RESOLVED.CONF(5)                                 RESOLVED.CONF(5)

     NAME
          resolved.conf, resolved.conf.d - Network Name Resolution
          configuration files

     SYNOPSIS
          /etc/systemd/resolved.conf

          /etc/systemd/resolved.conf.d/*.conf

          /run/systemd/resolved.conf.d/*.conf

          /usr/lib/systemd/resolved.conf.d/*.conf

     DESCRIPTION
          These configuration files control local DNS and LLMNR name
          resolution.

     CONFIGURATION DIRECTORIES AND PRECEDENCE
          The default configuration is set during compilation, so
          configuration is only needed when it is necessary to deviate
          from those defaults. Initially, the main configuration file
          in /etc/systemd/ contains commented out entries showing the
          defaults as a guide to the administrator. Local overrides
          can be created by editing this file or by creating drop-ins,
          as described below. Using drop-ins for local configuration
          is recommended over modifications to the main configuration
          file.

          In addition to the "main" configuration file, drop-in
          configuration snippets are read from
          /usr/lib/systemd/*.conf.d/,
          /usr/local/lib/systemd/*.conf.d/, and
          /etc/systemd/*.conf.d/. Those drop-ins have higher
          precedence and override the main configuration file. Files
          in the *.conf.d/ configuration subdirectories are sorted by
          their filename in lexicographic order, regardless of in
          which of the subdirectories they reside. When multiple files
          specify the same option, for options which accept just a
          single value, the entry in the file sorted last takes
          precedence, and for options which accept a list of values,
          entries are collected as they occur in the sorted files.

          When packages need to customize the configuration, they can
          install drop-ins under /usr/. Files in /etc/ are reserved
          for the local administrator, who may use this logic to
          override the configuration files installed by vendor
          packages. Drop-ins have to be used to override package
          drop-ins, since the main configuration file has lower
          precedence. It is recommended to prefix all filenames in
          those subdirectories with a two-digit number and a dash, to

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          simplify the ordering of the files.

          To disable a configuration file supplied by the vendor, the
          recommended way is to place a symlink to /dev/null in the
          configuration directory in /etc/, with the same filename as
          the vendor configuration file.

     OPTIONS
          The following options are available in the [Resolve]
          section:

          DNS=
              A space-separated list of IPv4 and IPv6 addresses to use
              as system DNS servers. Each address can optionally take
              a port number separated with ":", a network interface
              name or index separated with "%", and a Server Name
              Indication (SNI) separated with "#". When IPv6 address
              is specified with a port number, then the address must
              be in the square brackets. That is, the acceptable full
              formats are "111.222.333.444:9953%ifname#example.com"
              for IPv4 and "[1111:2222::3333]:9953%ifname#example.com"
              for IPv6. DNS requests are sent to one of the listed DNS
              servers in parallel to suitable per-link DNS servers
              acquired from systemd-networkd.service(8) or set at
              runtime by external applications. For compatibility
              reasons, if this setting is not specified, the DNS
              servers listed in /etc/resolv.conf are used instead, if
              that file exists and any servers are configured in it.
              This setting defaults to the empty list.

          FallbackDNS=
              A space-separated list of IPv4 and IPv6 addresses to use
              as the fallback DNS servers. Please see DNS= for
              acceptable format of addresses. Any per-link DNS servers
              obtained from systemd-networkd.service(8) take
              precedence over this setting, as do any servers set via
              DNS= above or /etc/resolv.conf. This setting is hence
              only used if no other DNS server information is known.
              If this option is not given, a compiled-in list of DNS
              servers is used instead.

          Domains=
              A space-separated list of domains optionally prefixed
              with "~", used for two distinct purposes described
              below. Defaults to the empty list.

              Any domains not prefixed with "~" are used as search
              suffixes when resolving single-label hostnames (domain
              names which contain no dot), in order to qualify them
              into fully-qualified domain names (FQDNs). These "search
              domains" are strictly processed in the order they are
              specified in, until the name with the suffix appended is

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              found. For compatibility reasons, if this setting is not
              specified, the search domains listed in /etc/resolv.conf
              with the search keyword are used instead, if that file
              exists and any domains are configured in it.

              The domains prefixed with "~" are called "routing
              domains". All domains listed here (both search domains
              and routing domains after removing the "~" prefix)
              define a search path that preferably directs DNS queries
              to this interface. This search path has an effect only
              when suitable per-link DNS servers are known. Such
              servers may be defined through the DNS= setting (see
              above) and dynamically at run time, for example from
              DHCP leases. If no per-link DNS servers are known,
              routing domains have no effect.

