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 <literal>[Resolve]</literal> 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. These domains are used as search suffixes when resolving
  71         single-label host names (domain names which contain no dot), in order to qualify them into fully-qualified
  72         domain names (FQDNs). Search domains are strictly processed in the order they are specified, until the name
  73         with the suffix appended is found. For compatibility reasons, if this setting is not specified, the search
  74         domains listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any domains
  75         are configured in it. This setting defaults to the empty list.</para>
  76
  77         <para>Specified domain names may optionally be prefixed with <literal>~</literal>. In this case they do not
  78         define a search path, but preferably direct DNS queries for the indicated domains to the DNS servers configured
  79         with the system <varname>DNS=</varname> setting (see above), in case additional, suitable per-link DNS servers
  80         are known. If no per-link DNS servers are known using the <literal>~</literal> syntax has no effect. Use the
  81         construct <literal>~.</literal> (which is composed of <literal>~</literal> to indicate a routing domain and
  82         <literal>.</literal> to indicate the DNS root domain that is the implied suffix of all DNS domains) to use the
  83         system DNS server defined with <varname>DNS=</varname> preferably for all domains.</para></listitem>
  84       </varlistentry>
  85
  86       <varlistentry>
  87         <term><varname>LLMNR=</varname></term>
  88         <listitem><para>Takes a boolean argument or
  89         <literal>resolve</literal>. Controls Link-Local Multicast Name
  90         Resolution support (<ulink
  91         url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
  92         the local host. If true, enables full LLMNR responder and
  93         resolver support. If false, disables both. If set to
  94         <literal>resolve</literal>, only resolution support is enabled,
  95         but responding is disabled. Note that
  96         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  97         also maintains per-link LLMNR settings. LLMNR will be
  98         enabled on a link only if the per-link and the
  99         global setting is on.</para></listitem>
 100       </varlistentry>
 101
 102       <varlistentry>
 103         <term><varname>MulticastDNS=</varname></term>
 104         <listitem><para>Takes a boolean argument or
 105         <literal>resolve</literal>. Controls Multicast DNS support (<ulink
 106         url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
 107         the local host. If true, enables full Multicast DNS responder and
 108         resolver support. If false, disables both. If set to
 109         <literal>resolve</literal>, only resolution support is enabled,
 110         but responding is disabled. Note that
 111         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 112         also maintains per-link Multicast DNS settings. Multicast DNS will be
 113         enabled on a link only if the per-link and the
 114         global setting is on.</para></listitem>
 115       </varlistentry>
 116
 117       <varlistentry>
 118         <term><varname>DNSSEC=</varname></term>
 119         <listitem><para>Takes a boolean argument or
 120         <literal>allow-downgrade</literal>. If true all DNS lookups are
 121         DNSSEC-validated locally (excluding LLMNR and Multicast
 122         DNS). If the response to a lookup request is detected to be invalid
 123         a lookup failure is returned to applications. Note that
 124         this mode requires a DNS server that supports DNSSEC. If the
 125         DNS server does not properly support DNSSEC all validations
 126         will fail. If set to <literal>allow-downgrade</literal> DNSSEC
 127         validation is attempted, but if the server does not support
 128         DNSSEC properly, DNSSEC mode is automatically disabled. Note
 129         that this mode makes DNSSEC validation vulnerable to
 130         "downgrade" attacks, where an attacker might be able to
 131         trigger a downgrade to non-DNSSEC mode by synthesizing a DNS
 132         response that suggests DNSSEC was not supported. If set to
 133         false, DNS lookups are not DNSSEC validated.</para>
 134
 135         <para>Note that DNSSEC validation requires retrieval of
 136         additional DNS data, and thus results in a small DNS look-up
 137         time penalty.</para>
 138
 139         <para>DNSSEC requires knowledge of "trust anchors" to prove
 140         data integrity. The trust anchor for the Internet root domain
 141         is built into the resolver, additional trust anchors may be
 142         defined with
 143         <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 144         Trust anchors may change at regular intervals, and old trust
 145         anchors may be revoked. In such a case DNSSEC validation is
 146         not possible until new trust anchors are configured locally or
 147         the resolver software package is updated with the new root
 148         trust anchor. In effect, when the built-in trust anchor is
 149         revoked and <varname>DNSSEC=</varname> is true, all further
 150         lookups will fail, as it cannot be proved anymore whether
 151         lookups are correctly signed, or validly unsigned. If
 152         <varname>DNSSEC=</varname> is set to
 153         <literal>allow-downgrade</literal> the resolver will
 154         automatically turn off DNSSEC validation in such a case.</para>
 155
 156         <para>Client programs looking up DNS data will be informed
 157         whether lookups could be verified using DNSSEC, or whether the
 158         returned data could not be verified (either because the data
 159         was found unsigned in the DNS, or the DNS server did not
 160         support DNSSEC or no appropriate trust anchors were known). In
