors may change at regular intervals, and old trust
 158         anchors may be revoked. In such a case DNSSEC validation is
 159         not possible until new trust anchors are configured locally or
 160         the resolver software package is updated with the new root
 161         trust anchor. In effect, when the built-in trust anchor is
 162         revoked and <varname>DNSSEC=</varname> is true, all further
 163         lookups will fail, as it cannot be proved anymore whether
 164         lookups are correctly signed, or validly unsigned. If
 165         <varname>DNSSEC=</varname> is set to
 166         <literal>allow-downgrade</literal> the resolver will
 167         automatically turn off DNSSEC validation in such a case.</para>
 168
 169         <para>Client programs looking up DNS data will be informed
 170         whether lookups could be verified using DNSSEC, or whether the
 171         returned data could not be verified (either because the data
 172         was found unsigned in the DNS, or the DNS server did not
 173         support DNSSEC or no appropriate trust anchors were known). In
 174         the latter case it is assumed that client programs employ a
 175         secondary scheme to validate the returned DNS data, should
 176         this be required.</para>
 177
 178         <para>It is recommended to set <varname>DNSSEC=</varname> to
 179         true on systems where it is known that the DNS server supports
 180         DNSSEC correctly, and where software or trust anchor updates
 181         happen regularly. On other systems it is recommended to set
 182         <varname>DNSSEC=</varname> to
 183         <literal>allow-downgrade</literal>.</para>
 184
 185         <para>In addition to this global DNSSEC setting
 186         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 187         also maintains per-link DNSSEC settings. For system DNS
 188         servers (see above), only the global DNSSEC setting is in
 189         effect. For per-link DNS servers the per-link
 190         setting is in effect, unless it is unset in which case the
 191         global setting is used instead.</para>
 192
 193         <para>Site-private DNS zones generally conflict with DNSSEC
 194         operation, unless a negative (if the private zone is not
 195         signed) or positive (if the private zone is signed) trust
 196         anchor is configured for them. If
 197         <literal>allow-downgrade</literal> mode is selected, it is
 198         attempted to detect site-private DNS zones using top-level
 199         domains (TLDs) that are not known by the DNS root server. This
 200         logic does not work in all private zone setups.</para>
 201
 202         <para>Defaults to <literal>allow-downgrade</literal></para>
 203         </listitem>
 204       </varlistentry>
 205
 206       <varlistentry>
 207         <term><varname>DNSOverTLS=</varname></term>
 208         <listitem>
 209         <para>Takes a boolean argument or <literal>opportunistic</literal>. If
 210         true all connections to the server will be encrypted. Note that this
 211         mode requires a DNS server that supports DNS-over-TLS and has a valid
 212         certificate. If the hostname was specified in <varname>DNS=</varname>
 213         by using the format format <literal>address#server_name</literal> it
 214         is used to validate its certificate and also to enable Server Name
 215         Indication (SNI) when opening a TLS connection. Otherwise
 216         the certificate is checked against the server's IP.
 217         If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>
 218
 219         <para>When set to <literal>opportunistic</literal>
 220         DNS request are attempted to send encrypted with DNS-over-TLS.
 221         If the DNS server does not support TLS, DNS-over-TLS is disabled.
 222         Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
 223         attacks, where an attacker might be able to trigger a downgrade
 224         to non-encrypted mode by synthesizing a response that suggests
 225         DNS-over-TLS was not supported. If set to false, DNS lookups
 226         are send over UDP.</para>
 227
 228         <para>Note that DNS-over-TLS requires additional data to be
 229         send for setting up an encrypted connection, and thus results
 230         in a small DNS look-up time penalty.</para>
 231
 232         <para>Note that in <literal>opportunistic</literal> mode the
 233         resolver is not capable of authenticating the server, so it is
 234         vulnerable to "man-in-the-middle" attacks.</para>
 235
 236         <para>In addition to this global DNSOverTLS setting
 237         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 238         also maintains per-link DNSOverTLS settings. For system DNS
 239         servers (see above), only the global DNSOverTLS setting is in
 240         effect. For per-link DNS servers the per-link
 241         setting is in effect, unless it is unset in which case the
 242         global setting is used instead.</para>
 243
 244         <para>Defaults to off.</para>
 245         </listitem>
 246       </varlistentry>
 247
 248       <varlistentry>
 249         <term><varname>Cache=</varname></term>
 250         <listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If
 251         <literal>yes</literal> (the default), resolving a domain name which already got queried earlier will
 252         return the previous result as long as it is still valid, and thus does not result in a new network
 253         request. Be aware that turning off caching comes at a performance penalty, which is particularly high
 254         when DNSSEC is used. If <literal>no-negative</literal>, only positive answers are cached.</para>
 255
 256         <para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address
 257         (such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.</para></listitem>
 258       </varlistentry>
 259
 260       <varlistentry>
 261         <term><varname>DNSStubListener=</varname></term>
 262         <listitem><para>Takes a boolean argument or one of <literal>udp</literal> and <literal>tcp</literal>. If
 263         <literal>udp</literal>, a DNS stub resolver will listen for UDP requests on address 127.0.0.53
 264         port 53. If <literal>tcp</literal>, the stub will listen for TCP requests on the same address and port. If
 265         <literal>yes</literal> (the default), the stub listens for both UDP and TCP requests.  If <literal>no</literal>, the stub
 266         listener is disabled.</para>
 267
 268         <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
 269         in use.</para></listitem>
 270       </varlistentry>
 271
 272       <varlistentry>
 273         <term><varname>ReadEtcHosts=</varname></term>
 274         <listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default),
 275         <command>systemd-resolved</command> will read <filename>/etc/hosts</filename>, and try to resolve
 276         hosts or address by using the entries in the file before sending query to DNS servers.
 277         </para></listitem>
 278       </varlistentry>
 279
 280       <varlistentry>
 281         <term><varname>ResolveUnicastSingleLabel=</varname></term>
 282         <listitem><para>Takes a boolean argument. When false (the default),
 283         <command>systemd-resolved</command> will not resolve A and AAAA queries for single-label names over
 284         classic DNS. Note that such names may still be resolved if search domains are specified (see
 285         <varname>Domains=</varname> above), or using other mechanisms, in particular via LLMNR or from
 286         <filename>/etc/hosts</filename>. When true, queries for single-label names will be forwarded to
 287         global DNS servers even if no search domains are defined.
 288         </para>
 289
 290         <para>This option is provided for compatibility with configurations where <emphasis>public DNS
 291         servers are not used</emphasis>. Forwarding single-label names to servers not under your control is
 292         not standard-conformant, see <ulink
 293         url="https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/">IAB
 294         Statement</ulink>, and may create a privacy and security risk.</para></listitem>
 295       </varlistentry>
 296     </variablelist>
 297   </refsect1>
 298
 299   <refsect1>
 300       <title>See Also</title>
 301       <para>
 302       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 303       <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 304       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 305       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 306       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 307       </para>
 308   </refsect1>
 309
 310 </refentry>