man/resolved.conf.xml

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   8
   9 <refentry id="resolved.conf" conditional='ENABLE_RESOLVE'
  10     xmlns:xi="http://www.w3.org/2001/XInclude">
  11   <refentryinfo>
  12     <title>resolved.conf</title>
  13     <productname>systemd</productname>
  14   </refentryinfo>
  15
  16   <refmeta>
  17     <refentrytitle>resolved.conf</refentrytitle>
  18     <manvolnum>5</manvolnum>
  19   </refmeta>
  20
  21   <refnamediv>
  22     <refname>resolved.conf</refname>
  23     <refname>resolved.conf.d</refname>
  24     <refpurpose>Network Name Resolution configuration files</refpurpose>
  25   </refnamediv>
  26
  27   <refsynopsisdiv>
  28     <para><filename>/etc/systemd/resolved.conf</filename></para>
  29     <para><filename>/etc/systemd/resolved.conf.d/*.conf</filename></para>
  30     <para><filename>/run/systemd/resolved.conf.d/*.conf</filename></para>
  31     <para><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></para>
  32   </refsynopsisdiv>
  33
  34   <refsect1>
  35     <title>Description</title>
  36
  37     <para>These configuration files control local DNS and LLMNR
  38     name resolution.</para>
  39
  40   </refsect1>
  41
  42   <xi:include href="standard-conf.xml" xpointer="main-conf" />
  43
  44   <refsect1>
  45     <title>Options</title>
  46
  47     <para>The following options are available in the [Resolve] section:</para>
  48
  49     <variablelist class='network-directives'>
  50
  51       <varlistentry>
  52         <term><varname>DNS=</varname></term>
  53         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. Each address can
  54         optionally take a port number separated with <literal>:</literal>, a network interface name or index separated with
  55         <literal>%</literal>, and a Server Name Indication (SNI) separated with <literal>#</literal>. When IPv6 address is
  56         specified with a port number, then the address must be in the square brackets. That is, the acceptable full formats
  57         are <literal>111.222.333.444:9953%ifname#example.com</literal> for IPv4 and
  58         <literal>[1111:2222::3333]:9953%ifname#example.com</literal> for IPv6. DNS requests are sent to one of the listed
  59         DNS servers in parallel to suitable per-link DNS servers acquired from
  60         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
  61         set at runtime by external applications.  For compatibility reasons, if this setting is not specified, the DNS
  62         servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
  63         are configured in it. This setting defaults to the empty list.</para>
  64
  65         <xi:include href="version-info.xml" xpointer="v213"/></listitem>
  66       </varlistentry>
  67
  68       <varlistentry>
  69         <term><varname>FallbackDNS=</varname></term>
  70         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Please see
  71         <varname>DNS=</varname> for acceptable format of addresses. Any per-link DNS servers obtained from
  72         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  73         take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
  74         <filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
  75         known. If this option is not given, a compiled-in list of DNS servers is used instead.</para>
  76
  77         <xi:include href="version-info.xml" xpointer="v216"/></listitem>
  78       </varlistentry>
  79
  80       <varlistentry>
  81         <term><varname>Domains=</varname></term>
  82         <listitem><para>A space-separated list of domains, optionally prefixed with <literal>~</literal>,
  83         used for two distinct purposes described below. Defaults to the empty list.</para>
  84
  85         <para>Any domains <emphasis>not</emphasis> prefixed with <literal>~</literal> are used as search
  86         suffixes when resolving single-label hostnames (domain names which contain no dot), in order to
  87         qualify them into fully-qualified domain names (FQDNs). These "search domains" are strictly processed
  88         in the order they are specified in, until the name with the suffix appended is found. For
  89         compatibility reasons, if this setting is not specified, the search domains listed in
  90         <filename>/etc/resolv.conf</filename> with the <varname>search</varname> keyword are used instead, if
  91         that file exists and any domains are configured in it.</para>
  92
  93         <para>The domains prefixed with <literal>~</literal> are called "route-only domains". All domains
