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 <literal>[Resolve]</literal> 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. DNS requests
  54         are sent to one of the listed DNS servers in parallel to suitable per-link DNS servers acquired from
  55         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry> or
  56         set at runtime by external applications.  For compatibility reasons, if this setting is not specified, the DNS
  57         servers listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any servers
  58         are configured in it. This setting defaults to the empty list.</para></listitem>
  59       </varlistentry>
  60
  61       <varlistentry>
  62         <term><varname>FallbackDNS=</varname></term>
  63         <listitem><para>A space-separated list of IPv4 and IPv6 addresses to use as the fallback DNS servers. Any
  64         per-link DNS servers obtained from
  65         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
  66         take precedence over this setting, as do any servers set via <varname>DNS=</varname> above or
  67         <filename>/etc/resolv.conf</filename>. This setting is hence only used if no other DNS server information is
  68         known. If this option is not given, a compiled-in list of DNS servers is used instead.</para></listitem>
  69       </varlistentry>
  70
  71       <varlistentry>
  72         <term><varname>Domains=</varname></term>
  73         <listitem><para>A space-separated list of domains. These domains are used as search suffixes when resolving
  74         single-label host names (domain names which contain no dot), in order to qualify them into fully-qualified
  75         domain names (FQDNs). Search domains are strictly processed in the order they are specified, until the name
  76         with the suffix appended is found. For compatibility reasons, if this setting is not specified, the search
  77         domains listed in <filename>/etc/resolv.conf</filename> are used instead, if that file exists and any domains
  78         are configured in it. This setting defaults to the empty list.</para>
  79
  80         <para>Specified domain names may optionally be prefixed with <literal>~</literal>. In this case they do not
  81         define a search path, but preferably direct DNS queries for the indicated domains to the DNS servers configured
  82         with the system <varname>DNS=</varname> setting (see above), in case additional, suitable per-link DNS servers
  83         are known. If no per-link DNS servers are known using the <literal>~</literal> syntax has no effect. Use the
  84         construct <literal>~.</literal> (which is composed of <literal>~</literal> to indicate a routing domain and
  85         <literal>.</literal> to indicate the DNS root domain that is the implied suffix of all DNS domains) to use the
  86         system DNS server defined with <varname>DNS=</varname> preferably for all domains.</para></listitem>
  87       </varlistentry>
  88
  89       <varlistentry>
  90         <term><varname>LLMNR=</varname></term>
  91         <listitem><para>Takes a boolean argument or
  92         <literal>resolve</literal>. Controls Link-Local Multicast Name
  93         Resolution support (<ulink
  94         url="https://tools.ietf.org/html/rfc4795">RFC 4795</ulink>) on
  95         the local host. If true, enables full LLMNR responder and
  96         resolver support. If false, disables both. If set to
  97         <literal>resolve</literal>, only resolution support is enabled,
  98         but responding is disabled. Note that
  99         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 100         also maintains per-link LLMNR settings. LLMNR will be
 101         enabled on a link only if the per-link and the
 102         global setting is on.</para></listitem>
 103       </varlistentry>
 104
 105       <varlistentry>
 106         <term><varname>MulticastDNS=</varname></term>
 107         <listitem><para>Takes a boolean argument or
 108         <literal>resolve</literal>. Controls Multicast DNS support (<ulink
 109         url="https://tools.ietf.org/html/rfc6762">RFC 6762</ulink>) on
 110         the local host. If true, enables full Multicast DNS responder and
 111         resolver support. If false, disables both. If set to
 112         <literal>resolve</literal>, only resolution support is enabled,
 113         but responding is disabled. Note that
 114         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 115         also maintains per-link Multicast DNS settings. Multicast DNS will be
 116         enabled on a link only if the per-link and the
 117         global setting is on.</para></listitem>
 118       </varlistentry>
 119
 120       <varlistentry>
 121         <term><varname>DNSSEC=</varname></term>
 122         <listitem><para>Takes a boolean argument or
 123         <literal>allow-downgrade</literal>. If true all DNS lookups are
 124         DNSSEC-validated locally (excluding LLMNR and Multicast
 125         DNS). If the response to a lookup request is detected to be invalid
 126         a lookup failure is returned to applications. Note that
 127         this mode requires a DNS server that supports DNSSEC. If the
 128         DNS server does not properly support DNSSEC all validations
 129         will fail. If set to <literal>allow-downgrade</literal> DNSSEC
 130         validation is attempted, but if the server does not support
 131         DNSSEC properly, DNSSEC mode is automatically disabled. Note
 132         that this mode makes DNSSEC validation vulnerable to
 133         "downgrade" attacks, where an attacker might be able to
 134         trigger a downgrade to non-DNSSEC mode by synthesizing a DNS
 135         response that suggests DNSSEC was not supported. If set to