 161         the latter case it is assumed that client programs employ a
 162         secondary scheme to validate the returned DNS data, should
 163         this be required.</para>
 164
 165         <para>It is recommended to set <varname>DNSSEC=</varname> to
 166         true on systems where it is known that the DNS server supports
 167         DNSSEC correctly, and where software or trust anchor updates
 168         happen regularly. On other systems it is recommended to set
 169         <varname>DNSSEC=</varname> to
 170         <literal>allow-downgrade</literal>.</para>
 171
 172         <para>In addition to this global DNSSEC setting
 173         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 174         also maintains per-link DNSSEC settings. For system DNS
 175         servers (see above), only the global DNSSEC setting is in
 176         effect. For per-link DNS servers the per-link
 177         setting is in effect, unless it is unset in which case the
 178         global setting is used instead.</para>
 179
 180         <para>Site-private DNS zones generally conflict with DNSSEC
 181         operation, unless a negative (if the private zone is not
 182         signed) or positive (if the private zone is signed) trust
 183         anchor is configured for them. If
 184         <literal>allow-downgrade</literal> mode is selected, it is
 185         attempted to detect site-private DNS zones using top-level
 186         domains (TLDs) that are not known by the DNS root server. This
 187         logic does not work in all private zone setups.</para>
 188
 189         <para>Defaults to <literal>allow-downgrade</literal></para>
 190         </listitem>
 191       </varlistentry>
 192
 193       <varlistentry>
 194         <term><varname>DNSOverTLS=</varname></term>
 195         <listitem>
 196         <para>Takes a boolean argument or <literal>opportunistic</literal>.
 197         If true all connections to the server will be encrypted. Note that
 198         this mode requires a DNS server that supports DNS-over-TLS and has
 199         a valid certificate for it's IP. If the DNS server does not support
 200         DNS-over-TLS all DNS requests will fail. When set to <literal>opportunistic</literal>
 201         DNS request are attempted to send encrypted with DNS-over-TLS.
 202         If the DNS server does not support TLS, DNS-over-TLS is disabled.
 203         Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
 204         attacks, where an attacker might be able to trigger a downgrade
 205         to non-encrypted mode by synthesizing a response that suggests
 206         DNS-over-TLS was not supported. If set to false, DNS lookups
 207         are send over UDP.</para>
 208
 209         <para>Note that DNS-over-TLS requires additional data to be
 210         send for setting up an encrypted connection, and thus results
 211         in a small DNS look-up time penalty.</para>
 212
 213         <para>Note as the resolver is not capable of authenticating
 214         the server, it is vulnerable for "man-in-the-middle" attacks.</para>
 215
 216         <para>In addition to this global DNSOverTLS setting
 217         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 218         also maintains per-link DNSOverTLS settings. For system DNS
 219         servers (see above), only the global DNSOverTLS setting is in
 220         effect. For per-link DNS servers the per-link
 221         setting is in effect, unless it is unset in which case the
 222         global setting is used instead.</para>
 223
 224         <para>Defaults to off.</para>
 225         </listitem>
 226       </varlistentry>
 227
 228       <varlistentry>
 229         <term><varname>Cache=</varname></term>
 230         <listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If <literal>yes</literal> (the default), resolving a domain name
 231         which already got queried earlier will return the previous result as long as it is still valid, and thus does
 232         not result in a new network request. Be aware that turning off caching comes at a performance penalty, which
 233         is particularly high when DNSSEC is used.</para>
 234         If <literal>no-negative</literal>, only positive answers are cached.
 235
 236         <para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address
 237         (such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.</para></listitem>
 238       </varlistentry>
 239
 240       <varlistentry>
 241         <term><varname>DNSStubListener=</varname></term>
 242         <listitem><para>Takes a boolean argument or one of <literal>udp</literal> and <literal>tcp</literal>. If
 243         <literal>udp</literal>, a DNS stub resolver will listen for UDP requests on address 127.0.0.53
 244         port 53. If <literal>tcp</literal>, the stub will listen for TCP requests on the same address and port. If
 245         <literal>yes</literal> (the default), the stub listens for both UDP and TCP requests.  If <literal>no</literal>, the stub
 246         listener is disabled.</para>
 247
 248         <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
 249         in use.</para></listitem>
 250       </varlistentry>
 251
 252       <varlistentry>
 253         <term><varname>ReadEtcHosts=</varname></term>
 254         <listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default), the DNS stub resolver will read
 255         <filename>/etc/hosts</filename>, and try to resolve hosts or address by using the entries in the file before
 256         sending query to DNS servers.</para></listitem>
 257       </varlistentry>
 258
 259     </variablelist>
 260   </refsect1>
 261
 262   <refsect1>
 263       <title>See Also</title>
 264       <para>
 265       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 266       <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 267       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 268       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 269       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 270       </para>
 271   </refsect1>
 272
 273 </refentry>