systemd/man/org.freedesktop.resolve1.xml

<?xml version="1.0"?>
<!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
<!ENTITY % entities SYSTEM "custom-entities.ent" >
%entities;
]>
<!-- SPDX-License-Identifier: LGPL-2.1+ -->

<refentry id="org.freedesktop.resolve1" conditional='ENABLE_RESOLVE'
    xmlns:xi="http://www.w3.org/2001/XInclude">
  <refentryinfo>
    <title>org.freedesktop.resolve1</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>org.freedesktop.resolve1</refentrytitle>
    <manvolnum>5</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>org.freedesktop.resolve1</refname>
    <refpurpose>The D-Bus interface of systemd-resolved</refpurpose>
  </refnamediv>

  <refsect1>
    <title>Introduction</title>

    <para>
    <citerefentry><refentrytitle>systemd-resolved.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    is a system service that provides hostname resolution and caching using DNS, LLMNR, and mDNS. It also
    does DNSSEC validation. This page describes the resolve semantics and the D-Bus interface.</para>

    <para>This page contains an API reference only. If you are looking for a longer explanation how to use
    this API, please consult
    <ulink url="https://wiki.freedesktop.org/www/Software/systemd/writing-network-configuration-managers">
    Writing Network Configuration Managers</ulink> and
    <ulink url="https://wiki.freedesktop.org/www/Software/systemd/writing-resolver-clients">Writing Resolver
    Clients</ulink>.
    </para>
  </refsect1>

  <refsect1>
    <title>The Manager Object</title>

    <para>The service exposes the following interfaces on the Manager object on the bus:</para>

    <programlisting>
$ gdbus introspect --system \
        --dest org.freedesktop.resolve1 \
        --object-path /org/freedesktop/resolve1

node /org/freedesktop/resolve1 {
  interface org.freedesktop.resolve1.Manager {
    methods:
      ResolveHostname(in  i ifindex,
                      in  s name,
                      in  i family,
                      in  t flags,
                      out a(iiay) addresses,
                      out s canonical,
                      out t flags);
      ResolveAddress(in  i ifindex,
                     in  i family,
                     in  ay address,
                     in  t flags,
                     out a(is) names,
                     out t flags);
      ResolveRecord(in  i ifindex,
                    in  s name,
                    in  q class,
                    in  q type,
                    in  t flags,
                    out a(iqqay) records,
                    out t flags);
      ResolveService(in  i ifindex,
                     in  s name,
                     in  s type,
                     in  s domain,
                     in  i family,
                     in  t flags,
                     out a(qqqsa(iiay)s) srv_data,
                     out aay txt_data,
                     out s canonical_name,
                     out s canonical_type,
                     out s canonical_domain,
                     out t flags);
      GetLink(in  i ifindex,
              out o path);
      SetLinkDNS(in  i ifindex,
                 in  a(iay) addresses);
      SetLinkDomains(in  i ifindex,
                     in  a(sb) domains);
      SetLinkDefaultRoute(in  i ifindex,
                          in  b enable);
      SetLinkLLMNR(in  i ifindex,
                   in  s mode);
      SetLinkMulticastDNS(in  i ifindex,
                          in  s mode);
      SetLinkDNSOverTLS(in  i ifindex,
                        in  s mode);
      SetLinkDNSSEC(in  i ifindex,
                    in  s mode);
      SetLinkDNSSECNegativeTrustAnchors(in  i ifindex,
                                        in  as names);
      RevertLink(in  i ifindex);
      RegisterService(in  s name,
                      in  s name_template,
                      in  s type,
                      in  q service_port,
                      in  q service_priority,
                      in  q serwise_weight,
                      in  aa{say} txt_datas,
                      out o service_path);
      UnregisterService(in  o service_path);
      ResetStatistics();
      FlushCaches();
      ResetServerFeatures();
    properties:
      readonly s LLMNRHostname = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly s LLMNR = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly s MulticastDNS = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly s DNSOverTLS = '...';
      readonly a(iiay) DNS = [...];
      @org.freedesktop.DBus.Property.EmitsChangedSignal("const")
      readonly a(iiay) FallbackDNS = [...];
      readonly (iiay) CurrentDNSServer = ...;
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly a(isb) Domains = [...];
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly (tt) TransactionStatistics = ...;
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly (ttt) CacheStatistics = ...;
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly s DNSSEC = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly (tttt) DNSSECStatistics = ...;
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly b DNSSECSupported = ...;
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly as DNSSECNegativeTrustAnchors = ['...', ...];
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly s DNSStubListener = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      @org.freedesktop.systemd1.Privileged("true")
      readwrite s LogLevel = '...';
  };
  interface org.freedesktop.DBus.Peer { ... };
  interface org.freedesktop.DBus.Introspectable { ... };
  interface org.freedesktop.DBus.Properties { ... };
};
    </programlisting>

