# resolved.conf

## NAME



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


## SYNOPSIS



               /etc/systemd/resolved.conf
               /run/systemd/resolved.conf
               /usr/local/lib/systemd/resolved.conf
               /usr/lib/systemd/resolved.conf
               /etc/systemd/resolved.conf.d/*.conf
               /run/systemd/resolved.conf.d/*.conf
               /usr/local/lib/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. The main configuration file is loaded from one of
           the listed directories in order of priority, only the first file
           found is used: /etc/systemd/, /run/systemd/,
           /usr/local/lib/systemd/ [1], /usr/lib/systemd/. The vendor version
           of the file contains commented out entries showing the defaults as
           a guide to the administrator. Local overrides can also be created
           by creating drop-ins, as described below. The main configuration
           file can also be edited for this purpose (or a copy in /etc/ if it
           is shipped under /usr/), however 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. This also defines a concept of
           drop-in priorities to allow OS vendors to ship drop-ins within a
           specific range lower than the range used by users. This should
           lower the risk of package drop-ins overriding accidentally
           drop-ins defined by users. It is recommended to use the range
           10-40 for drop-ins in /usr/ and the range 60-90 for drop-ins in
           /etc/ and /run/, to make sure that local and transient drop-ins
           take priority over drop-ins shipped by the OS vendor.
    
           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.
    
               Added in version 213.
    
           _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.
    
               Added in version 216.
    
           _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 "route-only domains".
               All domains listed here (_both search domains and route-only_
               _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, route-only domains have no effect.
    
               Use the construct "~."  (which is composed from "~" to
               indicate a route-only 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.
    
               See "Protocols and Routing" in systemd-resolved.service(8) for
               details of how search and route-only domains are used.
    
               Note that configuring the MulticastDNS domain "local" as
               search or routing domain has the effect of routing lookups for
               this domain to classic unicast DNS. This may be used to
               provide compatibility with legacy installations that use this
               domain in a unicast DNS context, against the IANA assignment
               of this domain to pure MulticastDNS purposes. Search and
               routing domains are a unicast DNS concept, they _cannot_ be used
               to resolve single-label lookups via MulticastDNS.
    
               Added in version 229.
    
           _LLMNR=_
               Takes a boolean argument or "resolve". Controls Link-Local
               Multicast Name Resolution support (**RFC 4795******[2]) 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 thatsystemd-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.
    
               Added in version 216.
    
           _MulticastDNS=_
               Takes a boolean argument or "resolve". Controls Multicast DNS
               support (**RFC 6762******[3]) 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 thatsystemd-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.
    
               Added in version 234.
    
           _DNSSEC=_
               Takes a boolean argument or "allow-downgrade".
    
               If set to 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 lookup 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".
    
               Added in version 229.
    
           _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 "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 "no".
    
               Added in version 239.
    
           _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.
    
               Added in version 231.
    
           _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 would not be cached in order to avoid
               potential duplicate local caching.
    
               Added in version 248.
    
           _DNSStubListener=_
               Takes a boolean argument or one of "udp" and "tcp". If "udp",
               a DNS stub resolver will listen for UDP requests on addresses
               127.0.0.53 and 127.0.0.54, port 53. If "tcp", the stub will
               listen for TCP requests on the same addresses and port. If
               "yes" (the default), the stub listens for both UDP and TCP
               requests. If "no", the stub listener is disabled.
    
               The DNS stub resolver on 127.0.0.53 provides the full feature
               set of the local resolver, which includes offering
               LLMNR/MulticastDNS resolution. The DNS stub resolver on
               127.0.0.54 provides a more limited resolver, that operates in
               "proxy" mode only, i.e. it will pass most DNS messages
               relatively unmodified to the current upstream DNS servers and
               back, but not try to process the messages locally, and hence
               does not validate DNSSEC, or offer up LLMNR/MulticastDNS. (It
               will translate to DNS-over-TLS communication if needed
               however.)
    
               Note that the DNS stub listener is turned off implicitly when
               its listening address and port are already in use.
    
               Added in version 232.
    
           _RefuseRecordTypes=_
               Takes a list of DNS record types separated by space. Specified
               record types in each query will be refused. This option can be
               specified multiple times. If an empty string is specified,
               then all previous assignments are cleared.
    
               Examples:
    
                   RefuseRecordTypes=AAAA SRV TXT
    
               Note that to refuse AAAA record (unlike SRV, TXT, etc.)
               completely, please also disable IPv6 stack in kernel with
               "sysctl -w net.ipv6.conf.all.disable_ipv6=1".
    
               Added in version 258.
    
           _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
    
               Added in version 247.
    
           _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.
    
               Added in version 240.
    
           _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******[4], and may create a
               privacy and security risk.
    
               Added in version 246._StaleRetentionSec=SECONDS_
               Takes a duration value, which determines the length of time
               DNS resource records can be retained in the cache beyond their
               Time To Live (TTL). This allows these records to be returned
               as stale records. By default, this value is set to zero,
               meaning that DNS resource records are not stored in the cache
               after their TTL expires.
    
               This is useful when a DNS server failure occurs or becomes
               unreachable. In such cases, systemd-resolved(8) continues to
               use the stale records to answer DNS queries, particularly when
               no valid response can be obtained from the upstream DNS
               servers. However, this does not apply to NXDOMAIN responses,
               as those are still perfectly valid responses. This feature
               enhances resilience against DNS infrastructure failures and
               outages.
    
               **systemd-resolved** always attempts to reach the upstream DNS
               servers first, before providing the client application with
               any stale data. If this feature is enabled, cache will not be
               flushed when changing servers.
    
               Added in version 254.
    


## NOTES



            1. 💣💥🧨💥💥💣 Please note that those configuration files must
               be available at all times. If /usr/local/ is a separate
               partition, it may not be available during early boot, and must
               not be used for configuration.
    
            2. RFC 4795
               https://tools.ietf.org/html/rfc4795
    
            3. RFC 6762
               https://tools.ietf.org/html/rfc6762
    
            4. IAB Statement
               https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/
    


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