              Use the construct "~."  (which is composed from "~" to
              indicate a routing domain and "."  to indicate the DNS
              root domain that is the implied suffix of all DNS
              domains) to use the DNS servers defined for this link
              preferably for all domains.

          LLMNR=
              Takes a boolean argument or "resolve". Controls
              Link-Local Multicast Name Resolution support (m[blue]RFC
              4795m[][1]) on the local host. If true, enables full
              LLMNR responder and resolver support. If false, disables
              both. If set to "resolve", only resolution support is
              enabled, but responding is disabled. Note that systemd-
              networkd.service(8) also maintains per-link LLMNR
              settings. LLMNR will be enabled on a link only if the
              per-link and the global setting is on.

          MulticastDNS=
              Takes a boolean argument or "resolve". Controls
              Multicast DNS support (m[blue]RFC 6762m[][2]) on the
              local host. If true, enables full Multicast DNS
              responder and resolver support. If false, disables both.
              If set to "resolve", only resolution support is enabled,
              but responding is disabled. Note that systemd-
              networkd.service(8) also maintains per-link Multicast
              DNS settings. Multicast DNS will be enabled on a link
              only if the per-link and the global setting is on.

          DNSSEC=
              Takes a boolean argument or "allow-downgrade". If true
              all DNS lookups are DNSSEC-validated locally (excluding
              LLMNR and Multicast DNS). If the response to a lookup
              request is detected to be invalid a lookup failure is
              returned to applications. Note that this mode requires a
              DNS server that supports DNSSEC. If the DNS server does
              not properly support DNSSEC all validations will fail.

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              If set to "allow-downgrade" DNSSEC validation is
              attempted, but if the server does not support DNSSEC
              properly, DNSSEC mode is automatically disabled. Note
              that this mode makes DNSSEC validation vulnerable to
              "downgrade" attacks, where an attacker might be able to
              trigger a downgrade to non-DNSSEC mode by synthesizing a
              DNS response that suggests DNSSEC was not supported. If
              set to false, DNS lookups are not DNSSEC validated.

              Note that DNSSEC validation requires retrieval of
              additional DNS data, and thus results in a small DNS
              look-up time penalty.

              DNSSEC requires knowledge of "trust anchors" to prove
              data integrity. The trust anchor for the Internet root
              domain is built into the resolver, additional trust
              anchors may be defined with dnssec-trust-anchors.d(5).
              Trust anchors may change at regular intervals, and old
              trust anchors may be revoked. In such a case DNSSEC
              validation is not possible until new trust anchors are
              configured locally or the resolver software package is
              updated with the new root trust anchor. In effect, when
              the built-in trust anchor is revoked and DNSSEC= is
              true, all further lookups will fail, as it cannot be
              proved anymore whether lookups are correctly signed, or
              validly unsigned. If DNSSEC= is set to "allow-downgrade"
              the resolver will automatically turn off DNSSEC
              validation in such a case.

              Client programs looking up DNS data will be informed
              whether lookups could be verified using DNSSEC, or
              whether the returned data could not be verified (either
              because the data was found unsigned in the DNS, or the
              DNS server did not support DNSSEC or no appropriate
              trust anchors were known). In the latter case it is
              assumed that client programs employ a secondary scheme
              to validate the returned DNS data, should this be
              required.

              It is recommended to set DNSSEC= to true on systems
              where it is known that the DNS server supports DNSSEC
              correctly, and where software or trust anchor updates
              happen regularly. On other systems it is recommended to
              set DNSSEC= to "allow-downgrade".

              In addition to this global DNSSEC setting systemd-
              networkd.service(8) also maintains per-link DNSSEC
              settings. For system DNS servers (see above), only the
              global DNSSEC setting is in effect. For per-link DNS
              servers the per-link setting is in effect, unless it is
              unset in which case the global setting is used instead.

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              Site-private DNS zones generally conflict with DNSSEC
              operation, unless a negative (if the private zone is not
              signed) or positive (if the private zone is signed)
              trust anchor is configured for them. If
              "allow-downgrade" mode is selected, it is attempted to
              detect site-private DNS zones using top-level domains
              (TLDs) that are not known by the DNS root server. This
              logic does not work in all private zone setups.

              Defaults to "allow-downgrade".

          DNSOverTLS=
              Takes a boolean argument or "opportunistic". If true all
              connections to the server will be encrypted. Note that
              this mode requires a DNS server that supports
              DNS-over-TLS and has a valid certificate. If the
              hostname was specified in DNS= by using the format
              format "address#server_name" it is used to validate its
              certificate and also to enable Server Name Indication
              (SNI) when opening a TLS connection. Otherwise the
              certificate is checked against the server's IP. If the
              DNS server does not support DNS-over-TLS all DNS
              requests will fail.