    <!--method SetLinkDefaultRoute is not documented!-->

    <!--method SetLinkDNSOverTLS is not documented!-->

    <!--method RegisterService is not documented!-->

    <!--method UnregisterService is not documented!-->

    <!--method FlushCaches is not documented!-->

    <!--method ResetServerFeatures is not documented!-->

    <!--property LLMNR is not documented!-->

    <!--property MulticastDNS is not documented!-->

    <!--property DNSOverTLS is not documented!-->

    <!--property FallbackDNS is not documented!-->

    <!--property CurrentDNSServer is not documented!-->

    <!--property DNSSEC is not documented!-->

    <!--property DNSSECNegativeTrustAnchors is not documented!-->

    <!--property DNSStubListener is not documented!-->

    <refsect2>
      <title>Methods</title>

      <para><function>ResolveHostname()</function> takes a hostname and resolves it to one or more IP addresses.
      As parameters it takes the Linux network interface index to execute the query on, or 0 if it may be
      done on any suitable interface. The <varname>name</varname> parameter specifies the hostname to
      resolve. Note that if required, IDNA conversion is applied to this name unless it is resolved via LLMNR or MulticastDNS. The <varname>family</varname> parameter
      limits the results to a specific address family. It may be <constant>AF_INET</constant>,
      <constant>AF_INET6</constant> or <constant>AF_UNSPEC</constant>. If <constant>AF_UNSPEC</constant> is specified (recommended), both kinds are retrieved, subject
      to local network configuration (i.e. if no local, routable IPv6 address is found, no IPv6 address is
      retrieved; and similarly for IPv4). A 64-bit <varname>flags</varname> field may be used to alter the
      behaviour of the resolver operation (see below). The method returns an array of address records. Each
      address record consists of the interface index the address belongs to, an address family as well as a
      byte array with the actual IP address data (which either has 4 or 16 elements, depending on the address
      family). The returned address family will be one of <constant>AF_INET</constant> or
      <constant>AF_INET6</constant>. For IPv6, the returned address interface index should be used to
      initialize the .sin6_scope_id field of a <structname>struct sockaddr_in6</structname> instance to permit
      support for resolution to link-local IP addresses. The address array is followed by the canonical name
      of the host, which may or may not be identical to the resolved hostname. Finally, a 64-bit
      <varname>flags</varname> field is returned that is defined similarly to the <varname>flags</varname>
      field that was passed in, but contains information about the resolved data (see below). If the hostname
      passed in is an IPv4 or IPv6 address formatted as string, it is parsed, and the result is returned. In
      this case, no network communication is done.</para>

      <para><function>ResolveAddress()</function> executes the reverse operation: it takes an IP address and
      acquires one or more hostnames for it. As parameters it takes the interface index to execute the query
      on, or <constant>0</constant> if all suitable interfaces are OK. The <varname>family</varname>
      parameter indicates the address family of the IP address to resolve. It may be either
      <constant>AF_INET</constant> or <constant>AF_INET6</constant>. The <varname>address</varname> parameter
      takes the raw IP address data (as either a 4 or 16 byte array). The <varname>flags</varname> input
      parameter may be used to alter the resolver operation (see below). The method returns an array of name
      records, each consisting of an interface index and a hostname.  The <varname>flags</varname> output
      field contains additional information about the resolver operation (see below).</para>

