man/resolved.conf.xml

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   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This file is part of systemd.
   9
  10   Copyright 2014 Tom Gundersen
  11 -->
  12
  13 <refentry id="resolved.conf" conditional='ENABLE_RESOLVE'
  14     xmlns:xi="http://www.w3.org/2001/XInclude">
  15   <refentryinfo>
  16     <title>resolved.conf</title>
  17     <productname>systemd</productname>
  18
  19     <authorgroup>
  20       <author>
  21         <contrib>Developer</contrib>
  22         <firstname>Tom</firstname>
  23         <surname>Gundersen</surname>
  24         <email>teg@jklm.no</email>
  25       </author>
  26     </authorgroup>
  27   </refentryinfo>
  28
  29   <refmeta>
  30     <refentrytitle>resolved.conf</refentrytitle>
  31     <manvolnum>5</manvolnum>
  32   </refmeta>
  33
  34   <refnamediv>
  35     <refname>resolved.conf</refname>
  36     <refname>resolved.conf.d</refname>
  37     <refpurpose>Network Name Resolution configuration files</refpurpose>
  38   </refnamediv>
  39
  40   <refsynopsisdiv>
  41     <para><filename>/etc/systemd/resolved.conf</filename></para>
  42     <para><filename>/etc/systemd/resolved.conf.d/*.conf</filename></para>
  43     <para><filename>/run/systemd/resolved.conf.d/*.conf</filename></para>
  44     <para><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></para>
  45   </refsynopsisdiv>
  46
  47   <refsect1>
  48     <title>Description</title>
  49
  50     <para>These configuration files control local DNS and LLMNR
  51     name resolution.</para>
  52
  53   </refsect1>
  54
  55   <xi:include href="standard-conf.xml" xpointer="main-conf" />
  56
  57   <refsect1>
  58     <title>Options</title>
  59
  60     <para>The following options are available in the <literal>[Resolve]</literal> section:</para>
  61
  62     <variablelist class='network-directives'>
  63
  64       <varlistentry>
  65         <term><varname>DNS=</varname></term>
  66         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. DNS requests
  67         are sent to one of the listed DNS servers in parallel to suitable per-link DNS servers acquired from
  68         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
  69         set at runtime by external applications.  For compatibility reasons, if this setting is not specified, the DNS
  70         servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
  71         are configured in it. This setting defaults to the empty list.</para></listitem>
  72       </varlistentry>
  73
  74       <varlistentry>
  75         <term><varname>FallbackDNS=</varname></term>
  76         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Any
  77         per-link DNS servers obtained from
  78         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  79         take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
  80         <filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
  81         known. If this option is not given, a compiled-in list of DNS servers is used instead.</para></listitem>
  82       </varlistentry>
  83
  84       <varlistentry>
  85         <term><varname>Domains=</varname></term>
  86         <listitem><para>A space-separated list of domains. These domains are used as search suffixes when resolving
  87         single-label host names (domain names which contain no dot), in order to qualify them into fully-qualified
  88         domain names (FQDNs). Search domains are strictly processed in the order they are specified, until the name
  89         with the suffix appended is found. For compatibility reasons, if this setting is not specified, the search
  90         domains listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any domains
  91         are configured in it. This setting defaults to the empty list.</para>
  92
  93         <para>Specified domain names may optionally be prefixed with <literal>~</literal>. In this case they do not
  94         define a search path, but preferably direct DNS queries for the indicated domains to the DNS servers configured
  95         with the system <varname>DNS=</varname> setting (see above), in case additional, suitable per-link DNS servers
  96         are known. If no per-link DNS servers are known using the <literal>~</literal> syntax has no effect. Use the
  97         construct <literal>~.</literal> (which is composed of <literal>~</literal> to indicate a routing domain and
  98         <literal>.</literal> to indicate the DNS root domain that is the implied suffix of all DNS domains) to use the
  99         system DNS server defined with <varname>DNS=</varname> preferably for all domains.</para></listitem>
 100       </varlistentry>
 101
 102       <varlistentry>
 103         <term><varname>LLMNR=</varname></term>
 104         <listitem><para>Takes a boolean argument or
