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><simplelist>
  29       <member><filename>/etc/systemd/resolved.conf</filename></member>
  30       <member><filename>/etc/systemd/resolved.conf.d/*.conf</filename></member>
  31       <member><filename>/run/systemd/resolved.conf.d/*.conf</filename></member>
  32       <member><filename>/usr/lib/systemd/resolved.conf.d/*.conf</filename></member>
  33     </simplelist></para>
  34   </refsynopsisdiv>
  35
  36   <refsect1>
  37     <title>Description</title>
  38
  39     <para>These configuration files control local DNS and LLMNR
  40     name resolution.</para>
  41
  42   </refsect1>
  43
  44   <xi:include href="standard-conf.xml" xpointer="main-conf" />
  45
  46   <refsect1>
  47     <title>Options</title>
  48
  49     <para>The following options are available in the [Resolve] section:</para>
  50
  51     <variablelist class='network-directives'>
  52
  53       <varlistentry>
  54         <term><varname>DNS=</varname></term>
  55         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as system DNS servers. Each address can
  56         optionally take a port number separated with <literal>:</literal>, a network interface name or index separated with
  57         <literal>%</literal>, and a Server Name Indication (SNI) separated with <literal>#</literal>. When IPv6 address is
  58         specified with a port number, then the address must be in the square brackets. That is, the acceptable full formats
  59         are <literal>111.222.333.444:9953%ifname#example.com</literal> for IPv4 and
  60         <literal>[1111:2222::3333]:9953%ifname#example.com</literal> for IPv6. DNS requests are sent to one of the listed
  61         DNS servers in parallel to suitable per-link DNS servers acquired from
  62         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
  63         set at runtime by external applications.  For compatibility reasons, if this setting is not specified, the DNS
  64         servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
  65         are configured in it. This setting defaults to the empty list.</para>
  66
  67         <xi:include href="version-info.xml" xpointer="v213"/></listitem>
  68       </varlistentry>
  69
  70       <varlistentry>
  71         <term><varname>FallbackDNS=</varname></term>
  72         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Please see
  73         <varname>DNS=</varname> for acceptable format of addresses. Any per-link DNS servers obtained from
  74         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  75         take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
  76         <filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
  77         known. If this option is not given, a compiled-in list of DNS servers is used instead.</para>
  78
  79         <xi:include href="version-info.xml" xpointer="v216"/></listitem>
  80       </varlistentry>
  81
  82       <varlistentry>
  83         <term><varname>Domains=</varname></term>
  84         <listitem><para>A space-separated list of domains, optionally prefixed with <literal>~</literal>,
  85         used for two distinct purposes described below. Defaults to the empty list.</para>
  86
  87         <para>Any domains <emphasis>not</emphasis> prefixed with <literal>~</literal> are used as search
  88         suffixes when resolving single-label hostnames (domain names which contain no dot), in order to
  89         qualify them into fully-qualified domain names (FQDNs). These "search domains" are strictly processed
  90         in the order they are specified in, until the name with the suffix appended is found. For
  91         compatibility reasons, if this setting is not specified, the search domains listed in
  92         <filename>/etc/resolv.conf</filename> with the <varname>search</varname> keyword are used instead, if
  93         that file exists and any domains are configured in it.</para>
  94
  95         <para>The domains prefixed with <literal>~</literal> are called "route-only domains". All domains
  96         listed here (<emphasis>both search domains and route-only domains</emphasis> after removing the
  97         <literal>~</literal> prefix) define a search path that preferably directs DNS queries to this
  98         interface. This search path has an effect only when suitable per-link DNS servers are known. Such