  94         listed here (<emphasis>both search domains and route-only domains</emphasis> after removing the
  95         <literal>~</literal> prefix) define a search path that preferably directs DNS queries to this
  96         interface. This search path has an effect only when suitable per-link DNS servers are known. Such
  97         servers may be defined through the <varname>DNS=</varname> setting (see above) and dynamically at run
  98         time, for example from DHCP leases. If no per-link DNS servers are known, route-only domains have no
  99         effect.</para>
 100
 101         <para>Use the construct <literal>~.</literal> (which is composed from <literal>~</literal> to
 102         indicate a route-only domain and <literal>.</literal> to indicate the DNS root domain that is the
 103         implied suffix of all DNS domains) to use the DNS servers defined for this link preferably for all
 104         domains.</para>
 105
 106         <para>See "Protocols and Routing" in
 107         <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 108         for details of how search and route-only domains are used.</para>
 109
 110         <para>Note that configuring the MulticastDNS domain <literal>local</literal> as search or routing
 111         domain has the effect of routing lookups for this domain to classic unicast DNS. This may be used to
 112         provide compatibility with legacy installations that use this domain in a unicast DNS context,
 113         against the IANA assignment of this domain to pure MulticastDNS purposes. Search and routing domains
 114         are a unicast DNS concept, they <emphasis>cannot</emphasis> be used to resolve single-label lookups
 115         via MulticastDNS.</para>
 116
 117         <xi:include href="version-info.xml" xpointer="v229"/>
 118         </listitem>
 119       </varlistentry>
 120
 121       <varlistentry>
 122         <term><varname>LLMNR=</varname></term>
 123         <listitem><para>Takes a boolean argument or
 124         <literal>resolve</literal>. Controls Link-Local Multicast Name
 125         Resolution support (<ulink
 126         url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
 127         the local host. If true, enables full LLMNR responder and
 128         resolver support. If false, disables both. If set to
 129         <literal>resolve</literal>, only resolution support is enabled,
 130         but responding is disabled. Note that
 131         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 132         also maintains per-link LLMNR settings. LLMNR will be
 133         enabled on a link only if the per-link and the
 134         global setting is on.</para>
 135
 136         <xi:include href="version-info.xml" xpointer="v216"/></listitem>
 137       </varlistentry>
 138
 139       <varlistentry>
 140         <term><varname>MulticastDNS=</varname></term>
 141         <listitem><para>Takes a boolean argument or
 142         <literal>resolve</literal>. Controls Multicast DNS support (<ulink
 143         url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
 144         the local host. If true, enables full Multicast DNS responder and
 145         resolver support. If false, disables both. If set to
 146         <literal>resolve</literal>, only resolution support is enabled,
 147         but responding is disabled. Note that
 148         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 149         also maintains per-link Multicast DNS settings. Multicast DNS will be
 150         enabled on a link only if the per-link and the
 151         global setting is on.</para>
 152
 153         <xi:include href="version-info.xml" xpointer="v234"/></listitem>
 154       </varlistentry>
 155
 156       <varlistentry>
 157         <term><varname>DNSSEC=</varname></term>
 158         <listitem><para>Takes a boolean argument or <literal>allow-downgrade</literal>.</para>
 159
 160         <para>If set to true, all DNS lookups are DNSSEC-validated locally (excluding LLMNR and Multicast
 161         DNS). If the response to a lookup request is detected to be invalid a lookup failure is returned to
 162         applications. Note that this mode requires a DNS server that supports DNSSEC. If the DNS server does
 163         not properly support DNSSEC all validations will fail.</para>
 164
 165         <para>If set to <literal>allow-downgrade</literal>, DNSSEC validation is attempted, but if the server
 166         does not support DNSSEC properly, DNSSEC mode is automatically disabled. Note that this mode makes
 167         DNSSEC validation vulnerable to "downgrade" attacks, where an attacker might be able to trigger a