      <para><function>ResolveRecord()</function> takes a DNS resource record (RR) type, class and name, and
      retrieves the full resource record set (RRset), including the RDATA, for it. As parameter it takes the
      Linux network interface index to execute the query on, or <constant>0</constant> if it may be done on
      any suitable interface. The <varname>name</varname> parameter specifies the RR domain name to look up
      (no IDNA conversion is applied), followed by the 16-bit class and type fields (which may be
      ANY). Finally, a <varname>flags</varname> field may be passed in to alter behaviour of the look-up (see
      below). On completion, an array of RR items is returned. Each array entry consists of the network interface
      index the RR was discovered on, the type and class field of the RR found, and a byte array of the raw
      RR discovered. The raw RR data starts with the RR's domain name, in the original casing, followed
      by the RR type, class, TTL and RDATA, in the binary format documented in
      <ulink url="https://www.ietf.org/rfc/rfc1035.txt">RFC 1035</ulink>. For RRs that support name
      compression in the payload (such as MX or PTR), the compression is expanded in the returned
      data.</para>

      <para>Note that currently, the class field has to be specified as IN or ANY. Specifying a different
      class will return an error indicating that look-ups of this kind are unsupported. Similarly, some
      special types are not supported either (AXFR, OPT, …). While <filename>systemd-resolved</filename> parses and validates resource
      records of many types, it is crucial that clients using this API understand that the RR data originates
      from the network and should be thoroughly validated before use.</para>

      <para><function>ResolveService()</function> may be used to resolve a DNS SRV service record, as well as the
      hostnames referenced in it, and possibly an accompanying DNS-SD TXT record containing additional
      service metadata. The primary benefit of using this method over <function>ResolveRecord()</function>
      specifying the SRV type is that it will resolve the SRV and TXT RRs as well as the hostnames referenced
      in the SRV in a single operation. As parameters it takes a Linux network interface index, a service
      name, a service type and a service domain. This method may be invoked in three different modes:</para>

      <orderedlist>
        <listitem><para>To resolve a DNS-SD service, specify the service name (e.g. <literal>Lennart's
        Files</literal>), the service type (e.g. <literal>_webdav._tcp</literal>) and the domain to search in
        (e.g. <literal>local</literal>) as the three service parameters. The service name must be in UTF-8
        format, and no IDNA conversion is applied to it in this mode (as mandated by the DNS-SD
        specifications). However, if necessary, IDNA conversion is applied to the domain parameter.</para>
        </listitem>

        <listitem><para>To resolve a plain SRV record, set the service name parameter to the empty string
        and set the service type and domain properly. (IDNA conversion is applied to the domain, if
        necessary.)</para></listitem>

        <listitem><para>Alternatively, leave both the service name and type empty and specify the full
        domain name of the SRV record (i.e. prefixed with the service type) in the domain parameter. (No IDNA
        coversion is applied in this mode.)</para></listitem>
      </orderedlist>

      <para>The <varname>family</varname> parameter of the <function>ResolveService()</function> method encodes
      the desired family of the addresses to resolve (use <constant>AF_INET</constant>,
      <constant>AF_INET6</constant>, or <constant>AF_UNSPEC</constant>). If this is enabled (Use the
      <constant>NO_ADDRESS</constant> flag to turn address resolution off, see below). The
      <varname>flags</varname> parameter takes a couple of flags that may be used to alter the resolver
      operation.</para>

      <para>On completion, <function>ResolveService()</function> returns an array of SRV record structures. Each
      items consisting of the priority, weight and port fields as well as the hostname to contact, as encoded in the SRV
      record. Immediately following is an array of the addresses of this hostname, with each item consisting
      of the interface index, the address family and the address data in a byte array. This address array is
      followed by the canonicalized hostname. After this array of SRV record structures an array of byte
      arrays follows that encodes the TXT RR strings, in case DNS-SD look-ups are enabled. The next parameters
      are the canonical service name, type and domain. This may or may not be identical to the parameters
      passed in. Finally, a <varname>flags</varname> field is returned that contains information about the
      resolver operation performed.</para>