 136         false, DNS lookups are not DNSSEC validated.</para>
 137
 138         <para>Note that DNSSEC validation requires retrieval of
 139         additional DNS data, and thus results in a small DNS look-up
 140         time penalty.</para>
 141
 142         <para>DNSSEC requires knowledge of "trust anchors" to prove
 143         data integrity. The trust anchor for the Internet root domain
 144         is built into the resolver, additional trust anchors may be
 145         defined with
 146         <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
 147         Trust anchors may change at regular intervals, and old trust
 148         anchors may be revoked. In such a case DNSSEC validation is
 149         not possible until new trust anchors are configured locally or
 150         the resolver software package is updated with the new root
 151         trust anchor. In effect, when the built-in trust anchor is
 152         revoked and <varname>DNSSEC=</varname> is true, all further
 153         lookups will fail, as it cannot be proved anymore whether
 154         lookups are correctly signed, or validly unsigned. If
 155         <varname>DNSSEC=</varname> is set to
 156         <literal>allow-downgrade</literal> the resolver will
 157         automatically turn off DNSSEC validation in such a case.</para>
 158
 159         <para>Client programs looking up DNS data will be informed
 160         whether lookups could be verified using DNSSEC, or whether the
 161         returned data could not be verified (either because the data
 162         was found unsigned in the DNS, or the DNS server did not
 163         support DNSSEC or no appropriate trust anchors were known). In
 164         the latter case it is assumed that client programs employ a
 165         secondary scheme to validate the returned DNS data, should
 166         this be required.</para>
 167
 168         <para>It is recommended to set <varname>DNSSEC=</varname> to
 169         true on systems where it is known that the DNS server supports
 170         DNSSEC correctly, and where software or trust anchor updates
 171         happen regularly. On other systems it is recommended to set
 172         <varname>DNSSEC=</varname> to
 173         <literal>allow-downgrade</literal>.</para>
 174
 175         <para>In addition to this global DNSSEC setting
 176         <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 177         also maintains per-link DNSSEC settings. For system DNS
 178         servers (see above), only the global DNSSEC setting is in
 179         effect. For per-link DNS servers the per-link
 180         setting is in effect, unless it is unset in which case the
 181         global setting is used instead.</para>
 182
 183         <para>Site-private DNS zones generally conflict with DNSSEC
 184         operation, unless a negative (if the private zone is not
 185         signed) or positive (if the private zone is signed) trust
 186         anchor is configured for them. If
 187         <literal>allow-downgrade</literal> mode is selected, it is
 188         attempted to detect site-private DNS zones using top-level
 189         domains (TLDs) that are not known by the DNS root server. This
 190         logic does not work in all private zone setups.</para>
 191
 192         <para>Defaults to <literal>allow-downgrade</literal></para>
 193         </listitem>
 194       </varlistentry>
 195
 196       <varlistentry>
 197         <term><varname>DNSOverTLS=</varname></term>
 198         <listitem>
 199         <para>Takes false or
 200         <literal>opportunistic</literal>. 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 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
 235         <para>Note that caching is turned off implicitly if the configured DNS server is on a host-local IP address
 236         (such as 127.0.0.1 or ::1), in order to avoid duplicate local caching.</para></listitem>
 237       </varlistentry>
 238
 239       <varlistentry>
 240         <term><varname>DNSStubListener=</varname></term>
 241         <listitem><para>Takes a boolean argument or one of <literal>udp</literal> and <literal>tcp</literal>. If
 242         <literal>udp</literal>, a DNS stub resolver will listen for UDP requests on address 127.0.0.53
 243         port 53. If <literal>tcp</literal>, the stub will listen for TCP requests on the same address and port. If
 244         <literal>yes</literal> (the default), the stub listens for both UDP and TCP requests.  If <literal>no</literal>, the stub
 245         listener is disabled.</para>
 246
 247         <para>Note that the DNS stub listener is turned off implicitly when its listening address and port are already
 248         in use.</para></listitem>
 249       </varlistentry>
 250
 251       <varlistentry>
 252         <term><varname>ReadEtcHosts=</varname></term>
 253         <listitem><para>Takes a boolean argument. If <literal>yes</literal> (the default), the DNS stub resolver will read
 254         <filename>/etc/hosts</filename>, and try to resolve hosts or address by using the entries in the file before
 255         sending query to DNS servers.</para></listitem>
 256       </varlistentry>
 257
 258     </variablelist>
 259   </refsect1>
 260
 261   <refsect1>
 262       <title>See Also</title>
 263       <para>
 264       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 265       <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 266       <citerefentry><refentrytitle>systemd-networkd.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 267       <citerefentry><refentrytitle>dnssec-trust-anchors.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 268       <citerefentry project='man-pages'><refentrytitle>resolv.conf</refentrytitle><manvolnum>4</manvolnum></citerefentry>
 269       </para>
 270   </refsect1>
 271
 272 </refentry>