  99         servers may be defined through the <varname>DNS=</varname> setting (see above) and dynamically at run
 100         time, for example from DHCP leases. If no per-link DNS servers are known, route-only domains have no
 101         effect.</para>
 102
 103         <para>Use the construct <literal>~.</literal> (which is composed from <literal>~</literal> to
 104         indicate a route-only domain and <literal>.</literal> to indicate the DNS root domain that is the
 105         implied suffix of all DNS domains) to use the DNS servers defined for this link preferably for all
 106         domains.</para>
 107
 108         <para>See "Protocols and Routing" in
 109         <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 110         for details of how search and route-only domains are used.</para>
 111
 112         <para>Note that configuring the MulticastDNS domain <literal>local</literal> as search or routing
 113         domain has the effect of routing lookups for this domain to classic unicast DNS. This may be used to
 114         provide compatibility with legacy installations that use this domain in a unicast DNS context,
 115         against the IANA assignment of this domain to pure MulticastDNS purposes. Search and routing domains
 116         are a unicast DNS concept, they <emphasis>cannot</emphasis> be used to resolve single-label lookups
 117         via MulticastDNS.</para>
 118
 119         <xi:include href="version-info.xml" xpointer="v229"/>
 120         </listitem>
 121       </varlistentry>
 122
 123       <varlistentry>
 124         <term><varname>LLMNR=</varname></term>
 125         <listitem><para>Takes a boolean argument or
 126         <literal>resolve</literal>. Controls Link-Local Multicast Name
 127         Resolution support (<ulink
 128         url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
 129         the local host. If true, enables full LLMNR responder and
 130         resolver support. If false, disables both. If set to
 131         <literal>resolve</literal>, only resolution support is enabled,
 132         but responding is disabled. Note that
 133         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 134         also maintains per-link LLMNR settings. LLMNR will be
 135         enabled on a link only if the per-link and the
 136         global setting is on.</para>
 137
 138         <xi:include href="version-info.xml" xpointer="v216"/></listitem>
 139       </varlistentry>
 140
 141       <varlistentry>
 142         <term><varname>MulticastDNS=</varname></term>
 143         <listitem><para>Takes a boolean argument or
 144         <literal>resolve</literal>. Controls Multicast DNS support (<ulink
 145         url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
 146         the local host. If true, enables full Multicast DNS responder and
 147         resolver support. If false, disables both. If set to
 148         <literal>resolve</literal>, only resolution support is enabled,
 149         but responding is disabled. Note that
 150         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 151         also maintains per-link Multicast DNS settings. Multicast DNS will be
 152         enabled on a link only if the per-link and the
 153         global setting is on.</para>
 154
 155         <xi:include href="version-info.xml" xpointer="v234"/></listitem>
 156       </varlistentry>
 157
 158       <varlistentry>
 159         <term><varname>DNSSEC=</varname></term>
 160         <listitem><para>Takes a boolean argument or <literal>allow-downgrade</literal>.</para>
 161
 162         <para>If set to true, all DNS lookups are DNSSEC-validated locally (excluding LLMNR and Multicast
 163         DNS). If the response to a lookup request is detected to be invalid a lookup failure is returned to
 164         applications. Note that this mode requires a DNS server that supports DNSSEC. If the DNS server does
 165         not properly support DNSSEC all validations will fail.</para>
 166
 167         <para>If set to <literal>allow-downgrade</literal>, DNSSEC validation is attempted, but if the server
 168         does not support DNSSEC properly, DNSSEC mode is automatically disabled. Note that this mode makes
 169         DNSSEC validation vulnerable to "downgrade" attacks, where an attacker might be able to trigger a
 170         downgrade to non-DNSSEC mode by synthesizing a DNS response that suggests DNSSEC was not
 171         supported.</para>
 172