      <para>The <function>ResetStatistics()</function> method resets the various statistics counters that
      <filename>systemd-resolved</filename> maintains to zero. (For details, see the statistics properties below.)</para>

      <para>The <function>GetLink()</function> method takes a network interface index and returns the object
      path to the <interfacename>org.freedesktop.resolve1.Link</interfacename> object corresponding to it.
      </para>

      <para>The <function>SetLinkDNS()</function> method sets the DNS servers to use on a specific
      interface. This method (and the following ones) may be used by network management software to configure
      per-interface DNS settings. It takes a network interface index as well as an array of DNS server IP
      address records. Each array item consists of an address family (either <constant>AF_INET</constant> or
      <constant>AF_INET6</constant>), followed by a 4-byte or 16-byte array with the raw address data. This
      method is a one-step shortcut for retrieving the Link object for a network interface using
      <function>GetLink()</function> (see above) and then invoking the <function>SetDNS()</function> method
      (see below) on it.
      </para>

      <para>Network management software integrating with <filename>systemd-resolved</filename> should
      call this method (and the five below) after the interface appeared in the kernel (and thus after a
      network interface index has been assigned), but before the network interfaces is activated
      (<constant>IFF_UP</constant> set) so that all settings take effect during the full time the network
      interface is up. It is safe to alter settings while the interface is up, however. Use
      <function>RevertLink()</function> (described below) to reset all per-interface settings.</para>

      <para>The <function>SetLinkDomains()</function> method sets the search and routing domains to use on a
      specific network interface for DNS look-ups. It takes a network interface index and an array of domains,
      each with a boolean parameter indicating whether the specified domain shall be used as a search domain
      (false), or just as a routing domain (true). Search domains are used for qualifying single-label names into
      FQDN when looking up hostnames, as well as for making routing decisions on which interface to send
      queries ending in the domain to. Routing domains are only used for routing decisions and not used for single-label
      name qualification. Pass the search domains in the order they should be used.</para>

      <para>The <function>SetLinkLLMNR()</function> method enables or disables LLMNR support on a specific
      network interface. It takes a network interface index as well as a string that may either be empty or one of
      <literal>yes</literal>, <literal>no</literal> or <literal>resolve</literal>. If empty, the systemd-wide
      default LLMNR setting is used. If <literal>yes</literal>, LLMNR is used for resolution of single-label
      names and the local hostname is registered on all local LANs for LLMNR resolution by peers. If
      <literal>no</literal>, LLMNR is turned off fully on this interface. If <literal>resolve</literal>, LLMNR
      is only enabled for resolving names, but the local host name is not registered for other peers to
      use.</para>

      <para>Similarly, the <function>SetLinkMulticastDNS()</function> method enables or disables MulticastDNS
      support on a specific interface. It takes the same parameters as <function>SetLinkLLMNR()</function>
      described above.</para>

      <para>The <function>SetLinkDNSSEC()</function> method enables or disables DNSSEC validation on a
      specific network interface. It takes a network interface index as well as a string that may either be
      empty or one of <literal>yes</literal>, <literal>no</literal>, or <literal>allow-downgrade</literal>. When
      empty, the system-wide default DNSSEC setting is used. If <literal>yes</literal>, full DNSSEC validation
      is done for all look-ups. If the selected DNS server does not support DNSSEC, look-ups will fail if this
      mode is used. If <literal>no</literal>, DNSSEC validation is fully disabled. If
      <literal>allow-downgrade</literal>, DNSSEC validation is enabled, but is turned off automatically if the
      selected server does not support it (thus opening up behaviour to downgrade attacks). Note that DNSSEC
      only applies to traditional DNS, not to LLMNR or MulticastDNS.</para>

