man/resolved.conf.xml

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