 173         <para>If set to false, DNS lookups are not DNSSEC validated. In this mode, or when set to
 174         <literal>allow-downgrade</literal> and the downgrade has happened, the resolver becomes
 175         security-unaware and all forwarded queries have DNSSEC OK (DO) bit unset.</para>
 176
 177         <para>Note that DNSSEC validation requires retrieval of additional DNS data, and thus results in a
 178         small DNS lookup time penalty.</para>
 179
 180         <para>DNSSEC requires knowledge of "trust anchors" to prove
 181         data integrity. The trust anchor for the Internet root domain
 182         is built into the resolver, additional trust anchors may be
 183         defined with
 184         <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 185         Trust anchors may change at regular intervals, and old trust
 186         anchors may be revoked. In such a case DNSSEC validation is
 187         not possible until new trust anchors are configured locally or
 188         the resolver software package is updated with the new root
 189         trust anchor. In effect, when the built-in trust anchor is
 190         revoked and <varname>DNSSEC=</varname> is true, all further
 191         lookups will fail, as it cannot be proved anymore whether
 192         lookups are correctly signed, or validly unsigned. If
 193         <varname>DNSSEC=</varname> is set to
 194         <literal>allow-downgrade</literal> the resolver will
 195         automatically turn off DNSSEC validation in such a case.</para>
 196
 197         <para>Client programs looking up DNS data will be informed
 198         whether lookups could be verified using DNSSEC, or whether the
 199         returned data could not be verified (either because the data
 200         was found unsigned in the DNS, or the DNS server did not
 201         support DNSSEC or no appropriate trust anchors were known). In
 202         the latter case it is assumed that client programs employ a
 203         secondary scheme to validate the returned DNS data, should
 204         this be required.</para>
 205
 206         <para>It is recommended to set <varname>DNSSEC=</varname> to
 207         true on systems where it is known that the DNS server supports
 208         DNSSEC correctly, and where software or trust anchor updates
 209         happen regularly. On other systems it is recommended to set
 210         <varname>DNSSEC=</varname> to
 211         <literal>allow-downgrade</literal>.</para>
 212
 213         <para>In addition to this global DNSSEC setting
 214         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 215         also maintains per-link DNSSEC settings. For system DNS
 216         servers (see above), only the global DNSSEC setting is in
 217         effect. For per-link DNS servers the per-link
 218         setting is in effect, unless it is unset in which case the
 219         global setting is used instead.</para>
 220
 221         <para>Site-private DNS zones generally conflict with DNSSEC
 222         operation, unless a negative (if the private zone is not
 223         signed) or positive (if the private zone is signed) trust
 224         anchor is configured for them. If
 225         <literal>allow-downgrade</literal> mode is selected, it is
 226         attempted to detect site-private DNS zones using top-level
 227         domains (TLDs) that are not known by the DNS root server. This
 228         logic does not work in all private zone setups.</para>
 229
 230         <para>Defaults to <literal>&DEFAULT_DNSSEC_MODE;</literal>.</para>
 231
 232         <xi:include href="version-info.xml" xpointer="v229"/>
 233         </listitem>
 234       </varlistentry>
 235
 236       <varlistentry>
 237         <term><varname>DNSOverTLS=</varname></term>
 238         <listitem>
 239         <para>Takes a boolean argument or <literal>opportunistic</literal>. If
 240         true all connections to the server will be encrypted. Note that this
 241         mode requires a DNS server that supports DNS-over-TLS and has a valid
 242         certificate. If the hostname was specified in <varname>DNS=</varname>
 243         by using the format <literal>address#server_name</literal> it
 244         is used to validate its certificate and also to enable Server Name
 245         Indication (SNI) when opening a TLS connection. Otherwise
 246         the certificate is checked against the server's IP.