      <para>The <function>SetLinkDNSSECNegativeTrustAnchors()</function> method may be used to configure DNSSEC
      Negative Trust Anchors (NTAs) for a specific network interface. It takes a network interface index and a
      list of domains as arguments.</para>

      <para>The <function>RevertLink()</function> method may be used to revert all per-link settings done with
      the six methods described above to the defaults again.</para>

      <refsect3>
        <title>The Flags Parameter</title>

        <para>The four methods above accept and return a 64-bit flags value. In most cases passing 0 is sufficient
        and recommended. However, the following flags are defined to alter the look-up:</para>

        <programlisting>
#define SD_RESOLVED_DNS           (UINT64_C(1) &lt;&lt; 0)
#define SD_RESOLVED_LLMNR_IPV4    (UINT64_C(1) &lt;&lt; 1)
#define SD_RESOLVED_LLMNR_IPV6    (UINT64_C(1) &lt;&lt; 2)
#define SD_RESOLVED_MDNS_IPV4     (UINT64_C(1) &lt;&lt; 3)
#define SD_RESOLVED_MDNS_IPV6     (UINT64_C(1) &lt;&lt; 4)
#define SD_RESOLVED_NO_CNAME      (UINT64_C(1) &lt;&lt; 5)
#define SD_RESOLVED_NO_TXT        (UINT64_C(1) &lt;&lt; 6)
#define SD_RESOLVED_NO_ADDRESS    (UINT64_C(1) &lt;&lt; 7)
#define SD_RESOLVED_NO_SEARCH     (UINT64_C(1) &lt;&lt; 8)
#define SD_RESOLVED_AUTHENTICATED (UINT64_C(1) &lt;&lt; 9)
        </programlisting>

        <para>On input, the first five flags control the protocols to use for the look-up. They refer to
        classic unicast DNS, LLMNR via IPv4/UDP and IPv6/UDP respectively, as well as MulticastDNS via
        IPv4/UDP and IPv6/UDP. If all of these five bits are off on input (which is strongly recommended) the
        look-up will be done via all suitable protocols for the specific look-up. Note that these flags
        operate as filter only, but cannot force a look-up to be done via a protocol. Specifically, <filename>systemd-resolved</filename>
        will only route look-ups within the .local TLD to MulticastDNS (plus some reverse look-up address
        domains), and single-label names to LLMNR (plus some reverse address lookup domains). It will route
        neither of these to Unicast DNS servers. Also, it will do LLMNR and Multicast DNS only on interfaces
        suitable for multicast.</para>

        <para>On output, these five flags indicate which protocol was used to execute the operation, and hence
        where the data was found.</para>

        <para>The primary use cases for these five flags are follow-up look-ups based on DNS data retrieved
        earlier. In this case it is often a good idea to limit the follow-up look-up to the protocol that was
        used to discover the first DNS result.</para>

        <para>The NO_CNAME flag controls whether CNAME/DNAME resource records shall be followed during the
        look-up. This flag is only available at input, none of the functions will return it on output. If a
        CNAME/DNAME RR is discovered while resolving a hostname, an error is returned instead. By default,
        when the flag is off, CNAME/DNAME RRs are followed.</para>

        <para>The NO_TXT and NO_ADDRESS flags only influence operation of the
        <function>ResolveService()</function> method. They are only defined for input, not output. If
        NO_TXT set, the DNS-SD TXT RR look-up is not done in the same operation. If NO_ADDRESS is specified,
        the hostnames discovered are not implicitly translated to their addresses.</para>

        <para>The NO_SEARCH flag turns off the search domain logic. It is only defined for input in
        <function>ResolveHostname()</function>. When specified, single-label hostnames are not qualified
        using defined search domains, if any are configured. Note that <function>ResolveRecord()</function>
        will never qualify single-label domain names using search domains. Also note that
        multi-label hostnames are never subject to search list expansion.</para>