 168         downgrade to non-DNSSEC mode by synthesizing a DNS response that suggests DNSSEC was not
 169         supported.</para>
 170
 171         <para>If set to false, DNS lookups are not DNSSEC validated. In this mode, or when set to
 172         <literal>allow-downgrade</literal> and the downgrade has happened, the resolver becomes
 173         security-unaware and all forwarded queries have DNSSEC OK (DO) bit unset.</para>
 174
 175         <para>Note that DNSSEC validation requires retrieval of additional DNS data, and thus results in a
 176         small DNS lookup time penalty.</para>
 177
 178         <para>DNSSEC requires knowledge of "trust anchors" to prove
 179         data integrity. The trust anchor for the Internet root domain
 180         is built into the resolver, additional trust anchors may be
 181         defined with
 182         <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 183         Trust anchors may change at regular intervals, and old trust
 184         anchors may be revoked. In such a case DNSSEC validation is
 185         not possible until new trust anchors are configured locally or
 186         the resolver software package is updated with the new root
 187         trust anchor. In effect, when the built-in trust anchor is
 188         revoked and <varname>DNSSEC=</varname> is true, all further
 189         lookups will fail, as it cannot be proved anymore whether
 190         lookups are correctly signed, or validly unsigned. If
 191         <varname>DNSSEC=</varname> is set to
 192         <literal>allow-downgrade</literal> the resolver will
 193         automatically turn off DNSSEC validation in such a case.</para>
 194
 195         <para>Client programs looking up DNS data will be informed
 196         whether lookups could be verified using DNSSEC, or whether the
 197         returned data could not be verified (either because the data
 198         was found unsigned in the DNS, or the DNS server did not
 199         support DNSSEC or no appropriate trust anchors were known). In
 200         the latter case it is assumed that client programs employ a
 201         secondary scheme to validate the returned DNS data, should
 202         this be required.</para>
 203
 204         <para>It is recommended to set <varname>DNSSEC=</varname> to
 205         true on systems where it is known that the DNS server supports
 206         DNSSEC correctly, and where software or trust anchor updates
 207         happen regularly. On other systems it is recommended to set
 208         <varname>DNSSEC=</varname> to
 209         <literal>allow-downgrade</literal>.</para>
 210
 211         <para>In addition to this global DNSSEC setting
 212         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 213         also maintains per-link DNSSEC settings. For system DNS
 214         servers (see above), only the global DNSSEC setting is in
 215         effect. For per-link DNS servers the per-link
 216         setting is in effect, unless it is unset in which case the
 217         global setting is used instead.</para>
 218
 219         <para>Site-private DNS zones generally conflict with DNSSEC
 220         operation, unless a negative (if the private zone is not
 221         signed) or positive (if the private zone is signed) trust
 222         anchor is configured for them. If
 223         <literal>allow-downgrade</literal> mode is selected, it is
 224         attempted to detect site-private DNS zones using top-level
 225         domains (TLDs) that are not known by the DNS root server. This
 226         logic does not work in all private zone setups.</para>
 227
 228         <para>Defaults to <literal>&DEFAULT_DNSSEC_MODE;</literal>.</para>
 229
 230         <xi:include href="version-info.xml" xpointer="v229"/>
 231         </listitem>
 232       </varlistentry>
 233
 234       <varlistentry>
 235         <term><varname>DNSOverTLS=</varname></term>
 236         <listitem>
 237         <para>Takes a boolean argument or <literal>opportunistic</literal>. If
 238         true all connections to the server will be encrypted. Note that this
 239         mode requires a DNS server that supports DNS-over-TLS and has a valid
 240         certificate. If the hostname was specified in <varname>DNS=</varname>
 241         by using the format <literal>address#server_name</literal> it
 242         is used to validate its certificate and also to enable Server Name
 243         Indication (SNI) when opening a TLS connection. Otherwise
 244         the certificate is checked against the server's IP.
