RESOLVED.CONF(5)



RESOLVED.CONF(5)                 resolved.conf                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 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
       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 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 for it'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.

           Server Name Indication (SNI) can be used when opening a TLS
           connection. Entries in DNS= should be in format
           "address#server_name".

           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), the DNS stub
           resolver will read /etc/hosts, and try to resolve hosts or address
           by using the entries in the file before sending query to DNS
           servers.

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

systemd 245                                                   RESOLVED.CONF(5)

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