        <para>The AUTHENTICATED bit is defined only in the output flags of the four functions. If set, the
        returned data has been fully authenticated. Specifically, this bit is set for all DNSSEC-protected data
        for which a full trust chain may be established to a trusted domain anchor. It is also set for locally
        synthesized data, such as <literal>localhost</literal> or data from
        <filename>/etc/hosts</filename>. Moreover, it is set for all LLMNR or mDNS RRs which originate from the
        local host. Applications that require authenticated RR data for operation should check this flag before
        trusting the data. Note that <filename>systemd-resolved</filename> will never return invalidated data, hence this flag
        simply allows to discern the cases where data is known to be trustable, or where there is proof that
        the data is "rightfully" unauthenticated (which includes cases where the underlying protocol or server
        does not support authenticating data).</para>
      </refsect3>

  </refsect2>

    <refsect2>
      <title>Properties</title>

      <para><varname>LLMNRHostname</varname> contains the hostname currently exposed on the network via
      LLMNR. It usually follows the system hostname as may be queried via
      <citerefentry project="man-pages"><refentrytitle>gethostname</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      but may differ if a conflict is detected on the network.</para>

      <para><varname>DNS</varname> contains an array of all DNS servers currently used by
      <filename>systemd-resolved</filename>. It contains similar information as the DNS server data written to
      /run/systemd/resolve/resolv.conf. Each structure in the array consists of a numeric network interface
      index, an address family, and a byte array containing the DNS server address (either 4 bytes in length
      for IPv4 or 16 bytes in lengths for IPv6). The array contains DNS servers configured system-wide,
      including those possibly read from a foreign <filename>/etc/resolv.conf</filename> or the
      <varname>DNS=</varname> setting in <filename>/etc/systemd/resolved.conf</filename>, as well as
      per-interface DNS server information either retrieved from
      <citerefentry><refentrytitle>systemd-networkd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      or configured by external software via <function>SetLinkDNS()</function> (see above). The network
      interface index will be 0 for the system-wide configured services and non-zero for the per-link
      servers.</para>

      <para>Similarly, the <varname>Domains</varname> property contains an array of all search and
      routing domains currently used by <filename>systemd-resolved</filename>. Each entry consists of a network interface index (again, 0
      encodes system-wide entries), the actual domain name, and whether the entry is used only for routing
      (true) or for both routing and searching (false).</para>

      <para>The <varname>TransactionStatistics</varname> property contains information about the number of
      transactions <filename>systemd-resolved</filename> has processed. It contains a pair of unsigned 64-bit counters, the first
      containing the number of currently ongoing transactions, the second the number of total transactions
      <filename>systemd-resolved</filename> is processing or has processed. The latter value may be reset using the
      <function>ResetStatistics()</function> method described above. Note that the number of transactions does
      not directly map to the number of issued resolver bus method calls. While simple look-ups usually require a
      single transaction only, more complex look-ups might result in more, for example when CNAMEs or DNSSEC
      are in use.</para>

      <para>The <varname>CacheStatistics</varname> property contains information about the executed cache
      operations so far. It exposes three 64-bit counters: the first being the total number of current cache
      entries (both positive and negative), the second the number of cache hits, and the third the number of
      cache misses. The latter counters may be reset using <function>ResetStatistics()</function> (see
      above). </para>

      <para>The <varname>DNSSECStatistics</varname> property contains information about the DNSSEC
      validations executed so far. It contains four 64-bit counters: the number of secure, insecure, bogus,
      and indeterminate DNSSEC validations so far. The counters are increased for each validated RRset, and
      each non-existance proof. The secure counter is increased for each operation that successfully verified
      a signed reply, the insecure counter is increased for each operation that successfully verified that an
      unsigned reply is rightfully unsigned. The bogus counter is increased for each operation where the
      validation did not check out and the data is likely to have been tempered with. Finally the
      indeterminate counter is increased for each operation which did not complete because the necessary keys
      could not be acquired or the cryptographic algorithms were unknown.</para>

