Provided by: systemd-resolved_253.5-1ubuntu6.1_amd64 bug

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 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

       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 "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.

       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 "no".

       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".

       Cache=
           Takes a boolean or "no-negative" as argument. If "yes", 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" (the default), 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 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.

       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

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

NOTES

        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/