 245         If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>
 246
 247         <para>When set to <literal>opportunistic</literal>
 248         DNS request are attempted to send encrypted with DNS-over-TLS.
 249         If the DNS server does not support TLS, DNS-over-TLS is disabled.
 250         Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
 251         attacks, where an attacker might be able to trigger a downgrade
 252         to non-encrypted mode by synthesizing a response that suggests
 253         DNS-over-TLS was not supported. If set to false, DNS lookups
 254         are send over UDP.</para>
 255
 256         <para>Note that DNS-over-TLS requires additional data to be
 257         send for setting up an encrypted connection, and thus results
 258         in a small DNS look-up time penalty.</para>
 259
 260         <para>Note that in <literal>opportunistic</literal> mode the
 261         resolver is not capable of authenticating the server, so it is
 262         vulnerable to "man-in-the-middle" attacks.</para>
 263
 264         <para>In addition to this global <varname>DNSOverTLS=</varname> setting
 265         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 266         also maintains per-link <varname>DNSOverTLS=</varname> settings. For system DNS servers (see above), only the global
 267         <varname>DNSOverTLS=</varname> setting is in effect. For per-link DNS servers the per-link setting is in effect, unless
 268         it is unset in which case the global setting is used instead.</para>
 269
 270         <para>Defaults to <literal>&DEFAULT_DNS_OVER_TLS_MODE;</literal>.</para>
 271
 272         <xi:include href="version-info.xml" xpointer="v239"/>
 273         </listitem>
 274       </varlistentry>
 275
 276       <varlistentry>
 277         <term><varname>Cache=</varname></term>
 278         <listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If
 279         <literal>yes</literal> (the default), resolving a domain name which already got queried earlier will
 280         return the previous result as long as it is still valid, and thus does not result in a new network
 281         request. Be aware that turning off caching comes at a performance penalty, which is particularly high
 282         when DNSSEC is used. If <literal>no-negative</literal>, only positive answers are cached.</para>
 283
 284         <para>Note that caching is turned off by default for host-local DNS servers.
 285         See <varname>CacheFromLocalhost=</varname> for details.</para>
 286
 287         <xi:include href="version-info.xml" xpointer="v231"/></listitem>
 288       </varlistentry>
 289
 290       <varlistentry>
 291         <term><varname>CacheFromLocalhost=</varname></term>
 292         <listitem><para>Takes a boolean as argument. If <literal>no</literal> (the default), and response cames from
 293         host-local IP address (such as 127.0.0.1 or ::1), the result wouldn't be cached in order to avoid
 294         potential duplicate local caching.</para>
 295
 296         <xi:include href="version-info.xml" xpointer="v248"/>
 297         </listitem>
 298       </varlistentry>
 299
 300       <varlistentry>
 301         <term><varname>DNSStubListener=</varname></term>
 302         <listitem><para>Takes a boolean argument or one of <literal>udp</literal> and
 303         <literal>tcp</literal>. If <literal>udp</literal>, a DNS stub resolver will listen for UDP requests
 304         on addresses 127.0.0.53 and 127.0.0.54, port 53. If <literal>tcp</literal>, the stub will listen for
 305         TCP requests on the same addresses and port. If <literal>yes</literal> (the default), the stub listens
 306         for both UDP and TCP requests. If <literal>no</literal>, the stub listener is disabled.</para>
 307
 308         <xi:include href="systemd-resolved.service.xml" xpointer="proxy-stub" />
 309
 310         <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
 311         in use.</para>
 312
 313         <xi:include href="version-info.xml" xpointer="v232"/></listitem>
 314       </varlistentry>
 315
 316       <varlistentry>
 317         <term><varname>DNSStubListenerExtra=</varname></term>
 318         <listitem><para>Takes an IPv4 or IPv6 address to listen on. The address may be optionally
 319         prefixed with a protocol name (<literal>udp</literal> or <literal>tcp</literal>) separated with
 320         <literal>:</literal>. If the protocol is not specified, the service will listen on both UDP and
 321         TCP. It may be also optionally suffixed by a numeric port number with separator
 322         <literal>:</literal>. When an IPv6 address is specified with a port number, then the address
 323         must be in the square brackets. If the port is not specified, then the service uses port 53.