      <para>The <varname>DNSSECSupported</varname> boolean property reports whether DNSSEC is enabled and
      the selected DNS servers support it. It combines information about system-wide and per-link DNS
      settings (see below), and only reports true if DNSSEC is enabled and supported on every interface for
      which DNS is configured and for the system-wide settings if there are any. Note that <filename>systemd-resolved</filename> assumes
      DNSSEC is supported by DNS servers until it verifies that this is not the case. Thus, the reported
      value may initially be true, until the first transactions are executed.</para>

      <para>The <varname>LogLevel</varname> property shows the (maximum) log level of the manager, with the
      same values as the <option>--log-level=</option> option described in
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>.</para>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Link Object</title>

    <programlisting>
$ gdbus introspect --system \
        --dest org.freedesktop.resolve1 \
        --object-path /org/freedesktop/resolve1/link/_34

node /org/freedesktop/resolve1/link/_34 {
  interface org.freedesktop.resolve1.Link {
    methods:
      SetDNS(in  a(iay) addresses);
      SetDomains(in  a(sb) domains);
      SetDefaultRoute(in  b enable);
      SetLLMNR(in  s mode);
      SetMulticastDNS(in  s mode);
      SetDNSOverTLS(in  s mode);
      SetDNSSEC(in  s mode);
      SetDNSSECNegativeTrustAnchors(in  as names);
      Revert();
    properties:
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly t ScopesMask = ...;
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly a(iay) DNS = [...];
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly (iay) CurrentDNSServer = ...;
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly a(sb) Domains = [...];
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly b DefaultRoute = ...;
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly s LLMNR = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly s MulticastDNS = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly s DNSOverTLS = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly s DNSSEC = '...';
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly as DNSSECNegativeTrustAnchors = ['...', ...];
      @org.freedesktop.DBus.Property.EmitsChangedSignal("false")
      readonly b DNSSECSupported = ...;
  };
  interface org.freedesktop.DBus.Peer { ... };
  interface org.freedesktop.DBus.Introspectable { ... };
  interface org.freedesktop.DBus.Properties { ... };
};
    </programlisting>

    <!--method SetDomains is not documented!-->

    <!--method SetDefaultRoute is not documented!-->

    <!--method SetLLMNR is not documented!-->

    <!--method SetMulticastDNS is not documented!-->

    <!--method SetDNSOverTLS is not documented!-->

    <!--method SetDNSSEC is not documented!-->

    <!--method SetDNSSECNegativeTrustAnchors is not documented!-->

    <!--method Revert is not documented!-->

    <!--property CurrentDNSServer is not documented!-->

    <!--property DefaultRoute is not documented!-->

    <!--property LLMNR is not documented!-->

    <!--property MulticastDNS is not documented!-->

    <!--property DNSOverTLS is not documented!-->

    <!--property DNSSEC is not documented!-->

    <!--property DNSSECNegativeTrustAnchors is not documented!-->

    <para>For each Linux network interface a "Link" object is created which exposes per-link DNS
    configuration and state. Use <function>GetLink()</function> on the Manager interface to retrieve the
    object path for a link object given the network interface index (see above).</para>

    <refsect2>
      <title>Methods</title>

      <para>The various methods exposed by the Link interface are equivalent to their similarly named
      counterparts on the Manager interface. e.g. <function>SetDNS()</function> on the Link object maps to
      <function>SetLinkDNS()</function> on the Manager object, the main difference being that the later
      expects an interface index to be specified. Invoking the methods on the Manager interface has the
      benefit of reducing roundtrips, as it is not necessary to first request the Link object path via
      <function>GetLink()</function> before invoking the methods. For further details on these methods see
      the <interfacename>Manager</interfacename> documentation above.</para>
    </refsect2>