              When set to "opportunistic" DNS request are attempted to
              send encrypted with DNS-over-TLS. If the DNS server does
              not support TLS, DNS-over-TLS is disabled. Note that
              this mode makes DNS-over-TLS vulnerable to "downgrade"
              attacks, where an attacker might be able to trigger a
              downgrade to non-encrypted mode by synthesizing a
              response that suggests DNS-over-TLS was not supported.
              If set to false, DNS lookups are send over UDP.

              Note that DNS-over-TLS requires additional data to be
              send for setting up an encrypted connection, and thus
              results in a small DNS look-up time penalty.

              Note that in "opportunistic" mode the resolver is not
              capable of authenticating the server, so it is
              vulnerable to "man-in-the-middle" attacks.

              In addition to this global DNSOverTLS= setting systemd-
              networkd.service(8) also maintains per-link DNSOverTLS=
              settings. For system DNS servers (see above), only the
              global DNSOverTLS= setting is in effect. For per-link
              DNS servers the per-link setting is in effect, unless it
              is unset in which case the global setting is used
              instead.

              Defaults to off.

          Cache=

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              Takes a boolean or "no-negative" as argument. If "yes"
              (the default), resolving a domain name which already got
              queried earlier will return the previous result as long
              as it is still valid, and thus does not result in a new
              network request. Be aware that turning off caching comes
              at a performance penalty, which is particularly high
              when DNSSEC is used. If "no-negative", only positive
              answers are cached.

              Note that caching is turned off implicitly if the
              configured DNS server is on a host-local IP address
              (such as 127.0.0.1 or ::1), in order to avoid duplicate
              local caching.

          DNSStubListener=
              Takes a boolean argument or one of "udp" and "tcp". If
              "udp", a DNS stub resolver will listen for UDP requests
              on address 127.0.0.53 port 53. If "tcp", the stub will
              listen for TCP requests on the same address and port. If
              "yes" (the default), the stub listens for both UDP and
              TCP requests. If "no", the stub listener is disabled.

              Note that the DNS stub listener is turned off implicitly
              when its listening address and port are already in use.

          DNSStubListenerExtra=
              Takes an IPv4 or IPv6 address to listen on. The address
              may be optionally prefixed with a protocol name ("udp"
              or "tcp") separated with ":". If the protocol is not
              specified, the service will listen on both UDP and TCP.
              It may be also optionally suffixed by a numeric port
              number with separator ":". When an IPv6 address is
              specified with a port number, then the address must be
              in the square brackets. If the port is not specified,
              then the service uses port 53. Note that this is
              independent of the primary DNS stub configured with
              DNSStubListener=, and only configures additional sockets
              to listen on. This option can be specified multiple
              times. If an empty string is assigned, then the all
              previous assignments are cleared. Defaults to unset.

              Examples:

                  DNSStubListenerExtra=192.168.10.10
                  DNSStubListenerExtra=2001:db8:0:f102::10
                  DNSStubListenerExtra=192.168.10.11:9953
                  DNSStubListenerExtra=[2001:db8:0:f102::11]:9953
                  DNSStubListenerExtra=tcp:192.168.10.12
                  DNSStubListenerExtra=udp:2001:db8:0:f102::12
                  DNSStubListenerExtra=tcp:192.168.10.13:9953
                  DNSStubListenerExtra=udp:[2001:db8:0:f102::13]:9953

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          ReadEtcHosts=
              Takes a boolean argument. If "yes" (the default),
              systemd-resolved will read /etc/hosts, and try to
              resolve hosts or address by using the entries in the
              file before sending query to DNS servers.

          ResolveUnicastSingleLabel=
              Takes a boolean argument. When false (the default),
              systemd-resolved will not resolve A and AAAA queries for
              single-label names over classic DNS. Note that such
              names may still be resolved if search domains are
              specified (see Domains= above), or using other
              mechanisms, in particular via LLMNR or from /etc/hosts.
              When true, queries for single-label names will be
              forwarded to global DNS servers even if no search
              domains are defined.

              This option is provided for compatibility with
              configurations where public DNS servers are not used.
              Forwarding single-label names to servers not under your
              control is not standard-conformant, see m[blue]IAB
              Statementm[][3], and may create a privacy and security
              risk.

     SEE ALSO
          systemd(1), systemd-resolved.service(8), systemd-
          networkd.service(8), dnssec-trust-anchors.d(5),
          resolv.conf(4)

     NOTES
           1. RFC 4795
              https://tools.ietf.org/html/rfc4795

           2. RFC 6762
              https://tools.ietf.org/html/rfc6762

           3. IAB Statement
              https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/

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