 324         Note that this is independent of the primary DNS stub configured with
 325         <varname>DNSStubListener=</varname>, and only configures <emphasis>additional</emphasis>
 326         sockets to listen on. This option can be specified multiple times. If an empty string is
 327         assigned, then the all previous assignments are cleared. Defaults to unset.</para>
 328
 329         <para>Examples:
 330         <programlisting>DNSStubListenerExtra=192.168.10.10
 331 DNSStubListenerExtra=2001:db8:0:f102::10
 332 DNSStubListenerExtra=192.168.10.11:9953
 333 DNSStubListenerExtra=[2001:db8:0:f102::11]:9953
 334 DNSStubListenerExtra=tcp:192.168.10.12
 335 DNSStubListenerExtra=udp:2001:db8:0:f102::12
 336 DNSStubListenerExtra=tcp:192.168.10.13:9953
 337 DNSStubListenerExtra=udp:[2001:db8:0:f102::13]:9953</programlisting>
 338         </para>
 339
 340         <xi:include href="version-info.xml" xpointer="v247"/></listitem>
 341       </varlistentry>
 342
 343       <varlistentry>
 344         <term><varname>ReadEtcHosts=</varname></term>
 345         <listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default),
 346         <command>systemd-resolved</command> will read <filename>/etc/hosts</filename>, and try to resolve
 347         hosts or address by using the entries in the file before sending query to DNS servers.
 348         </para>
 349
 350         <xi:include href="version-info.xml" xpointer="v240"/></listitem>
 351       </varlistentry>
 352
 353       <varlistentry>
 354         <term><varname>ResolveUnicastSingleLabel=</varname></term>
 355         <listitem><para>Takes a boolean argument. When false (the default),
 356         <command>systemd-resolved</command> will not resolve A and AAAA queries for single-label names over
 357         classic DNS. Note that such names may still be resolved if search domains are specified (see
 358         <varname>Domains=</varname> above), or using other mechanisms, in particular via LLMNR or from
 359         <filename>/etc/hosts</filename>. When true, queries for single-label names will be forwarded to
 360         global DNS servers even if no search domains are defined.
 361         </para>
 362
 363         <para>This option is provided for compatibility with configurations where <emphasis>public DNS
 364         servers are not used</emphasis>. Forwarding single-label names to servers not under your control is
 365         not standard-conformant, see <ulink
 366         url="https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/">IAB
 367         Statement</ulink>, and may create a privacy and security risk.</para>
 368
 369         <xi:include href="version-info.xml" xpointer="v246"/></listitem>
 370       </varlistentry>
 371       <varlistentry>
 372         <term>StaleRetentionSec=<replaceable>SECONDS</replaceable></term>
 373         <listitem><para>Takes a duration value, which determines the length of time DNS resource records can
 374         be retained in the cache beyond their Time To Live (TTL). This allows these records to be returned as
 375         stale records. By default, this value is set to zero, meaning that DNS resource records are not
 376         stored in the cache after their TTL expires.</para>
 377
 378         <para>This is useful when a DNS server failure occurs or becomes unreachable. In such cases,
 379         <citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 380         continues to use the stale records to answer DNS queries, particularly when no valid response can be
 381         obtained from the upstream DNS servers. However, this doesn't apply to NXDOMAIN responses, as those
 382         are still perfectly valid responses. This feature enhances resilience against DNS infrastructure
 383         failures and outages.</para>
 384
 385         <para><command>systemd-resolved</command> always attempts to reach the upstream DNS servers first,
 386         before providing the client application with any stale data. If this feature is enabled, cache will
 387         not be flushed when changing servers.</para>
 388
 389         <xi:include href="version-info.xml" xpointer="v254"/>
 390         </listitem>
 391       </varlistentry>
 392     </variablelist>
 393   </refsect1>
 394
 395   <refsect1>
 396       <title>See Also</title>
 397       <para>
 398       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 399       <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 400       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 401       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 402       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 403       </para>
 404   </refsect1>
 405
 406 </refentry>