 105         <literal>resolve</literal>. Controls Link-Local Multicast Name
 106         Resolution support (<ulink
 107         url="https://tools.ietf.org/html/rfc4795">RFC 4794</ulink>) on
 108         the local host. If true, enables full LLMNR responder and
 109         resolver support. If false, disables both. If set to
 110         <literal>resolve</literal>, only resolution support is enabled,
 111         but responding is disabled. Note that
 112         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 113         also maintains per-link LLMNR settings. LLMNR will be
 114         enabled on a link only if the per-link and the
 115         global setting is on.</para></listitem>
 116       </varlistentry>
 117
 118       <varlistentry>
 119         <term><varname>MulticastDNS=</varname></term>
 120         <listitem><para>Takes a boolean argument or
 121         <literal>resolve</literal>. Controls Multicast DNS support (<ulink
 122         url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
 123         the local host. If true, enables full Multicast DNS responder and
 124         resolver support. If false, disables both. If set to
 125         <literal>resolve</literal>, only resolution support is enabled,
 126         but responding is disabled. Note that
 127         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 128         also maintains per-link Multicast DNS settings. Multicast DNS will be
 129         enabled on a link only if the per-link and the
 130         global setting is on.</para></listitem>
 131       </varlistentry>
 132
 133       <varlistentry>
 134         <term><varname>DNSSEC=</varname></term>
 135         <listitem><para>Takes a boolean argument or
 136         <literal>allow-downgrade</literal>. If true all DNS lookups are
 137         DNSSEC-validated locally (excluding LLMNR and Multicast
 138         DNS). If the response to a lookup request is detected to be invalid
 139         a lookup failure is returned to applications. Note that
 140         this mode requires a DNS server that supports DNSSEC. If the
 141         DNS server does not properly support DNSSEC all validations
 142         will fail. If set to <literal>allow-downgrade</literal> DNSSEC
 143         validation is attempted, but if the server does not support
 144         DNSSEC properly, DNSSEC mode is automatically disabled. Note
 145         that this mode makes DNSSEC validation vulnerable to
 146         "downgrade" attacks, where an attacker might be able to
 147         trigger a downgrade to non-DNSSEC mode by synthesizing a DNS
 148         response that suggests DNSSEC was not supported. If set to
 149         false, DNS lookups are not DNSSEC validated.</para>
 150
 151         <para>Note that DNSSEC validation requires retrieval of
 152         additional DNS data, and thus results in a small DNS look-up
 153         time penalty.</para>
 154
 155         <para>DNSSEC requires knowledge of "trust anchors" to prove
 156         data integrity. The trust anchor for the Internet root domain
 157         is built into the resolver, additional trust anchors may be
 158         defined with
 159         <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 160         Trust anchors may change at regular intervals, and old trust
 161         anchors may be revoked. In such a case DNSSEC validation is
 162         not possible until new trust anchors are configured locally or
 163         the resolver software package is updated with the new root
 164         trust anchor. In effect, when the built-in trust anchor is
 165         revoked and <varname>DNSSEC=</varname> is true, all further
 166         lookups will fail, as it cannot be proved anymore whether
 167         lookups are correctly signed, or validly unsigned. If
 168         <varname>DNSSEC=</varname> is set to
 169         <literal>allow-downgrade</literal> the resolver will
 170         automatically turn off DNSSEC validation in such a case.</para>
 171
 172         <para>Client programs looking up DNS data will be informed
 173         whether lookups could be verified using DNSSEC, or whether the
 174         returned data could not be verified (either because the data
 175         was found unsigned in the DNS, or the DNS server did not
 176         support DNSSEC or no appropriate trust anchors were known). In
 177         the latter case it is assumed that client programs employ a
 178         secondary scheme to validate the returned DNS data, should
 179         this be required.</para>
 180
 181         <para>It is recommended to set <varname>DNSSEC=</varname> to
 182         true on systems where it is known that the DNS server supports
 183         DNSSEC correctly, and where software or trust anchor updates
 184         happen regularly. On other systems it is recommended to set
