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 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 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. 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 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 by default for host-local DNS
           servers. See CacheFromLocalhost= for details.

       CacheFromLocalhost=
           Takes a boolean as argument. If "no" (the default), and
           response cames from host-local IP address (such as 127.0.0.1
           or ::1), the result wouldn't be cached in order to avoid
           potential 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

       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 2021-06-20.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2021-06-19.)  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 249                                             RESOLVED.CONF(5)

Pages that refer to this page: dnssec-trust-anchors.d(5)org.freedesktop.resolve1(5)systemd.network(5)systemd.directives(7)systemd.index(7)systemd-resolved.service(8)