    <refsect2>
      <title>Properties</title>

      <para><varname>ScopesMask</varname> defines which resolver scopes are currently active on this
      interface. This 64-bit unsigned integer field is a bit mask consisting of a subset of the bits of the
      flags parameter describe above. Specifically, it may have the DNS, LLMNR and MDNS bits (the latter in
      IPv4 and IPv6 flavours) set. Each individual bit is set when the protocol applies to a specific
      interface and is enabled for it. It is unset otherwise. Specifically, a multicast-capable interface in
      the "UP" state with an IP address is suitable for LLMNR or MulticastDNS, and any interface that is UP and
      has an IP address is suitable for DNS. Note the relationship of the bits exposed here with the LLMNR
      and MulticastDNS properties also exposed on the Link interface. The latter expose what is *configured*
      to be used on the interface, the former expose what is actually used on the interface, taking into
      account the abilities of the interface.</para>

      <para><varname>DNSSECSupported</varname> exposes a boolean field that indicates whether DNSSEC is
      currently configured and in use on the interface. Note that if DNSSEC is enabled on an interface, it is
      assumed available until it is detected that the configured server does not actually support it. Thus,
      this property may initially report that DNSSEC is supported on an interface.</para>

      <para>The other properties reflect the state of the various configuration settings for the link which
      may be set with the various methods calls such as SetDNS() or SetLLMNR().</para>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Common Errors</title>

    <para>Many bus methods <filename>systemd-resolved</filename> exposes (in particular the resolver methods such
    as <function>ResolveHostname()</function> on the <interfacename>Manager</interfacename> interface) may return
    some of the following errors:</para>

    <variablelist>
      <varlistentry><term><constant>org.freedesktop.resolve1.NoNameServers</constant></term>
      <listitem><para>No suitable DNS servers were found to resolve a request.</para></listitem>
      </varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.InvalidReply</constant></term>
      <listitem><para>A response from the selected DNS server was not understood.</para></listitem>
      </varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.NoSuchRR</constant></term>
      <listitem><para>The requested name exists, but there is no resource record of the requested type for
      it. (This is the DNS NODATA case).</para></listitem></varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.CNameLoop</constant></term>
      <listitem><para>The look-up failed because a CNAME or DNAME loop was detected.</para></listitem>
      </varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.Aborted</constant></term>
      <listitem><para>The look-up was aborted because the selected protocol became unavailable while the
      operation was ongoing.</para></listitem>
      </varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.NoSuchService</constant></term>
      <listitem><para>A service look-up was successful, but the SRV record reported that the service is not
      available.</para></listitem></varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.DnssecFailed</constant></term>
      <listitem><para>The acquired response did not pass DNSSEC validation.</para></listitem>
      </varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.NoTrustAnchor</constant></term>
      <listitem><para>No chain of trust could be established for the response to a configured DNSSEC trust
      anchor.</para></listitem>
      </varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.ResourceRecordTypeUnsupported</constant></term>
      <listitem><para>The requested resource record type is not supported on the selected DNS servers. This
      error is generated for example when an RRSIG record is requested from a DNS server that does not
      support DNSSEC.</para></listitem>

      </varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.NoSuchLink</constant></term>
      <listitem><para>No network interface with the specified network interface index exists.
      </para></listitem></varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.LinkBusy</constant></term>
      <listitem><para>The requested configuration change could not be made because
      <citerefentry><refentrytitle>systemd-networkd</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
      already took possession of the interface and supplied configuration data for it.</para></listitem>
      </varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.NetworkDown</constant></term>
      <listitem><para>The requested look-up failed because the system is currently not connected to any
      suitable network.</para></listitem></varlistentry>

      <varlistentry><term><constant>org.freedesktop.resolve1.DnsError.NXDOMAIN</constant></term>
      <term><constant>org.freedesktop.resolve1.DnsError.REFUSED</constant></term>
      <term>...</term>
      <listitem><para>The look-up failed with a DNS return code reporting a failure. The error names used as
      suffixes here are defined in by IANA in
      <ulink url="https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-6">DNS RCODEs</ulink>.
      </para></listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Versioning</title>

    <para>These D-Bus interfaces follow <ulink url="http://0pointer.de/blog/projects/versioning-dbus.html">
    the usual interface versioning guidelines</ulink>.</para>
  </refsect1>
</refentry>