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/. 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. If multiple files specify the same option, the entry in the file with the lexicographically latest name takes 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. 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.
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" (the
default), 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 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
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