 247         If the DNS server does not support DNS-over-TLS all DNS requests will fail.</para>
 248
 249         <para>When set to <literal>opportunistic</literal>
 250         DNS request are attempted to send encrypted with DNS-over-TLS.
 251         If the DNS server does not support TLS, DNS-over-TLS is disabled.
 252         Note that this mode makes DNS-over-TLS vulnerable to "downgrade"
 253         attacks, where an attacker might be able to trigger a downgrade
 254         to non-encrypted mode by synthesizing a response that suggests
 255         DNS-over-TLS was not supported. If set to false, DNS lookups
 256         are send over UDP.</para>
 257
 258         <para>Note that DNS-over-TLS requires additional data to be
 259         send for setting up an encrypted connection, and thus results
 260         in a small DNS look-up time penalty.</para>
 261
 262         <para>Note that in <literal>opportunistic</literal> mode the
 263         resolver is not capable of authenticating the server, so it is
 264         vulnerable to "man-in-the-middle" attacks.</para>
 265
 266         <para>In addition to this global <varname>DNSOverTLS=</varname> setting
 267         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 268         also maintains per-link <varname>DNSOverTLS=</varname> settings. For system DNS servers (see above), only the global
 269         <varname>DNSOverTLS=</varname> setting is in effect. For per-link DNS servers the per-link setting is in effect, unless
 270         it is unset in which case the global setting is used instead.</para>
 271
 272         <para>Defaults to <literal>&DEFAULT_DNS_OVER_TLS_MODE;</literal>.</para>
 273
 274         <xi:include href="version-info.xml" xpointer="v239"/>
 275         </listitem>
 276       </varlistentry>
 277
 278       <varlistentry>
 279         <term><varname>Cache=</varname></term>
 280         <listitem><para>Takes a boolean or <literal>no-negative</literal> as argument. If
 281         <literal>yes</literal> (the default), resolving a domain name which already got queried earlier will
 282         return the previous result as long as it is still valid, and thus does not result in a new network
 283         request. Be aware that turning off caching comes at a performance penalty, which is particularly high
 284         when DNSSEC is used. If <literal>no-negative</literal>, only positive answers are cached.</para>
 285
 286         <para>Note that caching is turned off by default for host-local DNS servers.
 287         See <varname>CacheFromLocalhost=</varname> for details.</para>
 288
 289         <xi:include href="version-info.xml" xpointer="v231"/></listitem>
 290       </varlistentry>
 291
 292       <varlistentry>
 293         <term><varname>CacheFromLocalhost=</varname></term>
 294         <listitem><para>Takes a boolean as argument. If <literal>no</literal> (the default), and response cames from
 295         host-local IP address (such as 127.0.0.1 or ::1), the result wouldn't be cached in order to avoid
 296         potential duplicate local caching.</para>
 297
 298         <xi:include href="version-info.xml" xpointer="v248"/>
 299         </listitem>
 300       </varlistentry>
 301
 302       <varlistentry>
 303         <term><varname>DNSStubListener=</varname></term>
 304         <listitem><para>Takes a boolean argument or one of <literal>udp</literal> and
 305         <literal>tcp</literal>. If <literal>udp</literal>, a DNS stub resolver will listen for UDP requests
 306         on addresses 127.0.0.53 and 127.0.0.54, port 53. If <literal>tcp</literal>, the stub will listen for
 307         TCP requests on the same addresses and port. If <literal>yes</literal> (the default), the stub listens
 308         for both UDP and TCP requests. If <literal>no</literal>, the stub listener is disabled.</para>
 309
 310         <xi:include href="systemd-resolved.service.xml" xpointer="proxy-stub" />
 311
 312         <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
 313         in use.</para>
 314
 315         <xi:include href="version-info.xml" xpointer="v232"/></listitem>
 316       </varlistentry>
 317
 318       <varlistentry>
 319         <term><varname>DNSStubListenerExtra=</varname></term>
 320         <listitem><para>Takes an IPv4 or IPv6 address to listen on. The address may be optionally
 321         prefixed with a protocol name (<literal>udp</literal> or <literal>tcp</literal>) separated with
 322         <literal>:</literal>. If the protocol is not specified, the service will listen on both UDP and
 323         TCP. It may be also optionally suffixed by a numeric port number with separator
 324         <literal>:</literal>. When an IPv6 address is specified with a port number, then the address
 325         must be in the square brackets. If the port is not specified, then the service uses port 53.
