resolved.conf(5) — Linux manual page

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/ or
       /usr/local/lib/systemd/*.conf.d/. 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 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 with the
       lexicographically latest name takes precedence. For options which
       accept a list of values, entries are collected as they occur in files
       sorted lexicographically.

       Files in /etc/ are reserved for the local administrator, who may use
       this logic to override the configuration files installed by vendor
       packages. 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 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 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 (RFC 4795[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 "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=
           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.

       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 IAB Statement[3], and may create a
           privacy and security risk.

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 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/

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 2020-07-14.  (At that
       time, the date of the most recent commit that was found in the repos‐
       itory was 2020-07-14.)  If you discover any rendering problems in
       this HTML version of the page, or you believe there is a better or
       more up-to-date source for the page, or you have corrections or
       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 246                                                 RESOLVED.CONF(5)

Pages that refer to this page: dnssec-trust-anchors.d(5)systemd.negative(5)systemd.network(5)systemd.positive(5)30-systemd-environment-d-generator(7)systemd.directives(7)systemd.index(7)systemd-resolved(8)systemd-resolved.service(8)