Provided by: systemd_237-3ubuntu10_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 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/. Files in /etc/ are reserved for the local administrator,
       who may use this logic to override the configuration files installed by vendor packages.
       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 which of the subdirectories
       they reside in. 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. 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 4794[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 off.

       Cache=
           Takes a boolean 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.

           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.

SEE ALSO

       systemd(1), systemd-resolved.service(8), systemd-networkd.service(8), dnssec-trust-
       anchors.d(5), resolv.conf(4)

NOTES

        1. RFC 4794
           https://tools.ietf.org/html/rfc4795

        2. RFC 6762
           https://tools.ietf.org/html/rfc6762