 326         Note that this is independent of the primary DNS stub configured with
 327         <varname>DNSStubListener=</varname>, and only configures <emphasis>additional</emphasis>
 328         sockets to listen on. This option can be specified multiple times. If an empty string is
 329         assigned, then the all previous assignments are cleared. Defaults to unset.</para>
 330
 331         <para>Examples:
 332         <programlisting>DNSStubListenerExtra=192.168.10.10
 333 DNSStubListenerExtra=2001:db8:0:f102::10
 334 DNSStubListenerExtra=192.168.10.11:9953
 335 DNSStubListenerExtra=[2001:db8:0:f102::11]:9953
 336 DNSStubListenerExtra=tcp:192.168.10.12
 337 DNSStubListenerExtra=udp:2001:db8:0:f102::12
 338 DNSStubListenerExtra=tcp:192.168.10.13:9953
 339 DNSStubListenerExtra=udp:[2001:db8:0:f102::13]:9953</programlisting>
 340         </para>
 341
 342         <xi:include href="version-info.xml" xpointer="v247"/></listitem>
 343       </varlistentry>
 344
 345       <varlistentry>
 346         <term><varname>ReadEtcHosts=</varname></term>
 347         <listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default),
 348         <command>systemd-resolved</command> will read <filename>/etc/hosts</filename>, and try to resolve
 349         hosts or address by using the entries in the file before sending query to DNS servers.
 350         </para>
 351
 352         <xi:include href="version-info.xml" xpointer="v240"/></listitem>
 353       </varlistentry>
 354
 355       <varlistentry>
 356         <term><varname>ResolveUnicastSingleLabel=</varname></term>
 357         <listitem><para>Takes a boolean argument. When false (the default),
 358         <command>systemd-resolved</command> will not resolve A and AAAA queries for single-label names over
 359         classic DNS. Note that such names may still be resolved if search domains are specified (see
 360         <varname>Domains=</varname> above), or using other mechanisms, in particular via LLMNR or from
 361         <filename>/etc/hosts</filename>. When true, queries for single-label names will be forwarded to
 362         global DNS servers even if no search domains are defined.
 363         </para>
 364
 365         <para>This option is provided for compatibility with configurations where <emphasis>public DNS
 366         servers are not used</emphasis>. Forwarding single-label names to servers not under your control is
 367         not standard-conformant, see <ulink
 368         url="https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/">IAB
 369         Statement</ulink>, and may create a privacy and security risk.</para>
 370
 371         <xi:include href="version-info.xml" xpointer="v246"/></listitem>
 372       </varlistentry>
 373       <varlistentry>
 374         <term>StaleRetentionSec=<replaceable>SECONDS</replaceable></term>
 375         <listitem><para>Takes a duration value, which determines the length of time DNS resource records can
 376         be retained in the cache beyond their Time To Live (TTL). This allows these records to be returned as
 377         stale records. By default, this value is set to zero, meaning that DNS resource records are not
 378         stored in the cache after their TTL expires.</para>
 379
 380         <para>This is useful when a DNS server failure occurs or becomes unreachable. In such cases,
 381         <citerefentry><refentrytitle>systemd-resolved</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 382         continues to use the stale records to answer DNS queries, particularly when no valid response can be
 383         obtained from the upstream DNS servers. However, this doesn't apply to NXDOMAIN responses, as those
 384         are still perfectly valid responses. This feature enhances resilience against DNS infrastructure
 385         failures and outages.</para>
 386
 387         <para><command>systemd-resolved</command> always attempts to reach the upstream DNS servers first,
 388         before providing the client application with any stale data. If this feature is enabled, cache will
 389         not be flushed when changing servers.</para>
 390
 391         <xi:include href="version-info.xml" xpointer="v254"/>
 392         </listitem>
 393       </varlistentry>
 394     </variablelist>
 395   </refsect1>
 396
 397   <refsect1>
 398       <title>See Also</title>
 399       <para><simplelist type="inline">
 400       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 401       <member><citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 402       <member><citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
 403       <member><citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 404       <member><citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
 405       </simplelist></para>
 406   </refsect1>
 407
 408 </refentry>