NAME | SYNOPSIS | DESCRIPTION | CONFIGURATION DIRECTORIES AND PRECEDENCE | OPTIONS | SEE ALSO | NOTES | COLOPHON

RESOLVED.CONF(5)                resolved.conf               RESOLVED.CONF(5)

NAME         top

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

SYNOPSIS         top

       /etc/systemd/resolved.conf

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

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

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

DESCRIPTION         top

       These configuration files control local DNS and LLMNR name
       resolution.

CONFIGURATION DIRECTORIES AND PRECEDENCE         top

       The default configuration is defined during compilation, so a
       configuration file is only needed when it is necessary to deviate
       from those defaults. By default, the configuration file in
       /etc/systemd/ contains commented out entries showing the defaults as
       a guide to the administrator. This file can be edited to create local
       overrides.

       When packages need to customize the configuration, they can install
       configuration snippets in /usr/lib/systemd/*.conf.d/. Files in /etc/
       are reserved for the local administrator, who may use this logic to
       override the configuration files installed by vendor packages. The
       main configuration file is read before any of the configuration
       directories, and has the lowest precedence; entries in a file in any
       configuration directory override entries in the single configuration
       file. Files in the *.conf.d/ configuration subdirectories are sorted
       by their filename in lexicographic order, regardless of which of the
       subdirectories they reside in. If multiple files specify the same
       option, the entry in the file with the lexicographically latest name
       takes precedence. It is recommended to prefix all filenames in those
       subdirectories with a two-digit number and a dash, to 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         top

       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. 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. 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. These domains are used as
           search suffixes when resolving single-label host names (domain
           names which contain no dot), in order to qualify them into
           fully-qualified domain names (FQDNs). Search domains are strictly
           processed in the order they are specified, until the name with
           the suffix appended is found. For compatibility reasons, if this
           setting is not specified, the search domains listed in
           /etc/resolv.conf are used instead, if that file exists and any
           domains are configured in it. This setting defaults to the empty
           list.

           Specified domain names may optionally be prefixed with "~". In
           this case they do not define a search path, but preferably direct
           DNS queries for the indicated domains to the DNS servers
           configured with the system DNS= setting (see above), in case
           additional, suitable per-link DNS servers are known. If no
           per-link DNS servers are known using the "~" syntax has no
           effect. Use the construct "~."  (which is composed of "~" to
           indicate a routing domain and "."  to indicate the DNS root
           domain that is the implied suffix of all DNS domains) to use the
           system DNS server defined with DNS= preferably for all domains.

       LLMNR=
           Takes a boolean argument or "resolve". Controls Link-Local
           Multicast Name Resolution support (RFC 4794[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 (RFC 6762[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. 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.

           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 off.

       Cache=
           Takes a boolean 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.

           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" (the
           default), 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 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.

SEE ALSO         top

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

NOTES         top

        1. RFC 4794
           https://tools.ietf.org/html/rfc4795

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

COLOPHON         top

       This page is part of the systemd (systemd system and service manager)
       project.  Information about the project can be found at 
       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a bug
       report for this manual page, see
       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.  This
       page was obtained from the project's upstream Git repository
       ⟨https://github.com/systemd/systemd.git⟩ on 2017-11-25.  (At that
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       improvements to the information in this COLOPHON (which is not part
       of the original manual page), send a mail to man-pages@man7.org

systemd 234                                                 RESOLVED.CONF(5)

Pages that refer to this page: dnssec-trust-anchors.d(5)systemd.network(5)systemd.directives(7)systemd.index(7)systemd-resolved.service(8)