 185         <varname>DNSSEC=</varname> to
 186         <literal>allow-downgrade</literal>.</para>
 187
 188         <para>In addition to this global DNSSEC setting
 189         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 190         also maintains per-link DNSSEC settings. For system DNS
 191         servers (see above), only the global DNSSEC setting is in
 192         effect. For per-link DNS servers the per-link
 193         setting is in effect, unless it is unset in which case the
 194         global setting is used instead.</para>
 195
 196         <para>Site-private DNS zones generally conflict with DNSSEC
 197         operation, unless a negative (if the private zone is not
 198         signed) or positive (if the private zone is signed) trust
 199         anchor is configured for them. If
 200         <literal>allow-downgrade</literal> mode is selected, it is
 201         attempted to detect site-private DNS zones using top-level
 202         domains (TLDs) that are not known by the DNS root server. This
 203         logic does not work in all private zone setups.</para>
 204
 205         <para>Defaults to off.</para>
 206         </listitem>
 207       </varlistentry>
 208
 209       <varlistentry>
 210         <term><varname>DNSOverTLS=</varname></term>
 211         <listitem>
 212         <para>Takes false or
 213         <literal>opportunistic</literal>. When set to <literal>opportunistic</literal>
 214         DNS request are attempted to send encrypted with DNS-over-TLS.
 215         If the DNS server does not support TLS, DNS-over-TLS is disabled.
 216         Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
 217         attacks, where an attacker might be able to trigger a downgrade
 218         to non-encrypted mode by synthesizing a response that suggests
 219         DNS-over-TLS was not supported. If set to false, DNS lookups
 220         are send over UDP.</para>
 221
 222         <para>Note that DNS-over-TLS requires additional data to be
 223         send for setting up an encrypted connection, and thus results
 224         in a small DNS look-up time penalty.</para>
 225
 226         <para>Note as the resolver is not capable of authenticating
 227         the server, it is vulnerable for "man-in-the-middle" attacks.</para>
 228
 229         <para>In addition to this global DNSOverTLS setting
 230         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 231         also maintains per-link DNSOverTLS settings. For system DNS
 232         servers (see above), only the global DNSOverTLS setting is in
 233         effect. For per-link DNS servers the per-link
 234         setting is in effect, unless it is unset in which case the
 235         global setting is used instead.</para>
 236
 237         <para>Defaults to off.</para>
 238         </listitem>
 239       </varlistentry>
 240
 241       <varlistentry>
 242         <term><varname>Cache=</varname></term>
 243         <listitem><para>Takes a boolean argument. If "yes" (the default), resolving a domain name which already got
 244         queried earlier will return the previous result as long as it is still valid, and thus does not result in a new
 245         network request. Be aware that turning off caching comes at a performance penalty, which is particularly
 246         high when DNSSEC is used.</para>
 247
 248         <para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address
 249         (such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.</para></listitem>
 250       </varlistentry>
 251
 252       <varlistentry>
 253         <term><varname>DNSStubListener=</varname></term>
 254         <listitem><para>Takes a boolean argument or one of <literal>udp</literal> and <literal>tcp</literal>. If
 255         <literal>udp</literal> (the default), a DNS stub resolver will listen for UDP requests on address 127.0.0.53
 256         port 53. If <literal>tcp</literal>, the stub will listen for TCP requests on the same address and port. If
 257         <literal>yes</literal>, the stub listens for both UDP and TCP requests.  If <literal>no</literal>, the stub
 258         listener is disabled.</para>
 259
 260         <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
 261         in use.</para></listitem>
 262       </varlistentry>
 263
 264     </variablelist>
 265   </refsect1>
 266
 267   <refsect1>
 268       <title>See Also</title>
 269       <para>
 270       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 271       <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 272       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 273       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 274       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 275       </para>
 276   </refsect1>
 277
 278 </refentry>